bluuebunny/crossref_metadata_2025 huggingface dataset:
Potential therapeutic misadministration due to inappropriate electron beam field shaping
Authors: Olch Arthur J. , Fallen Robert , Conrad Jill , Lavey Robert S.
Published: February 2007

Lead or cerrobend blocking strips are used to shape electron treatment fields when an appropriate custom insert is not available. For the Varian 2100C accelerator, the structural supports of the electron applicators impede the free placement of these field‐shaping strips on the open custom insert frame while placement at the top of the applicator is unimpeded. We have investigated the dosimetric ramifications of placing field shaping strips at the top level of the applicator for 6, 9, and 16 MeV electrons. Our results demonstrate as much as a 30% dose decrease and 2 cm penumbral increase when this is done compared to field shaping at the insert level. The magnitude of this dosimetric error qualifies as a therapeutic misadministration in many states depending on how many treatments are delivered in this manner. Based on this finding, we recommend that routine use of lead strip blocking be discouraged in favor of custom inserts due to the potential for inappropriate placement on some linear accelerators.

PACS number(s): 87.53.–j, 87.52.–g
Entrance dose measurements for in‐vivo diode dosimetry: Comparison of correction factors for two types of commercial silicon diode detectors
Authors: Zhu X. R.
Published: February 2007

Silicon diode dosimeters have been used routinely for in‐vivo dosimetry. Despite their popularity, an appropriate implementation of an in‐vivo dosimetry program using diode detectors remains a challenge for clinical physicists. One common approach is to relate the diode readout to the entrance dose, that is, dose to the reference depth of maximum dose such as for the field. Various correction factors are needed in order to properly infer the entrance dose from the diode readout, depending on field sizes, target‐to‐surface distances (TSD), and accessories (such as wedges and compensate filters). In some clinical practices, however, no correction factor is used. In this case, a diode‐dosimeter‐based in‐vivo dosimetry program may not serve the purpose effectively; that is, to provide an overall check of the dosimetry procedure. In this paper, we provide a formula to relate the diode readout to the entrance dose. Correction factors for TSD, field size, and wedges used in this formula are also clearly defined. Two types of commercial diode detectors, ISORAD (n‐type) and the newly available QED (p‐type) (Sun Nuclear Corporation), are studied. We compared correction factors for TSDs, field sizes, and wedges. Our results are consistent with the theory of radiation damage of silicon diodes. Radiation damage has been shown to be more serious for n‐type than for p‐type detectors. In general, both types of diode dosimeters require correction factors depending on beam energy, TSD, field size, and wedge. The magnitudes of corrections for QED (p‐type) diodes are smaller than ISORAD detectors.

PACS number(s): 87.66.–a, 87.52.–g
Applying the equivalent uniform dose formulation based on the linear‐quadratic model to inhomogeneous tumor dose distributions: Caution for analyzing and reporting
Authors: McGary John. E. , III Walter Grant , Woo S. Y.
Published: February 2007

We apply the concept of equivalent uniform dose (EUD) to our data set of model distributions and intensity modulated radiotherapy (IMRT) treatment plans as a method for analyzing large dose inhomogeneities within the tumor volume. For large dose nonuniformities, we find that the linear‐quadratic based EUD model is sensitive to the linear‐quadratic model parameters, α and β, making it necessary to consider EUD as a function of these parameters. This complicates the analysis for inhomogeneous dose distributions. EUD provides a biological estimate that requires interpretation and cannot be used as a single parameter for judging an inhomogeneous plan. We present heuristic examples to demonstrate the dose volume effect associated with EUD and the correlation to statistical parameters used for describing dose distributions. From these examples and patient plans, we discuss the risk of incorrectly applying EUD to IMRT patient plans.

PACS number(s): 87.53.Tf
A clinical evaluation of setup errors for a prostate immobilization system
Authors: McGary John E. , Grant Walter
Published: February 2007

A prostate treatment immobilization system was evaluated with respect to setup errors and efficiency for a specific treatment setup. Prostate patients were treated in the prone position with a rectal catheter using the NOMOS intensity modulated radiotherapy system. Immobilization and setup consisted of a Vac‐Lok™ bag (MED‐TEC, Orange City, IO) fitted within a registration carrier box where patients were aligned to the bag using skin marks along the lower leg. Daily setup errors were analyzed using lateral portal films, registration plates mounted to the carrier box, and the pubic symphasis as a bony reference. Two studies were conducted to evaluate setup technique. In the first study, patient setup required 3–5 minutes for patient positioning and the corresponding superior/inferior errors were found to have a standard deviation of 3.5 mm. In the second study, the technique standards were reduced to allow for faster setup times and, consequently, larger errors; setup times were 1–2 minutes and the mean and standard deviation errors were and 5 mm, respectively.

PACS number(s): 87.53.–j, 87.90.+y
Use of a radioactive check device for redundancy check of ionization chambers
Authors: Sidhu N. P. S. , Rouvas Alkis , Cadman Patrick
Published: February 2007

The use of a radioactive check device containing Strontium 90 was investigated to carry out the redundancy checks of Farmer‐type ionization chambers. It was not possible to meet the recommended tolerance limits of the redundancy checks without taking into account the angular response of the ionization chambers. The ionization chambers exhibited a maximum variation of 1% in the angular response in this check device. After accounting for the angular response the maximum variation of the short‐term repeatability was 0.14% with a standard deviation of . The long‐term constancy checked over a period of nine months was less than for measurements, taking into account the angular response of the ionization chambers. No significant effect of the background radiation levels was observed on the measurements.

PACS number(s): 87.53.–j, 87.52.–g
Fast verification of Gamma Knife™ treatment plans
Authors: Zhang Pengpeng , Dean David , Wu Q. Jackie , Sibata Claudio
Published: February 2007

The Leksell stereotactic Gamma Knife™ uses radiation from 201 sources that are focused to the center of a collimator helmet to deliver a high dose of radiation with minimal irradiation of proximal structures. This paper presents a method for fast verification of the irradiation time as calculated by the Leksell Gamma Knife™ treatment planning software GammaPlan®. To obtain the irradiation time for each shot in the treatment plan, one must first accurately calculate the tissue maximum ratio (TMR) for each of the individual 201 beams. The algorithm presented in this paper begins with the determination of the geometrical relationship between the Gamma Knife™ collimator helmet and the stereotactic frame. A group of reference points is measured to build a head model simulating the patient skull geometry. During radiosurgery, the isocenter of the collimator helmet is moved to the shot center. A group of spatial vectors describing the reference points at the skull surface is obtained by converting the Cartesian coordinates to Polar coordinates. For each individual beam, the three nearest reference vectors are found by ranking the relative angles. The depth that each beam penetrates the patient's skull to the isocenter is obtained via linear interpolation. The TMR for each beam then is compared with the TMR for the calibration setup, which is done using a spherical 8 cm radius phantom. This algorithm is applied to verify the treatment time calculated in GammaPlan® Version 5.2. The results are shown to agree with GammaPlan® within 3%.

PACS number(s): 87.52.–g, 87.66.–a
Comparison of commercially available three‐dimensional treatment planning algorithms for monitor unit calculations in the presence of heterogeneities
Authors: Butts Joseph R. , Foster Alvis E.
Published: February 2007

This study uses an anthropomorphic phantom and its computed tomography (CT) data set to evaluate monitor unit (MU) calculations using the CMS Focus Clarkson, the CMS Focus Multigrid Superposition Model, the CMS Focus FFT Convolution Model, and the ADAC Pinnacle3 Collapsed Cone Convolution Superposition Algorithms. Using heterogeneity corrections, a treatment plan and corresponding MU calculations were generated for several typical clinical situations. A diode detector, placed in an anthropomorphic phantom, was used to compare the treatment planning algorithms' predicted doses with measured data. Differences between diode measurements and the algorithms' calculations were within reasonable levels of acceptability as recommended by Van Dyk et al. [Int. J. Rad. Onc. Biol. Phys. 26, 261–273 (1993)], except for the CMS Clarkson algorithm, which predicted too few MU for delivery of the intended dose to chest wall fields.

PACS number(s): 87.53.Bn, 87.53.Dq
A simple method for electron energy constancy measurement
Authors: King R. Paul , Anderson R. Scott
Published: February 2007

A device is described for use in confirming the energy constancy of clinical electron beams. A wedge shaped absorber is placed over an ionization chamber leading to an energy dependent response. A measurement under the energy filter is divided by a measurement in air to correct for the inherent energy dependence of the chamber. A nearly linear response is demonstrated.

PACS number(s): 87.52.–g, 87.53.–j, 87.66.–a
Verification of tangential breast treatment dose calculations in a commercial 3D treatment planning system
Authors: Baird Christopher T. , Starkschall George , Liu H. Helen , Buchholz Thomas A. , Hogstrom Kenneth R.
Published: February 2007

The accuracy of the photon convolution/superposition dose algorithm employed in a commercial radiation treatment planning system was evaluated for conditions simulating tangential breast treatment. A breast phantom was fabricated from machineable wax and placed on the chest wall of an anthropomorphic phantom. Radiographic film was used to measure the dose distribution at the axial midplane of the breast phantom. Subsequently, thermoluminescent dosimeters (TLDs) were used to measure the dose at four points within the midplane to validate the accuracy of the film dosimetry. Film measurements were compared with calculations performed using the treatment planning system for four types of treatment: optimized wedged beams at 6 and 18 MV and two‐dimensional compensated beams at 6 and 18 MV. Both the film‐ and TLD‐measured doses had a precision of approximately 0.6%. The film‐measured doses were approximately 1.5% lower than the TLD‐measured doses, ranging from 0–3 % at 6 MV and 0.5–1 % at 18 MV. Such results placed a high level of confidence in the accuracy and precision of the film data. The measured and calculated doses agreed to within for both the film and TLD measurements throughout the midplane exclusive of areas not having charged particle equilibrium. Good agreement was not expected within these regions due to the limitations in both film dosimetry and the dose‐calculation algorithm. These results indicated that the treatment planning system calculates doses at the midplane with clinically acceptable accuracy in conditions simulating tangential breast treatment.

PACS number(s): 87.53.–j, 87.66.–a
Response to “Comment on ‘Independent corroboration of monitor unit calculations performed by a 3D computerized planning system’” [J. Appl. Clin. Med. Phys. 2, 102 (2001)]
Authors: Leszczynski Konrad W. , Dunscombe Peter B.
Published: February 2007

PACS number(s): 87.53.–j, 87.52.–g
Comparison of two planning systems for HDR brachytherapy gynecological application
Authors: Elhanafy Osman A. , Migahed Mostafa D. , Sakr Hanim A. , Ellithy Mostafa , Das Rupak K. , Odau Heath J. , Thomadsen Bruce R.
Published: February 2007

Purpose: This report compares the Nucletron NPS and PLATO planning system for patients treated for cervix cancer.

Materials and Methods: This study compares calculations generated using the older NPS (version 11.43) planning system and the more recent PLATO (version 14.1) system for two cases: 1) a single dwell position and 2) an actual patient application using a tandem and ovoid.

Results: For one dwell position: for NPS planning the dose for points along the source axis forward of the cable was 9.85% more than for symmetrically placed points in the cable direction. For PLATO, the same test gave rise to a difference of 10.2%. Comparing the two systems, NPS calculated doses for points in the forward direction 14% greater than those calculated by PLATO. The entry of points using the digitizer accounted for less than 1% of any difference. For the patient case: the dose difference between NPS and PLATO planning for all patient reference points entered from films ranged from 1 to 4%. The difference in dose between optimized and nonoptimized planning was approximately 0.5% for prescription points (points A), while for the bladder and rectum the differences were 6% and 20%, respectively with NPS, and with PLATO, 8% and 22%, respectively.

Conclusion: This study highlighted the effects of the differences in the calculational algorithm between the older and newer planning systems from Nucletron. While the differences were minimal on the perpendicular bisector of the source, along the axis they become considerable. In a practical gynecological case, these differences mostly affect the dose to the rectum, since that organ receives the greatest proportion of its dose from rays near the same axis. Overall, the PLATO system plan required about 2.5% less integrated reference air kerma than the NPS plan for the same dose to point A. For either planning system, optimization is crucial in decreasing dose to bladder and rectal points. © 2001 American College of Medical Physics. [DOI: 10.1120/1.1384528]

PACS number(s): 87.53.–j
The overshoot phenomenon in step‐and‐shoot IMRT delivery
Authors: Ezzell Gary A. , Chungbin Suzanne
Published: February 2007

The control loop in the Varian DMLC system (V4.8) requires msec to monitor and halt the irradiation of a segment, causing an “overshoot” effect: the segment ends on a fractional monitor unit larger than that planned. As a result, the actual MU delivered may differ from that planned. In general, for step‐and‐shoot treatments, the first segment receives more, the last receives less, and intermediate segments vary. The overshoot for each segment (ΔMU) is small, approximately 0.6 MU at 600 MU/min Our IMRT planning system (Corvus) produces plans often having more than 20% of the segments with less than 1 MU/segment. Such segments may be skipped if the ΔMU exceeds the segments’ planned MU. Furthermore, QA filming often requires reducing the total MU by a factor of 4–6, increasing the potential for dosimetric error. This study measured ΔMU over a range of MU/min and MU/segment. At MU/segment, the ΔMU was stable, corresponding to a delay of 62 msec. ΔMU became larger and more variable at MU/segment. The behavior was modeled in a computer program that predicted the change in delivered MU/segment and total change in delivered MU to each beamlet. Beams were analyzed for patients receiving 5 field prostate or 9 field head and neck treatments. At 400 MU/min, 28% and 16%, respectively, of the planned segments were skipped. For QA filming, up to 75% of the segments were skipped. The cumulative error averaged MU/beamlet, but individual beamlets had errors exceeding 200%. The effect is most significant for low dose regions. Recommendations are given for deciding when to treat or do QA studies with lower MU/min. In general, treatments are not significantly affected, but QA films taken at reduced MU may be improved if irradiated at lowered MU/min.

PACS number(s): 87.53.–j, 87.90.+y
Shielding assessment of a mobile electron accelerator for intraoperative radiotherapy
Authors: Daves Jodi L. , Mills Michael D.
Published: February 2007

A new approach to intraoperative radiotherapy makes use of a mobile electron linear accelerator delivering therapeutic radiation doses in an operating room suite. This unconventional technology has raised questions concerning protection for personnel and the necessity of shielding the adjoining areas. In this study, the leakage and scatter radiation from the mobile electron accelerator is measured and characterized in a series of spherical projections. An analysis is performed to determine the need for shielding or, alternatively, patient‐based load restrictions in the operating room. This investigation provides a resource to assess shielding and/or patient load restrictions for any facility performing intraoperative radiotherapy with a similar unit. The data presented indicates that the mobile electron accelerator may be operated in an area with little or no shielding under nominal patient load expectations.

PACS number(s): 87.52.–g, 87.53.–j
b matrix errors in echo planar diffusion tensor imaging
Authors: Boujraf Saïd , Luypaert Robert , Osteaux Michel
Published: February 2007

Diffusion‐weighted magnetic resonance imaging (DW‐MRI) is a recognized tool for early detection of infarction of the human brain. DW‐MRI uses the signal loss associated with the random thermal motion of water molecules in the presence of magnetic field gradients to derive parameters that reflect the translational mobility of the water molecules in tissues. If diffusion‐weighted images with different values of b matrix are acquired during one individual investigation, it is possible to calculate apparent diffusion coefficient maps that are the elements of the diffusion tensor. The diffusion tensor elements represent the apparent diffusion coefficient of protons of water molecules in each pixel in the corresponding sample. The relation between signal intensity in the diffusion‐weighted images, diffusion tensor, and b matrix is derived from the Bloch equations. Our goal is to establish the magnitude of the error made in the calculation of the elements of the diffusion tensor when the imaging gradients are ignored.

PACS number(s): 87.57. –s, 87.61.–c
Book Review
Authors: Stovall Marilyn
Published: February 2007

Liver Cancer Risk from Internally‐Deposited Radionuclides. NCRP Report No. 135., 94 pp. National Council on Radiation Protection and Measurement, Bethesda, MD, 2001. ISBN: 0‐929600‐68‐1. Price: $20.00.

PACS number(s): 87.52.Px, 87.58.Ji
Ion chamber measurements of transverse gamma knife beam profiles
Authors: Bank Morris I.
Published: February 2007

A microchamber, PTW Pinpoint 31006, was used to measure transverse beam profiles for an Elekta Gamma Knife, Model B, and compared with profiles measured with film dosimetry. The microchamber sensitive volume has a diameter of 2 mm, which is smaller than the gamma knife beams, and a length of 5 mm. The chamber was mounted in a custom cassette in a spherical plastic phantom, supplied by Elekta, and oriented in a sagittal plane with the 2‐mm dimension at right angles to the transverse plane. The phantom was manually moved across the beam, using the gamma knife x‐coordinate trunnions, to measure the profiles. Profiles were also measured with V‐film placed in a cassette mounted in the spherical plastic phantom. The films were scanned with a Scanditronix film scanner and converted to dose with a density to dose calibration curve. The results were superimposed for comparison. The beam width at the 50% intensity was measured from the film profiles to give the dimensions of the beams in the orthagonal planes. The ion chamber measurements are compared with the film results for the transverse, x profiles. Good agreement between the film and ion chamber transverse profiles is observed.

PACS number(s): 87.66.–a, 87.53.–j, 87.53.Ly
Verification of the accuracy of a photon dose‐calculation algorithm
Authors: Gifford Kent A. , Followill David S. , Liu H. Helen , Starkschall George
Published: February 2007

An extensive set of measured data was developed for the purpose of verifying the accuracy of a photon dose‐calculation algorithm. Dose distributions from a linear accelerator were measured using an ion chamber in a water phantom and thermoluminescent dosimeters in a heterogeneous anthropomorphic phantom. Test cases included square fields, rectangular fields, fields having different source‐to‐surface distances, wedged fields, irregular fields, obliquely incident fields, asymmetrically collimated fields with wedges, multileaf collimator‐shaped fields, and two heterogeneous density cases. The data set was used to validate the photon dose‐calculation algorithm in a commercial radiation treatment planning system. The treatment planning system calculated photon doses to within the American College of Medical Physics (AAPM) Task Group 53 (TG‐53) criteria for 99% of points in the buildup region, 90% of points in the inner region, 88% of points in the outer region, and 93% of points in the penumbra. For the heterogeneous phantoms, calculations agreed with actual measurements to within . The monitor unit tests revealed that the 18‐MV open square fields, oblique incidence, oblique incidence with wedge, and mantle field test cases did not meet the TG‐53 criteria but were within of measurements. It was concluded that (i) the photon dose calculation algorithm used by the treatment planning system did not meet the TG‐53 criteria 100% of the time; (ii) some of the TG‐53 criteria may need to be modified, and (iii) the generally stated goal of accuracy in dose delivery of within 5% cannot be met in all situations using this beam model in the treatment planning system.

PACS number(s): 87.53.–j, 87.66.–a
Dosimetric verification of inverse planned step and shoot multileaf collimator fields from a commercial treatment planning system
Authors: MacKenzie M. A. , Lachaine M. , Murray B. , Fallone B. G. , Robinson D. , Field G. C.
Published: February 2007

An inverse treatment planning (ITP) module on a commercial treatment planning system (TPS) (Helax AB, Uppsala, Sweden) is being used for an in‐house clinical trial for treatment of nasopharyngeal cancer with contralateral parotid sparing. Intensity modulated radiation therapy (IMRT) fields are delivered by step and shoot multileaf collimator (MLC) with a DMLC enabled Varian 2300 CD (Varian Associates, Palo Alto, CA). A series of testing procedures have been devised to quantify the modeling and delivery accuracy of routine clinical inverse planned IMRT using Helax TMS and the Varian step and shoot MLC delivery option. Testing was done on specific aspects of the TPS modeling germane to DMLC. Measured relative dose factors (head scatter plus phantom scatter) for small MLC fields, normalized to a non‐MLC field, were found to differ by from the TPS values for the smallest of the fields tested. Relative distributions for small off axis fields were found to be in good agreement. A process for the routine clinical verification of IMRT fields has been implemented. Each IMRT field in an inverse plan is imported into a flat water tank plan and a “beam's eye view” (BEV) dose distribution is generated. This is compared to the corresponding measured BEV dose distribution. The IMRT verification process has also been performed using an anthropomorphic phantom. Large clinical fields (i.e., greater than 14.5 cm in the leaf direction) caused difficulties due to a vendor specific machine restriction, and several techniques for dealing with these were examined. These techniques were (i) the use of static stepping of closed junctions, (ii) the use of two separate IMRT fields for a given gantry angle, and (iii) restricting the overall maximum field size used. The overall process has allowed implementation of an in‐house protocol for IMRT use on an initial clinical site. Results of the verification measurements for the first ten patients treated at this center reveal an average maximum dose per IMRT field delivered of 71.0 cGy, with a mean local deviation from the planned dose of – 1.2 cGy, and a standard deviation of 2.4 cGy.

PACS number(s): 87.53.Dq, 87.53.Tf
Verification of prostate treatment setup using computed radiography for portal imaging
Authors: Whittington Richard , Bloch Peter , Hutchinson Della , Bjarngard Bengt E.
Published: February 2007

A non‐film‐based system was used to obtain high quality portal film images. Digital portal images were obtained with a computed radiography (CR) system, in which the film is replaced with a photostimulable phosphor plate. Digital processing of portal images enhanced the display contrast using regional histogram equalization. The images were compared to images on radiographic film, exposed in the same cassette. The contrast‐enhanced CR images of prostate treatment fields facilitated identification of the entire contour of the ischium, the location of the pubic symphysis, and the ischial tuberosity to determine the anterior and inferior locations of the prostate and bladder. Identifying the coccyx on the processed portal images permits the physician to locate accurately the posterior wall of the rectum. In each case the quality of the CR image was judged by the clinician to be superior to conventional portal film. The identification of these anatomical structures on the portal images is clinically important for verifying 3D conformal therapy of the prostate. With the same CR system one may acquire digital treatment portal and simulation images. This provides a foundation for a picture archival communication system for radiation oncology. Existing software can be used to register these digital portal and simulation images to facilitate verification of treatment setup.

PACS number(s): 87.53.–j, 87.57. –s
Video Review
Authors: Almond Peter R.
Published: February 2007

Is radiation as dangerous as they say? John Cameron, Medical Physics Publishing, Madison, WI 2002.

PACS number(s): 87.50.Gi, 87.52.Px, 87.52.Tr
A quality control program for MR‐guided focused ultrasound ablation therapy
Authors: Wu Tao , Felmlee Joel P.
Published: February 2007

In this study, we propose a quality control program for MR‐guided focused ultrasound (FUS) ablation treatment to assess FUS beam positioning accuracy, FUS power delivery accuracy, MR imaging quality, and FUS ablation system safety. A total of 353 sonication points in Lucite cards were measured, the average placement errors were in the SI direction and in the LR direction. Temperature elevation was calculated from MR phase difference images and the measured water proton chemical shift (WPCS) temperature coefficient. WPCS temperature calibration for phantoms yielded a temperature coefficient of 0.011 ppm/°C. Sixteen experiments were conducted using six different phantoms to test the reliability of FUS power delivery. SNR and RF power calculated from phantom images were analyzed and stored at the MR console. A computer program was developed to integrate the system power delivery and the MR image quality control into one automated process. In the clinical trial at our institution, we expect this quality control program to be carried out before each patient treatment. If measured quality control values exceeds or below the preset values, a system service and retest should be conducted before the treatment.

PACS number(s): 87.61.–c, 87.54.–n
Examination of Jeltrate®Plus as a tissue equivalent bolus material
Authors: Babic Steven , Kerr Andrew T. , Westerland Mary , Gooding Jim , Schreiner L. John
Published: February 2007

A product available commercially for making dental impressions, Jeltrate®Plus, was evaluated as a tissue equivalent bolus material. Jeltrate®Plus was found to be tissue equivalent in 6 and 15 MV photon energy beams and 6, 12, and 20 MeV electron energy beams. As a first step, different preparations for making the bolus material were investigated and an optimal mixture was determined to be two parts Jeltrate®Plus powder to three parts water by weight. A suitable method for storing the material was found to be in a water filled plastic container. Since the product is fairly inexpensive and is easily and quickly made and moulded into different shapes, it is an excellent bolus material to use when treating irregular patient contours.

PACS number(s): 87.53.–j, 87.66.–a
On the selection of optimization parameters for an inverse treatment planning replacement of a forward planning technique for prostate cancer
Authors: Hristov Dimitre H. , Moftah Belal A. , Charrois Colette , Parker William , Souhami Luis , Podgorsak Ervin B.
Published: February 2007

The influence of organ volume sampling, lateral scatter inclusion, and the selection of objectives and constraints on the inverse treatment planning process with a commercial treatment planning system is investigated and suitable parameters are identified for an inverse treatment planning replacement of a clinical forward planning technique for prostate cancer. For the beam geometries of the forward technique, a variable set of parameters is used for the calculation of dose from pencil beams. An optimal set is identified after the evaluation of optimized plans that correspond to different sets of pencil beam parameters. This set along with a single, optimized set of objectives and constraints is used to perform inverse planning on ten randomly selected patients. The acceptability of the resulting plans is verified by comparisons to the clinical ones calculated with the forward techniques. For the particular commercial treatment planning system, the default values of the pencil beam parameters are found adequate for inverse treatment planning. For all ten patients, the optimized, single set of objectives and constraints results in plans with target coverage comparable to that of the forward plans. Furthermore inverse treatment planning reduces the overall mean rectal and bladder doses by 4.8% and 5.8% of the prescription dose respectively. The study indicates that (i) inverse treatment planning results depend implicitly on the sampling of the dose distribution, (ii) inverse treatment planning results depend on the method used by the dose calculation model to account for scatter, and (iii) for certain sites, a single set of optimization parameters can be used for all patient plans.

PACS number(s): 87.53.–j, 87.90.+y
Calibration of a liquid I‐125 source in a syringe
Authors: Gillin Michael T.
Published: February 2007

The calibration of a liquid I‐125 source in an older standard dose calibration system is presented. The calibration factor agrees well with factors established by the NIST for newer dose calibration systems. The determination of the source activity is necessary to accurately calculate the time required to deliver the prescribed dose.

PACS number(s): 87.53.Dq, 87.53Jw
Optimized planning for intraoperative planar permanent‐seed implant
Authors: Fung Albert Y. C. , Amols Howard I. , Zaider Marco
Published: February 2007

We describe a fast, PC‐based optimization planning system for a planar permanent‐seed implant. Sites where this system is applicable include brain, lung, and head and neck. The system described here allowsplacing ribbons of different strengths and of different lengths along and across the implant plane. The program takes full advantage of the availability of different source strengths in inventory, and attempts to find configurations of ribbons that result in optimal dose uniformity over the prescription plane. Dosimetry is based on the AAPM TG 43 Report [R. Nath et al., Med. Phys. 22, 209–234 (1995)]. Compared with TG 43 parameters, the classical tables underestimate the I‐125 source strengths needed by 40%. The use of several source strengths improves the plan. Typical optimization yields dose uniformity of 10%, and computing times are within 2–3 min No further enhancement is obtained if ribbons are placed in a grid pattern as opposed to the (simpler) arrangement along parallel lines. Nor is it valuable to have variable ribbon lengths. For an I‐125 implant the optimization system described here is a practical alternative to the (strictly speaking inapplicable) classical systems. It calculates correctly the total source strengths, and ‐ most notably ‐ generates plans with optimal dose uniformity. The fast computing time is well suited for planning during surgery in the operating room.

PACS number(s): 87.53Jw, 87.53.Tf
A parameter optimization algorithm for intensity‐modulated radiotherapy prostate treatment planning
Authors: Barbiere J. , Chan M. F. , Mechalakos J. , Cann D. , Schupak K. , Burman C.
Published: February 2007

An iterative algorithm has been developed to analytically determine patient specific input parameters for intensity‐modulated radiotherapy prostate treatment planning. The algorithm starts with a generic set of inverse planning parameters that include dose and volume constraints for the target and surrounding critical structures. The overlap region between the target volume and the rectum is used to determine the optimized target volume coverage goal. Sequential iterations are performed to vary the numerous parameters individually or in sets while other parameters remain fixed. A coarse grid search is first used to avoid convergence on a local maximum. Linear interpolation is then used to define a region for a fine grid search. Selected parameters are also tested for possible improvements in target coverage. In several representative test cases investigated the coverage of the planning target volume improved with the use of the algorithm while still meeting the clinical acceptability criteria for critical structures. The algorithm avoids time‐consuming random trial and error variations that are often associated with difficult cases and also eliminates lengthy user learning curves. The methodology described in this paper can be applied to any treatment planning system that requires the user to select the input optimization parameters.

PACS number(s): 87.53.–j, 87.90.+y
Monitor unit calculation for tangential breast treatments: Verification in an anthropomorphic phantom
Authors: Howlett Stephen J. , Kron Tomas
Published: February 2007

This paper presents an anthropomorphic phantom study of dose delivered to a specific point during tangential breast irradiation to verify monitor unit calculations. Measurements were made using a 0.6 cc Farmer type cylindrical ionization chamber in the phantom and compared to calculations made on a three‐dimensional radiotherapy treatment planning system using single digitized contour through to multi slice CT data. A large breast phantom was used for a single field size with a combination of open and wedged fields for three different energies (4, 6, and 18 MV). Solid flat phantom measurements were also performed for comparison. Results showed a lower calculated dose than the dose measured for a fixed number of monitor units where the variations were within a range of 0.8% to 4.5%. Differences were larger for the anthropomorphic phantom than the flat phantom. We conclude that little accuracy is gained from CT based monitor unit calculations compared to those based on digitised contours for this breast treatment but that the dose distributions will be affected. This type of test is recommended as one of a large set, in the commissioning and testing procedures for treatment planning systems.

PACS number(s): 87.52.Df, 87.53.Dq, 87.53.Xd
Radiographic techniques in screen‐film mammography
Authors: LaVoy Thomas R. , Huda Walter , Ogden Kent M.
Published: February 2007

The objectives of this study were to document imaging physics parameters associated with mammography physics surveys, and investigate how the choice of tube potential affects average glandular dose (AGD) and x‐ray exposure time. Data from 60 mammography units were obtained pertaining to representative values of mAs, exposure time, half value layer, AGD and film density when acquiring phantom images. The survey of clinical systems showed that for a normal sized breast as represented by the mammography accreditation phantom, 60% of these units were operated at 25 kVp, and 33% at 26 kVp. Median exposure times were 1.14 s at 25 kVp and 0.73 s at 26 kVp. The median AGD was 1.62 mGy at 25 kVp and 1.51 mGy at 26 kVp. As expected, the choice of x‐ray tube potential did not significantly affect the median film density value of 1.5. Five clinical systems, all from different vendors, had measurements performed of the AGD and x‐ray exposure time as a function of x‐ray tube potential at a constant film density. For a typical clinical x‐ray unit, increasing the x‐ray tube potential from 25 to 28 kVp reduced the exposure time by 50%, and reduced the AGD by 26%.

PACS number(s): 87.57.–s, 87.59.Ek, 87.62.+n
A comparative evaluation of loading times and exposures for permanent prostate brachytherapy
Authors: Bice W. S. , Walker E. S. , Gearty S. , Walker A. V. , Marbach J. R. , Prestidge B. R.
Published: February 2007

The loading of needles for loose seed implantation of the prostate gland results requires a significant amount of effort and some radiation exposure to members of the medical staff. This study was performed to quantify the time spent and exposure levels associated with implant preparation, as well as to investigate any improvement in the time or exposure burden due to the introduction of a new loading device. The movements and radiation exposures for two single, highly experienced dosimetrists were monitored for ten conventionally loaded iodine implant cases. These same cases were reloaded with dummy sources using the sleeved system to determine time savings, if any. Two of these ten cases were then loaded with live sources using the sleeved system to determine relative exposure to the loading staff between the two methods. The results were then analyzed to generate per‐seed and per‐needle loading time and exposure burdens. Formulas are presented that may be used to determine the average time to load implants and the resultant staff exposure, both with the conventional technique and with the sleeved method. On the average, it takes an experienced loader 48 min to prepare an implant for the operating room, receiving a hand dose of about 10 mrem and a whole body dose of about 1 mrem. The sleeved system reduced these values by at least half. The time and exposure burden associated with the preparation of iodine loose seed implants has been characterized. The use of the sleeved needles resulted in significant time and exposure reductions for the medical staff.

PACS number(s): 87.53Jw, 87.53.Xd
Absorbed dose calculations in a brachytherapy pelvic phantom using the Monte Carlo method
Authors: Rodríguez Miguel L. , deAlmeida Carlos E.
Published: February 2007

Monte Carlo calculations of the absorbed dose at various points of a brachytherapy anthropomorphic phantom are presented. The phantom walls and internal structures are made of polymethylmethacrylate and its external shape was taken from a female Alderson phantom. A complete Fletcher‐Green type applicator with the uterine tandem was fixed at the bottom of the phantom reproducing a typical geometrical configuration as that attained in a gynecological brachytherapy treatment. The dose rate produced by an array of five 137Cs CDC‐J type sources placed in the applicator colpostats and the uterine tandem was evaluated by Monte Carlo simulations using the code penelope at three points: point A, the rectum, and the bladder. The influence of the applicator in the dose rate was evaluated by comparing Monte Carlo simulations of the sources alone and the sources inserted in the applicator. Differences up to 56% in the dose may be observed for the two cases in the planes including the rectum and bladder. The results show a reduction of the dose of 15.6%, 14.0%, and 5.6% in the rectum, bladder, and point A respectively, when the applicator wall and shieldings are considered.

PACS number(s): 87.53Jw, 87.53.Wz, 87.53.Vb, 87.66.Xa
Comparison of monitor unit calculations performed with a 3D computerized planning system and independent “hand” calculations: Results of three years clinical experience
Authors: Chan Jackson , Russell David , Peters Victor G. , Farrell Thomas J.
Published: February 2007

A comparison of the monitor unit calculations of a commercial 3D computerized treatment planning system (TPS) with “hand” calculations from lookup tables was made for a large number of clinical cases (greater than 13 500 treatment fields). Differences were analyzed by treatment site for prostate, rectum, cranium, and breast. The 3D TPS monitor unit calculation was systematically higher than the “hand” calculation by an amount that depended on the complexity of the treatment geometry. For simple geometries the mean difference was 1% and was as high as 3% for more complicated geometries. The higher value was attributed to an accumulation of differences introduced by multiple factors in the monitor unit calculation. Careful attention to factors such as patient contour could reduce the mean difference. “Hand” calculations were shown to be an accurate and useful tool for verification of TPS monitor unit calculations.

PACS number(s): 87.53.‐j, 87.52.‐g
Method to plan, administer, and verify supine craniospinal irradiation
Authors: Michalski Jeff M. , Klein Eric E. , Gerber Russell
Published: February 2007

Craniospinal irradiation remains an important technique in the management of malignancies of the central nervous system. It is technically demanding, with potential for treatment field overlap or gaps to yield unacceptable dosimetric heterogeneity. A method to accurately simulate and verify the three‐field junction is described. We use a comfortable supine position to minimize patient movement. The supine position provides airway access by anesthesiology in patients requiring sedation or anesthesia. Virtual simulation is performed with a dedicated computed tomography (CT) simulator. Multiplanar sagittal and coronal CT reconstructions allow visual confirmation of three‐field matching at the cervical region. The placement of isocenters for each field, table position, and collimator angles are determined by calculation of field sizes accommodating for beam divergence. At treatment, exact matching of the three fields is assured using the record and verify confirmation of beam collimator settings and rotation, digital couch readouts, and gantry parameters. Mini‐verification silver halide (Kodak XV) films are placed behind the patient's neck and are exposed by all treatment fields (posterior flash from the lateral cranial fields and entrance from the PA spine field). These films assess field placement accuracy at the junction of these three fields. Finally, placement of radio‐opaque markers at the junction is visualized in each clinical portal radiograph. Patients readily accept the supine position as their treatment setup is eased. Field placement using digital couch settings is efficient and accurate. Daily mini‐verification films are simple, inexpensive, and allow verification of each treatment field matching. Field placement errors of greater than 1 mm can be readily identified and corrected at subsequent treatment sessions. Virtual simulation and direct junction verification with mini‐verification films allow for simple and quantitative evaluation of the junction associated with the three‐field craniospinal irradiation technique. The supine patient position does not present any difficulties in field matching or verification.

PACS number(s): 87.53.–j, 87.53.–j
Technical Note: A new TLD‐phantom measurement system for determining dose distribution levels in the right and left breast from spiral CT chest imaging
Authors: Hall Jeffery L. , Navarrete Jorge L. , Surprenant Edgar , Sklansky Jack , Eisenman Jack I.
Published: February 2007

Two specially designed plastic/aluminum phantoms positioned thermoluminescent dosimeters (TLDs) at the right and left breast location of an anthrophomorophic chest torso. Imaging was performed on a spiral CT for a volume of the chest phantom through the breast area for a noncontiguous (pitch 1.5) helical chest scan. Conventional pencil beam ionization chamber measurements were made at the same operating parameters. The doses ranged from approximately 1 to 3 cGy. For both breast phantoms, the doses were highest for the medial inner quadrants near the mediastinum. The doses were lowest for the outer quadrants (lateral aspects) of both breasts.

PACS number(s): 87.66.–a, 87.52.–g
Surface and buildup dose characteristics for 6, 10, and 18 MV photons from an Elekta Precise linear accelerator
Authors: Klein Eric E. , Esthappan Jacqueline , Li Zuofeng
Published: February 2007

Understanding head scatter characteristics of photon beams is vital to properly commission treatment planning (TP) algorithms. Simultaneously, having definitive surface and buildup region dosimetry is important to optimize bolus. The Elekta Precise linacs have unique beam flattening filter configurations for each photon beam (6, 10, and 18 MV) in terms of material and location. We performed a comprehensive set of surface and buildup dose measurements with a thin window parallel‐plate (PP) chamber to examine effects of field size (FS), source‐to‐skin distance (SSD), and attenuating media. Relative ionization data were converted to fractional depth dose (FDD) after correcting for bias effects and using the Gerbi method to account for chamber characteristics. Data were compared with a similar vintage Varian linac. At short SSDs the surface and buildup dose characteristics were similar to published data for Varian and Elekta accelerators. The FDD at surface for 6, 10, and 18 MV photons was 0.171, 0.159, and 0.199, respectively, for a , 100 cm SSD field. A blocking tray increased to 0.200, 0.200, and 0.256, while the universal wedge decreased to 0.107, 0.124, and 0.176. increased linearly with FS . decreased exponentially for 10 and 18 MV with increasing SSD. However, the 6 MV actually increased slightly with increasing SSD. This is likely due to the unique distal flattening filter for 6 MV The measured buildup curves have been used to optimize TP calculations and guide bolus decisions. Overall the and buildup doses were very similar to published data. Of interest were the relatively low 10 MV surface doses, and the 6 MV 's dependence on SSD. © 2003 American College of Medical Physics.

PACS number(s): 87.53.–j, 87.66.–a
Patient specific quality assurance for the delivery of intensity modulated radiotherapy
Authors: Agazaryan Nzhde , Solberg Timothy D. , DeMarco John J.
Published: February 2007

A patient specific quality assurance program has been developed to facilitate the clinical implementation of intensity modulated radiotherapy (IMRT) delivered using a micro‐multileaf collimator. The methodology includes several dosimetric tasks that are performed prior to the treatment of each patient. Film dosimetry is performed for each individual field and for the multifield composite plan. Individual field measurements are performed at a depth of 5 cm in a water equivalent slab phantom; export of dose calculations from the treatment planning system is similarly specified. For the composite distribution, parameters from the patient plan are applied to an IMRT phantom, and film is exposed in an axial orientation. Distributions are compared with the aid of software developed for the specific tasks. The measured and calculated dose distributions can be superimposed and positioned graphically using move, rotate, and mirror tools, as well as by specifying isocenter coordinates and using fiducial marks. Horizontal and vertical profiles are available for analysis. Dose difference, distance‐to‐agreement, and γ index, the minimum scaled multidimensional distance between a measurement and a calculation point determined in combined dose and physical distance space, are calculated along a specified isodose line and displayed. γ provides an excellent measure of disagreement between measurement and calculation for complex intensity distributions. We specify 3% dose difference and 3 mm distance as our scaling acceptability criteria. Absolute dosimetry for each composite plan is performed using an ionization chamber. To date, excellent agreement between measurements and calculations has been observed. © 2003 American College of Medical Physics.

PACS number(s): 87.53.–j, 87.66.–a