bluuebunny/crossref_metadata_2025 huggingface dataset:
Dosimetric advantage of using 6 MV over 15 MV photons in conformal therapy of lung cancer: Monte Carlo studies in patient geometries
Authors: Wang Lu , Yorke Ellen , Desobry Gregory , Chui Chen‐Shou
Published: February 2007

Many lung cancer patients who undergo radiation therapy are treated with higher energy photons (15‐18 MV) to obtain deeper penetration and better dose uniformity. However, the longer range of the higher energy recoil electrons in the low‐density medium may cause lateral electronic disequilibrium and degrade the target coverage. To compare the dose homogeneity achieved with lower versus higher energy photon beams, we performed a dosimetric study of 6 and 15 MV three‐dimensional (3D) conformal treatment plans for lung cancer using an accurate, patient‐specific dose‐calculation method based on a Monte Carlo technique. A 6 and 15 MV 3D conformal treatment plan was generated for each of two patients with target volumes exceeding 200 cm3on an in‐house treatment planning system in routine clinical use. Each plan employed four conformally shaped photon beams. Each dose distribution was recalculated with the Monte Carlo method, utilizing the same beam geometry and patient‐specific computed tomography (CT) images. Treatment plans using the two energies were compared in terms of their isodose distributions and dose‐volume histograms (DVHs). The 15 MV dose distributions and DVHs generated by the clinical treatment planning calculations were as good as, or slightly better than, those generated for 6 MV beams. However, the Monte Carlo dose calculation predicted increased penumbra width with increased photon energy resulting in decreased lateral dose homogeneity for the 15 MV plans. Monte Carlo calculations showed that all target coverage indicators were significantly worse for 15 MV than for 6 MV; particularly the portion of the planning target volume (PTV) receiving at least 95% of the prescription dosedropped dramatically for the 15 MV plan in comparison to the 6 MV Spinal cord and lung doses were clinically equivalent for the two energies. In treatment planning of tumors that abut lung tissue, lower energy (6 MV) photon beams should be preferred over higher energies (15–18 MV) because of the significant loss of lateral dose equilibrium for high‐energy beams in the low‐density medium. Any gains in radial dose uniformity across steep density gradients for higher energy beams must be weighed carefully against the lateral beam degradation due to penumbra widening.

PACS number(s): 87.90.+y, 87.52.–g
Book Review
Authors: Lee Steve P.
Published: February 2007

Applied Radiobiology and Bioeffect Planning. David R. Wigg, Medical Publishing, Madison, WI, 2001.

PACS number(s): 87.50.–a
CT gel dosimetry technique: Comparison of a planned and measured 3D stereotactic dose volume
Authors: Audet C. , Hilts M. , Jirasek A. , Duzenli C.
Published: February 2007

This study presents a 3D dose mapping of complex dose distributions using an x‐ray computed tomography (CT) polymer gel dosimetry technique. Two polyacrylamide gels (PAGs) of identical composition were irradiated with the same four arc stereotactic treatment to maximum doses of 15 Gy (PAG1) and 8 Gy (PAG2). The PAGs were CT imaged using a previously defined protocol that involves image averaging and background subtraction to improve image quality. For comparison with the planned isodose distribution, the PAG images were converted to relative dose maps using a CT number‐dose calibration curve or simple division. The PAG images were then co‐registered with the planning CT images in the BrainLab® treatment planning software which automatically provides reconstructed sagittal and coronal images for 3D evaluation of measured and planned dose. The hypo‐intense high dose region in both sets of gel images agreed with the planned 80% isodose contour and was shifted by up to 1.5 and 3.0 mm in the axial and reconstructed planes, respectively. This demonstrates the ability of the CT gel technique to accurately localize the high dose region produced by the stereotactic treatment. The resulting agreement of the measured relative dose volume for PAG1 was within 3.0 mm for the 50% and 80% isodose surfaces. However, the dose contrast was too low in PAG2 to allow for accurate definition of measured relative dose surfaces. Thus, a PAG should be irradiated to higher doses if quantitative relative dose information is required. Unfortunately, this implies use of an additional PAG and its CT number dose response since doses greater than 8–10 Gy fall outside the linear regions of the response.

PACS number(s): 87.53.–j, 87.57.–s, 87.59.Fm
Dosimetric parameters of three new solid core I‐125 brachytherapy sources
Authors: Solberg Timothy D. , DeMarco John J. , Hugo Geoffrey , Wallace Robert E.
Published: February 2007

Monte Carlo calculations and TLD measurements have been performed for the purpose of characterizing dosimetric properties of new commercially available brachytherapy sources. All sources tested consisted of a solid core, upon which a thin layer of has been adsorbed, encased within a titanium housing. The PharmaSeed BT‐125 source manufactured by Syncor is available in silver or palladium core configurations while the ADVANTAGE source from IsoAid has silver only. Dosimetric properties, including the dose rate constant, radial dose function, and anisotropy characteristics were determined according to the TG‐43 protocol. Additionally, the geometry function was calculated exactly using Monte Carlo and compared with both the point and line source approximations. The 1999 NIST standard was followed in determining air kerma strength. Dose rate constants were calculated to be , and for the PharmaSeed BT‐125‐1, BT‐125‐2, and ADVANTAGE sources, respectively. TLD measurements were in excellent agreement with Monte Carlo calculations. Radial dose function, g(r), calculated to a distance of 10 cm, and anisotropy function F(r, θ), calculated for radii from 0.5 to 7.0 cm, were similar among all source configurations. Anisotropy constants, , were calculated to be 0.941, 0.944, and 0.960 for the three sources, respectively. All dosimetric parameters were found to be in close agreement with previously published data for similar source configurations. The MCNP Monte Carlo code appears to be ideally suited to low energy dosimetry applications.

PACS number(s): 87.53.–j
Quasi‐independent monitor unit calculation for intensity modulated sequential tomotherapy
Authors: Tsai Jen‐San , Engler Mark J. , Liu James
Published: February 2007

The number of linac monitor units (MU) from intensity modulated sequential tomotherapy (IMST) is substantially larger than the MU delivered in conventional radiation therapy, and the relation between MU and dose is obscure due to complicated variation of the beam intensities. The purpose of this work was to develop a practical method of verifying the MU and dose from IMST so that the MU of each arced beam could be double‐checked for accuracy. MU calculations for 41 arced beams from 14 IMST patients were performed using the variables of vane open fraction time, field size, target depth output factor, TMR, and derived intensity distribution. Discrepancy between planned and checked MU was quantified as percent. All 41 discrepancies were clustered between to , illustrated in a Gaussian‐shaped histogram centered at standard deviation indicating the present MU calculations are in agreement with the planned expectations. To confirm the correctness of the present calculated MUs of the IMST plans, eight of the calculated IMST plans are performed dose verifications using their hybrid plans, which are created by transporting patient's IMST plan beams onto a spherical polystyrene Phantom for dose distribution within the Phantom. The dose was measured with a 0.07 cc ionization chamber inserted in the spherical Phantom during the hybrid plan irradiation. Average discrepancy between planned and measured doses was found to be with single standard deviation uncertainty. The spread of the discrepancies of present calculated MUs relative to their planned ones are attributed to uncertainties of effective field size, effective planned dose corresponding to each arc, and inaccuracy of quantification of scattered dose from adjacent arced beams. Overall, the present calculation of MUs is consistent with what derived from treatment plans. Since the MUs are verified by actual dose measurements, therefore the present MU calculation technique is considered adequate for double‐checking planned IMST MUs.

PACS number(s): 87.53.–j, 87.66.–a
Verification of dynamic and segmental IMRT delivery by dynamic log file analysis
Authors: Litzenberg Dale W. , Moran Jean M. , Fraass Benedick A.
Published: February 2007

A program has been developed to evaluate the delivered fluence of step‐and‐shoot segmental and sliding window dynamic multileaf collimator (MLC) fields. To automate these checks, a number of tools have been developed using data available from the dynamic log files that can be created each time a dynamic delivery occurs. Experiments were performed with a Varian 2100EX with a 120 leaf MLC equipped with dynamic capabilities. A dynamic leaf sequence is delivered and measured with film or an amorphous silicon imager. After delivery, the dynamic log file is written by the accelerator control system. The file reports the expected and actual position for each leaf and the dose fraction every 0.055 seconds. Leaf trajectories are calculated from this data and expected and actual fluence images are created from the difference of opposing leaf trajectories. These images can be compared with the expected delivery, measurements, and calculations of fluence. Tools have been developed to investigate other aspects of the delivery, such as specific leaf errors, beam hold‐off flags sent by the control system to the MLC, and gap widths. This program is part of a semi‐automated quality assurance (QA) system for pretreatment fluence verification and daily treatment verification of dynamic multileaf collimation (DMLC) delivery.

PACS number(s): 87.53.–j, 87.52.–g
Verification of dose calculations with a clinical treatment planning system based on a point kernel dose engine
Authors: Weber Lars , Nilsson Per
Published: February 2007

Dose calculations with a collapsed cone algorithm implemented in a clinical treatment planning system have been studied. The algorithm has been evaluated in homogeneous as well as in heterogeneous media, and the results have been compared to measurements and Monte Carlo simulations. Commonly encountered clinical beam configurations as well as more complex geometries have been pursued to test the limitations of the model. The results show that the accuracy level reached allows for clinical use. Some situations, e.g., large wedge beams and dose calculations in the build up region, not specific to the collapsed cone model, show deviations (outside ) compared to measurements.

PACS number(s): 87.53.Bn, 87.53.–j, 87.66.–a
A new geometric and mechanical verification device for medical LINACs
Authors: Welsh Keith T. , Wlodarczyk Robert A. , Reinstein L. E.
Published: February 2007

The goal of quality assurance (QA) for a radiation oncology medical LINAC is to maintain an acceptable level of equipment performance and reliability. The increasing complexity of Radiation Oncology equipment and treatment techniques have led to increased demands on the work load of the medical physicist. Regular testing needs to be as efficient as possible. Generally, the QA tests, as recommended by the AAPM Task Group 40 for medical LINACs, can be grouped into two categories: dosimetry and mechanical checks. A new QA device has been developed that facilitates many of the daily and monthly mechanical QA checks. Its efficiency and speed is achieved through a set of QA tools that are mounted on a single platform, which is designed to fit into the accessory mount of the medical LINAC. Named Mini‐GARD (MG), it verifies the accuracy of the digital readouts for gantry angles, collimator angles, and field sizes. It also tests crosshair position, the optical distance indicator (ODI), and patient setup laser alignment. It uses two calibrated digital levels for the gantry and collimator angle verification, an electronic tape measure for ODI verification, and a calibrated transparent projection scale for the remaining tests. This paper evaluates the stability and accuracy of the device in clinical tests over a period of a year. Results show that the MG is reliable and capable of measuring gantry and collimator angle constancy to , ODI constancy to , and field size accuracies to .

PACS number(s): 87.56.Fc
PTV margin determination in conformal SRT of intracranial lesions
Authors: Parker Brent C. , Shiu Almon S. , Maor Moshe H. , Lang Frederick F. , Liu H. Helen , White R. Allen , Antolak John A.
Published: February 2007

The planning target volume (PTV) includes the clinical target volume (CTV) to be irradiated and a margin to account for uncertainties in the treatment process. Uncertainties in miniature multileaf collimator (mMLC) leaf positioning, CT scanner spatial localization, CT‐MRI image fusion spatial localization, and Gill‐Thomas‐Cosman (GTC) relocatable head frame repositioning were quantified for the purpose of determining a minimum PTV margin that still delivers a satisfactory CTV dose. The measured uncertainties were then incorporated into a simple Monte Carlo calculation for evaluation of various margin and fraction combinations. Satisfactory CTV dosimetric criteria were selected to be a minimum CTV dose of 95% of the PTV dose and at least 95% of the CTV receiving 100% of the PTV dose. The measured uncertainties were assumed to be Gaussian distributions. Systematic errors were added linearly and random errors were added in quadrature assuming no correlation to arrive at the total combined error. The Monte Carlo simulation written for this work examined the distribution of cumulative dose volume histograms for a large patient population using various margin and fraction combinations to determine the smallest margin required to meet the established criteria. The program examined 5 and 30 fraction treatments, since those are the only fractionation schemes currently used at our institution. The fractionation schemes were evaluated using no margin, a margin of just the systematic component of the total uncertainty, and a margin of the systematic component plus one standard deviation of the total uncertainty. It was concluded that (i) a margin of the systematic error plus one standard deviation of the total uncertainty is the smallest PTV margin necessary to achieve the established CTV dose criteria, and (ii) it is necessary to determine the uncertainties introduced by the specific equipment and procedures used at each institution since the uncertainties may vary among locations.

PACS number(s): 87.53.Kn, 87.53.Ly
Comparison of two algorithms for determining beam weights and wedge filters
Authors: Dai Jianrong , Zhu Yunping
Published: February 2007

This article compares two algorithms for determining beam weights and wedge filters for conformal treatment planning. One algorithm, which is based on dose‐gradient analysis, provides analytic formulas for determining beam weights, wedge angles, and collimator angles (i.e., wedge orientations) so that the dose distribution is homogeneous in the target volume. The second algorithm is based on the concept of the super‐omni wedge (i.e., the arrangement of two pairs of orthogonal nominal wedged beams), numerically optimize beam weights, wedge angles, and collimator angles so that the dose requirements to targets and organs at risk are satisfied to the best. Three clinical cases were tested. For the first case, both algorithms resulted in comparable homogeneous dose distributions in the target volume. For the second case, the second algorithm resulted in much lower doses to the eyes plus a better homogeneous dose distribution in the target volume. For the third case, only the second algorithm was applicable, and the treatment plan it developed met the prescribed requirements. The results show that the first algorithm is better in terms of feasibility, whereas the second is better in terms of applicability and the quality of treatment plans.

PACS number(s): 87.53.–j
Measured transverse‐axis dosimetric parameters of the model STM1251 interstitial source
Authors: Li Z. , Williamson J. F.
Published: February 2007

The recent popularity of permanent implants for treatment of prostate cancer has created a large demand for low energy seed sources. Vendors have introduced new source designs to meet this demand. The AAPM has recommended that all low energy interstitial brachytherapy seed sources be subjected to independent dosimetric evaluations, preferably using experimental measurements as well as Monte Carlo calculations. This work presents the results of Thermo‐Luminescence Dosimeter (TLD) measurements of dosimetric parameters on the transverse axis of a new seed source, the Source Tech Medical STM1251 seed. Experimental measurements were performed in a Solid Water phantom, with the results corrected to values for liquid water using Monte Carlo calculated correction factors. The parameters measured include the dose rate constant and values of the radial dose function at distances of 0.5 cm through 5 cm. The measured dose rate constant in liquid water for the STM1251 seed was 1.039 cGy/U‐hr. Measured radial dose function values agreed with Monte Carlo calculated ones to within 10%. These measurements therefore confirm the modeling and simulation of Monte Carlo calculations for this source design, within the statistical uncertainties of the calculation and measurement techniques.

PACS number(s): 87.53.–j, 87.66.–a
Letter to the Editor
Authors: Keall Paul , Siebers Jeffrey
Published: February 2007

PACS number(s): 87.90.+y, 87.52.–g
Improved method of magnification factor calculation for the angiographic measurement of neurovascular lesion dimensions
Authors: Wang Zhou , Rudin Stephen , Bednarek Daniel R. , Miskolczi Laszlo
Published: February 2007

Accurately evaluating the size of a neurovascular lesion is essential for properly devising treatment strategies. The magnification factor must be considered in order to measure the dimension of a lesion from an angiogram. Although a method to calculate the magnification of the lesion by linear interpolation of the measurable magnification factors of two markers has been in use, this paper shows that it can be inaccurate. By deriving the exact formula for calculating the magnification factor at the level of the lesion, the error generated by the linear interpolation of magnification factor has been evaluated. This error was found to depend on source‐to‐skin distance (SSD), the location of the lesion in the head, and the head size. The closer the head is to the focal spot and the nearer the lesion is to the center of the head, the larger is the error. Since clinicians tend to use high geometric magnification (i.e., small SSD) in interventional procedures, there exists a possible consequential error of more than 3% in lesion sizing if the linear‐interpolation calculation method is used. It is thus recommended that the exact formula derived here be used to calculate the magnification factor to improve accuracy.

PACS number(s): 87.57.–s, 87.57.Nk, 87.59.Dj
IMRT clinical implementation: Prostate and pelvic node irradiation using Helios and a 120‐leaf multileaf collimator
Authors: Clark C. H. , Mubata C. D. , Meehan C. A. , Bidmead A. M. , Staffurth J. , Humphreys M. E. , Dearnaley D. P.
Published: February 2007

Dynamic intensity modulated radiation therapy (IMRT) to treat prostate and pelvic nodes using the Varian 120‐leaf Millennium multileaf collimator (MLC) has been implemented in our clinic. This paper describes the procedures that have been undertaken to achieve this, including some of the commissioning aspects of Helios, verification of the dynamic dose delivery, and quality assurance (QA) of the dose delivered to the patient. Commissioning of Helios included measurements of transmission through the 120‐leaf MLC, which were found to be 1.7% for 6 mV and 1.8% for 10 MV. The rounded leaf edge effect, known as the dosimetric separation, was also determined using two independent methods. Values of 1.05 and 1.65 mm were obtained for 6 and 10 MV beams. Five test patients were planned for prostate and pelvic node irradiation to 70 and 50 Gy, respectively. Dose and fluence verification were carried out on specially designed phantoms and dose points in the prostate were measured to be within 2.0% (mean 0.9%, s.d. 0.6%) of the calculated dose and in the nodes within 3.0% (mean 1.6%, s.d. 1.1%). Following the results of this commissioning and implementation study, we have started to treat men with a target volume including the prostate and pelvic nodes using Helios optimized dynamic IMRT delivery in a dose escalation protocol.

PACS number(s): 87.53.–j, 87.90.+y
Peripheral dose distributions for a linear accelerator equipped with a secondary multileaf collimator and universal wedge
Authors: Mutic Sasa , Esthappan Jacqueline , Klein Eric E.
Published: February 2007

The American Association of Physicists in Medicine Task Group 36 (AAPM TG‐36) data can be used to estimate peripheral dose (PD) distributions outside the primary radiation field. However, the report data does not apply to linear accelerators equipped with a multileaf collimator (MLC) and universal wedge (UW). Tertiary multileaf collimators have been shown to significantly affect PD distributions and TG‐36 reported data. Measurements were performed to evaluate PD distributions for a linear accelerator equipped with a secondary MLC, backup diaphragms, and UW. This data can be used to compliment the TG‐36 report for estimation of doses to critical structures outside primary radiation fields. For the evaluated linear accelerator, an MLC is incorporated in the upper secondary collimator jaws. Backup shielding diaphragms are located underneath the MLC. At the nominal collimator position, the MLC and the backup diaphragm provide collimation primarily in the transverse direction. Conventional, solid tungsten‐alloy jaws, located underneath the backup diaphragms, provide secondary collimation in the longitudinal direction. The universal wedge provides dose modulation in the direction of the conventional jaws. Measurements were made with an ionization chamber inserted into a water‐equivalent plastic phantom with the secondary collimator and MLC settings of , and with and without UW. Data was acquired along the machine's longitudinal axis for 6, 10, and 18 MV photons. Peripheral dose distributions were measured with the collimator rotated to 0° and 270° for open field measurements and to 0°, 180°, and 270° for wedged fields (IEC 1217). This allowed evaluation of peripheral dose distributions as a function of collimator rotation. Wedged fields were normalized to deliver the same dose at the depth of maximum dose on the central axis as open fields. The measured PD distributions were generally comparable to data reported by TG‐36. At distances close to the field edge (less than 30 or 40 cm), the measured PD distributions were lower when the measurement point was shielded by solid jaws than with MLC and backup diaphragm. At longer distances, this trend reversed for all energies and evaluated field sizes. However, the difference in PD distribution with collimator rotation was not large enough to warrant strategic positioning of the collimator to reduce dose to critical structures outside the primary radiation field. Because internal scatter dominates close to the field edge, wedged PD distributions were comparable to open field doses at distances closer than 30 cm. However, at distances larger than 30 cm from the field edge, wedged PD distributions were significantly grater than those for open fields due to increased contribution of leakage radiation. Increased leakage radiation is due to the increase in wedged field monitor units, which is related to a small wedge factor (0.27 to 0.29).

PACS number(s): 87.53.–j, 87.66.–a
An immobilization and localization technique for SRT and IMRT of intracranial tumors
Authors: Leybovich Leonid B. , Sethi Anil , Dogan Nesrin , Melian Edward , Krasin Mathew , Emami Bahman
Published: February 2007

A noninvasive localization and immobilization technique that facilitates planning and accurate delivery of both intensity modulated radiotherapy (IMRT) and linac based stereotactic radiotherapy (SRT) of intracranial tumors has been developed and clinically tested. Immobilization of a patient was based on a commercially available Gill‐Thomas‐Cossman (GTC) relocatable frame. A stereotactic localization frame (LF) with the attached NOMOS localization device (CT pointer) was used for CT scanning of patients. Thus, CT slices contained fiducial marks for both IMRT and SRT. The patient anatomy and target(s) were contoured on a stand‐alone CT‐based imaging system. CT slices and contours were then transmitted to both IMRT and SRT treatment planning systems (TPSs) for concurrent development of IMRT and SRT plans. The treatment method that more closely approached the treatment goals could be selected. Since all TPSs used the same contour set, the accuracy of competing treatment plans comparison was improved. SRT delivery was done conventionally. For IMRT delivery patients used the SRT patient immobilization system. For the patient setup, the IMRT target box was attached to the SRT LF, replacing the IMRT CT Pointer. A modified and lighter IMRT target box compatible with SRT LF was fabricated. The proposed technique can also be used for planning and delivery of 3D CRT, thus improving its accuracy. Day‐to‐day reproducibility of the patient setup can be evaluated using a SRT Depth Helmet.

PACS number(s): 87.53.Kn, 87.53Ly, 87.56.Da
Assessment of consistency in contouring of normal‐tissue anatomic structures
Authors: Collier Dawn C. , Burnett Stuart S. C. , Amin Mayankkumar , Bilton Stephen , Brooks Christopher , Ryan Amanda , Roniger Dominique , Tran Danny , Starkschall George
Published: February 2007

The purpose of this work is to estimate the uncertainty in the manual contouring of normal anatomical structures. The heart, esophagus, and spinal cord were contoured manually on six sets of computed tomography images by six dosimetrists whose experience ranged from 1 year to over 15 years. To determine the differences between inter‐ and intraobserver variations, each data set was contoured by one of the dosimetrists five times and once each by the five other dosimetrists. The magnitude of the discrepancies in delineating the contours was assessed. Intradosimetrist contouring discrepancies were as follows: esophagus, average 0.3 cm and maximum 2.9 cm; heart, average 0.5 cm and maximum 7.6 cm; and spinal cord, average 0.1 cm and maximum 0.7 cm. Interdosimetrist contouring discrepancies were as follows: esophagus, average 0.4 cm and maximum 3.1 cm; heart, average 0.7 cm and maximum 8.1 cm; and spinal cord, average 0.2 cm and maximum 0.9 cm. Significant discrepancies can occur when normal anatomic structures are contoured manually. Interdosimetrist discrepancies are typically slightly greater than intradosimetrist discrepancies. The magnitude of the discrepancies does not appear to be correlated to the experience of the dosimetrist. © 2003 American College of Medical Physics.

PACS number(s): 87.53.–j, 87.66.–a
Dosimetric comparison of extended dose range film with ionization measurements in water and lung equivalent heterogeneous media exposed to megavoltage photons
Authors: Charland Paule M. , Chetty Indrin J. , Yokoyama Shigeru , Fraass Benedick A.
Published: February 2007

In this study, a dosimetric evaluation of the new Kodak extended dose range (EDR) film versus ionization measurements has been conducted in homogeneous solid water and water‐lung equivalent layered heterogeneous phantoms for a relevant range of field sizes (up to a field size of and a depth of 15 cm) for 6 and 15 MV photon beams from a linear accelerator. The optical density of EDR film was found to be linear up to about 350 cGy and over‐responded for larger fields and depths (5% for at depth of 15 cm compared to a , 5 cm depth reference value). Central axis depth dose measurements in solid water with the film in a perpendicular orientation were within 2% of the Wellhöfer IC‐10 measurements for the smaller field sizes. A maximum discrepancy of 8.4% and 3.9% was found for the field at 15 cm depth for 6 and 15 MV photons, respectively (with curve normalization at a depth of 5 cm). Compared to IC‐10 measurements, film measured central axis depth dose inside the lung slab showed a slight over‐response (at most 2%). At a depth of 15 cm in the lung phantom the over‐response was found to be 7.4% and 3.7% for the field for 6 and 15 MV photons, respectively. When results were presented as correction factors, the discrepancy between the IC‐10 and the EDR was greatest for the lowest energy and the largest field size. The effect of the finite size of the ion chamber was most evident at smaller field sizes where profile differences versus film were observed in the penumbral region. These differences were reduced at larger field sizes and in situations where lateral electron transport resulted in a lateral spread of the beam, such as inside lung material. Film profiles across a lung tumor geometry phantom agreed with the IC‐10 chamber within the experimental uncertainties. From this investigation EDR film appears to be a useful medium for relative dosimetry in higher dose ranges in both water and lung equivalent material for moderate field sizes and depths. © 2003 American College of Medical Physics.

PACS number(s): 87.53.Dq, 87.66.Cd, 87.66.Jj, 87.66.Xa
Differences in electron beam dosimetry using two commercial ionization chambers and the TG‐21 protocol: Another reason to switch to TG‐51
Authors: Followill David S. , Hanson William F. , Ibbott Geoffrey S. , Eglezopoulos Leon R. , Chui Chen‐Shou
Published: February 2007

Two of the most popular dosimetry systems used for calibration of megavoltage photon and electron beams in radiation therapy are (i) cylindrical Farmer‐type chambers in liquid water and (ii) Holt Memorial parallel‐plate chambers in clear polystyrene. Since implementation of the AAPM TG‐21 calibration protocol, the Radiological Physics Center (which uses the Farmer in‐water system) has compared machine calibrations on two occasions with those of Memorial Sloan‐Kettering Cancer Center (which uses the Holt in‐polystyrene system). Two years post publication of the TG‐51 protocol, 70% of the clinics monitored by the RPC still use TG‐21. Seventeen photon beams from cobalt‐60 to 18 MV and 31 electron beams from 6 to 20 MeV were compared using the TG‐21 protocol. These data represent the most comprehensive comparison of the two most popular systems in use. Based on the average percent difference, the two systems yielded the same absorbed dose to water at the reference point in phantom to within 1.5% for both modalities. No energy dependence was evident in the results; however, a systematic average percent difference between photons and electrons was seen, with the Farmer in‐water system consistently predicting a dose 1.3% lower for electrons than the Holt in‐polystyrene system. For photons both systems predicted the same dose to within 0.3% on average. When a physicist converts from TG‐21 to TG‐51, these data may be of assistance in explaining unexpected changes in output that are different from previously published values. Implementation of the TG‐51 protocol should eliminate any of the observed differences in electron beam dosimetry between the two dosimetry systems because the Holt system cannot be used with TG‐51.

PACS number(s): 87.53.‐j, 87.53.‐j
A fast, independent dose check of HDR plans
Authors: Lachaine Martin E. , Gorman Jason C. , Palisca Madeline G.
Published: February 2007

High dose rate (HDR) brachytherapy often involves optimization routines to calculate the dwell times and positions of a radioactive source along specified applicator paths. These routines optimize the dwells in such a way as to deliver the prescribed dose at one or more points while satisfying various constraints. The importance of independently verifying the doses calculated by the optimization software prior to treatment delivery has been recognized in various works, and is a requirement of various regulatory agencies. Most previous methods are specific to particular treatment configurations, or require a full replanning of the case. In this work we describe an in‐house software which provides an independent verification of dose calculations in less than 3 min, which adds negligible additional waiting time for the patient, regardless of the number of applicators, paths of the applicators, or complexity of the dwell times and positions. In order to verify errors which may occur between the planning and delivery stages, the verification code directly uses the treatment file used to control the HDR afterloader to compute the dose. Since this file references the source positions in the frame of reference of the catheters, an algorithm is described to convert these positions to Cartesian coordinates. We validate the code for various arbitrary cases ranging from a single catheter to complex multicatheter plans, and show results for various clinical plans. The maximum discrepancy observed for these clinical plans is 2%.

PACS number(s): 87.53.–j, 87.90.+y
Neutron and capture gamma along the mazes of linear accelerator vaults
Authors: Wu Raymond K. , McGinley Patton H.
Published: February 2007

Neutron and photon dose equivalents at various points along the mazes of the vaults for two 15 MV linear accelerators were measured. The measurements were made with the machines set at various gantry angles with and without the scattering phantom, and with the collimators set at the maximum and the minimum field sizes. Neutron dose equivalent measurements were made for five other accelerator vaults. Empirical equations were used to fit the dose data at points along the center of the maze, at 1 m above floor level, with the primary radiation beam pointing downward. It is reported here that both the capture gamma and the neutron dose attenuations along the maze are in agreement with the literature. The neutron dose is dependent on the square root of the ratio of the cross‐sectional areas of the inner maze entrance and the maze. The tenth value distance is proportional to the square root of the cross‐sectional area of the maze.

PACS number(s): 87.52.Ga
Book Review
Authors: Huda Walter
Published: February 2007

Applied Imaging Technology. 4th ed. J. C. P. Heggie, N. A. Liddell, and K. P. Maher. St. Vincent's Hospital, Melbourne, Australia, 2001. http://www.life.rmit.edu.au/mrs/kpm/AIT/

PACS number(s): 87.57.
Neutron source strength measurements for Varian, Siemens, Elekta, and General Electric linear accelerators
Authors: Followill David S. , Stovall Marilyn S. , Kry Stephen F. , Ibbott Geoffrey S.
Published: February 2007

The shielding calculations for high energy () linear accelerators must include the photoneutron production within the head of the accelerator. Procedures have been described to calculate the treatment room door shielding based on the neutron source strength (Q value) for a specific accelerator and energy combination. Unfortunately, there is currently little data in the literature stating the neutron source strengths for the most widely used linear accelerators. In this study, the neutron fluence for 36 linear accelerators, including models from Varian, Siemens, Elekta/Philips, and General Electric, was measured using gold‐foil activation. Several of the models and energy combinations had multiple measurements. The neutron fluence measured in the patient plane was independent of the surface area of the room, suggesting that neutron fluence is more dependent on the direct neutron fluence from the head of the accelerator than from room scatter. Neutron source strength, Q, was determined from the measured neutron fluences. As expected, Q increased with increasing photon energy. The Q values ranged from 0.02 for a 10 MV beam to neutrons per photon Gy for a 25 MV beam. The most comprehensive set of neutron source strength values, Q, for the current accelerators in clinical use are presented for use in calculating room shielding.

PACS number(s): 87.53.–j, 87.52.–g
Effectiveness of compensating filters in the presence of tissue inhomogeneities
Authors: Sethi A. , Leybovich L. , Dogan N. , Glasgow G.
Published: February 2007

CT based 3D treatment planning systems (3DTPS) can be used to design compensating filters that, in addition to missing tissue compensation, can account for tissue inhomogeneities. The use of computer‐driven systems provides a practical, convenient, and accurate method of fabricating compensating filters. In this work, we have evaluated a commercially available PAR Scientific DIGIMILL milling machine linked with FOCUS 3DTPS. Compensating filters were fabricated using refined gypsum material with no additives. Thus, filters were of manageable dimensions and were not sensitive to common machining errors. Compensating filters were evaluated using a homogeneous step phantom and step phantoms containing various internal inhomogeneities (air, cork, and bone). The accuracy of two planning algorithms used to design filters was experimentally evaluated. The superposition algorithm was found to produce better agreement with measurements than the Clarkson algorithm. Phantom measurements have demonstrated that compensating filters were able to produce a uniform dose distribution along the compensation plane in the presence of tissue inhomogeneity. However, the dose variation was greatly amplified in planes located beyond the inhomogeneity when a single compensated beam was used. The use of parallel‐opposed compensated beams eliminated this problem. Both lateral and depth‐dose uniformity was achieved throughout the target volume.

PACS number(s): 87.53.–j, 87.66.–a
A spreadsheet solution for off‐axis, noncentral enhanced dynamic wedge factors
Authors: Wichman B. D.
Published: February 2007

Enhanced dynamic wedges can be very useful in increasing radiation therapy treatment efficiency. A drawback to their use is finding a factor analogous to a conventional wedge factor to calculate the dose to a point in a dynamically wedged field. A formalism has previously been published [J. Gibbons, Med. Phys. 25, 1411–1418 (1998)] to determine this factor at the central point of a dynamically wedged field. This paper expands the use of the previous formalism to show efficacy at any point under a dynamically wedged field, provided the point is not in a penumbra region. Results of measurements taken with four clinically relevant field sizes, both symmetric and asymmetric, are given. These results show acceptable correlation between the original formalism and its expanded use to noncentral points.

PACS number(s): 87.53.–j, 87.66.–a
Comparison of dose calculated by an intensity modulated radiotherapy treatment planning system and an independent monitor unit verification program
Authors: Haslam J. J. , Bonta D. V. , Lujan A. E. , Rash C. , Jackson W. , Roeske J. C.
Published: February 2007

A comparison of isocenter dose calculated by a commercial intensity modulated radiation therapy treatment planning system and independent monitor unit verification calculation (MUVC) software was made. The percent disparity between the treatment plan and MUVC doses were calculated for 507 treatments (head and neck, prostate, abdomen, female pelvis, rectum and anus, and miscellaneous) from 303 patients. The MUVC calculated dose was, on average, 1.4% higher than the treatment planning dose, with a 1.2% standard deviation. The distribution of the disparities appeared to be Gaussian in shape with some variation by treatment site. Based on our analysis, disparities outside the range of about the mean value should be checked and resolved prior to treatment delivery.

PACS number(s): 87.53.–j, 87.66.–a
Shielding evaluation for IMRT implementation in an existing accelerator vault
Authors: Price R. A. , Chibani O. , Ma C.‐M.
Published: February 2007

A formalism is developed for evaluating the shielding in an existing vault to be used for IMRT. Existing exposure rate measurements are utilized as well as a newly developed effective modulation scaling factor. Examples are given for vaults housing 6, 10 and 18 MV linear accelerators. The use of an 18 MV Siemens linear accelerator is evaluated for IMRT delivery with respect to neutron production and the effects on individual patients. A modified modulation scaling factor is developed and the risk of the incurrence of fatal secondary malignancies is estimated. The difference in neutron production between 18 MV Varian and Siemens accelerators is estimated using Monte Carlo results. The neutron production from the Siemens accelerator is found to be approximately 4 times less than that of the Varian accelerator resulting in a risk of fatal secondary malignancy occurrence of approximately 1.6% when using the SMLC delivery technique and our measured modulation scaling factors. This compares with a previously published value of 1.6% for routine 3D CRT delivery on the Varian accelerator.

PACS number(s): 87.52.Ga, 87.52.Px, 87.53.Qc, 87.53.Wz
Automatic detection of small lung nodules on CT utilizing a local density maximum algorithm
Authors: Zhao Binsheng , Gamsu Gordon , Ginsberg Michelle S. , Jiang Li , Schwartz Lawrence H.
Published: February 2007

Increasingly, computed tomography (CT) offers higher resolution and faster acquisition times. This has resulted in the opportunity to detect small lung nodules, which may represent lung cancers at earlier and potentially more curable stages. However, in the current clinical practice, hundreds of such thin‐sectional CT images are generated for each patient and are evaluated by a radiologist in the traditional sense of looking at each image in the axial mode. This results in the potential to miss small nodules and thus potentially miss a cancer. In this paper, we present a computerized method for automated identification of small lung nodules on multislice CT (MSCT) images. The method consists of three steps: (i) separation of the lungs from the other anatomic structures, (ii) detection of nodule candidates in the extracted lungs, and (iii) reduction of false‐positives among the detected nodule candidates. A three‐dimensional lung mask can be extracted by analyzing density histogram of volumetric chest images followed by a morphological operation. Higher density structures including nodules scattered throughout the lungs can be identified by using a local density maximum algorithm. Information about nodules such as size and compact shape are then incorporated into the algorithm to reduce the detected nodule candidates which are not likely to be nodules. The method was applied to the detection of computer simulated small lung nodules (2 to 7 mm in diameter) and achieved a sensitivity of 84.2% with, on average, five false‐positive results per scan. The preliminary results demonstrate the potential of this technique for assisting the detection of small nodules from chest MSCT images.

PACS number(s): 87.57.–s, 87.90.+y
Use of intensity modulation for missing tissue compensation of pediatric spinal fields
Authors: Phillips M. H. , Cho P. S. , Parsai H. , Douglas J. G.
Published: February 2007

Irradiation of the cranio‐spinal axis is often one of the treatment modalities of certain childhood cancers, e.g., medulloblastoma. In order to achieve a uniform dose to the spinal cord, missing tissue compensators are required. In the past, our practice was to fabricate compensators out of strips of lead. We report on the use of intensity modulated fields to achieve the desired compensation. Seven cases of pediatric cancer whose treatment involved irradiation of the cranio‐spinal axis had compensators designed using a beam intensity modulation method rather than making mechanical compensators. The compensators only adjusted for missing tissue along the spinal axis. Comparisons between calculated and measured doses were made at depth in phantoms and on the surface of the patient. The intensity modulated fields were delivered using a step‐and‐shoot delivery on an Elekta SL20 accelerator equipped with multileaf collimator. The intensity‐modulated compensators provided more flexibility in design than the physical compensator method. Finer intensity steps were achievable, more accurate dose distributions were able to be calculated, and adjustments during treatment, e.g., junction changes, were more easily implemented. Convolution/superposition dose calculations were within of measurements. Intensity modulated fields are a practical and more efficient method of delivering uniform doses to the spine in pediatric cancer treatments. They provide many advantages over mechanical compensators with regard to time and flexibility.

PACS number(s): 87.53.–j, 87.90.+y
Reference photon dosimetry data: A preliminary study of in‐air off‐axis factor, percentage depth dose, and output factor of the Siemens Primus linear accelerator
Authors: Cho S. H. , Ibbott G. S.
Published: February 2007

The dosimetric characteristics for modern computer‐controlled linear accelerators with the same make, model, and nominal energy are known to be very similar, as long as the machines are unaltered from the manufacturer's original specifications. In this preliminary study, a quantitative investigation of the similarity in the basic photon dosimetry data from the Siemens Primus linear accelerators at eight different institutions is reported. The output factor, percentage depth dose (PDD), and in‐air off‐axis factor (OAF) for the 6 and 18 MV photon beams measured or verified by the Radiological Physics Center (RPC) were analyzed. The RPC‐measured output factors varied by less than about 2% for each field size. The difference between the maximum and minimum RPC‐verified PDD values at each depth was less than about 3%. The difference between the maximum and minimum RPC‐measured in‐air OAF was no more than 4% at all off‐axis distances considered in this study. These results strongly suggest that it is feasible to establish a reference photon dosimetry data set for each make, model, and nominal energy, universally applicable to those machines unaltered from the manufacturers' original specifications, within a clinically acceptable tolerance (e.g., ).

PACS number(s): 87.53.–j, 87.66.–a
Utilizing an electronic portal imaging device to monitor light and radiation field congruence
Authors: Prisciandaro J. I. , Herman M. G. , Kruse J. J.
Published: February 2007

A method to investigate light and radiation field congruence utilizing a commercially available amorphous silicon electronic portal imaging device (EPID) was developed. This method employed an EPID, the associated EPI software, and a diamond‐shaped template. The template was constructed from a block tray in which Sn/Pb wires, 1 mm in diameter, were embedded into a diamond shaped groove milled into the tray. The collimator jaws of the linac were aligned such that the light field fell directly on the corners of the diamond. A radiation detection algorithm within the EPI software determined the extent of the radiation field. The light and radiation field congruence was evaluated by comparing the vertexes of the diamond reference structure to the detected radiation field. In addition, the digital jaw settings were recorded and later compared to the light field detected on the films and EPIs. Three linear accelerators were tracked for a period ranging from 2–8 months. Light radiation field congruence tests with films and EPIs were comparable, yielding a difference of less than 0.6 mm, well within the allowed 2‐mm tolerance. A disparity was observed in the magnitude of the detected light field. The X and Y dimensions of the light field measured with film differed by less than or equal to 1.4 mm from the digital collimator settings, whereas the values extracted from the EPIs differed by up to 2.5 mm. Based on these findings, EPIs were found to be a quick and reliable alternative to film for qualitative and relative analyses.

PACS number(s): 87.53.Xd, 87.56.Fc, 87.53.Oq, 87.52.–g, 87.53.–j