bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐FF‐T‐335: MU Variation for Total Body Irradiation (TBI)
Authors: Zhu T , Zhou X , Glatstein E
Published: July 2006

Purpose: To determine a simplified formula to calculate monitor unit (MU) for total body irradiation (TBI) for 6 MV photon beams. Method and Materials: Patient data were accumulated for TBI with 6 MV photon beams between January 2002 and February 2006. The dependence of MU on SAD, patient thickness, and field sizes were examined for four different linear accelerator types. Results: MU depends strongly on SAD but is almost independent of the linear accelerator types. Among the 47 patients examined, the depth varied between 6.5 and 21.5 cm (mean and s.d.11.5 ± 3.6 cm), the field sizes varied between 20 and 50 cm (mean and s.d.35 ± 8 cm) and SAD varied between 492 and 591 cm. MU is linearly proportional to the thickness of patient separation and, to a lesser extent, linearly dependent on the equivalent patient field size. To deliver 200 cGy, the mean and standard deviation of MU*(100/SAD)2 is 113 ± 11 for all patients examined. Conclusion: To deliver 200 cGy with 6 MV photon beams, MU can be determined using a formula MU*(100/SAD)2 = 2.63 d + 82.6 to within an accuracy of 5.7% for most of patients examined. To increase the accuracy further, requires taking into account the field size dependence.
SU‐FF‐T‐336: Multi‐Slice Tomotherapy Delivery Methods Using a Novel Multileaf Collimator
Authors: Cadman P , Mackie T , Reckwerdt P
Published: July 2006

Purpose: In this study, we present an initial dosimetric investigation of multi‐slice tomotherapy delivery methods using a novel multileaf collimator design. Method and Materials: A compact multileaf collimator design for multi‐slice tomotherapy is under investigation. Intensity modulation is achieved through binary actuation of adjacent pivoting leaves with multiple leaf banks employed to deliver multi‐slice tomotherapy. Pitch may be determined by the ratio of the couch travel distance for a complete gantry rotation divided by the slice width at the axis of rotation. For multi‐slice tomotherapy the concept of pitch is complicated by the use of multiple collimated slices and differing fan beam divergences. We have studied the dosimetric effect of: (a) using regular pitch, where a given collimated slice does not irradiate beyond the adjacent slice and (b) using multi‐slice pitch, where different collimated slices irradiate the same region in the patient. Other novel aspects of tomotherapy delivery with the multi‐slice collimator are presented. Results: The multi‐slice pitch delivery method allows for treatment of extended volumes greater than the spacing between slices, without the junctioning effect of the regular pitch method. A pitch less than 1 may be achieved when considering irradiation from different collimated slices and is a determined by the slice spacing, slice width and couch travel. The difference in fan beam divergence in the axial direction may be clinically advantageous. Single‐slice techniques may also benefit from non‐coplanar delivery. Conclusions: A method for multi‐slice tomotherapy delivery is investigated. Dosimetric results are presented for various methods of dose delivery including a multi‐slice pitch method for extended treatment lengths. Multi‐slice delivery will result in shorter treatment times for both linac and cobalt‐based tomotherapy systems and development of novel radiation therapy techniques. Conflict of Interest: Research was supported in part through a licensing agreement with TomoTherapy Inc.
SU‐FF‐T‐337: Multiobjective Inverse Planning Optimization: Adjustment of Dose Homogeneity and Urethra Protection in HDR‐Brachytherapy of the Prostate
Authors: Lessard E , Hsu I , Aubry J , Pouliot J
Published: July 2006

Purpose: Multiobjective optimizations are performed to evaluate the inverse planning (IP) ability to adjust the dose homogeneity and the urethra protection in HDR brachytherapy of the prostate. Materials and Methods: An IP is an anatomy‐based optimization guided by dose objectives specified for each organ extracted from medical imaging. It selects automatically the active dwell positions and optimizes the dwell times to fulfill the dose objectives. It is setup to maximize the prostate dose coverage while taking into account other clinical objectives like the dose homogeneity and the organs at risk protection. Multiobjective optimizations are performed using the IP for one small (23cc), one intermediate (35cc) and one large (80cc) prostate. 10 inverse plans were generated with different compromises between dose coverage and dose homogeneity. This was performed first with the prostate alone and then with all the organs at risk. In addition, 10 inverse plans were generated with different compromises between dose coverage and urethra protection. 90 DVH were generated and analyzed. Results: When only the prostate is included, the prostate V100 varies from 100% to 97% and the homogeneity index (HI) from 0.06 to 0.68. When all the organs at risk are included, the prostate V100 varies from 97% to 91% and the HI from 0.52 to 0.72. When the urethra protection is increased, the prostate V100 varies from 100% to 89%, the urethra V100 from 100% to 89%, the urethra V120 from 87% to 0% and the urethra V150 from 3% to 0%. Conclusion: For simple cases where only a target is defined, the dose homogeneity is adjustable with the IP. For complex cases where organs at risk are added, this anatomy‐based optimization automatically adjusts the dose homogeneity to protect the urethra. Additional organ protection can be achieved with specific penalty. This research was supported by Nucletron Corporation.
SU‐FF‐T‐338: MVCT Superiority Over KVCT in Assessment of Electron Density for Treatment Planning
Authors: Shaw R , Mackie T
Published: July 2006

Purpose: Accurate assessment of electron density (ρe‐[electrons/cm3]) is essential for radiotherapy treatment planning, especially for protons. Measurements of ρe‐ are typically performed with kilovoltage CT (kVCT) and its calibration assumes that object is tissue equivalent. In this work, we demonstrate that kVCT presents confusing information when objects have varying atomic number (Z). At the same time, we demonstrate that megavoltage CT (MVCT) may be preferred over kVCT in treatment planning since its calibrated pixel value, or CT number, is proportional to ρe‐, independent of atomic number(ignoring pair production), and is suitable for non‐tissue equivalent materials(e.g. hip prostheses). Method and Materials: A dual‐energy kV & MVCT benchtop is used to image a tissue‐equivalent phantom as well as samples with various combinations of ρe‐ and Z. Results: The plot of CT number to ρe‐ for the tissue equivalent phantom is non‐linear for the kVCT, but linear for the MVCT. For the non‐tissue equivalent samples, the MVCT number increases linearly with ρe‐ for all the samples, however the kV CT number to electron density is complicated by the two material components of the kV attenuation: atomic number and electron density Conclusion: For all the samples presented the MVCT number increases with the electron density of the sample. The kVCT number assessment of electron density is only reliable for low atomic number materials. Beam hardening affects the kVCT number even for relatively small samples. Beam hardening effects for MVCT is slight and only apparent in large patients. Ignoring image‐quality to dose considerations, MVCT should be preferred for electron density assessment. Conflict of Interest: This work was partially supported by grant from the NIH (PO1 CA088960) and a contract from TomoTherapy Inc.
SU‐FF‐T‐348: Optimize Spike Patterns For An Embedded Boost Technique For Gamma Knife Radiosurgery
Authors: Ma L , Larson D , Petti P , Chuang C , Verhey L
Published: July 2006

Purpose: With the Gamma Knife Automatic Positioning System, high dose spikes can be placed inside a target using 4‐mm shots to boost the central target dose without affecting adjacent normal brain sparing. Potential applications of the technique include target boost when nearby critical structures such as optical structure limit the peripheral dose or increasing the dose to centrally hypoxic regions. This study investigates how the spatial distribution and height of the spikes can be optimized for this purpose. Method and Materials: A computer program was developed to optimize the pattern of spike boost placed over conventional Gamma Knife plans. The program first extracts the prescription isodose volume and then iteratively adjusts the positions and the heights of the spikes while accounting for the existing dose distribution. Different boost patterns were generated for both patient and phantom cases. The program calculated the stereotactic coordinates and weights of the spike shots, which were manually entered into the Leksell Gamma Plan for dose calculation and dose‐volume evaluations of the modified treatment plans. Results: The total number of dose spikes and the maximum dose for the spike shots correlates with the target size (R2> 0.9). For the cases studied, an increase in average dose to the target of 15–28% was noted while the conformity index of target volume coverage remained unchanged. The dose to the adjacent normal brain (defined as the volume enclosed by half of the prescription isodose line excluding the target) remained unchanged to better than 2% of the maximum dose. Overall, spike patterns arranged along the radial direction consistently give a higher average dose to the target than for spikes arranged along the translational directions. Conclusion: Optimizing the shot weights and the spike patterns enhances dose to the target volume while maintaining normal brain sparing for Gamma Knife radiosurgery.
SU‐FF‐T‐350: Optimizing the MLC Apertures for Conformal Radiotherapy with a Hybrid Algorithm
Authors: Li Y , Yao D
Published: July 2006

Purpose: To develop a feasible technique to simultaneously optimize the shapes and weights of the MLC apertures for conformal radiotherapy planning. Method and Materials: The shapes of the MLC apertures are optimized using the genetic algorithm (GA). There are several individuals in each generation of GA, and each individual includes a group of apertures, which number is equal to the number of pre‐selected beams, i.e., each individual is a trial conformal plan. The initial aperture shapes are obtained by adding different margins to the target on Beam's‐eye‐view (BEV). The margins range from −1.0 cm (i.e., shrink) to 1.0 cm (i.e., expand), with a step of 0.2 cm. The apertures of all beams with 1.0 cm margin constitute the first individual, and the apertures with 0.8 cm margin constitute the second, etc. For the selected two individuals (parents), the crossover and mutation operation are applied to the corresponding two apertures of the two parents. After the loop of shape optimization with GA, the aperture weights are optimized with a conjugate gradient (CG) method. The shapes and weights are iteratively optimized. Results: A five‐beam conformal plan is used for a head‐and‐neck tumor case. In the manual plan the weights of all apertures are set to unit, and the shapes are obtained by adding a 0.5 cm margin to the target on the BEV of each beam. About 2 minutes are taken to find the optimized plan. Both the dose distributions on CT slices and the DVH curves demonstrate the improvement of the optimized plan. Conclusion: This work indicates that the proposed GA and CG‐based algorithm is feasible and efficient for the optimization of the shapes and weight of MLC apertures for conformal radiotherapy planning, and a better dose distribution could be achieved within clinically acceptable computation time.
SU‐FF‐T‐354: Penumbra Measurement with the Use of a 2D Pixel Ionization Chamber
Authors: Peroni C , Brusasco C , Cirio R , Donetti M , Giordanengo S , Gomola I , Iliescu S , Marchetto F , Nastasi U , Stasi M
Published: July 2006

Purpose: To attest the accuracy of the penumbra measurement with the use of a 2D pixel ionization chamber. Method and Materials: Penumbra measurements were carried out for several clinical cases, treated with Intensity Modulated Radiation Therapy, with a pixel ionization chamber and with radiographic film as reference. The pixel chamber consists of a matrix 1020 detectors, each 4.5 mm diameter and 5 mm height at a pitch of 7.62 mm. Each of the 1020 ionization chambers was read out independently with no dead time. Standard radiographic films have been used, scanned with a pitch of 0.16 mm and 0.40 mm. Both detectors were irradiated with 6 MV photon beams, using a linac equipped with a 120‐leaves multileaf collimator. Measured dose profiles were fitted with a “Fermi‐like” function: with x0 and a as free parameters; the value for the penumbra could then be derived from the fit. The two sets of data were compared. Results: The data for the penumbra measured with the pixel chamber and the film for several IMRT fields show a strong linear correlation. This made it possible to correct the penumbra values obtained with the pixel ionization chamber to obtain a precise measurement even in a range well below the pixel dimensions. Conclusion: The 2D pixel ionization chamber has been shown to be able to measure the penumbra down to ∼ 2mm with an accuracy of ± 0.6 mm. This device can then efficiently replace radiographic film in some QA procedures with a significative gain in time as it gives results in real‐time and data are immediately available. Conflict of Interest: Research sponsored by MAESTRO, European project on Methods and Advanced Equipment for Simulation and Treatment in Radiation Oncology.
SU‐FF‐T‐355: Performance Evaluations of a Preemptive Approach in IMRT Planning
Authors: Jee K , McShan D , Vineberg K , Fraass B
Published: July 2006

Purpose: Preemptive Goal Programming (PGP) facilitates the use of soft‐constraints in a hierarchical manner for solving complex inverse planning problems. It is a highly effective multicriteria strategy for IMRT planning. However, in order to ensure good numerical performance, a robust constrained optimization algorithm with a practical strategy for handling degeneracy is required. This study identifies cases when PGP introduces challenging problems that do not hold a conventional constraint qualification. Numerical performance for such problems is evaluated using nonlinear planning metrics (DVH, EUD, and NTCP). Method and Materials: PGP, as implemented in our in‐house planning system, solves a series of nonlinear least‐squares problems by a sequential quadratic programming (SQP) algorithm provided with precise Jacobians by an automatic differentiation algorithm. In the presence of constraint degeneracy, an elastic mode (feasibility bound relaxation) is used to ensure a positive step taken at all iterations. Dosimetric as well as numerical performance is evaluated for dose (or EUD) escalation cases with critical organ dose constraints. Reformulation of constraints is also studied as a potential way of reducing degeneracy in these problems. Results: Optimization levels involving strongly active constraints showed degeneracy near optimality, thus requiring use of the elastic mode for convergence. This mode exhibited robust convergence (typically <∼100 iterations) for all IMRT cases studied. Highly nonlinear models (NTCP or EUD with high +/−powers) were handled well by SQP. A strategy of constraint reformulation showed a monotonic decrease in merit function value and its dosimetric performances were comparable to those without reformulation. Conclusion: The optimization task of escalating target dose is limited by binding dose constraints of OARs, which can also create degeneracy in PGP. This study shows SQP with an elastic mode exhibits a stable convergence using the dose‐volume, EUD and NTCP metrics. Numerical performances can be further stabilized by reformulating degenerate constraints.
SU‐FF‐T‐360: Planning Strategies to Reduce Unnecessary Skin Dose in Head and Neck IMRT, Including Experimental Verification
Authors: Court L , Tishler R
Published: July 2006

Purpose: To investigate IMRT planning techniques that reduce skin dose without adversely impacting important doses at depth and to experimentally confirm these findings.

Method and Materials: A semi‐cylindrical phantom was constructed with catheters at 3, 6, 9 and 12mm depths, allowing micro‐MOSFET dosimeters to be inserted to measure dose at multiple points on the surface and in the build‐up region. Critical structures, a node‐like CTV, and 2mm surface structure (‘skin’) were contoured on a CT image of the phantom with and without an immobilization mask. A PTV was generated by expanding the CTV 5mm in all directions up to the body contour; three modifications were then implemented, pulling back the PTV 0, 3 or 5mm from the body contour. Seven‐field IMRT plans were created using Eclipse to maximize PTV coverage, with one of the following strategies: (1) aim for maximum 110% hotspot, 115% allowed, (2) maximum 105% hotspot, (3) maximum 105% hotspot and 50% of skin to get maximum 70% of prescribed dose, (4) 99% of PTV volume to receive 90–93% of prescribed dose, maximum 105% hotspot, and dose to skin structure minimized. All twelve PTV and planning strategy combinations were investigated, with and without immobilization mask. The plans were then delivered using a Varian 21Ex, and dose to skin and build‐up region was measured.

Results: From highest to lowest skin dose, the planning strategies were (3), (4), (2), (1), with little dependence on the PTV expansion approach. Technique (3), however, showed a tendency to underdose tissues at depth. Conclusion: The best strategy to reduce unnecessary skin dose, while maintaining target dose, is to pull the PTV back 3–5mm from the skin surface, and plan such that PTV coverage is maximized, with 99% of PTV volume receiving 90–93% of prescribed dose, maximum 105% hotspot, and the skin structure dose minimized.
SU‐FF‐T‐361: Planning Study of Intensity‐Modulated and 3D Conformal Radiotherapy of Whole Pelvis Including Inguinal Lymphatics: Radiobiological Considerations for Designing New Fractionation Schemes
Authors: Ahmad M , Song H , Lund M , Niemierko A , Moran M , Weidhaas J , Higgins S , Chen Z , Deng J , Nath R
Published: July 2006

Purpose: To examine the possibility of using “simultaneous integrated boost” intensity‐modulated radiotherapy (SIB‐IMRT), for the management of pelvic malignancies, with potential clinical advantage for simultaneous delivery of high doses to the primary disease and lower doses to the subclinical disease or electively treated regions.

Method and Materials: For this study, a sample patient diagnosed with anal cancer was randomly selected from our database. Four treatment plans were developed: (1) the 3D conformal plan for initial pelvis/inguinal node irradiation and subsequent conventional 3D boost to the primary tumor, (2) conventional 3D initial pelvis/inguinal node irradiation and subsequent IMRT boost plan, (3) Two‐phase IMRT plan for initial pelvis/inguinal node irradiation and subsequent IMRT boost plan, (4) A single‐phase SIB‐IMRT plan to deliver simultaneous boost. DVHs for the target volumes were transformed into equivalent uniform dose (EUD) using the LQ model to evaluate relative merits of these plans using tumor control probability (TCP), and normal tissue complication probability (NTCP) for normal tissues.

Results: We have found that dose distributions for SIB‐IMRT are similar in conformality to dose distributions when IMRT is divided into a large‐field phase and a boost phase. As shown in Fig. 1(a–d), both techniques produce significantly superior dose distributions compared to those obtained with 3DCRT as well as to dose distributions obtained using conventional beams for the large‐field phase followed by IMRT boost phase. We also found that normal tissues outside the target volume receive significantly lower dose due to higher dose conformality of IMRT plans (Table 1). Conclusions: SIB‐IMRT offers unique ability to produce highly conformal dose distributions with better sparing of organs at risk, with more efficient delivery, and shorter treatment duration. This new modality also facilitates the prospect of dose escalation to highly advanced tumors for better radiobiological effectiveness potentially leading to improved outcome.
SU‐FF‐T‐365: Practical Reconstruction Method for 3D CT‐Based Brachytherapy with Shielded Colpostats
Authors: Perez‐Calatayud J , Carmona Meseguer V , Lliso‐Valverde F , Ballester F , Candelad F , Gimeno J , Granero D
Published: July 2006

Introduction and purpose.‐ The most extended worldwide HDR and PDR afterloading machines are from Nucletron, using the Plato Treatment Planning System (TPS) to perform the clinical dosimetry. The Fletcher‐Williamson (FW) vaginal colpostat applicators are widely used with these machines, they are made of a very dense material (densimet‐17) which allows shielding up to 50% of the dose. Almost all the TPS have incorporated the CT‐Based Brachytherapy (BT), in which the CT catheters and applicators reconstruction is based on contiguous CT transverse slices where the user points the catheter position. The problem with these applicators is the production of artefacts that makes the reconstruction impracticable. CT Orthogonal Scout Views or scanograms (OSV) to reconstruct sources in Brachytherapy, have been well described (Meli 1995, Yue 1999). The purpose of this work is to incorporate the OSV reconstruction method to the PLATO TPS. Material and methods.‐ We have taken profit of the fact that the TPS keeps the CT coordinates. A spread sheet has been developed to reconstruct the FW from OSV. The advantage of this method is that the obtained catheter coordinates are referred to the CT coordinates. The obtained OSV points are introduced on TPS as markers in the contouring routine because the TPS does not allow direct catheter coordinate input. Even in the catheter reconstruction routine it is easy to follow the markers to recognize the catheter within the FW. Some phantoms have been built with pellets to check the procedure accuracy. Results:.‐ The global OSV accuracy obtained with the phantom tests is estimated to be within 1 mm. Significant step by step examples will be shown. Conclusions.‐ The method is easy and feasible for Plato TPS users, and not only for shielded colpostats but also for implants quasi parallel to the CT slices as breast implant.
SU‐FF‐T‐366: Prediction of Collimator Scatter Factor and Phantom Scatter Factor for Kilovoltage X‐Ray Radiation Fields
Authors: Karabrahimi V , Blais N
Published: July 2006

Purpose: Kilovoltage x‐ray tube therapy beams are used for treatment of keloids and localized superficial malignancies. The purpose of this work is to predict the collimator scatter factor and phantom scatter factor for kilovoltage x‐ray radiation fields. Method and Materials: The radiation field is defined by a variable collimator with or without lead sheets on the patient's skin. Measurement and prediction were done at tube potentials of 60, 100, 120 and 250 kVp. Our approach uses three different sets of field types: square fields defined by the collimator and square and circular fields defined by lead sheets. Two calculation methods are employed: the equivalent field and Clarkson's method. Calculation and measurement were also done for rectangular and irregular shape fields. The relative difference between predicted and measured values is given in the form of percentage error as follows: 100% (predicted value − measured value) / (measured value). Results: The error ranges between calculated and measured collimator scatter factors for the equivalent field method and Clarkson's method are, respectively, 2.41% and 1.84%. All errors are within ±1% with Clarkson's method. The results show that Clarkson's method is more accurate at predicting collimator scatter factors. The error ranges between calculated and measured phantom scatter factors for the equivalent field method and Clarkson's method are, respectively, 6.3% and 3.5%. The spread of errors is narrower for Clarkson's method. Clarkson's method is therefore more accurate at predicting phantom scatter factors. Conclusion: Using the measured data for square fields defined by the collimator together with Clarkson's method is recommended. The implementation of this method requires a minimum number of measurements which are acquired during the commissioning of the unit and can be applied in dose calculation for a variety of field shapes and sizes.
SU‐FF‐T‐369: Propagation of Linac Output and Fluence Discretization Error to Dose Distributions in IMRT
Authors: Flynn R , Kissick M , Jeraj R , Mackie T , Mehta M , Olivera G , Srinavasan S
Published: July 2006

Purpose: Linac output variation and the fluence discretization from MLC leaf sequencing are sources of IMRT delivery error. We investigate a theory describing the propagation of fluence errors to dose distributions. Method and Materials: We assume that the fluence from each beam port is the planned fluence plus a gantry angle dependent error. We assume linac output variation contributes to systematic and random error whereas fluence discretization error is purely random. Expressions for the random and systematic dose error distributions were derived with standard error propagation theory. We investigated hepatocellular carcinoma (HCC) and prostate cases planned with step‐and‐shoot IMRT and tomotherapy. Linac output errors were estimated from measurements from a linac with a malfunctioning magnetron, thus representing higher‐than‐normal output errors. The maximum relative random and systematic linac output errors were both around 2%. Results: For both patients and both modalities, the relative dose error in the PTV due to systematic linac output variations was around 1% and the random component was negligible. The dose error in the PTV due to fluence discretization was between 0.1% and 0.2% for tomotherapy and about 1% for step‐and‐shoot. The relative dose error threshold such that 90% of the voxels in the PTV had a 90% chance of error less than the threshold was 1.5% for step‐and‐shoot and 0.5% for tomotherapy. Conclusions: The relative random dose error at a point decreases as the inverse square root of the number of beams that affect that point, making the relative random dose error orders of magnitude lower than the relative systematic error. Systematic dose error changes little with the number of beams used in the treatment since it is not propagated in quadrature. The dose errors from the malfunctioning magnetron had little effect on the treatment. Conflict of Interest: TR Mackie has financial interest in TomoTherapy Inc.
SU‐FF‐T‐370: Properties of the Iso‐NTCP Envelope
Authors: Stavrev P , Schinkel C , Stavreva N , Markov K , Fallone B G
Published: July 2006

Purpose: To analytically investigate a property of the integral dose‐volume histogram (DVH) space. Method and Materials: A curve called an α‐iso‐NTCP envelope is constructed by connecting points belonging to step‐like integral DVHs each corresponding to partial organ homogeneous irradiation of relative volumes να to dose levels Dα such that the resulting NTCP is in all cases α %. The two subspaces into which the envelope divides the DVH space are analytically explored through comparing the values of the equivalent uniform doses (EUDs) corresponding to the different DVHs and using the fact that NTCP is a monotonic function of EUD as well as the monotonic nature of the integral DVH itself. Results: It is theoretically proven that any DVH passing through a point (Dα, να) from the α‐iso‐NTCP envelope, i.e. any DVH that crosses or is tangential to the envelope, will result in an NTCP⩾α%, the equality being valid only for the step‐like DVH corresponding to the partial organ homogeneous irradiation of να to Dα,. Thus, it is proven that any DVH that at least partially lies above the envelope result in NTCP>α%. For some of the DVHs lying under the envelope, e.g. those that are tangential to the envelope, it is also true that the resulting NTCP>α%. However, it was numerically demonstrated elsewhere that there exist DVHs lying entirely in the lower subspace that result in NTCP<α%. Conclusion: Since there is a chance that a DVH lying under the α‐iso‐NTCP envelope will result in NTCP<α%, it would therefore be preferable in the treatment optimization process to seek solutions for DVHs lying entirely under an iso‐NTCP envelope.
SU‐FF‐T‐374: Quality Assurance of the On‐Board Imager and Cone‐Beam Computed Tomography Systems
Authors: Yoo S , Kim G , Hammoud R , Elder E , Pawlicki T , Guan H , Fox T , Luxton G , Yin F , Munro P
Published: July 2006

Purpose: To develop a comprehensive quality assurance (QA) program for the On‐Board Imager (OBI) used for patient positioning verification and to present the results of QA tests over extended periods from multiple institutions. Radiographic and cone‐beam computed tomography (CBCT) mode have been evaluated. Methods and Materials: The QA program consists of three parts; (1) safety and functionality (2) geometry and (3) image quality. Safety checks evaluate the functionality of collision interlocks and audible alarms. Functional QA checks the flow of clinical operation during tube warm‐up. Geometry QA checks geometrical accuracy and variation of the OBI/CBCT and hardware/software. Image quality QA includes measurements to monitor accuracy and stability of the OBI/CBCT imaging system. Results: All test items show stable functionality for safety and functional QA on a daily bases. The precision of arm positioning is very high with variation of 1mm and average accuracy of < 1mm over 8 months. The average accuracy of the isocenter and couch shift is < 1mm with variation of 1mm over 11 months. Arm position was performed when the results were > 2 mm. Results of geometry QA have been stable within tolerances through the test periods. All tests for Image quality tests also show consistent results within tolerances. Conclusion: We have developed a practical yet comprehensive set of QA tests for the OBI system. Data collected for these tests over the extended period show that the OBI system has mechanical reliability and stable image quality. Nevertheless, the tests were useful in detecting performance deficits in the OBI system that needed recalibration. After accumulating enough experience and confidence on stability and accuracy, some institutions have reduced the frequency of QA checks. However, it is important that all tests are performed on a regular basis within a suggested period to establish a guideline.
SU‐FF‐T‐375: Quality Contra Complexity in IVD. A Comparative Study Between Three Semiconductor Detectors
Authors: Rikner G , Grusell E
Published: July 2006

Semiconductor detectors have frequently been used in vivo dosimetry (IVD) to confirm the dose given to a patient in external radiotherapy, due to the high mechanical stability, small size, high signal and no need of external bias. As it is important that the complexity of these measurements should be as low as possible, semiconductor detectors from three different manufactures have been compared with respect to the need of correction factors, where effects of radiation damage has been studied.

Detectors from Sun Nuclear (SN), PTW and Scanditronix Wellhöfer (SW) have been investigated in 15 MV x‐rays. The sensitivity drop, dose rate linearity, the sensitivity variation with temperature (SVWT) and “dark current” have been studied after different doses of irradiation. Parameters, which are related to the mechanical construction, the field size and directional dependence have also been studied.

It was shown that the SW and SN detectors showed small variations in the sensitivity drop and the linearity properties did not change after moderate doses, about 1 kGy of 15 MV x‐rays. PTW, however, showed a three times larger sensitivity drop and an increasingly non‐linear response. The SVWT was stable for the SW, 0,2% per centigrade, and for the SN detectors, 0,5 % respectively. The PTW detector showed a low value in the beginning, but increased after irradiation. Also the “dark current” changed after irradiation, and was in favour for the SW detector.

The directional dependence was about the same for all detectors, but the field size dependence deviated for the PTW detector.

To obtain an accuracy better than 1,5% in each parameter the complexity is rather low for the SW and SN, except for the temperature dependence for SN. It is, however, rather complicated for the PTW detector, as large corrections must be applied for some parameters.

“Research sponsored by Scanditronix‐Wellhoepher corporation.”
SU‐FF‐T‐377: Radiation Shielding Calculations and Measurement for a Helical TomoTherapy Unit in An Existing Treatment Room
Authors: Wu C , Guo F , Yang C , Stern R , Purdy J
Published: July 2006

Purpose: To quantitatively evaluate the appropriateness of an existing LINAC treatment room shielding for a Helical TomoTherapy (HT) (TomoTherapy Inc., Madison, WI, USA) unit operating under typical radiation oncology department workload. Methods and Materials: A treatment room previously designed for a Varian Clinac 600C LINAC was considered for housing a HT unit. Shielding calculations for primary and secondary radiations (leakage and scatter) were performed to evaluate whether additional shielding was needed. A typical radiation oncology patient load (40,000 rem/wk) was used. Use factors were derived and calculated based on unique HT rotational beam delivery. Leakage and scatter was included in the calculation based on corresponding measurements as documented by TomoTherapy Inc. After installation of the HT unit, a radiation survey was performed to verify calculation results. Results: Calculations showed that a typical 600C shielding design is generally sufficient for a HT unit for typical workloads. However, in our situation, additional shielding was required for a small area of one wall due to positioning of the HT unit isocenter. The radiation survey carried out after HT installation showed all barrier calculation locations satisfied radiation safety requirements. Conclusions: It is recognized that leakage radiation levels for IMRT treatments on HT are increased significantly due to increased beam‐on time to deliver modulated fields. This investigation showed that a typical 6 MV room design provides adequate shielding barriers for a HT unit operating under typical radiation oncology workload in most cases. Under some circumstances, such as placement of unit in the room away from original isocenter design, extra shielding materials may be required. A caution is thus needed for institutions considering installing new HT unit within an existing LINAC treatment room.
SU‐FF‐T‐378: Radiation Transport Software for Medical Physics Studies
Authors: Kirk B , Haghigat A , Jeraj R , Newhauser W , Rivard M , Goorley J T
Published: July 2006

Purpose: To present a summary of radiation transport software for medical physics applications. Method and Materials: The Radiation Safety Information Computational Center (RSICC), a center at Oak Ridge National Laboratory (ORNL), is the Department of Energy software center for radiation transport and safety software. The center houses over 1600 software packages and nuclear cross section data of importance to nuclear science applications. The different software packages have been applied to the following topics:

• Dosimetry calculations for radiation therapy

• Treatment planning in radiation oncology

• Design of photon and secondary neutron shielding for therapy rooms

• Evaluating and estimating patient and staff radiation dose

• Electron beam transport and energy deposition

• Secondary neutron and gamma transport and energy deposition

• Cancer brachytherapy dosimetry

• Medical diagnostic imaging applications, including SPECT, PET, and x‐ray imaging

• Error evaluations for accelerator particle delivery systems

• Modalities of treatment and exploration of alternatives

• Licensing and safety analysis for medical radiation facilities

• Medical diagnostics and therapy

Examples of software in the RSICC collection include MCNP/MCNPX, ITS, ANISN, TORT, EGS4, PARTISN, SERA, and PENELOPE. There are other software packages (not available through RSICC), which have been applied to the above topics ‐ for example, PENTRAN, A3MCNP, ATTILA, COMET, EGSnrc, TransMED, EVENT, FLUKA, PEREGRINE. Results: Studies on selected software is presented, particularly on the above applications. Conclusion: As the field of medical physics advances, computer software technology continues on the road to improvement and efficiency. Conflict of Interest: Work was supported by the Department of Energy under contract DE‐AC05‐00OR22725.
SU‐FF‐T‐379: Radiochromic Film Dosimetry for a New Type of I‐125 Seed: Determination of TG43 Parameters
Authors: Chiu‐Tsao S , Munro J , Harrison L
Published: July 2006

Purpose: A new prototype of I‐125 seed was studied using radiochromic EBT film in solid water for the dose distributions from 0.06cm out to 5cm. The high activity seed was designed for intravascular brachytherapy of lesions in peripheral arteries. The active source core was produced with Xenon ion implantation into a solid quartz core (without silver marker) and then encapsulated in titanium to form a sealed source.

Materials and methods: A multiple film technique was employed. Each film (EBT lot #35076) was in contact geometry with the seed at the center of a solid water phantom, 30×30×20 cm. Totally 46 films were separately exposed to 12 seeds, with the product of activity and exposure time between 8 and 4910 mCi‐hr. The seed activity ranged from 12.5 to 2.4 mCi during the experimental runs. 30 calibration films were exposed to one seed at 0.5 cm above or below the seed. All experimental, calibration and background films were scanned (pixel resolution 0.2mm) using a PeC CCD100 densitometer, with red and green light sources at least one day after exposures. Conversion from optical densities to doses was achieved based on the calibration curve established for each light source used in scanning. The 2‐d dose values in cylindrical coordinates were converted to polar coordinates, and the TG43 parameters were generated. Results: The radial dose function and anisotropy function were plotted. Compared with the Implant Sciences model 3500 seed, the anisotropy function values are similar, while the radial dose function values (at distances > 1cm) are higher, possibly due to the absence of silver fluorescent x‐rays. An interim value of dose rate constant was determined. Conclusion: TG43 parameters for a new type of high activity I‐125 seed have been obtained using radiochromic EBT film for distances from 0.06 to 5 cm in solid water phantom.
SU‐FF‐T‐380: Radiological Dependence of Electronic Brachytherapy Simulation On Input Parameters
Authors: Rivard MJ , Rusch TW , Axelrod S
Published: July 2006

Purpose: In comparison to 125I or 192Ir, characterization of dose rate distributions from electronic brachytherapy is subject to the additional challenge of unforeseen photon energy spectra. Towards simulating photon energy spectra and resultant dose rate distribution, Monte Carlo investigators first generate electrons which bombard the x‐ray tube anode and subsequently create photons via bremsstrahlung. Modeling techniques for this endeavor are largely unexplored. Therefore, sensitivities of spectra and dose rate distributions were assessed through varying modeling parameters for the Xoft Axxent x‐ray source. Materials & Methods: MCNP5 was used to simulate photon spectra and dose rate distributions, with comparisons to experimental measurements (PTW model 34013 chamber in liquid water) for 1<r⩽7 cm and 0°⩽θ⩽150° with simulations covering 0.3 ⩽r⩽ 15 cm and all available angles. The following source modeling parameters were evaluated for impact on in‐water spectra and dose: electron beam radius (R), electron beam annularity (R′) like a doughnut, and anode film thickness (t). Since simulations of electron:photon transport are inefficient in comparison to Monte Carlo modeling of radionuclides, MCNP variance reduction techniques such as cell importances (IMP), electron cutoff energies (PHYS:E), high‐energy biasing of bremsstrahlung spectrum (BBREM), and bremsstrahlung photon multiplicity (BNUM) were assessed. Results: Due to the complex anode shape, F(r,θ) was highly‐dependent on R, varying a factor of 2 when changing R from 0 to 0.084 cm. This effect was more pronounced when varying R′ due to less radial volume averaging. Through comparison with experimental measurements, the optimal electron beam shape had the largest spot size which could fit within the anode and no annularity; it was a uniform pencil beam. Altering MCNP variance reduction techniques did not significantly alter results, but greatly hastened simulation efficiency. Conflict of Interest: Research was sponsored in part by Xoft, Inc.
SU‐FF‐T‐385: Reproducibility of the Optical CT‐Based Polymer Gel Dosimetry
Authors: Xu Y , Wuu C S
Published: July 2006

Purpose: To study the reproducibility of the reconstructed dose distributions from optical CT scanning of polymer gel dosimeters with variation of time between irradiation and scanning using phantoms of the same and different geometry. Method and Materials: The commercial BANG®3 polymer gels from the same batch were housed in cylindrical phantoms of 3 types: a) PET bottles of 15 cm diameter and 20 cm height; b) Barex cylinders of 17 cm diameter and 15 cm height; c) Acrylic cylinders of 25 cm diameter and 25 cm height. Four PET and Acrylic gel phantoms were irradiated with a 5 field IMRT and a 3DCRT once each. Six Barex cylinders were irradiated with the same IMRT and 3DCRT plans three times each. One additional Barex gel phantom was irradiated with a 6 cm × 6cm, 12 MeV electron field for calibration purpose.

The gel phantoms were scanned using the OCTOPUS™ optical scanner one day after irradiation. One Barex phantom irradiated with IMRT fields was scanned 1 and 8 weeks later. All reconstructed images were calibrated using the dose response curve from the electron field irradiation. Root mean square of the pixel dose differences in a cylindrical region of 10 cm diameter and 10 cm height were compared for each pair of phantoms irradiated with the same plan. Results: The reconstructed dose distributions from scanning the same IMRT gel phantom 8 weeks apart change less than 1%. The root mean square dose difference among the 3 different kinds of phantoms irradiated with the IMRT and 3DCRT plans are all less than 3%. Conclusion: The optical CT‐based polymer gel dosimetry is reproducible within the experimental uncertainty (3%) when different gel phantoms are irradiated and scanned at different time after irradiation.

This work was supported by MGS research Inc. (Madison, CT; www.mgsresearch.com).
SU‐FF‐T‐386: Respiratory Gating in the Treatment of Liver Tumors
Authors: Briere T , Krishnan S , Dong L , Balter P , Crane C , Mohan R , Gillin M , Beddar A
Published: July 2006

Purpose: To determine if respiratory gated radiotherapy is feasible for the treatment of liver tumors. Method and Materials: The patient was a 70 year‐old woman with metastatic liver cancer. We used the RPM system (Varian Medical Systems) to track the respiratory cycle of the patient and gate the beam at the end of the respiratory cycle. Three implanted gold fiducials were used for daily patient positioning with AP and Lateral electronic portal images. The fiducials were also used to verify the location of the tumor during treatment. Prior to the initial treatment, the patient underwent a mock treatment in which a series of portal images were taken to verify the respiratory cycle correlated with the position of the tumor at the end of expiration. Results: The patient's mock treatment was successful, with nearly all fiducial locations lying within a sphere of 2.5 mm. Analysis of the data obtained over the course of treatment (∼600 data points) showed that 95% of the fiducial locations stayed within a sphere of about 7 mm. The uncertainty due to set‐up was ±4.6 mm, while the precision in the gating window was ±2.2 mm. Conclusion: The location of the liver tumor was well reproduced throughout the course of treatment. Our results suggest that we can use respiration to successfully track the motion of a liver tumor and gate the beam at the end of the respiratory cycle.
SU‐FF‐T‐389: Sensitivity of Document Scanners Used for Radiochromic Film Dosimetry
Authors: Wang Y , Lewis D , Poon E , Verhaegen F , Podgorsak E , Devic S
Published: July 2006

Purpose: The use of color CCD‐based document scanners, having an ability to perform scans in the transmission mode, has been suggested as an inexpensive option in the process of radiochromic film dosimetry. These scanners allow acquisition of transmission scans in up to a 48‐bit red‐greenblue (RGB) mode. Since the absorption spectrum of the radiochromic film exhibits a maximum in the red region of the visible spectrum, extraction of the red channel from the RGB image improves document scanner sensitivity when used in combination with radiochromic films. However; transmission scanners use a linear CCD array that suffers from a difference in response along the array. In this work, we present our measurements of the spatial response of an AGFA Arcus II document scanner used for radiochromic film dosimetry.

Method and Materials: To create various pixel value levels, we employed 5 neutral density filters and their combinations. The profiles were fitted with the sixth order polynomial and consequently used for image correction. The sensitivity curves have been created by normalizing their values to the central profile value.

Results: In the regions close to scanner bed edges, the non‐uniformity of the raw pixel data depends on the pixel level detected by the CCD array and can be as large as 15%. Over the 80% of the scanning region, the uniformity is of the order of 2% after corrections are applied. However in areas close to the left and right edges of the scan deviations can reach 5% even after the correction. Conclusions: We have demonstrated that in transmission scanners, using linear CCD array detectors, a considerable change in sensitivity may occur along the detector array direction. If not properly corrected for, this spatial sensitivity may obscure a 2D accuracy of the radiochromic film dosimetry. Conflict of interests: Research is sponsored by ISP.
SU‐FF‐T‐390: Setting Apart the Intensity Delivered to Moving Target, and Intensity Delivered to Moving Organ at Risk, in DMLC IG IMRT
Authors: Papiez L , McMahon R , Rangaraj D
Published: July 2006

Purpose: To utilize tissue motion in order to benefit radiation therapy delivery. The approach makes possible to deliver the required by plan intensity to the target while sparing sensitive organs beyond what is attainable during delivery to static geometry of the patient body. Method and Materials: Few parameters can be identified which control the delivery of the intensity maps during DMLC IMRT that do not need to be unambiguously specified in order to paint a given, treatment plan derived, intensity over the target. In particular, the initial moment of DMLC delivery, speeds of the leading leaves of MLC assembly and the intensity rate of the beam can all be varied in some interdependent manner to minimize intensity to organs at risk without compromising the delivery of a given intensity map over the moving target. Formulas that define algorithms achieving these goals are presented. Special attention is given to simplified methods of multi‐parameter optimizations for which subsequent parameters are treated independently. Results: Two examples of optimal sparing of organs at risk are provided for DMLC IGIMRT. In the first case the moment of initiation of radiation delivery as well as the speed of leading leaf is appropriately regulated with the aim of minimizing the cumulative dose delivered to the sensitive organ that performs periodic movement with respect to target. The second example minimizes the delivery to sensitive organ for identically moving target and organ at risk in the case when variation of the beam intensity rate is permitted during exposure. Conclusion: The method of DMLC IGIMRT allows improving the quality of radiation therapy when patient organs are moving at delivery. The required ingredients are: the a priori information about the motion of the target and organs at risk and the algorithm for the optimization of leaf trajectories for moving targets.