Block Tray Research Articles

Errors in manual radiotherapy treatment procedures and their evolution in a low resource setting: Uganda's experience

PurposeIn Uganda, two-dimensional (2D) radiotherapy treatments have been in use since the establishment of radiotherapy in 1995. Preliminary investigations of treatment records in November 2019 showed evidence of gaps requiring urgent attention. The purpose of this study was to improve the safety of the treatments. MethodsRecords of 1164 patients treated in 1387 courses (1412 sites) on Cobalt-60 units were reviewed todetermine the frequency and dosimetric implications of events that occurred at different stepsof the radiotherapy process. The results were presented and discussed with the differentprofessionals for learning purposes. ResultsMost common dosimetric eventswere omission of block tray, bolus and couch transmission factors in time calculations, incorrect field sizes and depths, wrong beam weighting, independent calculations and prescription doses contributing 28.6 %, 10.1 %, 6.0 %,11.9 %, 10.1 %, 5.4 %, 4.8 % and 8.9 % to the 168 observed errors. Comparison of the calculated treatment doses with the prescribed doses showed that 88 % of the 1412 sites were treated with radiation doses within an accuracy of ± 5 %. However, an analysis of the evolution along the years demonstrated an improvement from 82.8 % in 2018 to 86.1 % in 2019, and 93.2 % in 2020. Most common procedural events were incomplete setup instructions and missing patient data in the record and verify system of the Co-60 units for 57 % and 60.1 % of the 1164 patients. ConclusionsOpportunities for improvement of safety in the delivery of radiotherapy treatments were identified. Learning from these past errors should raise awareness in the team leading to a safer treatments.

Inter-fractional entrance dose monitoring as quality assurance using Gafchromic EBT3 film.

This study describes a simple method of inter-fractional photon beam monitoring to measure the entrance dose of radiation treatment using Gafchromic EBT3 film. The film was placed at the center of a 1-cm thick phantom shaped like a block tray and fixed on the accessory tray of the gantry. The entrance dose was measured following the placement of the film in the accessory tray. The dose distribution calculated with the treatment planning system was compared with the dose distribution on the irradiated EBT3 films. The effectiveness of this methodology, as determined by gamma passing rates, was evaluated for the 22 fields of eight three-dimensional conformal radiotherapy (3D-CRT) plans and the 41 fields of nine intensity-modulated radiotherapy (RT) plans. The plans for three-dimensional conformal RT included treatments of the rectum, liver, breast, and head and neck, whereas the plans for intensity-modulated RT included treatments of the liver, brain, and lung. The gamma passing rates for 3D-CRT ranged from 96.4% to 99.5%, with the mean gamma passing rate for 22 fields being 98.0%. The gamma passing rate for intensity-modulated RT ranged from 96.1% to 98.9%, with the mean gamma passing rate for 41 fields being 97.7%. All gamma indices were over the 95% tolerance level. The methodology described in this study, based on Gafchromic EBT3 film, can be utilized for inter-fractional entrance dose monitoring as quality assurance during RT. Clinical application of this method to patients can verify the accuracy of beam delivery in the treatment room.

Effect of block tray on Build-up dose in γ- ray radiotherapy

Effect of block tray on Build-up dose in γ- ray radiotherapy

A customizable aluminum compensator system for total body irradiation.

PurposeTo develop a simplified aluminum compensator system for total body irradiation (TBI) that is easy to assemble and modify in a short period of time for customized patient treatments.MethodsThe compensator is composed of a combination of 0.3 cm thick aluminum bars, two aluminum T‐tracks, spacers, and metal bolts. The system is mounted onto a plexiglass block tray. The design consists of 11 fixed sectors spanning from the patient's head to feet. The outermost sectors utilize 7.6 cm wide aluminum bars, while the remaining sectors use 2.5 cm wide aluminum bars. There is a magnification factor of 5 from the compensator to the patient treatment plane. Each bar of aluminum is interconnected at each adjacent sector with a tongue and groove arrangement and fastened to the T‐track using a metal washer, bolt, and nut. Inter‐bar leakage of the compensator was tested using a water tank and diode. End‐to‐end measurements were performed with an ion chamber in a solid water phantom and also with a RANDO phantom using internal and external optically stimulated luminescent detectors (OSLDs). In‐vivo patient measurements from the first 20 patients treated with this aluminum compensator were compared to those from 20 patients treated with our previously used lead compensator system.ResultsThe compensator assembly time was reduced to 20–30 min compared to the 2–4 h it would take with the previous lead design. All end‐to‐end measurements were within 10% of that expected. The median absolute in‐vivo error for the aluminum compensator was 3.7%, with 93.8% of measurements being within 10% of that expected. The median error for the lead compensator system was 5.3%, with 85.1% being within 10% of that expected.ConclusionThis design has become the standard compensator at our clinic. It allows for quick assembly and customization along with meeting the Task Group 29 recommendations for dose uniformity.

A method for generating intensity-modulated radiation therapy fields for small animal irradiators utilizing 3D-printed compensator molds.

The purpose of this study was to investigate the feasibility of using fused deposition modeling (FDM) three-dimensional (3D) printer to generate radiation compensators for high-resolution (~1mm) intensity-modulated radiation therapy (IMRT) for small animal radiation treatment. We propose a novel method incorporating 3D-printed compensator molds filled with NaI powder. The inverse planning module of the computational environment for radiotherapy research (CERR) software was adapted to simulate the XRAD-225Cx irradiator, both geometry and kV beam quality (the latter using a phase space file provided for XRAD-225Cx). A nine-field IMRT treatment was created for a scaled-down version of the imaging and radiation oncology core (IROC) Head and Neck IMRT credentialing test, recreated on a 2.2-cm-diameter cylindrical phantom. Optimized fluence maps comprising nine fields and a total of 2564 beamlets were calculated at resolution of 1.25×1.25mm2 . A hollow compensator mold was created (using in-house software and algorithm) for each field using 3D printing with polylactic acid (PLA) filaments. The molds were then packed with sodium iodide powder (NaI, measured density ρNaI =2.062g/cm3 ). The mounted compensator mold thickness was limited to 13.8mm due to clearance issues with couch collision. At treatment delivery, each compensator was manually mounted to a customized block tray attached to the reference 40×40mm2 collimator. Compensator reproducibility among three repeated 3D-printed molds was measured with Radiochromic EBT2 film. The two-dimensional (2D) dose distributions of the nine fields were compared to calculated 2D doses from CERR using gamma comparisons with distance-to-agreement criteria of 0.5-0.25mm and dose difference criteria of 3-5%. Good reproducibility of 3D-printed compensator manufacture was observed with mean error of ±0.024Gy and relative dose error of ±4.2% within the modulated part of the beam. Within the limit of 13.8mm compensator height, a maximum radiation blocking efficiency of 91.5% was achieved. Per field, about 45.5g of NaI powder was used. Gamma analysis on each of the nine delivered IMRT fields using radiochromic films resulted in eight of nine treatment fields with >90% pass rate with 5%/0.5mm tolerances. However, low gamma passing rate of 49-66% (3%/0.25mm to 5%/0.5mm) was noted in one field, attributed to fabrication errors resulting in over-filling the mold. The nine-field treatment plan was delivered in under 30min with no mechanical or collisional issues. We show the feasibility of high spatial resolution IMRT treatment on a small animal irradiator utilizing 3D-printed compensator shells packed with NaI powder. Using the PLA mold with NaI powder was attractive due to the ease of 3D printing a PLA mold at high geometric resolution and the well-balanced attenuation properties of NaI powders that prevented the mold from becoming too bulky. IMRT fields with 1.25-mm resolution are capable with significant fluence modulation with relative dose accuracy of ±4.2%.

Surface Dose Measurements on an Indigenously Made Inhomogeneous Female Pelvic Phantom Using Metal-Oxide-Semiconductor-Field-Effect-Transistor Based Dosimetric System

Introduction: Megavoltage X-ray beams are used to treat cervix cancer due to their skin-sparing effect. Preferably, the radiation surface doses should be negligible; however, it increases due to electron contamination produced by various field parameters. Therefore, it is essential to provide proper knowledge about the effect of different field parameters on radiation doses. This study sought to find out the effect of various physical parameters on the surface doses. Materials and Methods: The effects of field size, source-to-surface distance, and open or acrylic block tray fields on surface doses were determined. Metal-Oxide-Semiconductor-Field-Effect-Transistor based dosimetric system was used for dose measurements for 6 MV photon beam. The directly measured radiation surface doses on pelvic phantom were compared to surface dose values computed by treatment planning system in the similar field parameters. Results: The measured results for the percentage depth dose (PDD0) in field size of 10x10 cm2 were 13.32%, 12.95% and 13.87% for open field and 36.87%, 36.31% and 35.88% for acrylic block tray field. In addition, the computed doses were 7.83%, 7.73% and 7.65% for open field and 16.33%, 16.12% and 15.88% for acrylic block tray field at 80 cm, 100 cm, and 120 cm SSDs, respectively. Conclusion: The surface dose increases along with the size of the field and decreases with increasing SSD. The surface doses in acrylic block tray fields were significantly higher than the open ones. The treatment planning system computed a lesser radiation doses in same field parameters.

Effect of scattered electrons on the ‘Magic Plate’ transmission array detector response

Transmission type detectors can provide a measure of the energy fluence and if they are real-time systems that do not significantly attenuate the radiation beam have a distinct advantage over the current method as Quality Assurance (QA) could in principle be done during the actual patient treatment. The use of diode arrays in QA holds much promise due to real-time operation and feedback when compared to other methods e.g. films which are not real-time. The goal of this work is to describe the characterization of the radiation response of a silicon diode array called the Magic Plate (MP) when operated in transmission mode (MPTM). The response linearity of MPTM was excellent (R2=1). When the MP was placed in linac block tray position; the change in PDD at phantom surface (SSD 100 cm) for a 10 × 10 cm2 was -0.037 %, -0.178 % and -0.949 % for 6 MV, 10 MV and 18 MV beams. Therefore, MP does not provide a significant increase in skin dose to the patient and the percentage depth doses showed an excellent agreement with and without MPTM for 6 MV, 10 MV and 18 MV beams.

Surface Dose Assessment for Different Clinical set up Parameters from High Energy Photon Beams

Introduction: The purpose of this study was to investigate surface doses of 6 and 10 MV energies of linear accelerator for different clinical setup parameters including (field size, gantry angle, SSD (source skin distance), PW (physical wedge), acrylic block tray and bolus), Comparison between measured surface dose by P.P (parallel plate) and calculated surface dose by TPS (Treatment Planning system). Comparison between surface doses measured by TLD (Thermo luminescence dosimeter) and P.P ion chamber, Comparison between the surface dose of 3DCRT and IMRT techniques. Materials and Methods: Surface dose measurements were carried out using a (PTW) Markus parallel-plate ion chamber in a plastic water plastic water phantom for various setup parameters using Primus Siemens (6, 10 MV) linear accelerator. For the normalization depth, i.e., the depth of maximum dose 1.5, 2.5 cm were chosen for 6, 10 MV photon beams, respectively. Results and Discussion: The measured skin dose values for 10 MV were lower than those of 6 MV, the skin dose increased as field size increased. The measured surface dose by P.P for 6 and 10 MV are 16%, 25%, for 10 × 10 cm2 square field size, within the first 2 millimetres of the build-up region, at field size 10 × 10 cm2 the PDD for a 6 and 10 MV photon beam increases from 24% to 62%, 16% to 44% respectively. With increasing the gantry angles (0 to 30) produces a minimal effect of dose, (40-70) gantry angles produce a significant increase. When studying the surface dose with different SSD found that the percentage of surface dose is nearly stable, the absolute surface dose (cGy) increased with decreasing SSD. The absolute skin dose for wedge fields were lower than for open fields, for field size 10 × 10 cm2, while the values of surface dose of wedge 30 are 20% and 14% for 6 and 10 MV respectively. The skin dose for a wedge field increased as field size increased. Bolus 1 cm material increased the surface dose for 10 × 10 cm2 from 24% to 96%, and from 16% to 87%, for 6 and 10 MV respectively. With the use of an acrylic block tray, the surface dose increased to the open fields for all field sizes higher than 10 cm2, but the increase was dominant for large fields, for field size 20 × 20 cm2 the surface dose increased from 29% to 41%, from 22% to 34% for 6 and 10 MV respectively. Conclusion: Agreement between skin doses calculated by multidata TPS and those measured by P.P ion chamber in water plastic water phantom was better than 20% for 96% of measurement points and this is indicate that the dose calculation in the build-up region using multidata TPS is good. For irregular tumor shape and closed OAR the IMRT is a good solution to cover the tumor and spare the critical organs without increasing the skin dose.

Beam perturbation characteristics of a 2D transmission silicon diode array, Magic Plate.

The main objective of this study is to demonstrate the performance characteristics of the Magic Plate (MP) system when operated upstream of the patient in transmission mode (MPTM). The MPTM is an essential component of a real‐time QA system designed for operation during radiotherapy treatment. Of particular interest is a quantitative study into the influence of the MP on the radiation beam quality at several field sizes and linear accelerator potential differences. The impact is measured through beam perturbation effects such as changes in the skin dose and/or percentage depth dose (PDD) (both in and out of field). The MP was placed in the block tray of a Varian linac head operated at 6, 10 and 18 MV beam energy. To optimize the MPTM operational setup, two conditions were investigated and each setup was compared to the case where no MP is positioned in place (i.e., open field): (i) MPTM alone and (ii) MPTM with a thin passive contamination electron filter. The in‐field and out‐of‐field surface doses of a solid water phantom were investigated for both setups using a Markus plane parallel (Model N23343) and Attix parallel‐plate, MRI model 449 ionization chambers. In addition, the effect on the 2D dose distribution measured by the Delta4 QA system was also investigated. The transmission factor for both of these MPTM setups in the central axis was also investigated using a Farmer ionization chamber (Model 2571A) and an Attix ionization chamber. Measurements were performed for different irradiation field sizes of 5×5 cm2 and 10×10 cm2. The change in the surface dose relative to dmax was measured to be less than 0.5% for the 6 MV, 10 MV, and 18 MV energy beams. Transmission factors measured for both set ups (i & ii above) with 6 MV, 10 MV, and 18 MV at a depth of dmax and a depth of 10 cm were all within 1.6% of open field. The impact of both the bare MPTM and the MPTM with 1 mm buildup on 3D dose distribution in comparison to the open field investigated using the Delta4 system and both the MPTM versions passed standard clinical gamma analysis criteria. Two MPTM operational setups were studied and presented in this article. The results indicate that both versions may be suitable for the new real‐time megavoltage photon treatment delivery QA system under development. However, the bare MPTM appears to be slightly better suited of the two MP versions, as it minimally perturbs the radiation field and does not lead to any significant increase in skin dose to the patient.PACS number(s): 87.50.up, 87.53.Bn, 87.55.N, 87.55.Qr, 87.56.Fc.

2D mapping of the MV photon fluence and 3D dose reconstruction in real time for quality assurance during radiotherapy treatment

Summary: the photon irradiation response of a 2D solid state transmission detector array mounted in a linac block tray is used to reconstruct the projected 2D dose map in a homogenous phantom along rays that diverge from the X-ray source and pass through each of the 121 detector elements. A unique diode response-to-dose scaling factor, applied to all detectors, is utilised in the reconstruction to demonstrate that real time QA during radiotherapy treatment is feasible. Purpose: to quantitatively demonstrate reconstruction of the real time radiation dose from the irradiation response of the 11×11 silicon Magic Plate (MP) detector array operated in Transmission Mode (MPTM). Methods and Materials: in transmission mode the MP is positioned in the block tray of a linac so that the central detector of the array lies on the central axis of the radiation beam. This central detector is used to determine the conversion factor from measured irradiation response to reconstructed dose at any point on the central axis within a homogenous solid water phantom. The same unique conversion factor is used for all MP detector elements lying within the irradiation field. Using the two sets of data, the 2D or 3D dose map is able to be reconstructed in the homogenous phantom. The technique we have developed is illustrated here for different depths and irradiation field sizes, (5 × 5 cm2 to 40 × 40 cm2) as well as a highly non uniform irradiation field.Results: we find that the MPTM response is proportional to the projected 2D dose map measured at a specific phantom depth, the sweet depth. A single factor, for several irradiation field sizes and depths, is derived to reconstruct the dose in the phantom along rays projected from the photon source through each MPTM detector element. We demonstrate that for all field sizes using the above method, the 2D reconstructed and measured doses agree to within ± 2.48% (2 standard deviation) for all in-field MP detector elements. Conclusions: a 2D detector system and method to reconstruct the dose in a homogeneous phantom and in real time has been demonstrated. The success of this work is an exciting development toward real time QA during radiotherapy treatment.

2D mapping of the MV photon fluence and 3D dose reconstruction in real time for quality assurance during radiotherapy treatment

Summary: the photon irradiation response of a 2D solid state transmission detector array mounted in a linac block tray is used to reconstruct the projected 2D dose map in a homogenous phantom along rays that diverge from the X-ray source and pass through each of the 121 detector elements. A unique diode response-to-dose scaling factor, applied to all detectors, is utilised in the reconstruction to demonstrate that real time QA during radiotherapy treatment is feasible. Purpose: to quantitatively demonstrate reconstruction of the real time radiation dose from the irradiation response of the 11×11 silicon Magic Plate (MP) detector array operated in Transmission Mode (MPTM).Methods and Materials: in transmission mode the MP is positioned in the block tray of a linac so that the central detector of the array lies on the central axis of the radiation beam. This central detector is used to determine the conversion factor from measured irradiation response to reconstructed dose at any point on the central axis within a homogenous solid water phantom. The same unique conversion factor is used for all MP detector elements lying within the irradiation field. Using the two sets of data, the 2D or 3D dose map is able to be reconstructed in the homogenous phantom. The technique we have developed is illustrated here for different depths and irradiation field sizes, (5 × 5 cm2 to 40 × 40 cm2) as well as a highly non uniform irradiation field.Results: we find that the MPTM response is proportional to the projected 2D dose map measured at a specific phantom depth, the “sweet depth”. A single factor, for several irradiation field sizes and depths, is derived to reconstruct the dose in the phantom along rays projected from the photon source through each MPTM detector element. We demonstrate that for all field sizes using the above method, the 2D reconstructed and measured doses agree to within ± 2.48% (2 standard deviation) for all in-field MP detector elements.Conclusions: a 2D detector system and method to reconstruct the dose in a homogeneous phantom and in real time has been demonstrated. The success of this work is an exciting development toward real time QA during radiotherapy treatment.

Dosimetric measurements of Onyx embolization material for stereotactic radiosurgery

Arteriovenous malformations are often treated with a combination of embolization and stereotactic radiosurgery. Concern has been expressed in the past regarding the dosimetric properties of materials used in embolization and the effects that the introduction of these materials into the brain may have on the quality of the radiosurgery plan. To quantify these effects, the authors have taken large volumes of Onyx 34 and Onyx 18 (ethylene-vinyl alcohol copolymer doped with tantalum) and measured the attenuation and interface effects of these embolization materials. The manufacturer provided large cured volumes (∼28 cc) of both Onyx materials. These samples were 8.5 cm in diameter with a nominal thickness of 5 mm. The samples were placed on a block tray above a stack of solid water with an Attix chamber at a depth of 5 cm within the stack. The Attix chamber was used to measure the attenuation. These measurements were made for both 6 and 16 MV beams. Placing the sample directly on the solid water stack and varying the thickness of solid water between the sample and the Attix chamber measured the interface effects. The computed tomography (CT) numbers for bulk material were measured in a phantom using a wide bore CT scanner. The transmission through the Onyx materials relative to solid water was approximately 98% and 97% for 16 and 6 MV beams, respectively. The interface effect shows an enhancement of approximately 2% and 1% downstream for 16 and 6 MV beams. CT numbers of approximately 2600-3000 were measured for both materials, which corresponded to an apparent relative electron density (RED) ρ(e) (w) to water of approximately 2.7-2.9 if calculated from the commissioning data of the CT scanner. We performed direct measurements of attenuation and interface effects of Onyx 34 and Onyx 18 embolization materials with large samples. The introduction of embolization materials affects the dose distribution of a MV therapeutic beam, but should be of negligible consequence for effective thicknesses of less than 8 mm. The measured interface effects are also small, particularly at 6 MV. Large areas of high-density artifacts and low-density artifacts can cause errors in dose calculations and need to be identified and resolved during planning.

Poster - Thur Eve - 56: Verification of buildup region dose calculation of a treatment planning system.

Buildup region dose calculation of Pinnacle3 (version 9.0), a treatment planning system (TPS) commissioned using cylindrical ionization chamber measurements, have been verified experimentally. Dose values measured using Attix parallel plate ionization chamber were compared with those calculated by Pinnacle3 for a variety of clinical setups involving: 6 MV and 15 MV photon beams, open fields, enhanced dynamic wedges, physical wedges, block tray, 85, 100 and 120 cm source-to-surface distances, and square field sizes ranging from 3 × 3 to 30 × 30 cm2 . The dose difference (DD) and distance-to-agreement (DTA) techniques were used to evaluate the discrepancy between measured and calculated dose values. Based on the criteria of DD less than 2% or DTA less than 2mm, 93.7% of 1,710 dose points for the 6 MV and 96.1% of 2,244 dose points for 15 MV passed. Dose points that failed were mostly for open fields, block tray fields, and physical wedges (15 and 30 degrees) fields; this is attributed to high electron contamination (EC) associated with these fields. The levels of discrepancies between measured and calculated dose values were greatly reduced after remodeling the EC in Pinnacle3 using Attix chamber measurements, an indication that the EC equation in Pinnacle3 may be adequate for modeling EC in the dose buildup region, and the commissioning of a TPS using cylindrical ionization chamber measurements may not provide accurate buildup region dose calculation. Attix chamber measurements were validated using GafChromic EBT2 film; the disagreement was less than 3% for 89.9% of dose values compared.

Skin dose during radiotherapy: a summary and general estimation technique

The skin dose associated with radiotherapy may be of interest for clinical evaluation or investigating the risk of late effects. However, skin dose is not intuitive and is difficult to measure. Our objectives were to develop and evaluate a general estimation technique for skin dose based on treatment parameters. The literature on skin dose was supplemented with measurements and Monte Carlo simulations. Using all available data, a general dosimetry system was developed (in the form of a series of equations) to estimate skin dose based on treatment parameters including field size, the presence of a block tray, and obliquity of the treatment field. For out‐of‐field locations, the distance from the field edge was also considered. This dosimetry system was then compared to TLD measurements made on the surface of a phantom. As compared to measurements, the general dosimetry system was able to predict skin dose within, on average, 21% of the local dose (4% of the Dmax dose). Skin dose for patients receiving radiotherapy can be estimated with reasonable accuracy using a set of general rules and equations.PACS numbers: 87.53.‐j, 87.53.Bn, 87.55.ne

SU‐C‐BRA‐01: Efficient Generation of Beamlet Arrays with Hybrid Multileaf Collimator for Grid Therapy

Purpose: To develop a simple method of generating megavoltage beamlet arrays for efficient grid therapy. Methods: Multileaf collimators (MLC) were constructed by mounting equally‐spaced, rectangular, lead sheets on Varian linac block trays, in a divergent geometry. The lead leaves were positioned orthogonal to the same alternating leaf pattern beneath a Varian MLC. Lead sheets with nominal thicknesses of 6.4, 3.2 and 1.6 mm resulted in projected beamlets at linac isocenter of 10×10, 5×5 and 5×2.5 mm respectively. Relative to an open beam, the symmetric grid beam area is 25% open, 50% shadow of single MLC leaf and 25% shadow of overlapping MLC leaves. Kodak EDR‐2 film, ISP Gafchromic EBT film and leuco crystal violet micelle gels with optical cone beam CT readout were used to characterize dose distributions. The impact of the x‐ray beam flattening filter was measured by operating the Varian 2100 series linac in 6 MV mode with and without the filter. Modulation ratios were recorded at water depths of 15 to 200 mm. Orthogonal 5×5 mm grid beams offset by 5 mm were delivered to a 95 mm diameter cylinder of gel in a cubic water phantom at 100 mm depths. Results: Dose rates from beamlet arrays and single beamlets, defined by linac jaws, were similar. Removing the flattening filter doubled the dose rate but had a minor effect on dose modulation. Optical density modulation ratio (5×5 mm beamlet / single leaf shadow) decreased from 7 at 15 mm to 4 at 200 mm depth in water. The gel 3D distribution demonstrated the anticipated features of intersecting diverging beamlet arrays with central dose modulations of 3.5. Conclusions: A hybrid system of orthogonal linac and block tray MLC leaves provides a simple method of generating arrays of megavoltage photon beamlets for efficient delivery of grid therapy.W Francis was partially supported by a London Regional Cancer Program Small Grants for grid therapy development.

A Monte Carlo investigation of contaminant electrons due to a novel in vivo transmission detector

A novel transmission detector (IBA Dosimetry, Germany) developed as an IMRT quality assurance tool, intended for in vivo patient dose measurements, is studied here. The goal of this investigation is to use Monte Carlo techniques to characterize treatment beam parameters in the presence of the detector and to compare to those of a plastic block tray (a frequently used clinical device). Particular attention is paid to the impact of the detector on electron contamination model parameters of two commercial dose calculation algorithms. The linac head together with the COMPASS transmission detector (TRD) was modeled using BEAMnrc code. To understand the effect of the TRD on treatment beams, the contaminant electron fluence, energy spectra, and angular distributions at different SSDs were analyzed for open and non-open (i.e. TRD and block tray) fields. Contaminant electrons in the BEAMnrc simulations were separated according to where they were created. Calculation of surface dose and the evaluation of contributions from contaminant electrons were performed using the DOSXYZnrc user code. The effect of the TRD on contaminant electrons model parameters in Eclipse AAA and Pinnacle3 dose calculation algorithms was investigated. Comparisons of the fluence of contaminant electrons produced in the non-open fields versus open field show that electrons created in the non-open fields increase at shorter SSD, but most of the electrons at shorter SSD are of low energy with large angular spread. These electrons are out-scattered or absorbed in air and contribute less to surface dose at larger SSD. Calculated surface doses with the block tray are higher than those with the TRD. Contribution of contaminant electrons to dose in the buildup region increases with increasing field size. The additional contribution of electrons to surface dose increases with field size for TRD and block tray. The introduction of the TRD results in a 12% and 15% increase in the Gaussian widths used in the contaminant electron source model of the Eclipse AAA dose algorithm. The off-axis coefficient in the Pinnacle3 dose calculation algorithm decreases in the presence of TRD compared to without the device. The electron model parameters were modified to reflect the increase in electron contamination with the TRD, a necessary step for accurate beam modeling when using the device.

Imaging Innovations Lead to Advances in Radiation Therapy

Imaging Innovations Lead to Advances in Radiation Therapy

SU‐GG‐T‐514: Dosimetric Measurements of Onyx Embolization Glue for Stereotactic Radiosurgery

Purpose: Arterio‐venous malformations (AVM) are often treated with a combination of embolization and stereotactic radiosurgery (SRS). Concern has been expressed in the past regarding the dosimetric properties of glues used in embolization and the effects that the introduction of these glues into the brain may have on the quality of the radiosurgery plan. To quantify these effects we have taken large volumes of Onyx©35 and Onyx©18 (ethylene‐vinyl alcohol copolymer doped with tantalum) and measured the attenuation and interface effects of these glues. Methods and Materials: The manufacturer provided large cured volumes (∼28 cc) of both Onyx glues. These samples were 8.5 cm in diameter with a nominal thickness of 5 mm. The samples were placed on a block tray above a stack of solid water with an Attix chamber at a depth of 5 cm within the stack. The Attix chamber was used to measure the attenuation. These measurements were made for 6 MV and 16 MV beams. The interface effects were measured by placing the sample directly on the solid water stack and varying the thickness of solid water between the sample and the Attix chamber. CT numbers of approximately 2700 were measured for both glues. Results: The transmission through the glues relative to solid water was approximately 98% and 97% for 16 MV and 6 MV beams respectively. The interface effect shows an enhancement of approximately 2% and 1% downstream for a 16 MV and 6 MV beams respectively. The largest effect that must be considered is the correction for the apparent attenuation due to the large CT numbers of the glue. Conclusions: The introduction of embolization glues has negligible affect on the dose distribution of MV therapeutic beams. These results are in quantitative agreement with previous Monte Carlo calculations by others. Research sponsored by the MTI Corporation.

SU-GG-T-610: Skin Dose during Radiotherapy: A Summary and General Estimation Technique

Purpose: The skindose associated with radiotherapy is often of interest for clinical evaluation or examining the risk of late effects. Because the skindose is difficult to measure, it is of interest to have a general dosimetry system for estimating the skindose based on treatment parameters. Our objectives were to review the literature, conduct measurements to supplement the literature, and develop a general dosimetry system suitable for estimating the skindose for clinical situations or epidemiologic evaluation. Methods: The literature was supplemented with measurements and Monte Carlo simulations. In particular, the impact of a block tray on skindoses from a cobaltbeam was evaluated with TLD measurements. The skindose outside of the treatment field for MV beams as a function of field size and distance from the edge of the treatment field was examined with Monte Carlo. Using the entirety of available data, a general dosimetry system was developed in the form of a series of equations to evaluate the skindose based on treatment parameters including the Dmaxdose,treatment modality, and field size.Results: While there was little difference in skindose between MV beam energies, cobalt,electron beams, and orthovoltage produced substantially different skindoses. Within the treatment field, it was found that the skindose increased with field size, the presence of a block tray, and the obliquity of the incident beam to the patient's surface. Outside the treatment field, the skindose was highly elevated relative to the dose at depth, increasing with increasing field size and distance from the field edge to as much as a factor of 7. Conclusions: On the basis of the literature and the findings of this study we developed a set of general rules and equations that estimate the skindose for patients receiving radiotherapy.

SU-GG-T-395: Monitor Unit Calculation for Electron Arc Therapy Using Monte Carlo Simulations

Purpose: This study reported a monitor unit (MU) calculation method for electron arc therapy. This MU calculation was carried out using Monte Carlo simulation and verified by measurement. Methods and Materials: The 9 MeV phase space electron beam (2.6 × 21 cm2) attached to the block tray holder of a Varian 21 EX linac was generated using the EGSnrc‐based BEAMnrc code, and was verified by dosimetric measurements using ionization chamber and radiographic film. The relative output factor (ROF), source‐to‐surface (SSD) offset and dwell factor (DF), required in the MU calculation, were determined using measurement and Monte Carlo simulation. Solid water rectangular and cylindrical phantom made of polystyrene were used in the measurements. Results: Deviations of ROF, SSD offset and DF between measured and Monte Carlo results were 1.2%, 0.18% and 1.5% respectively. Our results show that the DF decreased with an increase of the treatment arc angle (α), and such decrease of DF was more significant in the α range of 0 – 60 deg than 60 – 120 deg. Comparing the DF determined using the rectangular and cylindrical phantom through measurement and Monte Carlo simulation, we found that the DF determined by the rectangular agreed well with that by the cylindrical phantom within ±1.2%. It shows that a simple setup of solid water rectangular phantom was sufficient to replace the cylindrical phantom to determine the DF associated with the electron arc. Conclusions:Monte Carlo simulation was proved to be an alternative way to determine the MU for electron arc therapy. Since Monte Carlo simulation can generate pre‐calculated database of ROF, SSD offset and DF for the MU calculation with a reduction of human effort and linac beam‐on time, it is suggested to integrate Monte Carlo simulation into the commissioning of electron arc therapy.