Cm Of Solid Water Research Articles

Validation of an in vivo transit dosimetry algorithm using Monte Carlo simulations and ionization chamber measurements.

Transit dosimetry is a safety tool based on the transit images acquired during treatment. Forward-projection transit dosimetry software, as PerFRACTION, compares the transit images acquired with an expected image calculated from the DICOM plan, the CT, and the structure set. This work aims to validate PerFRACTION expected transit dose using PRIMO Monte Carlo simulations and ionization chamber measurements, and propose a methodology based on MPPG5a report. The validation process was divided into three groups of tests according to MPPG5a: basic dose validation, IMRT dose validation, and heterogeneity correction validation. For the basic dose validation, the fields used were the nine fields needed to calibrate PerFRACTION and three jaws-defined. For the IMRT dose validation, seven sweeping gaps fields, the MLC transmission and 29 IMRT fields from 10 breast treatment plans were measured. For the heterogeneity validation, the transit dose of these fields was studied using three phantoms: 10, 30, and a 3cm cork slab placed between 10cm of solid water. The PerFRACTION expected doses were compared with PRIMO Monte Carlo simulation results and ionization chamber measurements. Using the 10cm solid water phantom, for the basic validation fields, the root mean square (RMS) of the difference between PerFRACTION and PRIMO simulations was 0.6%. In the IMRT fields, the RMS of the difference was 1.2%. When comparing respect ionization chamber measurements, the RMS of the difference was 1.0% both for the basic and the IMRT validation. The average passing rate with a γ(2%/2mm, TH=20%) criterion between PRIMO dose distribution and PerFRACTION expected dose was 96.0%±5.8%. We validated PerFRACTION calculated transit dose with PRIMO Monte Carlo and ionization chamber measurements adapting the methodology of the MMPG5a report. The methodology presented can be applied to validate other forward-projection transit dosimetry software.

Halide Perovskites for Direct Conversion Megavoltage X‐Ray Detectors

AbstractMegavoltage (MV) X‐ray detectors used in cancer treatment either suffer from low sensitivity (scintillators) or prohibitively high cost (direct conversion). Here solution‐processed direct‐conversion MV X‐ray detectors are demonstrated based on halide perovskites. The authors’ prototype devices show a sensitivity of ≈0.7 µC Gyair−1 cm–2, high photon‐to‐carrier conversion efficiency of 42 500%, and a signal‐to‐noise ratio of ≈1750 to 6 MV X‐ray beam of a medical linear accelerator. The detector shows a contrast of over −1.5% per cm of solid water, comparable to state‐of‐art commercial gadolinium oxysulfide (GOS) MV X‐ray scintillators. This work demonstrates the first prototype of low‐cost and efficient direct‐conversion MV X‐ray detectors.

Technical note: Bremsstrahlung dose in the electron beam at extended distances in total skin electron therapy.

Electron beam from a linear accelerator is commonly used in total skin electron therapy (TSET) at extended distances. Since Das et al (Med Phys 21, p.1733, 1994) reported 5% bremsstrahlung dose for a 6 MeV electron beam at extended distance of 500cm it has been accepted as common knowledge. However, measurements by Chen et al (Int J. Rad Onc Biol Phys 59 p.872, 2004) and Monte Carlo simulations by Ding et al (Phys. Med. Biol. 66, 075010, 2021) were unable to reproduce such high bremsstrahlung dose. As bremsstrahlung dose contributes to whole-body dose, which could produce bone marrow toxicity with serious complications for the outcome of the TSET, it is important to re-evaluate the magnitude of bremsstrahlung dose accurately. The EGSnrc Monte Carlo system is used to investigate bremsstrahlung doses from 6 MeV high dose rate total skin electron (HDTSe) beams from Varian TrueBeam and Clinac Accelerators. The measurements were carried out at a depth of dmax and 5cm in Solid Water and Acrylic phantoms at extended distances using a parallel-plate chamber and a cylindrical ion chamber. We were able to reproduce previously reported high bremsstrahlung dose at extended distances by using a parallel plate ionization chamber. However, both the measurements made by using a cylindrical chamber and Monte Carlo simulations showed an insignificant bremsstrahlung dose (∼1%) even at SSD=500cm. The bremsstrahlung doses of a 6 MeV electron beam are 0.5% to 1% for SSD from 100 to 700cm, although it increases with the increasing extended distance. The common belief of up to 5% bremsstrahlung dose at large extended distances is incorrect. Previously reported high bremsstrahlung doses might be due to poor signal-to-noise ratio of using a parallel plate chamber for measuring very low dose or particular setup.

Patient-specific pixel-based weighting factor dual-energy x-ray imaging system using a priori CT data.

The purpose of this study was to develop a novel patient-specific pixel-based weighting factor dual-energy (PP-DE) algorithm to effectively suppress bone throughout the image and overcome the limitation of the conventional DE algorithm with constant weighting factor which is restricted to regions with uniform patient thickness. Additionally, to derive theoretical expressions to describe the dependence of the weighting factors on several imaging parameters and validate them with measurement. A step phantom was constructed consisting of slabs of solid water and bone materials. Thicknesses of bone ranged [0-6]cm in one direction and solid water [5-30]cm in the other direction. Projection images at 60 and 140kVp were acquired using a clinical imaging system. Optimal weighting factors were found by iteratively varying it in the range [0-1.4], where bone and soft-tissue contrast-to-noise ratio (CNR) reached zero. Bone and soft-tissue digitally reconstructed thicknesses were created using computed tomography (CT) images of a Rando phantom and ray tracing techniques. A weighting factor image (ω) was calculated using digitally reconstructed thicknesses (DRTs) and precalculated weighting factors from the step phantom. This ω image was then used to generate a PP-DE image. The PP-DE image was compared to the conventional DE image which uses a constant weighting factor throughout the image. The effect of the misaligned ω image on PP-DE images was investigated by acquiring LE and HE images at various shifts of Rando phantom. A rigid registration was used based on mutual information algorithm in Matlab. The signal-to-noise ratios (SNR) were calculated in the step phantom for the PP-DE image and compared to that of conventional DE technique. Analytical expressions for theoretical weighting factors were derived which included various effects such as beam hardening, scatter, and detector response. The analytical expressions were simulated in Spektr3.0 for different bone and solid water thicknesses as per the step phantom. A tray of steel pins was constructed and used with the step phantom to remove the scattered radiation. The simulated theoretical weighting factors were validated by comparing to those from the step phantom measurement. Optimal weighting factor values for the step phantom varied from 0.633 to 1.372 depending on region thickness. Thicker regions required larger weighting factors for bone cancellation. The PP-DE image of the Rando phantom favorably cancelled both ribs and spine, whereas in the conventional DE image, only one could be cancelled at a time. The misaligned ω image was less effective in removing all bones indicating the importance of alignment as part of the PP-DE algorithm implementation. The SNRs for the PP-DE image was larger than those of the conventional DE images for regions which required smaller weighting factors for bone suppression. Comparisons of measured and simulated weighting factors demonstrated a 3% agreement for all bone overlapped regions except for the thickest region with 30cm of solid water overlapped with 6cm bone where the signal was lost due to excess attenuation. A novel PP-DE algorithm was developed which can create higher quality DE images with enhanced bone cancellation and improved noise characteristics compared to conventional DE technique. In addition, theoretical weighting factor expressions were derived and validated against measurement.

Optimal planar X-ray imaging soft tissue segmentation using a photon counting detector

A rigorous method for automated soft tissue segmentation using planar kilovoltage (kV) imaging, a photon counting detector (PCD), and a convolutional neural network is presented. The goal of the project was to determine the optimum number of energy bins in a PCD for soft tissue segmentation. Planar kV X-ray images of solid water (SW) phantoms with varying depth of cartilage were generated with a cone-beam analytical method and parallel-beam Monte Carlo simulations. Simulations were preformed using 2 to 5 PCD energy bins with equal photon fluence distribution. Simulated image signal to noise ratio (SNR) was varied between 10 to 250 measured after transmission through 4 cm of SW. Algorithms using non-linear as well as linear regression were used to predict the amount of cartilage for every pixel of the phantom. These algorithms were evaluated based on the mean squared error (MSE) between their prediction and the ground truth. The best algorithm was used to decompose randomly generated SW and cartilage images with an SNR of 100. These randomly generated images trained a U-Net convolutional neural network to segment the cartilage in the image. The results indicated the smallest MSE occurred for non-linear regression with 4 energy bins over all SNR. The trained U-Net was able to correctly segment all regions of cartilage for the smallest amount of cartilage used (4 mm) and segmented the region with > 99% categorical accuracy by pixel.

SU‐C‐201‐05: Silicon Array Dosimeter in Situ with Electronic Portal Image Device for Simultaneous Transit Dose and Image Verification in Radiotherapy

Purpose:To investigate an Electronic Portal Imaging Device (EPID) coupled to a 2D array dosimeter to provide simultaneous imaging and dose verification.Methods:The novel dual detector configuration comprised of a 2D diode array dosimeter, referred to as a Magic Plate (MP) placed directly on a standard EPID. Dose response of the MP was evaluated by measuring the detector's response with respect to off‐axis position and field size with 30 cm of solid water (SW) acting as a transit object in the beam. Measurements were performed with 3, 5, 10 and 15 mm SW build‐up and compared to 2D ionisation chamber array (ICA) measurements and the PinnacleTM treatment planning system (TPS) at a source to detector distance of 150 cm with a 6 MV beam. Clinical dosimetric performance was evaluated by measuring a number of intensity‐modulated radiation therapy (IMRT) beams in transit geometry. Imaging performance of the EPID was quantified by measuring the contrast‐to‐noise ratio (CNR) and spatial resolution. Images of a Rando phantom were used for qualitative assessment.Results:Measured MP off‐axis and field size response agreed within 2% of TPS and ICA responses when measured using 15 mm SW build‐up. Clinical IMRT beams had gamma pass rates of ≥95% at 3%/3mm criteria. Measured CNR and spatial resolution (f50) were 264.96, 210.6, and 0.41, 0.40 with build‐up of 5 and 15 mm respectively for the dual detector configuration. CNR and spatial resolution of 643.9 and 0.41 were measured for standard EPID. CNR was quantitatively worse in the dual detector configuration. Differences in imaging performance were not visible in a qualitative assessment using a Rando phantom.Conclusion:Combining a prototype MP 2D dosimeter with a conventional EPID did not significantly detract from the performance of either device and has the potential for simultaneous on‐line patient transit dosimetry and image assessment in radiation therapy.Cancer Institute NSW Australia(Research Equipment Grant 10/REG/1‐20) and Cancer Council NSW (Grant ID RG 1‐06)

SU‐F‐T‐319: The Impact of Radiation Beam Obliquity and Air Gap Thickness On Optically Stimulated Luminescent in Vivo Dosimetry for Radiation Therapy

Purpose:Optically‐stimulated luminescent dosimeters (OSLDs) are increasingly utilized for in vivo dosimetry of complex radiation delivery techniques. Measured doses, however, underestimate planned doses for plans that utilize thermoplastic mask immobilization. The purpose of this work was to quantify the effect of beam obliquity and air gap span between the mask and backscatter material, on measured‐to‐planned OSLD dose agreement.Methods:A previously‐used thermoplastic mask was cut, reheated, and flattened to form a 33 by 9 cm2 stage approximately 2 mm thick. Two OSLDs were placed on the stage on 5 cm of solid water, covered with 50 by 50 by 5 mm3 square of bolus, and scanned in the CT simulator. Plans were created with 10 by 10 cm2 open fields using 4, 6, 10, and 15 MV photon beams at 0°, 45°, and 90° incidence. The isocenter was placed between the OSLDs at 5 mm depth. Dose was calculated and averaged for two OSLDs. Artificial air gaps of 3, 5, 10, and 20 mm were introduced in the plan and dose was recalculated for each energy/angle/gap combination. The experimental setup was replicated on a linear accelerator and air gaps were introduced by “bridging” the thermoplastic stage across solid water plastic of varying thickness. Fields were delivered as planned. OSLDs were read 12–15 hours after irradiation.Results:Measured‐toplanned percent differences were constant with increasing gap thickness for 0° and 45° beam angles. At 90° and 0 cm gap, planned dose underestimated measured dose by 10–23% for all energies. This discrepancy decreased linearly to 0% with a 20 mm gap. OSLD signal did not decrease more than 6% for any gap span and energy.Conclusion:With the exception of parallel beam incidence, beam obliquity and air gap thickness did not have a substantial effect on measured‐to‐planned dose agreement.

SU-E-T-312: Dosimetric Consideration for the Agility MLC When Planning Rotational SRT/SBRT Treatments

Purpose: To analyze the radiation transmission of the Agility MLC and make recommendation based on the MLC dosimetric characteristics for SRT, SBRT and VMAT planning Method and Materials: Agility MLC is the newest model from Elekta and has no back up diaphragm behind leaves for this generation. Leaves are single focused with rounded end; composed of leafs each 0.5cm wide, 9cm thick, constructed from tungsten alloy and provide low transmission <0.5%. Total radiation transmission from leaves and diaphragm is <0.13%. A 360degree arc was generated using iCom tools; leaves were programmed closed while keeping the diaphragm fully open to maximize the MLC transmission effect. Gafchromic EBT films were sandwiched between 4cm of solid water and situated at midplane to take dose measurement. 5000MU was delivered using 6MV VersaHD, various collimator angles, and a 5cm central axis offset was tested also. Films were scanned with Epson 10000XL scanner and analyzed using DoseLab Pro. Results: Due to the rounded leaf end and nature of rotation therapy, dose accumulation through the leaf gap is significant. By offsetting the leaf gap from central axis, this accumulation can be greatly reduced. There are dark bands showing accumulation of interleaf transmission which is improved by increasing collimator angle from 0 to 45dgree. However for 45 degree, in most cases, there are larger volumes sweeping under MLC alone, which needs considered planning. Conclusions: While inter-leaf leakage is minimized by using collimator angles greater than 0 degrees, the location of the leaf gap must also be managed. The leaf gap position becomes critically important when the treatment area is off axis such is the case when more than one PTV is being treated. With VMAT for SRT, SBRT becoming a more popular planning technique, special attention needs to be paid when initially setting up the field geometry.

Characterization of a fiber-taper charge-coupled device system for plastic scintillation dosimetry and comparison with the traditional lens system

PurposeTo compare the signal-to-noise ratio (SNR), dose sensitivity and stability, and reproducibility of a lens-less charge-coupled device (CCD) photon-counting system with those of a traditional CCD + lens photon-counting system for plastic scintillation detectors (PSDs). MethodsThe PSD used in this study was made from a 1-mm diameter, 2-mm long BCF60 scintillating fiber (emission peak at 530nm) coupled to a 2.6-m Eska GH-4001 clear plastic fiber. This PSD was coupled to either a fiber-taper-based photon-counting system (FTS) or a lens-based photon-counting system (LS). In the FTS, the fiber-taper was attached to a 2048×2048 pixel, uncooled Alta 4020 polychromatic CCD camera. The LS consisted of a 1600×1200 pixel Alta 2020 polychromatic CCD camera (cooled to −18°C) with a 50-mm lens with f/#=1. Dose measurements were made under the same conditions for each system (isocentric setup; depth of 1.5cm in solid water using a 10×10cm2 field size and 6-MV photon beam). The performance of each system was determined and compared, using the chromatic Čerenkov removal method to account for the stem effects produced in the clear plastic fiber. ResultsThe FTS increased the light collected by a factor of 4 compared with the LS, for the same dose measurements. This gain was possible because the FTS was not limited by the optical aberration that comes with a lens system. Despite a 45°C operating temperature difference between the systems, the SNR was 1.8–1.9 times higher in the FTS than in the LS, for blue and green channels respectively. Low-dose measurements of 1.0 and 0.5 cGy were obtained with an accuracy of 3.4% and 5.6%, respectively, in the FTS, compared with 5.8% and 15.9% in the LS. The FTS provided excellent dose measurement stability as a function of integration time, with at most a 1% difference at 5 cGy. Under the same conditions, the LS system produced a measurement difference between 2 and 3%. ConclusionOur results showed that the FTS could measure doses more accurately than the LS and that low-dose measurements were feasible without the complexity of a lens-based system. The FTS would therefore be better adapted for routine clinical usage of fiber arrays. The increased SNR in the FTS suggests that water-equivalent PSDs with smaller radii could be used to obtain measurements with greater spatial resolution, further simplifying the use of PSDs for “in-vivo” monitoring and small-field dosimetry.

SU-E-T-49: Implementation of Optically Stimulated Luminescent Detectors in the Clinical Setting

Purpose: To perform experiments to determine how Optically Stimulated Luminescent Detectors (OSLDs) can be used for in vivo dosimetry in the clinical environment. For clinical use, OSLDs should have stable, reproducible behavior in measuring dose and be easy to use. Methods: OSLDs were placed in solid water at dmax, with 12cm of solid water below for backscatter, and were irradiated with a TG51 calibrated linac at 10MV, 100SSD and with a 10x10 cm2 field. The OSLDs were of the Nanodot type from Landauer; the Landauer microStar reader was used to measure photomultiplier counts. No modifications were made to the reader, e.g., use of special filters. OSLDs were irradiated to a variety of dose levels which could be encountered in the clinic, as well as levels of 200Gy or more. OSLDs were bleached after each irradiation with a compact fluorescent light source. Results: Without preirradiation to relatively high doses, OSLDs displayed non-stabilized supralinear behavior in plots of counts versus dose; dosing histories where cumulative dose is acquired solely in amounts comparable to clinical levels, e.g., <10Gy per irradiation, do not yield useful detectors; the counts versus dose curve continues to change significantly with acquired cumulative irradiation until cumulative dose reaches 250Gy or more. However, irradiation of OSLDs to as little as 200Gy before use yields stable detectors that followed supralinear behavior characterized with a quadratic polynomial fit. Pre-irradiation of 1kGy or more, as reported by Jursinic*, is not needed to yield stable detector performance. Conclusion: Nanodot OSLDs can be used in a clinical setting after receiving one shot preirradiations of ∼200Gy. Dose measuring performance is stable after the preirradiation treatment. One shot irradiations of 1kGy or more are not needed to obtain stable counts versus dose performance.* P. Jursinic, Med. Phys. 37, p. 132 to 140 (2010)

Evaluation of Flattening Filter-free Lung/Tissue Heterogeneity Dose Calculations for 2 Commercial Planning Systems

To evaluate the accuracy of two commercial treatment planning systems (TPS) to calculate dose distributions produced by a flattening filter free (FFF) beam incident on a heterogeneous lung/tissue phantom. Two TPS were evaluated using phantom geometries constructed from Solid Water and lung equivalent material (0.28g/cm3). The phantom consisted of 3cm Solid Water, 11cm lung equivalent material, and 4 cm of Solid Water, with a 2cm x 2cm cylinder of Solid Water located at 10cm depth along the central axis to represent a lung tumor. Planar dose measurements with film for a 10MV FFF beam were taken at four lung/tissue interfaces (3cm, 9cm, 11cm, and 14cm depth) and at mid-tumor (10cm depth) for two field sizes (MLC shaped 2x2cm2, and a jaw collimated 4x4cm2). A CT scan of the phantom was transferred to the TPSs for dose calculations. Planar dose distributions corresponding to the measurement planes were exported from the TPSs and compared to measurements in RIT113 software. Ionization chamber point dose measurements at mid-tumor depth along the central axis confirmed the film measurements. Penumbra width (90%-10%) and gamma analysis (3%, 3mm) were used to evaluate the accuracy of the calculated dose distributions which were normalized relative to the central axis for each depth. The effect of the dose grid was evaluated by calculating both TPSs at 3mm, 2mm, and 1mm resolution. The largest discrepancies existed in the 50%-10% penumbra region at mid-tumor depth and at the tissue/lung interface distal to the tumor, for the 2x2cm2 field, with the calculated dose for both planning systems exceeding the measured dose by more than the gamma criteria (Table). Other calculated planes had less than 3% of pixels exceeding the gamma criteria, with the TPS1 calculated dose marginally more accurate than TPS2 dose calculations. One notable exception was the Eclipse calculated dose exceeding measured dose by 7% along the central axis for the 2x2cm2 field at the plane distal to the lung tissue. The 1mm resolution was used for comparison to film measurements for best accuracy. Current clinical TPSs over-estimate the penumbral width for small FFF fields incident on tissue equivalent material located within lung, resulting in incorrect dose calculations to the neighboring healthy lung tissue.Poster Viewing Abstract 3741; TablePenumbra size and percent of pixels failing gamma criteria for mid and distal tumor planesMeasurement 2x2cm2 Field SizeTPS1 2x2cm2 Field SizeTPS2 2x2cm2 Field SizeLocationPenumbra 90%-10% (cm)Penumbra 90%-10% (cm)Gamma(% Fail)Penumbra 90%-10%(cm)Gamma(% Fail)Mid tumor0.811.124.881.24.37Distal to tumor0.881.1715.221.227.12 Open table in a new tab

In vivo dosimetry with radiochromic films in low‐voltage intraoperative radiotherapy of the breast

EBT2 radiochromic films were studied and used for in vivo dosimetry in targeted intraoperative radiotherapy (TARGIT), a technique in which the Intrabeam system (Carl Zeiss, Oberkochen, Germany) is used to perform intraoperative partial breast irradiation with x-rays of 50 kV(p). The energy of the radiation emitted by the Intrabeam with the different spherical applicators, under 1 and 2 cm of solid water, and under the tungsten impregnated rubber used for shielding of the heart in TARGIT of the breast, was characterized with measurements of half-value layer (HVL). The stability of response of EBT2 was verified inside this range of energies. EBT2 films were calibrated using the red and green channels of the absorption spectrum in the 0-20 Gy dose range delivered by the Intrabeam x-rays. The dependence of film response on temperature during irradiation was measured. For in vivo dosimetry, pieces of radiochromic films wrapped in sterile envelopes were inserted after breast conserving surgery and before TARGIT into the excision cavity, on the skin and on the shielded pectoralis fascia for treatments of the left breast. HVLs of the Intrabeam in TARGIT of the breast correspond to effective energies of 20.7-36.3 keV. The response of EBT2 was constant inside this range of energies. We measured the dose to the target tissue and to organs at risk in 23 patients and obtained an average dose of 13.52 ± 1.21 Gy to the target tissue. Dose to the skin in close proximity to the applicator was 2.22 ± 0.97 Gy, 0.29 ± 0.17 Gy at 5-10 cm from the applicator, and 0.08 ± 0.07 Gy at more than 10 cm from the applicator. Dose to the pectoral muscle for left breast treatment was 0.57 ± 0.23 Gy. Our results show that EBT2 films are accurate at the beam energies, dose range, and irradiation temperature found in TARGIT and that in vivo dosimetry in TARGIT with EBT2 films wrapped in sterile envelopes is a feasible procedure. Measured dose to the organs at risk indicates that the technique is safe from side effects to the skin and the heart.

Physics Contributions Dose correction in lung for HDR breast brachytherapy

PurposeTo evaluate the dosimetric impact of lung tissue in Ir-192 APBI.Material and methodsIn a 40 × 40 × 40 cm3 water tank, an Accelerated Partial Breast Irradiation (APBI) brachytherapy balloon inflated to 4 cm diameter was situated directly below the center of a 30 × 30 × 1 cm3 solid water slab. Nine cm of solid water was stacked above the 1 cm base. A parallel plate ion chamber was centered above the base and ionization current measurements were taken from the central HDR source dwell position for channels 1, 2, 3 and 5 of the balloon. Additional ionization data was acquired in the 9 cm stack at 1 cm increments. A comparable data set was also measured after replacing the 9 cm solid water stack with cork slabs. The ratios of measurements in the two phantoms were calculated and compared to predicted results of a commercial treatment planning system.ResultsLower dose was measured in the cork within 1 cm of the cork/solid water interface possibly due to backscatter effects. Higher dose was measured beyond 1 cm from the cork/solid water interface, increasing with path length up to 15% at 9 cm depth in cork. The treatment planning system did not predict either dose effect.ConclusionsThis study investigates the dosimetry of low density material when the breast is treated with Ir-192 brachytherapy. HDR dose from Ir-192 in a cork media is shown to be significantly different than in unit density media. These dose differences are not predicted in most commercial brachytherapy planning systems. Empirical models based on measurements could be used to estimate lung dose associated with HDR breast brachytherapy.

SU‐E‐T‐339: Target Change Effect on Surface Dose of Treatment and Imaging Helical Tomotherapy Beams

Purpose: The target of the TomoTherapy unit is replaced at regular intervals. This work studies surface dose both before and after a target change for the treatment and imaging beams.Methods: Attix parallel plate chamber, as well as LiF powder thermoluminescent dosimeter (TLD) monolayer measurements were performed on the surface and at depths up to 1.5 cm in solid water before and after the target change. Due to the 1.0 cm diameter collecting volume of the Attix chamber, broad imaging beam measurements were performed with a 5 cm jaw width at isocenter; TLD measurements were performed using the narrow jaw width used for clinical MVCT imaging. Measurements at depths of 10 and 20 cm in solid water were also performed using A1SL cylindrical ionization chambers with a 5 cm field width.Results: Attix chamber measurements in the broad imaging beam showed surface dose versus dose at 5 mm to be constant before (31.3 ± 0.6%) and after (31.4 ± 0.3%) the target change. TLD monolayer measurements showed no change in the narrow imaging beam surface dose (25.0 ± 0.8% before and 24.6 ± 0.5% after). The treatment beam showed a small decrease in surface dose after the target change (18.2 ± 0.7% before and 17.0 ± 0.3% after). The ratio of dose at depths of 10 and 20 cm for the imaging beam was 2% greater after the target change whereas the the treatment beam ratio was 0.6% greater after the target change as compared to the ratio measured before the target change Conclusions: Even though degradation of the thin tungsten target occurs with use, little to no change is observed in the surface dose for either the imaging or the treatment beam. This is likely due to a thick, low atomic number metal block located immediately after the target.

TU‐G‐BRB‐05: Dosimetric Characterization of Motion Effect in Delivering Uniform Target Dose Distribution Using Scanning Proton Beams

Purpose: The purpose of this study is to evaluate the impact of motion on scanning beam delivery in synchrotron‐based spot‐scanning proton therapy.Methods: A MatriXX 2D ion chamber array detector was placed on a moving platform with 19 cm of solid water for buildup. The platform was moved perpendicular to the beam direction in a simulated breathing pattern using a computer controlled stepping motor. A 10×10cm2 uniform field was delivered using a 173.7MeV beam with an approximately 20 mm spot size (FWHM in water) and 8 mm spot spacing. The delivery time was 1.2 sec/paint, 1–4 repaints were studied. The treatment field was delivered at different motion amplitudes: A=0 (reference), 3, 5, 10 and 20 mm and motion cycles T=3s and 5s. We also tested the starting phase effect by starting the beam at different phases of the motion cycle. In each case the dose distributions under motion were compared to the reference. Results: In the presence of motion, the delivered dose pattern to a moving phantom is typically skewed and non‐uniform. However, if the motion amplitude was less than 5mm, there was less dosimetric impact (&lt;1% difference in homogeneity) regardless of motion cycle, number of repainting, and the beam starting phase conditions. However there were 6.9% and 27.9% dose difference in homogeneity compared to the reference for 10mm and 20mm motion amplitudes. We found that the starting phase had an impact on the shape of the delivered dose pattern. Multiple repainting minimized but not eliminated the dose pattern distortion caused by different starting phases. Conclusions: Delivered dose pattern can be distorted by the interplay effect of patient motion and scanning beam delivery. However, when the spot size (20mm FWHM) and spacing (8mm) are appropriate, the delivered dose pattern can tolerate non‐anticipated motion up to 5mm.

SU‐E‐T‐204: Real‐Time Monitoring of Age‐Related Macular Degeneration Radiosurgery Using Plastic Scintillation Dosimetry

Purpose: Age‐related macular degeneration (AMD) is the leading cause of vision loss and blindness among those over age 65. In its wet form, blood vessels grow from the choroid, causing macular tension and sometimes retinal displacement. As an alternative to monthly direct‐injection methods, Oraya Therapeutics has developed the IRayTM stereotactic radiosurgery device. This table‐top system has been validated in Monte Carlo simulations, human cadaver eye studies, pre‐clinical animal studies, and a phase 1 clinical trial. A plastic scintillation dosimetry (PSD) system was developed to monitor dose delivery in real‐time and provide measurement of total dose delivered. Methods: The IRayTM uses 100 keV x‐rays to deliver 24 Gy of macular dose, divided into three positions along the inferior pars plana. The PSDˈs small volume (0.5 mm diameter and 2 mm height), allows placement within the machine head. The PSD is fiber‐optically coupled to a photomultiplier tube via a plastic waveguide. Before direct characterization of the IRayTM, the preliminary PSD performance was tested using a portable x‐ray unit, mobile c‐arm fluoroscopy unit, and an ion chamber. Results: In order to avoid buffering issues, real‐time measurements used 250 ms bins, but can be adjustable to 10 ms. A calibration factor (CF) from PSD percent‐depth‐dose to ion chamber percent‐depth‐dose was 1.06 at a depth of 1.5 cm in solid water, the typical macular target depth. Counts‐to‐exposure CF varied less than 1% from 0.862 to 15.26R. Counts‐to‐exposure CF remained linear with R2 of 0.999 up to 16R even with varying dose rate (.002 R/s to 1.0 R/s). Conclusions: The PSD demonstrated linear dose measurements even at high dose and high dose rates. This study demonstrates the feasibility, accuracy, and precision of a PSD for clinical implantation in the IRayTM, a device for treatment of AMD.This research was supported by a grant from Oraya Therapeutics, Inc.

SU-GG-T-61: Dose Correction in Lung for HDR Breast Brachytherapy

Purpose: To evaluate and compare the dose in water equivalent medium to a lung equivalent medium from HDR Ir‐192. Methods and Materials: In a 40×40×40 cm3 water tank, a Contura brachytherapy device, inflated to 4 cm diameter was situated directly below the center of a 30 cm×30 cm×1 cm solid water slab. In the first phase, 9 cm of solid water was stacked above the 1 cm base. A Scanditronix parallel plate ion chamber (Type NACP‐02) was centered above the 1 cm of solid water base. Ionization current measurements established the central HDR source dwell position for channels 1, 2, 3, and 5 of the Contura. Additional data was acquired in the 9 cm stack at 1 cm increments. The second measurement phase was performed after replacing the 9cm solid water stack with cork slabs. The ratios of measurements in the two phantoms were calculated. The Oncentra Masterplan treatment planning system was used to compute comparable ratios based on CT studies of the two experimental setups. Results: Lower dose was measured in the cork within 1 cm of the cork/solid water interface due to backscatter effects. Higher dose was measured beyond 1cm from the cork/solid water interface, increasing with path length up to 15% at 9 cm depth in cork. The treatment planning computations did not predict either of these dose changes. Conclusions:Dose from HDR Ir‐192 in a cork media has been shown to be significantly different than in unit density media. This model should represent the dose effects in the lung when the breast is treated with Ir‐192 brachytherapy. Commercial brachytherapy planning systems do not consider heterogeneous media and therefore do not compute accurate lungdose. Empirical corrections based on measurements from this study may be used to correct lungdose computations associated with HDR breast brachytherapy.

SU‐GG‐T‐367: Quantification and Reduction of Peripheral Dose from Electron Applicator Leakage

Purpose: To quantify peripheral dose due to electron applicator leakage and to propose techniques for minimizing the leakage. Method and Materials: Peripheral dose was measured as a function of off‐axis distance from field edge and of depth in water using an electron diode and 10×10−25×25 cm2 applicators on Siemens primus accelerators. Second, the effect of oblique beam incidence on peripheral dose was investigated by comparing peripheral doses measured for normal beam incidence to those measured for obliquely incident beams. Third, leakage transmitted through various thicknesses of Xenolite‐NL was measured at depth 0.2 cm in solid water for 6×18 MeV electron beams. Finally, to reduce leakage escaping through applicator sidewalls, each applicator was modified by lining the outer surface of its sidewalls with a 1mm thick sheet of lead. Results: The applicator leakage gave rise to a broad peripheral dose‐off‐axis distance peak in patient plane. Maximum peripheral dose from normally incident beams at depth 1 cm in water was observed to be 5% of central‐axis dose maximum and as high as 9% for obliquely incident beams with angles of obliquity ≤40°. Measured depth‐peripheral dose curves showed the “practical range” of leakage electrons in water varies from approximately 1.4 to 5.7 cm for 6 to 18 MeV beams. The leakage transmission measurements showed a 4 mm thick sheet of Xenolite‐NL is required to attenuate the leakage from 9 MeV beams by two‐thirds. For each additional 3 MeV increase in electron energy, an additional Xenolite‐NL thickness of 2 mm is needed for shielding on skin. Finally, modification of applicator sidewalls resulted in reduction of peripheral dose by up to 80%. Conclusion: Peripheral dose from leakage can be reduced by shielding with an appropriate thickness of Xenolite‐NL. Alternatively, electron applicator leakage can be minimized by modifying the applicator sidewalls with a sheet of lead.

SU-GG-T-460: Liquid Scintillator Dosimetry for Passive Scattering Proton Beam Quality Assurance

Purpose: A liquid scintillator (LS) detection system has been developed for volumetric characterization of radiation beams. We describe the utility of this system for quality assurance of passively scattered proton beams. Materials and Methods: The detector system consists of a 20×20×20 cm3 volume of LS surrounded by a light tight housing. One side of the tank is transparent to allow viewing by a CCD camera, which is placed 80 cm from the center of the tank. Irradiations were performed at the Proton Therapy Center Houston (PTCH). The tank was irradiated with two spread-out Bragg peaks (SOBPs), one with 13 cm range and 6 cm width, and one with 28.5 cm range and 10 cm width. For the latter, 14 cm of solid water was added before the tank. A median filter was used to correct for radiation noise in the image. The light signal along the beam's central axis was compared to the percent depth dose data measured with an ionization chamber in a water phantom. Results: Light-depth profiles obtained with the LS device are proportional to measured dose in the proximal build-up region of the beam. LET-dependent quenching artifacts decrease the light signal in the last few cm of the proton range, and optical artifacts decrease the signal near the edges of the tank. As a result, the light signal in the SOBP is not proportional to dose. However, the relative light signal can be obtained during quality assurance measurements and compared with baseline data. Conclusion: SOBP light-depth profiles can be obtained by irradiating an LS device with passive proton beams. Despite a non-linear dose response in the SOBP, the device provides sufficient information to measure reliably beam range and SOBP width for daily quality assurance. Supported by the NCI (1R01CA120198-01A2, 2P01CA021239-29A1)

TU-D-BRB-07: Determination of Neutron Dose Due to a Therapeutic Proton Beam Incident On a Closed Tungsten MLC Using the Dual Hydrogenous/non-Hydrogenous Ionization Chamber Method

Purpose: To measure neutron dose around a tungsten MLC used for proton beam therapy using the dual ionization chamber (IC) method. Method and Materials: ICs were constructed with large 10cc volumes to enable low dose measurements in the mGy range. The chambers are identical aside from wall, electrode, and guard-ring material with one constructed of A150 tissue-equivalent plastic (T) and filled with methane based tissue-equivalent gas and the other magnesium (U) and filled with argon gas. IC(T) has approximately the same sensitivity to neutrons and photons while the non-hydrogenous IC(U) exhibits a lower sensitivity to neutrons. The dose from neutrons and photons can be expressed as DN=(RT-RU)/(1-kU) and DG=(RU- kURT)/(1-kU) where RU,RT are IC readings and kU is the relative sensitivity of IC(U) to neutrons which varies with energy. Measurements were made with 230MeV proton beams incident on a closed MLC with the ICs on the beam axis 21cm from the MLC. Open field measurements were made with attenuation by 0, 0.5, 2, 5, 9, and 10cm of solid water. The ICs were calibrated against a Farmer chamber in air with a 6MV beam and suitable buildup caps. Results: Initial analysis of the measurements suggests an average neutron energy of 2.6MeV corresponding to kU=0.033. Measured neutron dose with the MLC closed and detector geometry stated above is 1.67mGy/Gy where IC(T) was used to determine dose in the open field. Conclusion: Initial measurements of neutron dose from a proton therapy beam incident on a MLC were performed which indicate the validity of the dual chamber method for determining neutron dose around a proton beam. This work was supported by the US Army Medical Research and Materiel Command under Contract Agreement No. DAMD17-W81XWH-07-2-0121. Opinions, interpretations, conclusions and recommendations are those of the author and are not necessarily endorsed by the US Army.