Radiological Physics Center Research Articles

End-to-End Test for a Radiotherapy Program Based on the Medical Linear Accelerator Installed in a Resource-Limited Oncology Centre in Sub-Saharan Africa

Several challenges including the availability of necessary funds and expertise hinder the development and modernization of radiotherapy in resource-scarce countries like those in Sub-Saharan Africa (SSA). This work presents the findings of an end-to-end audit on independent verification of radiation doses delivered by a commercially available medical linear accelerator (linac) installed at Cameroon Oncology Center, a resource-constrained oncology centre in SSA. The medical linac with 6 MV and 18 MV x-rays, and five electron energies ranging from 6–20 MeV was commissioned for clinical use. The mailed TLD dosimetry irradiation systems based on the American Radiological Physics Center technique were used to check the output of the photon beams and electron energies. The end-to-end test was achieved by requesting, imaging and treating the MD Anderson anthropomorphic head and neck phantom using an IMRT technique on our linac. The phantom was irradiated and sent back to the USA for analysis. Evaluation criteria require that an institution’s treatment plan agree within ± 7% of measured TLD doses and that ≥ 85% of pixels pass ± 7%/4 mm gamma analysis for film. Beam output met the required criteria within ± 3%, and our institution’s treatment plan satisfied the established criteria of measured TLD doses and film dose distributions. The gamma-passing rate was ≥ 91%. A resource-constrained oncology centre in SSA has met the MD Anderson humanoid phantom irradiation criteria generally used for credentialing institutions to assure quality and safety of complex radiation treatments. Despite the various challenges faced by resource-constrained countries in SSA, this work demonstrates the practicability of implementing a modern radiotherapy program based on linear accelerator technology in a resource-limited region.

Development of a Monte Carlo multiple source model for inclusion in a dose calculation auditing tool.

The Imaging and Radiation Oncology Core Houston (IROC-H) (formerly the Radiological Physics Center) has reported varying levels of agreement in their anthropomorphic phantom audits. There is reason to believe one source of error in this observed disagreement is the accuracy of the dose calculation algorithms and heterogeneity corrections used. To audit this component of the radiotherapy treatment process, an independent dose calculation tool is needed. Monte Carlo multiple source models for Elekta 6MV and 10MV therapeutic x-ray beams were commissioned based on measurement of central axis depth dose data for a 10×10cm2 field size and dose profiles for a 40×40cm2 field size. The models were validated against open field measurements consisting of depth dose data and dose profiles for field sizes ranging from 3×3cm2 to 30×30cm2 . The models were then benchmarked against measurements in IROC-H's anthropomorphic head and neck and lung phantoms. Validation results showed 97.9% and 96.8% of depth dose data passed a ±2% Van Dyk criterion for 6MV and 10MV models respectively. Dose profile comparisons showed an average agreement using a ±2%/2mm criterion of 98.0% and 99.0% for 6MV and 10MV models respectively. Phantom plan comparisons were evaluated using ±3%/2mm gamma criterion, and averaged passing rates between Monte Carlo and measurements were 87.4% and 89.9% for 6MV and 10MV models respectively. Accurate multiple source models for Elekta 6MV and 10MV x-ray beams have been developed for inclusion in an independent dose calculation tool for use in clinical trial audits.

Development of a flattening filter free multiple source model for use as an independent, Monte Carlo, dose calculation, quality assurance tool for clinical trials.

The Imaging and Radiation Oncology Core-Houston (IROC-H) Quality Assurance Center (formerly the Radiological Physics Center) has reported varying levels of compliance from their anthropomorphic phantom auditing program. IROC-H studies have suggested that one source of disagreement between institution submitted calculated doses and measurement is the accuracy of the institution's treatment planning system dose calculations and heterogeneity corrections used. In order to audit this step of the radiation therapy treatment process, an independent dose calculation tool is needed. Monte Carlo multiple source models for Varian flattening filter free (FFF) 6 MV and FFF 10 MV therapeutic x-ray beams were commissioned based on central axis depth dose data from a 10 × 10 cm2 field size and dose profiles for a 40 × 40 cm2 field size. The models were validated against open-field measurements in a water tank for field sizes ranging from 3 × 3 cm2 to 40 × 40 cm2 . The models were then benchmarked against IROC-H's anthropomorphic head and neck phantom and lung phantom measurements. Validation results, assessed with a ±2%/2 mm gamma criterion, showed average agreement of 99.9% and 99.0% for central axis depth dose data for FFF 6 MV and FFF 10 MV models, respectively. Dose profile agreement using the same evaluation technique averaged 97.8% and 97.9% for the respective models. Phantom benchmarking comparisons were evaluated with a ±3%/2 mm gamma criterion, and agreement averaged 90.1% and 90.8% for the respective models. Multiple source models for Varian FFF 6 MV and FFF 10 MV beams have been developed, validated, and benchmarked for inclusion in an independent dose calculation quality assurance tool for use in clinical trial audits.

Robert J. Shalek, Ph.D., J.D.

We are saddened to report the death of Dr. Robert J. Shalek, who passed away at the age of 93 on April 20, 2015. Robert J. Shalek, Ph.D. was born in Chicago, IL in 1922. He earned his undergraduate degree in physics from the University of Illinois in Champaign. After graduating, he served in the US Army for 3 years as a Signal Corp officer. He returned home to Texas where his parents had moved and entered the graduate program at the Rice Institute (now Rice University). He earned two masters degrees (physics and mathematics) and went on to earn a Ph.D. in biophysics in 1953. Dr. Shalek was the second Rice Physics Fellow in the Department of Physics at the MD Anderson Hospital [currently The University of Texas MD Anderson Cancer Center (MDACC)]. During this time, he worked with Leonard Grimmett on one of the world's first 60Co units. Upon receiving his Ph.D. he spent a postdoctoral year at the Royal Cancer Hospital (now called the Royal Marsden Hospital) in London, England under the guidance of W. V. Mayneord, the Head of the Physics Department and author of the “Mayneord F factor.” After this postdoctoral year he returned to the Department of Physics at MDACC. He published over 100 scientific papers before retiring in 1985 after working at MDACC for 35 years. Among his many contributions to Medical Physics, he was Chairman of the Department of Radiation Physics (1961–1984) at MDACC following Dr. Leonard Grimmett and Dr. Warren Sinclair, both very experienced medical physicists from England. He and Dr. Gilbert Fletcher, the chairman of the Department of Radiation Therapy at MDACC from 1948 to 1981, recognized the need for medical physics quality assurance for national multi-institutional clinical trials and as such, along with the AAPM, proposed a center of physics quality assurance for clinical trials. He ultimately was awarded the first National Cancer Institute grant to create the Radiological Physics Center (RPC) and was its first Director from 1968 until 1985. He was one of AAPM's founding charter members and served as the AAPM's seventh president in 1966. He became a fellow of the AAPM and was awarded ASTRO's Gold Medal in 1996. He was the second recipient of the AAPM's William D. Coolidge Award in 1972 and the sixth recipient of the Marvin M. D. Williams Professional Achievement Award in 1994. His career has encompassed both basic research and the application of physics to clinical problems. His basic research included studies of oxygen reactions following radiation interactions1 and the proportion of direct and indirect radiation action in living cells.2 His clinically related work included the dosimetry of external treatment beams, brachytherapy dosimetry, and quality assurance in radiotherapy physics.3 In the 1960s he wrote one of the seminal papers4 on ferrous sulfate (Fricke) dosimetry that made chemical dosimetry available in the clinic. Also in the 1960s, he and his trainee (and later, colleague) Marilyn Stovall published a series of articles describing the MD Anderson method for implant dosimetry using computerized techniques.5–9 These publications helped disseminate these methods and enabled many institutions to introduce what were for the time very advanced brachytherapy calculation techniques. From right to left, Dr. Shalek, Dr. Hanson, and Dr. Ibbott at a reception in 2001 for Geoff Ibbott, the new RPC Director. While working at MDACC, Dr. Shalek studied law at the University of Houston during the evenings and received his J.D. in 1983. He lectured regularly on legal questions in medical physics and regularly gave professional legal advice to physicists. He also participated in medical malpractice suits, usually as an expert witness. Bob was especially proud of his book, Medical Physicists and Malpractice10 (authored with David Gooden) that outlines the legal framework of liability risk in radiation therapy, the impact on clinical physicists, and practical approaches for avoiding and managing such lawsuits. Dr. Shalek is survived by his wife Elaine V. Shalek, 6 children, 9 grandchildren, and 4 great grandchildren. His accomplishments in medical physics will be long remembered. Equally well known is the way he conducted himself throughout his career, always as a man of high principle and character. He truly was a credit to our profession and is remembered fondly by so many whose lives he touched during his incredibly successful and meaningful career.

WE‐AB‐BRB‐02: Methods and Applications of 3D Radiochromic Dosimetry

Despite widespread IMRT treatments at modern radiation therapy clinics, precise dosimetric commissioning of an IMRT system remains a challenge. In the most recent report from the Radiological Physics Center (RPC), nearly 20% of institutions failed an end‐to‐end test with an anthropomorphic head and neck phantom, a test that has rather lenient dose difference and distance‐to‐agreement criteria of 7% and 4 mm. The RPC report provides strong evidence that IMRT implementation is prone to error and that improved quality assurance tools are required. At the heart of radiation therapy dosimetry is the multidimensional dosimeter. However, due to the limited availability of water‐equivalent dosimetry materials, research and development in this important field is challenging. In this session, we will review a few dosimeter developments that are either in the laboratory phase or in the pre‐commercialization phase. 1) Radiochromic plastic. Novel formulations exhibit light absorbing optical contrast with very little scatter, enabling faster, broad beam optical CT design. 2) Storage phosphor. After irradiation, the dosimetry panels will be read out using a dedicated 2D scanning apparatus in a non‐invasive, electro‐optic manner and immediately restored for further use. 3) Liquid scintillator. Scintillators convert the energy from x‐rays and proton beams into visible light, which can be recorded with a scientific camera (CCD or CMOS) from multiple angles. The 3D shape of the dose distribution can then be reconstructed. 4) Cherenkov emission imaging. Gated intensified imaging allows video‐rate passive detection of Cherenkov emission during radiation therapy with the room lights on. Learning Objectives: To understand the physics of a variety of dosimetry techniques based upon optical imaging To investigate the strategies to overcome respective challenges and limitations To explore novel ideas of dosimeter designSupported in part by NIH Grants R01CA148853, R01CA182450, R01CA109558. Brian Pogue is founder and president of the company DoseOptics LLC, dedicated to developing and commercializing the first dedicated Cerenkov imaging camera and system for radiation dose imaging. Work reported in this talk does not involve the use of DoseOptics technology.; H. Li, this work was supported in part by NIH Grant No. R01CA148853; S. Beddar, NIH funding R01‐CA182450

WE‐AB‐BRB‐03: Real‐Time Volumetric Scintillation Dosimetry for Radiation Therapy

Despite widespread IMRT treatments at modern radiation therapy clinics, precise dosimetric commissioning of an IMRT system remains a challenge. In the most recent report from the Radiological Physics Center (RPC), nearly 20% of institutions failed an end‐to‐end test with an anthropomorphic head and neck phantom, a test that has rather lenient dose difference and distance‐to‐agreement criteria of 7% and 4 mm. The RPC report provides strong evidence that IMRT implementation is prone to error and that improved quality assurance tools are required. At the heart of radiation therapy dosimetry is the multidimensional dosimeter. However, due to the limited availability of water‐equivalent dosimetry materials, research and development in this important field is challenging. In this session, we will review a few dosimeter developments that are either in the laboratory phase or in the pre‐commercialization phase. 1) Radiochromic plastic. Novel formulations exhibit light absorbing optical contrast with very little scatter, enabling faster, broad beam optical CT design. 2) Storage phosphor. After irradiation, the dosimetry panels will be read out using a dedicated 2D scanning apparatus in a non‐invasive, electro‐optic manner and immediately restored for further use. 3) Liquid scintillator. Scintillators convert the energy from x‐rays and proton beams into visible light, which can be recorded with a scientific camera (CCD or CMOS) from multiple angles. The 3D shape of the dose distribution can then be reconstructed. 4) Cherenkov emission imaging. Gated intensified imaging allows video‐rate passive detection of Cherenkov emission during radiation therapy with the room lights on. Learning Objectives: To understand the physics of a variety of dosimetry techniques based upon optical imaging To investigate the strategies to overcome respective challenges and limitations To explore novel ideas of dosimeter designSupported in part by NIH Grants R01CA148853, R01CA182450, R01CA109558. Brian Pogue is founder and president of the company DoseOptics LLC, dedicated to developing and commercializing the first dedicated Cerenkov imaging camera and system for radiation dose imaging. Work reported in this talk does not involve the use of DoseOptics technology.; H. Li, this work was supported in part by NIH Grant No. R01CA148853; S. Beddar, NIH funding R01‐CA182450

WE‐AB‐BRB‐00: Session in Memory of Robert J. Shalek: High Resolution Dosimetry from 2D to 3D to Real‐Time 3D

Despite widespread IMRT treatments at modern radiation therapy clinics, precise dosimetric commissioning of an IMRT system remains a challenge. In the most recent report from the Radiological Physics Center (RPC), nearly 20% of institutions failed an end‐to‐end test with an anthropomorphic head and neck phantom, a test that has rather lenient dose difference and distance‐to‐agreement criteria of 7% and 4 mm. The RPC report provides strong evidence that IMRT implementation is prone to error and that improved quality assurance tools are required. At the heart of radiation therapy dosimetry is the multidimensional dosimeter. However, due to the limited availability of water‐equivalent dosimetry materials, research and development in this important field is challenging. In this session, we will review a few dosimeter developments that are either in the laboratory phase or in the pre‐commercialization phase. 1) Radiochromic plastic. Novel formulations exhibit light absorbing optical contrast with very little scatter, enabling faster, broad beam optical CT design. 2) Storage phosphor. After irradiation, the dosimetry panels will be read out using a dedicated 2D scanning apparatus in a non‐invasive, electro‐optic manner and immediately restored for further use. 3) Liquid scintillator. Scintillators convert the energy from x‐rays and proton beams into visible light, which can be recorded with a scientific camera (CCD or CMOS) from multiple angles. The 3D shape of the dose distribution can then be reconstructed. 4) Cherenkov emission imaging. Gated intensified imaging allows video‐rate passive detection of Cherenkov emission during radiation therapy with the room lights on. Learning Objectives: To understand the physics of a variety of dosimetry techniques based upon optical imaging To investigate the strategies to overcome respective challenges and limitations To explore novel ideas of dosimeter designSupported in part by NIH Grants R01CA148853, R01CA182450, R01CA109558. Brian Pogue is founder and president of the company DoseOptics LLC, dedicated to developing and commercializing the first dedicated Cerenkov imaging camera and system for radiation dose imaging. Work reported in this talk does not involve the use of DoseOptics technology.; H. Li, this work was supported in part by NIH Grant No. R01CA148853; S. Beddar, NIH funding R01‐CA182450

WE‐AB‐BRB‐04: Cherenkov Imaging for Radiation Therapy Dose Verification On Patients

Despite widespread IMRT treatments at modern radiation therapy clinics, precise dosimetric commissioning of an IMRT system remains a challenge. In the most recent report from the Radiological Physics Center (RPC), nearly 20% of institutions failed an end‐to‐end test with an anthropomorphic head and neck phantom, a test that has rather lenient dose difference and distance‐to‐agreement criteria of 7% and 4 mm. The RPC report provides strong evidence that IMRT implementation is prone to error and that improved quality assurance tools are required. At the heart of radiation therapy dosimetry is the multidimensional dosimeter. However, due to the limited availability of water‐equivalent dosimetry materials, research and development in this important field is challenging. In this session, we will review a few dosimeter developments that are either in the laboratory phase or in the pre‐commercialization phase. 1) Radiochromic plastic. Novel formulations exhibit light absorbing optical contrast with very little scatter, enabling faster, broad beam optical CT design. 2) Storage phosphor. After irradiation, the dosimetry panels will be read out using a dedicated 2D scanning apparatus in a non‐invasive, electro‐optic manner and immediately restored for further use. 3) Liquid scintillator. Scintillators convert the energy from x‐rays and proton beams into visible light, which can be recorded with a scientific camera (CCD or CMOS) from multiple angles. The 3D shape of the dose distribution can then be reconstructed. 4) Cherenkov emission imaging. Gated intensified imaging allows video‐rate passive detection of Cherenkov emission during radiation therapy with the room lights on. Learning Objectives: To understand the physics of a variety of dosimetry techniques based upon optical imaging To investigate the strategies to overcome respective challenges and limitations To explore novel ideas of dosimeter designSupported in part by NIH Grants R01CA148853, R01CA182450, R01CA109558. Brian Pogue is founder and president of the company DoseOptics LLC, dedicated to developing and commercializing the first dedicated Cerenkov imaging camera and system for radiation dose imaging. Work reported in this talk does not involve the use of DoseOptics technology.; H. Li, this work was supported in part by NIH Grant No. R01CA148853; S. Beddar, NIH funding R01‐CA182450

WE‐AB‐BRB‐01: Memorial Introduction; Storage Phosphor Panels for Radiation Therapy Dosimetry

Despite widespread IMRT treatments at modern radiation therapy clinics, precise dosimetric commissioning of an IMRT system remains a challenge. In the most recent report from the Radiological Physics Center (RPC), nearly 20% of institutions failed an end‐to‐end test with an anthropomorphic head and neck phantom, a test that has rather lenient dose difference and distance‐to‐agreement criteria of 7% and 4 mm. The RPC report provides strong evidence that IMRT implementation is prone to error and that improved quality assurance tools are required. At the heart of radiation therapy dosimetry is the multidimensional dosimeter. However, due to the limited availability of water‐equivalent dosimetry materials, research and development in this important field is challenging. In this session, we will review a few dosimeter developments that are either in the laboratory phase or in the pre‐commercialization phase. 1) Radiochromic plastic. Novel formulations exhibit light absorbing optical contrast with very little scatter, enabling faster, broad beam optical CT design. 2) Storage phosphor. After irradiation, the dosimetry panels will be read out using a dedicated 2D scanning apparatus in a non‐invasive, electro‐optic manner and immediately restored for further use. 3) Liquid scintillator. Scintillators convert the energy from x‐rays and proton beams into visible light, which can be recorded with a scientific camera (CCD or CMOS) from multiple angles. The 3D shape of the dose distribution can then be reconstructed. 4) Cherenkov emission imaging. Gated intensified imaging allows video‐rate passive detection of Cherenkov emission during radiation therapy with the room lights on. Learning Objectives: To understand the physics of a variety of dosimetry techniques based upon optical imaging To investigate the strategies to overcome respective challenges and limitations To explore novel ideas of dosimeter designSupported in part by NIH Grants R01CA148853, R01CA182450, R01CA109558. Brian Pogue is founder and president of the company DoseOptics LLC, dedicated to developing and commercializing the first dedicated Cerenkov imaging camera and system for radiation dose imaging. Work reported in this talk does not involve the use of DoseOptics technology.; H. Li, this work was supported in part by NIH Grant No. R01CA148853; S. Beddar, NIH funding R01‐CA182450

Dosimetric comparison among different head and neck radiotherapy techniques using PRESAGE® dosimeter

Purpose: The purpose of this analysis was to investigate dose distribution of Three Dimensional Conformal Radiation Therapy (3DCRT), Intensity Modulated Radiation Therapy (IMRT) and Volumetric Modulated Arc Therapy (VMAT) for Head and Neck cancer using 3-dimensional PRESAGE® dosimeter. Method : Computer Tomography (CT) scans of Radiological Physics Center (RPC) Head and Neck anthropomorphic phantom with both RPC standard insert and PRESAGE® insert were acquired separated with Philipp’s CT scanner and both CT scans were exported via DICOM to the pinnacle treatment planning system (TPS). Each plan was delivered twice to the RPC phantom first containing the RPC standard insert having Thermoluminescent detectors (TLD) and film dosimeters and then again containing the PRESAGE® insert having three dimensional dosimeter (PRESAGE®) by using a Varian True beam linear accelerator. After irradiation, the standard insert including point dose measurement (TLD) and planner GafChromic® EBT film measurement was read using RPC standard procedure. The 3D dose distribution from PRESAGE® was read out with the Duke Midsized optical scanner dedicated to RPC (DMOS-RPC). Dose volume histogram (DVH), mean and maximal doses for organ-at-risk (OARs) were calculated and compared among each Head and Neck technique. The prescription dose was same for all Head and Neck radiotherapy techniques which was 6.60 Gy per friction. Beam profile comparison and gamma analysis were used to quantify agreement among film measurement, PRESAGE® measurement and calculated dose distribution. Quality assurances of all plans were performed by using ArcCHECK method. Results: VMAT delivered the lowest mean and maximal doses to organ at risk (spinal cord and parotid) than IMRT and 3DCRT. Such dose distribution was verified by absolute dose distribution using TLD system. 2D gamma 5%/3 mm criteria of Pinnacle vs. EBT2 film 3DCRT (92.34%), IMRT (92.3%) and VMAT (96.63%) in axial plan respectively. It was also found that agreement between PRESAGE® and pinnacle along the axial, sagittal and coronal plans VMAT agreement was better than IMRT and 3DCRTplan excludes a 7 mm rim at the edge of the dosimeter using 2D gamma map criteria (±5%/3 mm) with 5% threshold dose. Profile showed good agreement for all plans between film, PRESAGE® and pinnacle. 3D gamma was performed for planning target volume (PTV) and organ at risks (OARs) VMAT and 3DCRT endow with better agreement than IMRT. Conclusion: VMAT delivered lowered mean and maximal doses to organ at risk and better PTV coverage. TLD, EBT film and PRESAGE® dosimeter has suggested that VMAT would be superior modality for the treatment of Head and Neck cancer than IMRT and 3DCRT.

Evaluations of secondary cancer risk in spine radiotherapy using 3DCRT, IMRT, and VMAT: A phantom study

This study evaluated the secondary cancer risk from volumetric-modulated arc therapy (VMAT) for spine radiotherapy compared with intensity-modulated radiotherapy (IMRT) and 3-dimensional conformal radiotherapy (3DCRT). Computed tomography images of an Radiological Physics Center spine anthropomorphic phantom were exported to a treatment planning system (Pinnacle3, version 9.4). Radiation treatment plans for spine were prepared using VMAT (dual-arc), 7-field IMRT (beam angles: 110°, 130°, 150°, 180°, 210°, 230°, and 250°), and 4-field 3DCRT technique. The mean and maximum doses, dose-volume histograms, and volumes receiving more than 2 and 4Gy to organs at risk (OARs) were calculated and compared. The lifetime risk for secondary cancers was estimated according to the National Cancer Registry Programme Report 116. VMAT delivered the lowest maximum dose to the esophagus (4.03Gy), bone (8.11Gy), heart (2.11Gy), spinal cord (6.45Gy), and whole lung (5.66Gy) as compared with other techniques (IMRT and 3DCRT). The volumes of OAR (esophagus) receiving more than 4Gy were 0% for VMAT, 27.06% for IMRT, and up to 32.35% for 3DCRT. The estimated risk for secondary cancer in the respective OAR is considerably lower in VMAT compared with other techniques. The results of maximum doses and volumes of OARs suggest that the risk of secondary cancer induction for the spine in VMAT is lower than IMRT and 3DCRT, whereas VMAT has the best target coverage compared with the other techniques.

VMAT Head and Neck Treatment Delivery: Benchmarking Accuracy of Patient DVH Based QA With Radiological Physics Center (RPC) Head and Neck Phantom Results

VMAT Head and Neck Treatment Delivery: Benchmarking Accuracy of Patient DVH Based QA With Radiological Physics Center (RPC) Head and Neck Phantom Results

On the feasibility of comprehensive high-resolution 3D remote dosimetry.

This study investigates the feasibility of remote high-resolution 3D dosimetry with the PRESAGE®/Optical-CT system. In remote dosimetry, dosimeters are shipped out from a central base institution to a remote institution for irradiation, then shipped back to the base institution for subsequent readout and analysis. Two nominally identical optical-CT scanners for 3D dosimetry were constructed and placed at the base (Duke University) and remote (Radiological Physics Center) institutions. Two formulations of PRESAGE® (SS1, SS2) radiochromic dosimeters were investigated. Higher sensitivity was expected in SS1, which had higher initiator content (0.25% bromotrichloromethane), while greater temporal stability was expected in SS2. Four unirradiated PRESAGE® dosimeters (two per formulation, cylindrical dimensions 11 cm diameter, 8.5-9.5 cm length) were imaged at the base institution, then shipped to the remote institution for planning and irradiation. Each dosimeter was irradiated with the same simple treatment plan: an isocentric 3-field "cross" arrangement of 4 × 4 cm open 6 MV beams configured as parallel opposed laterals with an anterior beam. This simple plan was amenable to accurate and repeatable setup, as well as accurate dose modeling by a commissioned treatment planning system (Pinnacle). After irradiation and subsequent (within 1 h) optical-CT readout at the remote institution, the dosimeters were shipped back to the base institution for remote dosimetry readout 3 days postirradiation. Measured on-site and remote relative 3D dose distributions were registered to the Pinnacle dose calculation, which served as the reference distribution for 3D gamma calculations with passing criteria of 5%/2 mm, 3%/3 mm, and 3%/2 mm with a 10% dose threshold. Gamma passing rates, dose profiles, and color-maps were all used to assess and compare the performance of both PRESAGE® formulations for remote dosimetry. The best agreements between the Pinnacle plan and dosimeter readout were observed in PRESAGE® formulation SS2. Under 3%/3 mm 3D gamma passing criteria, passing rates were 91.5% ± 3.6% (SS1) and 97.4% ± 2.2% (SS2) for immediate on-site dosimetry, 96.7% ± 2.4% (SS1) and 97.6% ± 0.6% (SS2) for remote dosimetry. These passing rates are well within TG119 recommendations (88%-90% passing). Under the more stringent criteria of 3%/2 mm, there is a pronounced difference [8.0 percentage points (pp)] between SS1 formulation passing rates for immediate and remote dosimetry while the SS2 formulation maintains both higher passing rates and consistency between immediate and remote results (differences ≤ 1.2 pp) at all metrics. Both PRESAGE® formulations under study maintained high linearity of dose response (R(2) > 0.996) for 1-8 Gy over 14 days with response slope consistency within 4.9% (SS1) and 6.6% (SS2), and a relative dose distribution that remained stable over time was demonstrated in the SS2 dosimeters. Remote 3D dosimetry was shown to be feasible with a PRESAGE® dosimeter formulation (SS2) that exhibited relative temporal stability and high accuracy when read off-site 3 days postirradiation. Characterization of the SS2 dose response demonstrated linearity (R(2) > 0.998) over 14 days and suggests accurate readout over longer periods of time would be possible. This result provides a foundation for future investigations using remote dosimetry to study the accuracy of advanced radiation treatments. Further work is planned to characterize dosimeter reproducibility and dose response over longer periods of time.

SU‐C‐BRD‐07: The Radiological Physics Center (RPC): 45 Years of Improving Radiotherapy Dosimetry

Purpose:The RPC, established in 1968 has contributed to the development, conduct, and QA of NCI funded multi‐institutional cooperative group clinical trials and institutions, primarily in the USA/Canada and 242 other countries, participating in trials.Methods:The RPC QA program components were designed to audit the radiation dose calculation chain from the NIST traceable reference beam calibration, to inclusion of dosimetry parameters used to calculate tumor doses, to the delivery of the radiation dose. The QA program included: 1) remote TLD/OSLD audit of machine output, 2) on‐site dosimetry review visits, 3) credentialing for advanced technologies, and 4) review of patient treatment records. The RPC presented and published their findings to the radiation oncology community.Results:The number of institutions monitored by the RPC increased from around 1200 in the late 90s, to ∼2000 in 2013. There were over 4000 megavoltage therapy machines and ∼28,000 therapy beams in the 1991 institutions monitored by the RPC by the end of 2013. Within the 14,000 photon, electron and proton beam outputs remotely monitored with TLD/OSLD annually, between 10‐20% of the institutions have one or more beams outside the RPC 5% criterion. Dosimetry site visits to photon and proton centers continue to result in 2‐4 recommendations affecting key dosimetry parameters that impact patient treatment times. One in four patient treatment records reviewed by the RPC have their dose data corrected by >5% before trial groups use them for outcomes analysis. Twelve of fourteen clinically active proton centers are approved to participate in NCI funded clinical trials. The RPC published 222 peer reviewed articles since 1972.Conclusion:Findings from the RPC suggest that human errors continue to play a role in radiotherapy discrepancies and without the RPC independent QA program, the number of undetected errors and time elapsed before their discovery would have been greater.Work supported by MGH C06 CA059267 and grants CA10953, CA081647 awarded by NCI, DHHS.

SU-E-T-583: Optimizing the MLC Model Parameters for IMRT in the RayStation Treatment Planning System

Purpose: To optimize the MLC model parameters for IMRT in the RayStation v.4.0 planning system and for a Varian C-series Linac with a 120-leaf Millennium MLC. Methods: The RayStation treatment planning system models rounded leaf-end MLC with the following parameters: average transmission, leaf-tip width, tongue-and-groove, and position offset. The position offset was provided by Varian. The leaf-tip width was iteratively evaluated by comparing computed and measured transverse dose profiles of MLC-defined fields at dmax in water. The profile comparison was also used to verify the MLC position offset. The transmission factor and leaf tongue width were derived iteratively by optimizing five clinical patient IMRT QA Results: brain, lung, pancreas, head-and-neck (HN), and prostate. The HN and prostate cases involved splitting fields. Verifications were performed with Mapcheck2 measurements and Monte Carlo calculations. Finally, the MLC model was validated using five test IMRT cases from the AAPM TG119 report. Absolute gamma analyses (3mm/3% and 2mm/2%) were applied. In addition, computed output factors for MLC-defined small fields (2×2, 3×3, 4×4, 6×6cm) of both 6MV and 18MV were compared to those measured by the Radiological Physics Center (RPC). Results: Both 6MV and 18MV models were determined to have the same MLC parameters: 2.5% transmission, tongue-and-groove 0.05cm, and leaftip 0.3cm. IMRT QA analysis for five cases in TG119 resulted in a 100% passing rate with 3mm/3% gamma analysis for 6MV, and >97.5% for 18MV. With 2mm/2% gamma analysis, the passing rate was >94.6% for 6MV and >90.9% for 18MV. The difference between computed output factors in RayStation and RPC measurements was less than 2% for all MLCdefined fields, which meets the RPC's acceptance criterion. Conclusion: The rounded leaf-end MLC model in RayStation 4.0 planning system was verified and IMRT commissioning was clinically acceptable. The IMRT commissioning was well validated using guidance from the AAPMTG119 protocol.

SU‐E‐T‐471: Small Field Jaw/MLC Reference Data

Purpose:In recent years the need for small field data of MLCs has increased due to the use of intensity‐modulated radiation (IMRT), but moreover the use of stereotactic body radiation (SBRT) has increased, which uses not simply small field sizes, but small jaw and field sizes together. Having reference data for these small fields that is reliable would be invaluable to the physics community. Our study has gathered these values and the data distributions from the Radiological Physics Center's (RPC) site visits between 1990 and the present.Methods:For all measurements, the RPC used a 25 × 25 × 25cm water phantom placed at 100cm SSD. All measurements were made with an Exradin A16 cylindrical ion chamber at an effective depth of 10 cm. A total of 42 Varian machine measurements were used to compose the data for a 6 MV beam and 5 TrueBeam 6 MV flattening filter free (FFF) beams were used for FFF data.Results:Jaw/MLC fields were measured for both 6 MV and 6 MF FFF beams with the jaws and MLCs both at the following field sizes: 6×6, 4×4, 3×3, and 2×2cm. Measurements were normalized to the 10×10 field readings (defined by the jaws and MLC). Spread in the data was minimal and demonstrates a high level of accuracy of acquired data.Conclusion:Small field Jaw/MLC reference data for Varian 6MV and 6 MV FFF beams has been analyzed and presented here, composed of the aggregation of numerous RPC site visits. Obtaining reliable small field data remains difficult, however the RPC has collected high fidelity small field Jaw/MLC data. The data are presented as a reference along with their distributions, in such a way that the physicist can act based upon their own desired agreement with the reference data.

IMRT credentialing for prospective trials using institutional virtual phantoms: results of a joint European Organization for the Research and Treatment of Cancer and Radiological Physics Center project.

Background and purposeIntensity-modulated radiotherapy (IMRT) credentialing for a EORTC study was performed using an anthropomorphic head phantom from the Radiological Physics Center (RPC; RPCPH). Institutions were retrospectively requested to irradiate their institutional phantom (INSTPH) using the same treatment plan in the framework of a Virtual Phantom Project (VPP) for IMRT credentialing.Materials and methodsCT data set of the institutional phantom and measured 2D dose matrices were requested from centers and sent to a dedicated secure EORTC uploader. Data from the RPCPH and INSTPH were thereafter centrally analyzed and inter-compared by the QA team using commercially available software (RIT; ver.5.2; Colorado Springs, USA).ResultsEighteen institutions participated to the VPP. The measurements of 6 (33%) institutions could not be analyzed centrally. All other centers passed both the VPP and the RPC ±7%/4 mm credentialing criteria. At the 5%/5 mm gamma criteria (90% of pixels passing), 11(92%) as compared to 12 (100%) centers pass the credentialing process with RPCPH and INSTPH (p = 0.29), respectively. The corresponding pass rate for the 3%/3 mm gamma criteria (90% of pixels passing) was 2 (17%) and 9 (75%; p = 0.01), respectively.ConclusionsIMRT dosimetry gamma evaluations in a single plane for a H&N prospective trial using the INSTPH measurements showed agreement at the gamma index criteria of ±5%/5 mm (90% of pixels passing) for a small number of VPP measurements. Using more stringent, criteria, the RPCPH and INSTPH comparison showed disagreement. More data is warranted and urgently required within the framework of prospective studies.

MO‐G‐BRE‐02: A Survey of IMRT QA Practices for More Than 800 Institutions

Purpose:A wide range of techniques and measurement devices are employed for IMRT QA, causing a large variation of accepted action limits and potential follow up for failing plans. Such procedures are not well established or accepted in the medical physics community. To achieve the goal of proving insight into current IMRT QA practices, we created an electronic IMRT QA survey. The survey was open to a variety of the most common QA devices and assessed the type of comparison to measurement, action limits, delivery methods, and clinical action for failing QA plans.Methods:We conducted an online survey through the Radiological Physics Center's (RPC) annual survey with the goal of ascertaining elements of routine patient‐specific IMRT QA. A total of 874 institutions responded to the survey. The questions ranged from asking for action limits, dosimeter type(s) used, delivery techniques, and actions taken when a plan fails IMRT QA.Results:The most common (52%) planar gamma criteria was 3%/3 mm with a 95% of pixels passing criteria. The most common QA device were diode arrays (48%). The most common first response to a plan failing QA was to re‐measure at the same point the point dose (89%), second was to re‐measure at a new point (13%), and third was to analyze the plan in relative instead of absolute mode (10%) (Does not add to 100% as not all institutions placed a response for each QA follow‐up option). Some institutions, however, claimed that they had never observed a plan failure.Conclusion:The survey provided insights into the way the community currently performs IMRT QA. This information will help in the push to standardize action limits among dosimeters.

SU‐F‐BRE‐14: Uncertainty Analysis for Dose Measurements Using OSLD NanoDots

Purpose:Optically stimulated luminescent dosimeters (OSLD) are an increasingly popular dosimeter for research and clinical applications. It is also used by the Radiological Physics Center for remote auditing of machine output. In this work we robustly calculated the reproducibility and uncertainty of the OSLD nanoDot.Methods:For the RPC dose calculation, raw readings are corrected for depletion, element sensitivity, fading, linearity, and energy. System calibration is determined for the experimental OSLD irradiated at different institutions by using OSLD irradiated by the RPC under reference conditions (i.e., standards): 1 Gy in a Cobalt beam. The intra‐dot and inter‐dot reproducibilities (coefficient of variation) were determined from the history of RPC readings of these standards. The standard deviation of the corrected OSLD signal was then calculated analytically using a recursive formalism that did not rely on the normality assumption of the underlying uncertainties, or on any type of mathematical approximation. This analytical uncertainty was compared to that empirically estimated from >45,000 RPC beam audits.Results:The intra‐dot variability was found to be 0.59%, with only a small variation between readers. Inter‐dot variability was found to be 0.85%. The uncertainty in each of the individual correction factors was empirically determined. When the raw counts from each OSLD were adjusted for the appropriate correction factors, the analytically determined coefficient of variation was 1.8% over a range of institutional irradiation conditions that are seen at the RPC. This is reasonably consistent with the empirical observations of the RPC, where the coefficient of variation of the measured beam outputs is 1.6% (photons) and 1.9% (electrons).Conclusion:OSLD nanoDots provide sufficiently good precision for a wide range of applications, including the RPC remote monitoring program for megavoltage beams.This work was supported by PHS grant CA10953 awarded by the NIH (DHHS).

An anthropomorphic spine phantom for proton beam approval in NCI-funded trials.

As part of the approval process for the use of scattered or uniform scanning proton therapy in National Cancer Institute (NCI)‐sponsored clinical trials, the Radiological Physics Center (RPC) mandates irradiation of two RPC anthropomorphic proton phantoms (prostate and spine). The RPC evaluates these irradiations to ensure that they agree with the institutions' treatment plans within criteria of the NCI‐funded cooperative study groups. The purpose of this study was to evaluate the use of an anthropomorphic spine phantom for proton matched‐field irradiation, and to assess its use as a credentialing tool for proton therapy beams. We used an anthropomorphic spine phantom made of human vertebral bodies embedded in a tissue substitute material called Muscle Substitute/Solid Rigid Number 4 (MS/SR4) comprising three sections: a posterior section containing the posterior surface and the spinous processes, and left and right (L/R) sections containing the vertebral bodies and the transverse processes. After feasibility studies at three institutions, the phantom, containing two thermoluminescent dosimeters (TLDs) for absolute dose measurements and two sheets of radiochromic film for relative dosimetry, was shipped consecutively to eight proton therapy centers participating in the approval study. At each center, the phantom was placed in a supine or prone position (according to the institution's spine treatment protocol) and imaged with computed tomography (CT). The images then were used with the institution's treatment planning system (TPS) to generate two matched fields, and the phantom was irradiated accordingly. The irradiated phantom was shipped to the RPC for analysis, and the measured values were compared with the institution's TPS dose and profiles using criteria of ± 7% for dose agreement and 5 mm for profile distance to agreement. All proton centers passed the dose criterion with a mean agreement of 3% (maximum observed agreement, 7%). One center failed the profile distance‐to‐agreement criterion on its initial irradiation, but its second irradiation passed the criterion. Another center failed the profile distance‐to‐agreement criterion, but no repeat irradiation was performed. Thus, seven of the eight institutions passed the film profile distance‐to‐agreement criterion with a mean agreement of 1.2 mm (maximum observed agreement 5 mm). We conclude that an anthropomorphic spine phantom using TLD and radiochromic film adequately verified dose delivery and field placement for matched‐field treatments.PACS number: 87.55.‐x, 87.55.N‐