ESTRO 2022 - Abstract Book

S827

Abstract book

ESTRO 2022

OC-0936 The value of measurement-based proton peer review

P. Taylor 1 , S. Kry 1 , J. Lowenstein 1

1 UT MD Anderson Cancer Center, Radiation Physics, Houston, USA

Purpose or Objective To highlight the value and key findings of on-site proton audits.

Materials and Methods The IROC QA Center has performed 38 on-site measurement-based peer reviews of proton centers participating in clinical trials in the USA. The audits covered dosimetric measurements, treatment planning, and QA practices. The measurements consisted of beam output calibration using the auditor's independent equipment, lateral profile measurements using an ion chamber array, depth dose measurements using a multi-layer ion chamber, proton vs. imaging isocenter coincidence using GafChromic film, and evaluation of the CT Number to Relative Linear Stopping Power (RLSP) conversion using a phantom with known RLSP values, and by comparing the institution’s curve to other proton centers. Results Institutions received an average of 3 [1, 8] recommendations for practice improvements. The number of deficiencies did not decrease over time, highlighting the continued need for this type of peer review. The most common deficiencies were for QA (97% of centers), CT Number to RLSP conversion (50%), and treatment planning (45%). 32% of institutions failed at least one lateral beam profile measurement, despite passing internal QA measurements. These failures occurred for several different plan configurations (large, small, shallow, and deep targets), and at different depths in the beam path (proximal to target, central, and distal), as shown in Table 1. CT Number to RLSP conversion curves showed deviations at low, mid, and high CT Numbers. This test highlights areas of inconsistency between proton centers, with many centers falling outside 2 sigma of the mean curve of their peers. All deficiencies from the peer review were discussed with the institutions, and many implemented practice changes to improve the accuracy of their system and consistency with other institutions. Table 1. Audit results outside tolerance for lateral profile measurements (gamma agreement between measured and calculated doses showed <90% of pixels passing 3%/3 mm, with a 10% low dose threshold). PBS = pencil beam scanning; US = uniform scanning

Modality Gamma Range (%) Failure Depth Location Failure Disease Site PBS 88 - 99 central head & neck PBS 76 - 100 proximal, central breast PBS 68 - 93 proximal, central sarcoma, mesothelioma PBS 79 - 100 central head & neck PBS 84 - 100 central, distal brain PBS 88 - 100 central reference PBS 86 - 95 central, distal brain, prostate Scattered 88 - 98 central sinus Scattered 89 - 98 central patch brain US 87 - 92 central prostate, spine

Conclusion The IROC peer review program minimizes deviations between proton centers enrolling in trials, letting outcomes data speak for itself. This program serves as a model for international clinical trial groups who are starting their own peer review programs. Institutions that don’t participate in clinical trials would also benefit from this site visit program; reduction in treatment inaccuracies and alignment with national and international standards would clearly benefit patients treated at all institutions.

OC-0937 Optimising tissue-equivalent materials for proton therapy

H. Cook 1 , M. Simard 2 , N. Niemann 3 , C. Gillies 4 , H. Palmans 5 , M. Hussein 6 , H. Bouchard 7 , G. Royle 1 , A. Lourenço 6

1 University College London , Medical Physics and Biomedical Engineering Department , London , United Kingdom; 2 University of Montreal , Department of Physics , Montreal , Canada; 3 Bartholomew Hospital , Medical Physics Department , London , United Kingdom; 4 University College London Hospital , Medical Physics Department , London , United Kingdom; 5 MedAustron Ion Therapy Centre , Medical Physics Group , Wiener Neustadt, Austria; 6 National Physical Laboratory , Medical Radiation Science , London , United Kingdom; 7 University of Montreal , Department of Physics, Montreal , Canada Purpose or Objective Phantoms are a vital tool for both reference and end-to-end audits in radiotherapy. Phantom-based dosimetric audits have been shown to improve confidence and consistency of radiotherapy treatments at clinical centres. Recently, there has been an increase in the number of proton therapy centres within the UK and worldwide. However, due to differences in radiation interaction with matter between photon and proton beams, previously used X-ray phantoms and phantom materials are suboptimal for proton therapy. Research shows current tissue-equivalent materials cause large uncertainties in proton therapy measurements. Consequently, there is a need for photon and proton optimised tissue-equivalent materials.