Session Item

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

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.

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

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.