An on-site audit system for dosimetry credentialing of intensity-modulated radiotherapy in Japanese Clinical Oncology Group (JCOG) clinical trials

• A first multi-institutional JCOG clinical trial using IMRT was started.
• We developed a phantom for IMRT dosimetry credentialing.
• Credentialing was performed on-site on a total of 44 treatment machines.
• All participating institutions accurately delivered IMRT to the phantom.

PurposeThis study was undertaken to analyze the results of intensity-modulated radiotherapy (IMRT) dosimetry credentialing using a phantom in the Japanese Clinical Oncology Group clinical trials.MethodsAll measurements were performed on-site. The IMRT phantom consisted of a phantom shell and a module. Two types of structures, including a C-shaped planning target volume (PTV) around a column-shaped organ at risk (OAR), were included in the module. Each participating institution was asked to image, plan, and treat the phantom. A prescription dose of 2 Gy should cover 95% of the PTV. The plan should limit the maximum doses to the PTV and OAR to less than 110% and 60%, respectively. The pass criteria were ±3% in terms of chamber dosimetry and a difference in profile position ⩽2 mm in the high-dose gradient area of film dosimetry. The positional difference was defined as the largest distance between the measured and calculated positions at doses of 60% or 80%. These tolerances were based on the Japanese Society for Radiation Oncology IMRT guidelines.ResultsCredentialing was performed on a total of 44 treatment machines in 32 institutions from 2009 to 2015. All differences between measured and planned doses at the measurement points of the PTV were within 3%. The means ± standard deviations of the positional differences were 1.0 ± 0.4 mm and 0.9 ± 0.3 mm without and with the phantom shell, respectively.ConclusionsThe dose differences and positional differences met the desired criteria in all institutions.