[Determination and Evaluation of a Tolerance Limit for In Vivo Dosimetry Based on Dose Variations]

We investigated the relationship between dose variation and gamma analysis pass rate in in vivo dosimetry (IVD) and verified the detectivity of errors by using the tolerance of the pass rate determined on a dose variation basis.

We used a treatment planning system and water-equivalent phantoms to simulate discrepancies between treatment planning and actual irradiation. These discrepancies were created by either deforming structures outlined in the phantom's computed tomography (CT) images or by modifying the CT values within those structures. We performed gamma analysis to compare the intensity distributions of the planned data (including discrepancies) with the distributions acquired by irradiating real phantoms using an electronic portal imaging device (EPID). We investigated the relationship between the gamma analysis pass rate and dose variation with and without the discrepancies.

For gamma analysis pass rates below 70%, a dose variation of approximately 5% was estimated. The accuracy of using the pass rate to predict a dose variation of 5% was 86%. In validation of the prediction, when the gamma analysis pass rate was below 70%, differences were detected in clinical cases with an 82% accuracy.

A method for determining the tolerance of gamma analysis pass rate based on dose variation in phantoms is useful.