Sum signal dosimetry: A new approach for high dose quality assurance with Gafchromic EBT 3

Abstract Gafchromic EBT 3 film dosimetry in radiosurgery ( RS ) and hypofractionated radiotherapy ( HRT ) is complicated by the limited film accuracy at high fractional doses. The aim of this study is to develop and evaluate sum signal ( SS ) film dosimetry to increase dose resolution at high fractional doses, thus allowing for use of EBT 3 for dose distribution verification of RS / HRT treatments. To characterize EBT 3 dose–response, a calibration was performed in the dose range 0.44–26.43 Gy. Red ( RC ) and green ( GC ) channel net optical densities were linearly added to produce the SS . Dose resolution and overall accuracy of the dosimetric protocol were estimated and compared for SS , RC , and GC . A homemade Matlab software was developed to compare, in terms of gamma analysis, dose distributions delivered by a Cyberknife on EBT 3 films to dose distributions calculated by the treatment planning system. The new SS and conventional single channel ( SC ) methods were compared, using 3%/1 and 4%/1 mm acceptance criteria, for 20 patient plans. Our analysis shows that the SS dose–response curve is characterized by a steeper trend in comparison with SC , with SS providing a higher dose resolution in the whole dose range investigated. Gamma analysis confirms that the percentage of points satisfying the agreement criteria is significantly higher for SS compared to SC : 95.03% vs. 88.41% ( P = 0.014) for 3%/1 mm acceptance criteria and 97.24% vs. 93.58% ( P = 0.048) for 4%/1 mm acceptance criteria. This study demonstrates that the SS approach is a new and effective method to improve dosimetric accuracy in the framework of the RS ‐ HRT patient‐specific quality assurance protocol.