Derivative-based gamma index: a novel methodology for stringent patient-specific quality assurance in the stereotactic treatment planning of liver cancer

Objective: The development of a stringent derivative-based gamma (DBG) index for patient-specific QA in stereotactic radiotherapy treatment planning (SRTP) to account for the spatial change in dose. Methods: Twenty-five patients of liver SBRT were selected retrospectively for this study. Deliberately, two different kinds of treatment planning approaches were used for each patient. Firstly, the treatment plans were generated using a conventional treatment planning (CTP) approach in which the target was covered with a homogeneous dose along with the nominal dose fall-off around the treatment field. Subsequently, the other treatment plans were generated using an SRTP approach with the intent of heterogeneous dose within the target region along with a steeper dose gradient outside the treatment field as much as possible. For both kinds of treatment plans, two dimensional (2D) conventional gamma (CG) and DBG analysis were performed using the 2D ion chamber array and radiochromic film. Results: Difference in the DBG index was statistically significant whereas, for CG analysis, the difference in CG index was insignificant for both types of treatment plans (CTP and SRTP). A significant positive correlation was observed between the difference in the DBG index and the difference in HI for high gamma criteria. Conclusion: The DBG evaluation is found to be more rigorous, and sensitive to the only SRTP. The proposed method could be opted-in the routine clinical practice in addition to CG. Advances in knowledge: DBG is more sensitive to detect the spatial change of dose, especially in high dose gradient regions.