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JOURNAL NAME: International Journal of Medical Physics, Clinical Engineering and Radiation Oncology, Vol.9 No.2, March 26, 2020
ABSTRACT: Purpose: Standardization of tumor dosimetric coverage is essential for the evaluation of radiotherapy treatment plan quality. National clinical trials network RTOG protocols include tumor target dosimetric criteria that specify the prescription dose and minimum and maximum dose (Dmin and Dmax) coverages. This study investigated the impact of various minimum and maximum dose definitions using tumor control probability (TCP) models. Methods and Materials: Three disease sites (head and neck, lung, and prostate) were studied using target volume dosimetric criteria from the RTOG 0920, 1308, and 0938 protocols. Simulated target dose-volume histograms (DVHs) of Dmin and Dmax were modeled using the protocol specifications. Published TCP models for the three disease sites were applied to the DVH curves. The effects of various dose definitions on TCP were studied. Results: While the prescription dose coverage was maintained, a -3.7% TCP difference was observed for head and neck cancer when the target doses varied by 3.5% of the tumor volume from the point dose. For prostate and lung cancers, -3.3% and -2.2% TCP differences were observed, respectively. The TCPs for head and neck and prostate cancers were more negatively affected by deviations in the Dmin than the TCP for lung cancer. The lung TCP increased to a greater extent with a change in the Dmax compared with the head and neck and prostate TCPs. Conclusions: These results can be used to evaluate plan quality when the target dose only slightly deviates from the dosimetric criteria. When the overall target prescription dose coverage is maintained, the Dmax is recommended to be within 3% of the target volume: 98% (for head and neck and prostate) and 97% (for lung) of the target volume, satisfying the Dmin needed to maintain TCP variations at less than 2.1%. Using 0.03 cc instead of a point dose for Dmin and Dmax criteria minimally impacts TCPs.