Retrospective analysis of multi-institutional, patient-specific treatment planning results of high-dose-rate intracavitary brachytherapy for gynecological cancer using V_100%

The objective of this study was to clarify the usefuleness of the K parameters of the independent verification method using V_100% (the volume of water receiving 100% of the prescription dose) for institutions implementing the high-dose-rate (HDR) intracavitary brachytherapy for gynecological cancer. The data of 249 plans of 11 institutions in Japan were used, and the constant K value obtained by a parameter fit for single-¹⁹²Ir, two-¹⁹²Ir, and three-¹⁹²Ir systems was calculated. The predicted total dwell time calculated using the constant K value was defined as T_pr, and the total dwell time calculated using a radiation treatment planning system was defined as T_RTP. The ratio of T_pr and T_RTP for each plan was calculated. The constant K values (95% CI) obtained for each system outlined above were 1233 (1227–1240), 1205 (1199–1211), and 1171 (1167–1175), respectively. Regarding the T_pr/T_RTP, the entire data were within 0.9–1.1. For accurate verification, it was clarified that constant K values should be calculated for each system. The Nuclear Regulatory Commission considers a difference of 20% between the prescribed total dose and the administered total dose as a reportable medical event. There is a need for a quick method to verify the accuracy with a minimum of 10% threshold of a plan. The constant K values in this study were obtained from multiple institutions, and the variation in the values among these institutions was small. The data obtained by this study may be used as a parameter of this verification method employed by numerous institutions, particularly those who have recently initiated HDR brachytherapy. In addition, for institutions already using this method, this data might be useful for the validation of the parameters which were used in such institutions.