Plan-Optimization Method for Central-shielding Pelvic Volumetric-modulated Arc Therapy for Cervical Cancer

Aim: To establish a method of plan optimization in pelvic volumetric-modulated arc therapy (VMAT) for cervical cancer using the central-shielding (CS) principle. Patients and Methods: We created external beam VMAT plans for eight cases with non-bulky stage I-IIb using the CS principle based on the Japanese standard guideline. Clinical target volumes (CTVs) for whole-pelvis (WP) irradiation were created using published guidelines, and CTVs for CS irradiation were created by subtracting the uterus corpus and 4 cm-wide regions centered at the cervix and vagina from the CTVs for WP irradiation. For plan optimization of CS irradiation, a 4-cm diameter cylindrical volume centered in the cervix and vagina was created as the volume receiving a high dose in brachytherapy, and the volume overlapping with the rectum was defined as the Ant-Rectum. Dose-volume histogram (DVH) parameters of two CS VMAT plans with and without (VMAT(OPT)/VMAT(NO)) dose optimization to the Ant-Rectum were compared. Results: VMAT(OPT) plans resulted in significantly lower DVH parameters of the Ant-Rectum and rectum compared to VMAT(NO) plans. These were comparable to the DVH parameters of three-dimensional conformal radiotherapy (3DCRT) plans. Both VMAT plans resulted in significantly better coverage of planning target volumes than did the 3DCRT plans. Conclusion: In the implementation of IMRT/VMAT as the standard treatment for cervical cancer in Japan, our optimization method may be an essential step toward fully benefitting from the CS principle.