Planning Target Volume Margin Determination for Volumetric Modulated Arc Therapy Planning in Cervix Cancers Using an Empty Bladder Protocol

OBJECTIVE To evaluate the planning target volume (PTV) margin and bladder volume variation of volumetric modulated arc therapy (VMAT) in cervical cancer using an empty bladder protocol. METHODS Ten patients with cervical cancer who had indications to receive whole pelvic radiotherapy and who had been identified to receive whole pelvic irradiation with a dose of 46-55.2 Gy in 23 fractions planned by VMAT were included. International Commission on Radiation Units and Measurements (ICRU) Report 83 was used to report plan parameters. The empty bladder protocol is designed for use with all patients before simulation and irradiation. From June 2020 to February 2021, 215 fractions from the 10 patients were evaluated. The ‘all fraction’ set-up errors were recorded using cone-beam computed tomography (CBCT) and were interpreted as error margins using the Van Herk Formula. RESULTS The calculated PTV margins were 0.75, 0.84, and 0.98 cm on the X, Y, and Z-axes, respectively. The median volume of the bladder before irradiation was 40.6 cc, with an interquartile range of 31.9 to 59.2 cc. The average change in bladder volume from the planning volume was 23.56%. CONCLUSIONS Using the empty bladder protocol, the clinical target volume (CTV) to PTV margin was 1 cm following the Van Herk formula. No patients experienced side effects of grade 3 or greater. The empty bladder protocol is a method which can reduce target placement error and reduce patient discomfort without causing serious side effects. KEYWORDS PTV margin, VMAT, IGRT, Cervix cancer, Empty bladder