In Vivo Dosimetry in the Urethra During Prostate Carbon Ion Radiotherapy

Background/Aim: In vivo dosimetry can prevent dose delivery errors by directly measuring the dose of radiation administered to a patient. However, a method for in vivo dosimetry during carbon ion radiotherapy (CIRT) has not been established. Therefore, we investigated data from in vivo dosimetry of the urethra during CIRT for prostate cancer using small spherical diode dosimeters (SSDDs). Patients and Methods: This study included five patients enrolled in a clinical trial (jRCT identifier: jRCTs032190180) on which the use of four-fraction CIRT for prostate cancer was examined. The urethral dose during CIRT for prostate cancer was measured using the SSDDs inserted into the ureteral catheter. The relative error between the in vivo and calculated doses obtained using the Xio-N treatment planning system was determined. Additionally, a dose-response stability test for the in vivo dosimeter was performed under clinical conditions. Results: The relative error between the in vivo and calculated urethral doses ranged from 6 to 12%. The dose-response stability under clinical conditions of the measured dose was <= 1%. Therefore, an error >1% would be due to an interfractional patient setup error in the large dose gradient in the urethra. Conclusion: The usefulness of in vivo dosimetry using SSDDs in CIRT and SSDDs' potential for detecting dose delivery errors during CIRT is herein highlighted.