Evaluation of Interfractional Movement of the Seminal Vesicle and Dose Variations with IGRT Throughout the Prostate Cancer Radiotherapy

Abstract

To validate the actual cumulative doses delivered to the seminal vesicle (SV) depending on the organ movement using kV-Cone Beam CT (kV-CBCT) datasets acquired during prostate image-guided radiation therapy. Ten patients with intermediate risk prostate cancer treated with IMRT were included in this study. Both the prostate and the first 2 cm of the SV were included in the CTV. PTV margin was 8 mm in all directions except posterior, where it was 5 mm. Sixteen kV-CBCT scans per patient were used for assessments. After each kV-CBCT was matched with the planning CT based on prostate, the SV were recontoured as proximal, distal SV and both of them together (the whole SV). Actual delivered doses to the SV were assessed by means of DVH. The variations in certain dose-volume parameters were analyzed using the Student's test. Systematic and random errors of the SV and margins were assessed using the van-Herk formula. We observed that the SV movement in the anterior-posterior direction was significantly more compared to superior-inferior and left-right direction. The dose volume variations for the whole SV (D98%, D50%, D2%) and the distal SV (D98%, D95%, D50%) were significantly different than the planned doses. However, both the whole SV and the distal SV received the prescribed dose. Although it seems that there is a need to give larger margin to the SV in posterior direction, the actual cumulative doses delivered to SV throughout the treatment were within the prescribed dose.