Effect of incorporation of preoperative CT scan on bladder doses and irradiated volume in postprostatectomy radiation therapy (pPRT).

Abstract

e561 Background: pPRT increases local control, biochemical progression free survival, and even overall survival in patients with adverse features undergoing prostatectomy. The Radiation Therapy Oncology Group (RTOG) consensus definition of the clinical target volume (CTV) in 2010 was based on patterns of failure and anatomy without consideration of pre-operative imaging. This results in large volumes of bladder in the treatment field. Our study evaluates whether incorporation of pre-operative prostate volume can reduce the post-operative CTV and minimize dose to adjacent normal tissue. Methods: We reviewed records of all patients with available pre-operative pelvic CT scans treated at our institution with pPRT. The pre-operative CT scan was fused to the simulation CT. Post-operative CTV (CTV1) was delineated based on RTOG guidelines. A separate CTV (CTV2) was constructed, based on the intact prostate and proximal seminal vesicles. Plans were constructed for each CTV and doses to rectum, bladder, and penile bulb calculated, as well as concordance between the two CTVs and planning target volumes (PTVs). Paired student’s t-test was used to calculate difference between doses in the two different plans. Results: 10 patients’ plans were analyzed. Dosimetric parameters are shown in table 1. Volume of the bladder receiving 65 Gy or higher (V65) was significantly higher in CTV1. As would be expected, there were no significant differences in dose to either the rectum or penile bulb. Additionally, there was on average only 39% overlap between the CTVs and 60% between the PTVs in the two plans. Conclusions: Utilization of the pre-operative pelvic CT scan can aid in more accurate delineation of the CTV/PTV in prostate bed radiation therapy and decrease the bladder dose. As many patients at risk for pPRT have had this imaging performed preoperatively, in accordance with guidelines, incorporation of this data appears prudent. These findings need to be validated in a larger cohort. [Table: see text]