Impact of radioimmunoscintigraphy on definition of clinical target volume for radiotherapy after prostatectomy

The goal of this study was to evaluate the role of radioimmunoscintigraphy (RIS) directed against prostate-specific membrane antigen in modifying postprostatectomy prostate fossa clinical target volume (CTV) definition. Methods: The records of 25 postprostatectomy patients who received external-beam radiotherapy after prostatectomy and who underwent vessel-based RIS/planning CT registration were reviewed. For each patient, the CTV that would have been treated (CTV_pre) before this registration was compared with that defined after the registration (CTV_post). In addition, using a standard dose of 66 Gy in 2-Gy fractions, the corresponding bladder and rectum dose volume histograms were compared using 2 endpoints: volume receiving ≥60 Gy (V60) and area under the curve (AUC). Results: The mean CTV_pre vs. CTV_post volumes were 24.4 vs. 35.0 cm³, respectively (P = 0.032). The V60 results for CTV_pre and CTV_post were 32.7 vs. 41.0 cm³, respectively, for the rectum (P = 0.168) and 33.9 vs. 46.6 cm³, respectively, for the bladder (P = 0.015). The AUC results for CTV_pre and CTV_post were 4,027 vs. 4,516 Gy x cm ³, respectively, for the rectum (P = 0.396) and 4,782 vs. 5,561, respectively, for the bladder (P = 0.119). No Radiation Therapy Oncology Group grade 3, 4, or 5 (acute or late, gastrointestinal, or genitourinary) toxicity was observed. Two-year biochemical failure-free survival (with failure defined as 2 consecutive prostate-specific antigen rises above 0.2 ng/mL) was 87% for the cohort. Conclusion: Incorporating RIS uptake resulted in significant modifications in CTV definition. The consequences of these modifications on the rectum V60 or AUC or on the bladder AUC were not significant, although the bladder V60 did increase. However, observed toxicity was low, with acceptable short-term biochemical control, suggesting that treatment to the modified CTV was tolerable.