Radioimmunoscintigraphy for postprostatectomy radiotherapy: Analysis of toxicity and biochemical control

Our goal was to evaluate the role of radioimmunoscintigraphy (RIS) directed against prostate-specific membrane antigen (PSMA) in influencing postprostatectomy radiotherapy (RT) toxicity and biochemical control. Methods: The records of 107 postprostatectomy RT patients were reviewed. The group for whom no RIS scan was obtained (group A, n = 54) was identified as was the group for whom a RIS scan was obtained (group B, n = 53). Group B was further subdivided into those who had a RIS and CT-scan correlation to aid in treatment planning (subgroup B1, n = 40) versus those who did not (subgroup B2, n = 13). Gastrointestinal (GI) and genitourinary (GU) toxicities were reviewed for each of these groups and subgroups and compared. Biochemical failures (defined as 2 successive PSA rises after a nadir of ≥0.2 ng/mL) were identified to generate biochemical failure-free survival (BFFS) curves for each of the groups and subgroups. Results: No significant differences in late toxicity were observed between any group or subgroup. However, acute GI toxicity was higher in group B versus group A (P = 0.026), and acute GU toxicity was higher in subgroup B2 versus subgroup B1 (P = 0.050). Overall, most toxicity was grade 1 or 2; only one case of grade 3 toxicity and no cases of grade 4 or 5 toxicity were observed. Three-year BFFS was higher for group B versus group A (80.7% vs. 75.5%) and for subgroup B1 versus subgroup B2 (84.5% vs. 71.6%). On multivariate analysis of pretreatment (age, race), surgical/staging (stage, grade, margin status, extracapsular extension, lymph node status, seminal vesicle invasion, post-radical retropubic prostatectomy [RRP] prostate-specific antigen [PSA] nadir, maximum post-RRP PSA, and RRP-to-RT interval), and treatment (hormone therapy, RT dose, RT technique, RIS scan, and RIS/CT correlation) factors on BFFS, the only covariate reaching significance was RIS/CT correlation (P = 0.042). Conclusion: A small BFFS advantage was observed in patients for whom RIS was used to guide RT decision making and treatment planning; however, this advantage only reached significance in this study for those for whom the RIS/CT correlation was used to guide target definition. The improved PSA control using RIS was achieved with a small increase in acute toxicity but with no observed change in late toxicity. These findings can serve as the basis for prospective studies in this area of investigation.