3256 - Treatment Outcomes of External Beam Radiotherapy with or without Focal Boost to Intraprostatic Lesions in Patients with Localized Prostate Cancer

H. C. Onal^1,2, O. C. Guler², G. Erbay³, A. Elmali Dogan⁴, and M. N. Yavuz⁴; ¹Department of Radiation Oncology, Baskent University Faculty of Medicine, Ankara, Turkey, ²Baskent University Faculty of Medicine, Adana Dr Turgut Noyan Research and Treatment Center, Department of Radiation Oncology, Adana, Turkey, ³Baskent University Faculty of Medicine, Adana Dr Turgut Noyan Research and Treatment Center, Department of Radiology, Adana, Turkey, ⁴Baskent University Faculty of Medicine, Department of Radiation Oncology, Ankara, Turkey

Purpose/Objective(s): We aimed to compare the treatment outcomes and toxicity of prostate radiotherapy (RT) at a dose of 78 Gy administered in 39 fractions, with or without focal boost (FB) to intraprostatic lesions (IPL) at a maximum of 86 Gy, using the simultaneous integrated boost (SIB) technique in patients with prostate cancer (PCa). Materials/

Methods: We analyzed prognostic factors for biochemical disease-free survival (bDFS), prostate cancer-specific survival (PCSS), and treatment-related toxicity in 1166 patients with PCa who received RT between March 2012 and September 2021. All patients were given 78 Gy for their prostate and seminal vesicles with or without 86 Gy for IPL using the SIB technique. High-risk patients had additional pelvic nodal irradiation, with a dose of 46–54 Gy of radiation. The primary endpoints were bDFS and PCSS, and the secondary endpoint was acute and late gastrointestinal (GI) and gentiourinarty (GU) toxicities.
Results: A total of 1166 patients, 606 (52%) who underwent SIB and 540 (48%) without SIB, were analyzed. With a median follow-up time of 84.7 months, the 7-year bDFS and PCSS rates were 88.9% and 94.6%, respectively. In the univariate analysis, serum PSA level, clinical T stage, ISUP grade, risk group, and FB to IPL were significant prognostic factors for bDFS and PCSS. Patients who underwent SIB had significantly higher 7-year bDFS (91.2% vs. 86.4%; p = 0.005) and PCSS (97.9% vs. 90.9%; p</em> < 0.001) than those who did not. The SIB technique significantly improved bDFS and PCSS in high-risk patients, but not in low- and intermediate-risk patients. In the multivariate analysis, advanced clinical T stage and higher ISUP grade were independent predictors of worse bDFS and PCSS. Other significant prognostic factors in the multivariate analysis were higher serum PSA levels [HR = 3.25 (95% CI, 1.99–5.32); p<0.001] for poor bDFS and the absence of FB to IPL [HR = 4.18 (95% CI, 2.03–8.59); p<0.001] for worse PCSS. Furthermore, high-risk disease and absence of FB [HR = 1.39 (95% CI, 0.96–2.04); p = 0.08] were independent predictors of worse bDFS, although only marginally significant. Adding FB to IPL significantly improved local control and distant metastases in high-risk patients. Acute grade =2 GU and GI toxicities were significantly higher in patients receiving SIB compared to those without (15.8% vs. 23.9%; p=0.001 and 8.3% vs. 13.9%; p=0.002, respectively). However, the incidence of late grade =2 GU (7.1% vs. 8.7%; p=0.32) and grade =2 GI toxicity (5.0% vs. 5.0%; p=0.99) did not differ significantly between those who did and did not receive SIB treatment.
Conclusion: The results of our study showed that the SIB technique was effective in terms of bDFS and PCSS, particularly in high-risk PCa patients. Nevertheless, a longer duration of follow-up is required in order to conduct a comprehensive analysis of survival. The study findings challenge the hypothesis that employing the prostate SIB technique for IPL leads to higher occurrences of delayed GU and GI toxicity.