Patient and treatment features are reported in Table 1. The median age of PCa diagnosis was 74.6 years (interquartile [IQR], 70.8–77.6 years), the median PSA was 7.93 ng/ml (IQR, 5.78–12 ng/ml), and 68.5% of the patients had a T1–T2 disease. The Gleason score was ≥ 8 in 278 (21%) patients. Based on NCCN risk grouping, 373 (28.1%) patients had low-risk, 492 (37.3%) intermediate risk, 419 (31.6%) high-risk, and 41 (3%) very high-risk disease. The median EBRT total dose was 70.2 Gy (IQR, 62–70.2 Gy), and only 5.1% were treated with 3DCRT (Table 1), whereas all the others received IMRT (27.6%) or VMAT (67.3%). At the time of EBRT, 698 (52.7%) of the patients were on androgen deprivation therapy (ADT).

The median follow-up was 5.2 years (IQR 3.2–7.5). A total of 57 patients died (from any cause) during follow-up: the OS was 92.6% (95%CI 90.3–95.0%) and 90.8% (95%CI 86.7–95.1%), at 10 and 15 years of follow-up, respectively (Fig. 1a), and the median OS was 17.3 years (IQR 17.3—not reached). Only 11 patients died from prostate cancer during follow-up: the median CSS was not reached, and the 10- and 15-year valueswere both 98.5% (95%CI 97.3–99.6%) (Fig. 1b). Distant metastases were detected during follow-up among 71 patients: the median MFS was not reached, and the rate of MFS was 90.5% both at 10 and at 15 years (95%CI 87.2–93.9%) (Fig. 1c). Finally, b-RFS (median 17.1 years, IQR, 17.1—not reached) was 85.5% (95%CI 81.9–89.4%) at 10 and 15 years, with a total of 107 patients experiencing a biochemical relapse during follow-up (Fig. 1d).

Survival curves. a Overall survival (OS). b Cancer-specific survival (CSS). c Metastasis-free survival (MFS). d Biochemical relapse-free survival (b-RFS)

The Kaplan–Meier curves for the entire cohort and stratified by risk group are reported in Fig. 2. All the survival curves show a behavior concordant to their respective risk classes, associating higher risk classes to lower survival rates (with minor differences most likely attributable to the limited number of patients in each risk class). Moreover, the p-value from the log-rank test was below 0.05 for all the analyses, thus confirming that patients’ survival significantly differed across risk classes.

Kaplan–Meier curves stratified by risk class (low: green; favorable-intermediate: red; unfavorable-intermediate: light blue; high: yellow-green; very-high: purple). a Overall survival (OS). b Cancer-specific survival (CSS). c Metastasis-free survival (MFS). d biochemical relapse-free survival (b-RFS)

On univariate analysis, increasing baseline PSA, higher Gleason score, increasing risk class, and increasing International Society of Urological Pathology (ISUP) grade were associated with worse b-RFS, MFS, and OS (Tables 2, 3). The multivariate analysis confirmed that baseline PSA and Gleason score were significant variables for all the outcomes. Regarding the use of ADT, in both univariate and multivariate analysis there was an association with worse oncologic outcomes (Tables 2, 3).

The toxicity analysis showed that only 6 patients (0.45%) developed acute gastrointestinal (GI) grade 3 (G3) toxicity (consisting of rectal bleeding [1 patient] and proctitis [5 patients]), and 93 (7.0%) patients a grade 2. Acute genitourinary (GU) toxicity of grade ≥ 2 was registered in 225 (16.98%), with only 12 (0.9%) having a G3 (consisting of hematuria [3 patient], cystitis [8 patients], and urinary obstruction [1 patient]). No patient experienced grade 4 GI and/or GU acute toxicity.

Late GI toxicity of grade ≥ 2 was registered in 54 (4%) patients, with 12 (0.9%) having a G3 (rectorrhagia 8, proctitis 4). The 5-, 10- and 15-year late grade ≥ 2 GI toxicity was 5% (95%CI, 4–6%) since no patient developed high-grade GI toxicity between 5 and 15 years of follow-up (Fig. 3a).

Late toxicity. a Late grade ≥ 2 GI toxicity. b Late grade ≥ 2 GU toxicity

Late GU toxicity of grade ≥ 2 was registered in 61 (4.6%), with 21 (1.6%) having a G3 (consisting of hematuria [5 patients], cystitis [4 patients], and urinary obstruction [12 patients]). The 5-year late grade ≥ 2 GU toxicity was 5% (95%CI, 4–7%), whereas the rate was 6% (95%CI, 4–8%) at 10 and 15 years since no patient developed late grade  ≥ 2 GU toxicity between 10 and 15 years of follow-up (Fig. 3b).