Background/Objectives: Carbon-ion radiotherapy (CIRT) offers precise dose distribution and enhanced biological effectiveness in localized prostate cancer. However, the safety of CIRT in patients with a history of surgery for benign prostatic hyperplasia (BPH), such as transurethral resection of the prostate (TURP), remains unclear. This study aimed to evaluate the long-term safety and oncological outcomes of CIRT in this population. Methods: A retrospective analysis was conducted in 74 of 3848 patients with prostate cancer and a history of surgery for BPH who underwent CIRT combined with risk-adapted androgen deprivation therapy between 2007 and 2023. Adverse events were assessed using CTCAE v5.0. Biochemical recurrence-free survival was estimated using the Kaplan–Meier method and risk factors for hematuria with multivariate logistic regression and receiver operation characteristic (ROC) analysis. Results: CIRT was generally well-tolerated. Early Grade 2 genitourinary (GU) adverse events occurred in 5.4% of patients, and late-Grade 2 or higher GU events occurred in 8.1%. The cumulative incidence of Grade 2 ≥ GU events remained 10% at 36 months. Compared to holmium laser enucleation of the prostate, a shorter interval between BPH surgery and CIRT initiation and a history of TURP were independently associated with an increased risk of hematuria. Notably, 5-year bRFS was 100% in low- and intermediate-risk groups and 88.6% in the high-risk group. Conclusions: CIRT demonstrates acceptable oncological outcomes and urinary complication rates in patients with prostate cancer and a history of BPH surgery. These findings suggest that CIRT can be a feasible treatment option in this surgically altered population, but careful patient selection, individualized treatment planning, and long-term follow-up are essential. Given the absence of a non-BPH control group, oncological efficacy should be interpreted with caution.

To evaluate the efficacy and patterns of failure after definitive radiotherapy (RT) using an elective nodal irradiation (ENI) dose of ≤40 Gy for head and neck cancer. We retrospectively analyzed 290 patients with stage I–IV oropharyngeal cancer (OPC), hypopharyngeal cancer (HPC), or laryngeal cancer (LYC) who underwent definitive RT with an ENI dose of ≤40 Gy at three institutions between 2008 and 2024. A total of 278 patients received 40 Gy of ENI, whereas ENI was omitted in 12 patients (ENI-omission). The study endpoints included patterns of failure, local control (LC), regional control (RC), progression-free survival (PFS) and overall survival (OS). With a median follow-up of 35 months among surviving patients, the 2-year LC, RC, PFS and OS rates were 79.8%, 91.2%, 63.1% and 79.8%, respectively. Regional recurrence within the ENI field occurred in only four patients (1.4%), including no patients in the ENI-omission group and four in the 40-Gy ENI group. Only one patient experienced an isolated ENI field recurrence. In the multivariable analysis, advanced N category, systemic therapy and prescribed dose were associated with OS. Advanced N category was also an independent predictor of both RC and PFS. Definitive RT using an ENI dose of ≤40 Gy achieved favorable oncologic outcomes with low incidence of regional recurrence within the ENI field. These findings support the feasibility of an ENI dose of ≤40 Gy while maintaining enough elective nodal control.