Bone Only Oligometastatic Renal Cell Carcinoma Patients Treated with Stereotactic Body Radiotherapy: A MULTI-Institutional Study

Abstract

<h3>Purpose/Objective(s)</h3> Previous studies demonstrated the efficacy of stereotactic-body radiotherapy (SBRT) for oligometastatic renal cell carcinoma (RCC) patients. However, it has not been well-studied in patients with bone-only metastatic RCC. This study aimed to analyze the prognostic factors associated with overall survival (OS) and progression-free survival (PFS) in patients with bone-only metastatic RCC who have five or fewer lesions treated with SBRT. <h3>Materials/Methods</h3> The clinical data of 54 patients with 70 bone metastases undergoing SBRT treated between 2013 and 2020 with a fraction dose of at least 6 Gy per fraction, and a biologically effective dose (BED) of at least 90 Gy was retrospectively evaluated. The prognostic factors associated with OS and PFS were assessed. <h3>Results</h3> The majority of lesions were located in the spine (57.4%), and had only one metastasis (64.8%). The median SBRT fraction and total doses were 10 Gy (range: 6–18 Gy) and 20 Gy (range: 12–40 Gy), and SBRT was administered with a median fraction of 3 (range, 1–5). The median BED of SBRT was 108.4 Gy (range: 90.0–202.2 Gy). After a median follow-up of 22.4 months, the 1- and 2-year OS rates were 84.6% and 67.3%, respectively, and median OS was 43.1 months. The 1- and 2-year PFS rate, and median PFS were 63.0%, 38.9%, and 15.3 months, respectively. After completion of metastasis-directed treatment (MDT), disease progression occurred in 26 patients (48.1%) at a median of 12.7 months (IQR 7.1–29.9 months). There were 22 (84.6%) with distant metastasis, one (3.8%) with in-field local recurrence, and three (11.6%) with both distant metastasis and local recurrence. In SBRT-treated lesions, only 4 patients (7.4%) experienced local recurrence, and the 1- and 2-year LC rates per lesion were 94.9% and 92.0% per lesion. Age, metastasis localization, and number of fractions for SBRT were significant prognostic factors for OS in univariate analysis. In multivariate analysis, patients with spine metastasis had a better OS [HR=3.1 (95% CI, 1.3–7.7; p = 0.001)] compared to their counterparts, and patients who received single fraction SBRT [HR=2.2 (95%, 1.2–4.2; p = 0.01)] had a better PFS than those who did not. No patient experienced acute or late toxicities of grade 3 or greater. <h3>Conclusion</h3> To the best of our knowledge, this is the first multicenter study of bone-only oligometastatic RCC patients treated with SBRT. Despite excellent LC at the oligometastatic site treated with SBRT, disease progression was observed in nearly half of patients 13 months after MDT, particularly as distant disease progression other than treated lesion, necessitating an effective systemic treatment to improve treatment outcomes. Prospective trials of SBRT for oligometastatic bone sites are warranted because they may shed light on disease control, survival, and quality of life after such therapy.