Systemic Therapy Postponement after Stereotactic Body Radiation Therapy for Oligometastatic Prostate Cancer

Abstract

<h3>Purpose/Objective(s)</h3> To evaluate the oncological outcome after stereotactic body radiation therapy (SBRT) for oligometastatic (OM) prostate cancer (PC) patients. <h3>Materials/Methods</h3> In this retrospective observational multi-institutional study (Health-data-hub N°F20210402112942), patients with OM (≤ 5) PC underwent SBRT. While patients were enrolled irrespective of PC initial treatment, castration sensitive (CS)/resistant (CR) and synchronous (SM)/metachronous (MM) metastatic disease status were taken into account. Primary endpoint was systemic therapy postponement (STP) after SBRT. Oncological outcome was assessed by post-SBRT biochemical response, local (LR; in SBRT field), metastatic (MR), prostatic (PR) and isolated biochemical (iBR) relapses. PSA kinetic (PSAdt; velocity) and prognostic factors for STP were investigated. Toxicity was reported (CTCAE v5). <h3>Results</h3> From 01/07 to 09/19, 169 pts (median age 71.4 y [48.7 – 100.1]) underwent SBRT. At the time of SBRT, 151 pts (89.3%) and 18 pts (10.7%) were MM and SM respectively and 119 pts (70.4%) and 50 pts (29.6%) were CS and CR respectively. Median PSA, PSAdt and velocity were 2.4 ng/ml [0.01 – 277], 5.2 months [0.9 – 45.2] and 2.3 ng/ml/an [0.1 – 25.9] respectively. Median time between initial PC/metastatic disease (TPCMD) and metastatic disease/SBRT were 68.9 months [1.9 – 273.4] and 3.1 months [0.1 – 97.9]. Metastatic anatomic sites were pelvic (PLN) and extra-pelvic lymph nodes (EPLN), bone and brain for 58 pts (34.4%), 15 pts (8.9%), 92 pts (54.4%) and 4 pts (2.4%) respectively. SBRT regimens were 27 Gy/3f (29%), 35 Gy/5f (24.3%), 36 Gy/6f (18.3%) and other (28.4%). With a MFU of 27.5 months [1 – 100.5], median time between SBRT/1^st PSA control was 2 months [0.5 – 29]. PSA decreased in 67.4%, was stable in 9.4% and increased in 23.2%. Eighty-eight pts (52.1%) underwent post-SBRT systemic therapy after a median STP of 15.4 months [2.6 – 86.9]. One hundred and thirty events (LR, MR, PR, iBR) were observed in 106 pts (62.7%), among whom 13 (12.3%) relapsed in the irradiated area (local control rate: 92.3%). MR, PR, iBR were observed in 82 pts (77.4%), 33 pts (31.1 %) and 2 pts (1.9 %) respectively. Prognostic factors for STP after SBRT were: hormonal status (CS vs CR: 20.9 vs 10.8 months; p < 0.001), post-SBRT biochemical response ([declined + stable] PSA vs increased PSA: 21 vs 11 months; p = 0.004) and TPCMD (< 42 vs ≥ 42 months; p < 0.001). G≥3 toxicity occurred in 4 pts: 2 pts with G3 post-SBRT fracture (1 surgery), 1 pt with G3 radiation pneumonitis and 1 pt with G4 intracranial hypertension (emergency surgery). <h3>Conclusion</h3> SBRT for oligometastatic prostate cancer is an option to postpone systemic therapy, mainly for CS and MM patients with an acceptable toxicity profile. Patient selection criteria remain a crucial goal in order to refine SBRT indications, while the impact of combination with systemic therapy remains under investigation.