Background: Peritoneal metastasis (PM) from colorectal cancer carries a dismal prognosis. Improved survival can be achieved by extensive cytoreductive surgery (CRS), frequently used together with hyperthermic intraperitoneal chemotherapy (HIPEC), with median time to recurrence around 12 months. Radspherin is a novel treatment principle based on the delivery of short range and cytotoxic alpha particles emitted during the decay of 224Ra. Alpha particles have high linear energy transfer and a radiation range less than 100 µm (3-10 cell diameters), generating highly localized and effective radiation with non-repairable double-strand DNA breaks in affected cells. Our hypothesis is that Radspherin produces an alpha-particle radiation field exclusively to the surfaces and liquid volumes of the peritoneum, delivering lethal doses to remaining micrometastasis in the peritoneal linings and eradicate free-floating tumor cells after surgical resection, thus assumingly prolonging time to recurrence and potentially overall survival. Methods: A phase 1/2a study (EudraCT 2018-002803-33) is ongoing to evaluate safety, tolerability and signal of efficacy of Radspherin injected intraperitoneally two days after CRS-HIPEC. After completion of dose escalation (1-2-4-7 MBq), an activity-dose of 7 MBq was recommended. Assessment of safety and efficacy (diagnostic imaging) was performed every three months. Safety data and progression-free survival (PFS) at 15 months are presented. Results: Twenty-three patients (pts) were enrolled across cohorts (safety population), of these 12 pts received the recommended dose of 7MBq, 9 pts single dose and 3 pts split dose (3,5 MBq x2). Twelve pts had synchronous PM and 11 metachronous PM. Median age was 64 years (28-78), 70 % were female and median peritoneal cancer index was 7 (3-19). To date, 271 adverse events (AE) were reported, whereof only 7 (all grade 1-2) were deemed related to Radspherin. Fourteen serious adverse events (SAEs) in 8 pts have been reported, none considered related to Radspherin. At 15 months, 9 out of 23 pts (39 %) had recurred, whereof 4 pts recurred in the peritoneum. In the expansion cohort (7MBq), 3 out of 12 pts (25 %) had recurred and none of these pts had peritoneal recurrences. Median PFS was not reached in the two populations. Conclusions: Radspherin was well tolerated with no related SAEs reported. At 15 months median PFS has not been reached and none of the patients at recommended dose had peritoneal recurrences. The results are encouraging and warrant further exploration of Radspherin as a novel treatment principle in clinical trials. Clinical trial information: NCT03732781.