Comparison of tumour size on outcomes for patients with unresectable locally advanced pancreatic adenocarcinoma (LAPC) receiving P-32 microparticles with standard-of-care chemotherapy (SoC CT)

Unresectable LAPC (uLAPC) has a poor prognosis. SoC is CT or chemoradiotherapy. Brachytherapy enables beta radiation emitting Phosphorus-32 (P-32) microparticles to be implanted into pancreatic tumours via endoscopic ultrasound (EUS) guidance. We report a post hoc analysis of tumour size in PanCO, a single-arm, multicentre study of P-32 microparticles with SoC CT in uLAPC. Eligible patients (pts) with uLAPC and ECOG 0-1 received either gemcitabine/nab-paclitaxel (GNP) or FOLFIRINOX (FFX) CT. P-32 microparticles (OncoSil™; OncoSil Medical) implantation was planned in week 4. P-32 activity was calculated from tumour volume (TV) to deliver 100 Gy absorbed dose. Implantation was assessed by EUS and SPECT/CT imaging. The primary endpoint was safety and tolerability. CT (RECIST 1.1) and PET response was assessed by independent central reader. 50 pts were enrolled (Intention-to-Treat); 42 were implanted with P-32 microparticles (Per Protocol [PP]) at a median of 31 days. 40 pts received GNP and 10 FFX CT (PP: 34/8, respectively). Median follow-up was 31.6 months. Median tumour longest diameter (LD) was 4.5 cm; 16 PP pts had tumours ≤4 cm (median 3.5 cm; range 3.0–4.0 cm) and 26 had tumours >4 cm (median 5.1 cm; range 4.2–7.1 cm). Baseline characteristics were similar in the ≤4 cm and >4 cm cohorts (age: 68 v 65 years; ECOG 0: 50.0% v 61.5%), although the >4 cm cohort had more males (43.8% v 76.9%), higher median baseline CA 19-9 (133 v 405 U/mL) and pancreatic body location (6.2% v 26.9%), respectively. Outcomes were similar for objective response (25.0% v 34.6%), median decrease in tumour LD (-16.2% v -21.6%) and TV (-51.8% v -54.0%), and overall survival (median 15.5 v 15.2 months). Surgical resection was slightly more frequent in ≤4 cm tumours (31.3% v 19.2%). Median maximal decrease in CA 19-9 (-67.0% v -89.5%) and PET (TLG -44.5% v -65.2%) was greater in >4 cm tumours. Although cohort size is limited, the results suggest that the potential benefits of adding P-32 microparticles to SoC CT for unresectable LAPC may be similar for patients treated with smaller (≤4 cm) or larger (4–7 cm) tumours. Further clinical studies are in development.