Background: Most newly diagnosed oro- & hypopharngeal cancers (OPC, HPC) are treated with (chemo)RT with curative intent but at the consequence of adverse effects on quality of life. We investigated if using DO-IMRT to reduce RT dose to the dysphagia/aspiration related structures (DARS) improved swallowing function compared to S-IMRT. Methods: Patients with T1-4, N0-3, M0 OPC/HPC were randomised 1:1 to S-IMRT (65 Gray (Gy)/30 fractions (f) to primary & nodal tumour; 54Gy/30f to remaining pharyngeal subsite & nodal areas at risk of microscopic disease) or DO-IMRT. The volume of the superior & middle pharyngeal constrictor muscle (PCM) (OPC) or inferior PCM (HPC) lying outside the high-dose target volume was set a mandatory mean dose constraint in DO-IMRT. Treatment allocation was by minimisation balanced by centre, use of induction/concomitant chemotherapy, tumour site & AJCC stage. Primary endpoint was mean MD Anderson Dysphagia Inventory (MDADI) composite score 12 months after RT. Secondary endpoints included University of Washington (UW)-Qol, Performance Status Scale Head & Neck (PSS-HN) domain scores (range: 0-100), swallow volume, swallow capacity and local control. Results: 112 patients (56 S-IMRT, 56 DO-IMRT) were randomised from 22 UK & Ireland centres from 06/2016 - 04/2018. 111/112 had RT doses as prescribed (1 patient died before RT). Outcome measures at 12 and 24 months are summarised below. DO-IMRT had higher MDADI scores at 12 (p = 0.04) and 24 (p = 0.07) months. Clinically important improvements in swallowing function were seen in patients receiving DO-IMRT using PSS-HN domains and the UW-QoL tool. Conclusions: DO-IMRT improved patient reported swallowing function compared with S-IMRT. Improvements were seen in overall MDADI as well as functional scores in both PSS-HN and UW-QoL. Clinical trial information: 25458988.