ICHNO-ECHNO 2022 - Abstract Book

S16

ICHNO-ECHNO 2022

Purpose or Objective Proton treatment for head and neck cancer has the potential to reduce dose to several organs at risk (OARs) due to the physical properties. However, there is little clinical evidence to support the theoretical gain observed in treatment plan comparisons between conventional photon IMRT and proton IMPT. The Danish Head and Neck Cancer Group (DAHANCA) is conducting a randomized controlled trial with an enriched patient population (clinicaltrials.gov NCT04607694). Patients with a change in normal tissue complication probability ( Δ NTCP) of 5%-point or more in favour of protons for xerostomia grade 2+ and/or dysphagia grade 2+ are randomized. Photon and a proton dose plans were prepared at the referring centre. If fulfilling the inclusion criteria, the patient was referred to the national proton centre (NPC). At the NPC, new immobilization, CT, MR scans, and proton treatment plan were made. Identical parameters for robust optimization, setup and range uncertainties were used. The purpose of this study was to compare the proton plan from the referring centre with the clinical proton plan at the NPC to ensure the patient selection is adequate. Materials and Methods Treatment plans for the 53 patients enrolled in the pilot phase of DAHANCA35 were collected. This included the comparative photon and proton plans generated in the local treatment centre and clinical proton plan from the NPC. Oropharyngeal cancer was primary in 45 patients. The Δ NTCP stability was evaluated between the comparative and clinical proton plans. Results Of the 53 patients, 20 and 43 had Δ NTCP gain above 5%-point for xerostomia and dysphagia, respectively. Ten patients were selected for both trial arms. The median Δ NTCP for the planning comparison was 6.8% [range 5.0 to 13.3%] and 6.3% [5.3 to 10.2%] for xerostomia and dysphagia, respectively. The local proton plans matched the clinical proton plans well, illustrated in figure 1, where most patients are close to the identity line indicating the same NTCP risk in the two proton plans.

Few patients had a substantial change in their clinical target volumes, which contributed to the change in Δ NTCP. In general, the clinical proton plans did not spare the OARs to the same level as the comparative proton plan (figure 2, boxplots).