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DAHANCA 35 (DAH35) is the first Danish study of proton radiotherapy in squamous cell carcinoma (SCC) of the pharynx or larynx. In the pilot phase patients (pts) were offered proton therapy if the risk of late dysphagia or xerostomia could be potentially reduced with proton therapy using Normal Tissue Complication Probability (NTCP) models. The present study aimed to describe the severity of a range of acute toxicities for patients in the pilot phase compared to a matched group of patients treated with photon therapy.

Between April 2019 and October 2020, 62 pts with histologically verified SCC of the pharynx or larynx received proton therapy in the pilot phase of DAH35. A matched control group consisting of 124 pts, who completed standard photon therapy according to DAHANCA guidelines between 2012 and 2020 was selected. Patients in the control group were matched to proton-treated pts in ratio 1:2 on the treatment center, tumor site, TNM stage according to UICC version 7, p16 status for oropharynx cancers (OPC), concurrent chemotherapy, and radiation dose. Data on toxicity was registered at the treating facility during treatment. Data on follow up were all registered at the referring photon clinic.

In total 186 pts were included, of which 77% were males. The median age for the whole cohort was 62 years (range 40-85). The majority (143, 89%) had p16-pos OPC with 155 (83%) receiving concurrent chemotherapy and 139 (75%) bilateral neck irradiation. Baseline characteristics for both groups are shown in Table 1. Proton therapy was associated with significantly less weight loss at the end of RT (p=0.01, mean weight loss 2.6 kg vs 4.5kg). Equal proportions of proton (34%) and photon pts (37%) required hospitalization during and up to 60 days day after treatment completion (p=0.7), mean days spent in hospital were 4 days and 3.5 days respectively (p=0.7). No difference between groups was observed for opioids use in the observation period. At 8-weeks post-treatment follow-up 5 pts in protons and 7 pts in photons still had persisting grade 3 mucositis (p=0.6). There were statistically significant differences in skin reactions between groups, with more grade 3 toxicity in protons at the end of RT (26% vs 14%, p=0.04). However, at 8 weeks post-RT no grade 3 skin reactions were registered in protons. At 8-weeks post-RT feeding tube was used for 6 (10%) of proton-treated pts and 8 (7%) for photon (p= 0.5).

Slight differences with more skin toxicity but less weight loss was observed with proton therapy in this case control study. Regarding acute toxicity, proton treatment seemed safe.