Olfactory Sensations During Proton and Photon Radiotherapy: A Multicenter Prospective Observational Study

Purpose: Patients often report a sense of smell during radiation therapy (RT), but the details of these events are not well understood. The purpose of the study was to evaluate events of smell during photon RT and proton beam therapy (PBT). Methods and materials: The subjects were all adult patients (≥20 years old) treated with photon RT or PBT at two centers from January 2019 to August 2020, with the exclusion of those with communication difficulties or olfactory abnormality. The presence of smell, odor type, intensity (five levels), and time period was examined prospectively using a weekly checklist. Results: A total of 649 courses were examined in 620 patients who received photon RT (n=415) or PBT (n=205). A smell during the procedure was sensed by 51 patients (8.2%). In multivariate logistic regression analysis, nasal cavity dose (p=0.002), age (p<0.001), and photon RT (p=0.018) were identified as significant factors associated with a sense of smell. Smell occurred in only 23/515 patients (4.5%) in whom the nasal cavity was not irradiated, but in 4/19 (21.1%) and 24/86 (27.9%) with nasal cavity maximum isodose lines of 10%-50% and 60%-100%, respectively. Patients who received photon RT sensed a smell (43/415; 10.4%) more frequently than those treated with PBT (8/205; 3.9%). Of the 51 patients who sensed a smell, 32 (63%) reported a burnt smell, eight (16%) a chemical smell, two (4%) a sour smell, and nine another smell (copier machine, sweet, garbage, etc.). Conclusions: The sense of a smell appears to be common during RT and this sensation is significantly associated with the nasal cavity dose, younger age, and photon RT.