S601
Clinical – Head & neck
ESTRO 2026
Floriana Pasini, Marco Liotta, Roberto Orecchia, Daniela Alterio, Barbara Alicja Jereczek-Fossa Radiation Oncology, European Institute of Oncology IRCCS, Milan, Italy Purpose/Objective: Xerostomia is one of the most debilitating late toxicities in head and neck cancer (HNC) patients treated with radiotherapy. The Pro-SPART study evaluated volumetric, dosimetric, and positional changes of the parotid glands and their stem cell–rich regions (SCRs) during proton therapy (PT), aiming to assess treatment robustness and organ-sparing performance. Material/Methods: HNC patients treated with curative or postoperative intent using intensity-modulated PT (IMPT) at the European Institute of Oncology were retrospectively analyzed. Patients who underwent replanning during the treatment as well as re-irradiation were excluded. Both parotid glands (PGs) and their SCRs were delineated1 on the planning computed tomography (CT) and on up to six weekly re-evaluation CTs (revCT). The ipsilateral PG was defined as the parotid receiving the higher dose. For PGs and SCRs, variations in volume ( Δ Volume) during the treatment course were quantified. Dosimetric parameters variations were evaluated recalculating the planned dose on all available revCTs. The centroid displacement for PGs during the treatment was evaluated. Results: Among 56 screened patients, 19 met the inclusion criteria. Median age was 64 years (range 24–78). Treatment intent was curative in 63% and postoperative in 37%. Primary tumor sites included the nasopharynx (26%), oropharynx (26%), and paranasal sinuses (10%), with the remaining cases involving the nasal cavity, salivary glands, orbit, and pituitary region. A total of 252 parotid glands and 252 corresponding SCRs were analyzed. Figure 1 shows the Δ Volume for the ipsilateral PG and SCR at each time-point. Median PG and SCR Δ Volume at the last revCT were -13.3% and +0.2%, respectively. Reduction in PG volume was mainly recorded during the first half of treatment. Considering contralateral PG and SCR, median Δ Volume at the last revCT was –6.3% and – 2.0%, respectively. Despite progressive PGs shrinkage, the considered dosimetric parameters (Table 1) remained stable, with variations ranged between -0.12 and +0.35 Gy (RBE). Regarding centroid displacements, only a limited subset of patients (~25%) showed measurable motion beyond voxel thresholds of 2 mm, mostly on the ipsilateral side.
Conclusion: PGs Δ Volume and variations in dosimetric parameters during the treatment were lower compared to those reported in the literature for photon therapy cohort. These findings suggest that IMPT may offer improved stability in dosimetric parameters for HNC patients. Further analysis on larger patient cohorts is necessary to validate these preliminary results and better understand the potential benefits of IMPT. References: 1. Steenbakkers RJHM, van Rijn-Dekker MI, Stokman MA, et al. Parotid Gland Stem Cell Sparing Radiation Therapy for Patients With Head and Neck Cancer: A Double-Blind Randomized Controlled Trial. Int J Radiat Oncol Biol Phys. 2022;112(2):306-316. doi:10.1016/j.ijrobp.2021.09.023 Keywords: Stem cell–rich regions, Parotid gland
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