Proton Therapy Specific Salivary Gland Volume Changes After Head and Neck Radiotherapy

Abstract

Salivary glands are exquisitely sensitive to radiotherapy. Patients treated with radiotherapy to the head and neck region commonly develop salivary gland volume loss and associated xerostomia. Salivary volume loss is associated with salivary function loss, making it a useful surrogate indicator of xerostomia. Previous studies have correlated the degree of salivary gland volume loss with mean radiation doses in patients undergoing photon head and neck radiation. However, change in salivary gland volume relative to radiation dose has not been evaluated in patients undergoing proton therapy. Currently, a constant value of 1.1 is used to relate physical dose (PD) to biological dose (BD) for proton therapy. However, specific tissues may respond differently and thus require a different PD to BD conversion. Herein, we evaluate changes in salivary gland volumes relative to radiation dose in head and neck cancer patients receiving proton therapy to examine the validity of current PD to BD conversions for head and neck salivary glands.Dosimetric data and pre-radiation parotid gland and submandibular gland (SMG) volumes were collected for 20 head and neck cancer patients that received proton therapy at our institution between 2018-2020. Pre-radiation volumes of the SMGs and parotids were collected from treatment planning scans. Post-treatment salivary gland volumes were contoured on 3-month post-treatment diagnostic scans. Mean doses to the glands were gathered from dose-volume histograms generated from treatment plans. Linear logistical regression was performed to measure the effect of dose on volume change.Volume changes of 27 parotids and 38 SMGs were evaluated. Mean doses were 10.0 Gy (standard deviation (SD) 13.2) and 25.2 Gy (SD 25.0) for parotids and SMGs, respectively. Mean dose was < 1 Gy, < 30 Gy, and < 50 Gy in 59.3%, 92.6%, and 100% of parotids compared to 39.5%, 55.3%, and 81.6% of SMGs, respectively. Mean ratios of gland volumes at 3 months post-treatment relative to pre-treatment were 0.962 (SD 0.210) and 1.106 (SD 0.273) for parotids and SMGs, respectively. Effect of mean dose on gland volume change was not statistically significant for either parotids or SMGs.The greater relative size reduction in parotids versus SMGs was consistent with previous photon data. The mean increase in SMG volume could represent post-treatment functional compensation in preserved glands, but this requires additional investigation. Also, determination of the effect of proton dose on salivary gland volume change requires further analysis. Currently, no conclusions can be drawn whether salivary glands are equally or differentially sensitive to proton therapy relative to photon therapy.