ESTRO 2022 - Abstract Book

S742

Abstract book

ESTRO 2022

The median duration of follow-up was 58.0 months. There were 174 (66.7%), 19 (7.3%), and 68 (26.0%) patients with low-, intermediate-, and high-risk histology, respectively. Multivariate analysis revealed a significant improvement in 5-year LC (77.8% to 96.0%, p=0.002), LRC (72.2% to 92.0; p=0.026), and DFS (61.1 to 80.0%; p=0.016) with ART in patients with high- risk (HR) histology when adjusted for various adverse factors. In patients with low-to-intermediate risk (LIR) histology, the 5-year LC, LRC, and DFS was 97.4% (ART, 100.0%; no ART, 96.0%), 97.4% (ART, 100.0%; no ART, 96.0%), and 95.3% (ART, 97.1%; no ART 94.4%), respectively. ART did not affect all three endpoints on multivariate analyses (all p>0.100). However, those receiving ART (33.3%) had significantly higher T-stage (p=0.003) and more positive resection margin (p<0.001). The 5-year LC in LIR histology patients who did not receive ART, and had advanced T-stage, perineural invasion, lymphovascular invasion, positive resection margin, or close resection margin (<1mm) were 88.9%, 100.0%, 100.0%, 87.5%, and 90.9%, respectively. Conclusion ART should be recommended in resected HR node-negative parotid gland cancer patients for disease control. The role of ART in patients with LIR histology remains unclear although the LC is extremely high following surgery plus ART. Further investigation to identify subgroups that would benefit with ART among LIR histology is warranted. 1 Albert Einstein College of Medicine and Montefiore Medical Center, Radiation Oncology, Bronx , USA; 2 Albert Einstein College of Medicine and Montefiore Medical Center, Radiation Oncology, Bronx, USA; 3 Albert Einstein College of Medicine, Radiation Oncoloy, Bronx, USA; 4 Montefiore Medical Center, Radiation Oncology, Bronx, USA Purpose or Objective Clinical trials investigating parotid gland stem cell (PGSC) sparing have not shown added protection from radiation-induced xerostomia, and we hypothesize that this is due to not adequately accounting for changes in the anatomy and location of the PGs during treatment. Here we evaluate the change in PG volume at mid-treatment and the association between dose to the PG or PGSC region and patient-reported xerostomia based on pre- and mid-treatment computed tomography (CT) scans for patients treated with IMRT for head and neck cancer (HNC). Materials and Methods Patients treated from 2006-2012 at our institution with prospective patient-reported xerostomia outcomes available at least 9 months following RT were included. The PG and PGSC regions were delineated and the dose was estimated from the treatment plan 3D dose distribution, on both pre- and mid-treatment CT scans. The association between radiation dose and volumetric changes at mid-treatment was assessed using linear regression. Logistic regression and logistic dose- response models were used to examine the association between dose to the PG or PGSC region and the risk of patient- reported xerostomia. Results Sixty-three patients were included, most treated with 70 Gy in 33 fractions; 34 of them had mid-treatment CT scans. Figure 1 shows that both the left and right PGs had a significant volume reduction from baseline to mid-treatment (p<0.001), and that change in PG volume was significantly associated with mean PG dose (p<0.01). The logistic regression models in Table 1 show a strong association between xerostomia and radiation dose to the spared (contralateral) PG and spared PGSC region (p=0.001 and p=0.002, respectively), when assessed based on pre-treatment contours. Using the dose delivered by the time of mid-treatment CT and the adapted mid-treatment contours revealed a larger OR/Gy of 1.13 (p=0.050) for the whole PG and 1.17 (p=0.012) for the PGSC region. In line with negative results from clinical trials of PGSC sparing, our dose-response models based on pre-treatment contours estimated very similar risks of xerostomia at the 20 Gy mean dose QUANTEC- recommended constraint, with 32.5% using whole spared PG vs. 31.4% using the spared PGSC region. PD-0824 Parotid gland volumetric change during IMRT and implications for stem cell sparing strategies M. Gjini 1 , P. Brodin 2 , S. Ahmed 3 , W. Tomé 2 , S. Kalnicki 4 , C. Guha 2 , R. Kabarriti 2 , M. Garg 4

Figure 1. Linear regression showing PG volume change of left (a) and right (b).

Table 1 . Xerostomia logistic regression with dose from pre-treatment CT (n=63) or dose delivered by time of mid-treatment CT with mid-treatment contours (n=34).