ESTRO 37 Abstract book

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ESTRO 37

Material and Methods We utilized dose-response data of 200 head-and-neck patients, treated between 1999 and January 2017 in prospective salivary gland function studies. SMG flow rates were measured 6 weeks, and 1 year after therapy, and converted into the percentage of baseline. SMG data were fitted to log regression and to the Lyman-Kutcher- Burman (LKB) model, with a complication defined as <25% of the flow before therapy 1 . Thirty patients were treated after unilateral neck dissection. Patients with N2c or N3 nodes were excluded, 38% were N 0 , 32% T 3-4 . Tumors were localized in the oropharynx in 69%, larynx in 14%, nasopharynx in 9%, oral cavity in 4%, and hypopharynx in 3%. Treatment consisted of conventional RT, non SMG sparing IMRT, SMG sparing IMRT, in 34, 84, 82 patients, respectively. The mean dose to the ipsilateral submandibular gland was 62Gy (94%>40 Gy), for the contralateral submandibular gland 47 Gy, range 0-72 Gy. Flow rates were converted to the baseline unilateral SMG flow rate, and compared with the contralateral SMG mean dose. Results There was a highly significant correlation between increasing mean SMG dose and decreasing ab solute and relative SMG flow after 6 weeks and 1 year. In 5 mean contralateral SMG dose groups:≤30 Gy (n=30), 30-40 Gy (n=28) 40-50 Gy (n=32), 50-60 Gy (n=57), > 60 Gy (n=53), the complication rate at 6 weeks was 22%, 50%, 73%, 76% and 86%, respectively; at 1 year 30%, 31%, 75%, 86% and 94%, respectively (p<0.0001). The D50 after 6 weeks was 31 Gy (95% CI 24-37) and 38 Gy after one year (95% CI 35-42) after analysis with the LKB model; 33 Gy (27-40) and 36 Gy (31-42), using logistic regression. Submandibular dose response curve 6 weeks and one year after therapy:

the impact of the difference between the planned and the actual delivered dose on NTCP models for head and

neck cancer patients. Material and Methods

143 head and neck cancer patients planned to receive a prescribed dose of 70 Gy in 35 fractions with daily CBCT guidance were included in this retrospective study. The actual delivered dose was estimated by 1) deforming the planning CT to the anatomy of the daily CBCT, 2) recalculating the treatment plan on this synthetic CT, 3) deforming the recalculated dose back to the planning CT, 4) accumulating all daily dose distributions. The parameters (m, TD 50 , n) of the Lyman-Kutcher-Burman (LKB) NTCP model were estimated with the maximum likelihood method by relating the equivalent uniform dose (EUD) of organs at risk (OAR) (oral cavity, contralateral parotid gland, and constrictor muscle) with toxicities (oral mucositis (≥G3), xerostomia (≥G2) and dysphagia (≥G3)) respectively. Due to the prior irradiation, baseline toxicity and missing contours, 11, 16, and 34 patients were excluded from the analysis for oral mucositis, xerostomia, and dysphagia respectively. To compare the model parameters, we performed the deviance test. To evaluate the performance, receiver- operator characteristic (ROC) analysis was performed and the area under the curve (AUC) was compared with z- test. The statistical significance was set to p<0.05. Results In this cohort the incidence for oral mucositis (≥G3), xerostomia (≥G2) and dysphagia (≥G3) was 25%, 26.8% and 30.3% respectively. The median EUD difference (0.31 Gy) between planned and delivered dose to the OAR were significantly larger for patients with dysphagia ≥Grade 3 than without (Figure 1). For oral mucositis and xerostomia, no significant differences were observed. Since there were no significant differences between 2 parameters (n=1) and 3 parameters model, the simpler model using mean dose (n=1) was used for the rest of the analysis. The delivered dose provided an equal or better fit for all toxicities and the steepness of the NTCP curve was equal or higher (smaller m) (Table 1). These differences, however were not significant. Similarly, no significant differences were found in AUC.

Conclusion This NTCP-curve for submandibular gland function is based on the largest database of objective measurements in the literature. External validation of the NTCP-curve will be performed. For planning purposes usually a threshold of 40 Gy is chosen. Based on our results the planning constraint for mean submandibular gland dose should be set to less than 30 Gy, preferably 20 Gy. 1: Dijkema et al. Int J Radiat Oncol Biol Phys 2010:78;449-453. PV-0315 Comparison of NTCP models between using planned and actual delivered dose for head and neck cancer T. Kanehira 1 , S.R. Van Kranen 1 , O. Hamming-Vrieze 1 , T. Janssen 1 , J.J. Sonke 1 1 Netherlands Cancer Institute, Department of Radiation Oncology, Amsterdam, The Netherlands Purpose or Objective Normal tissue complication probability (NTCP) models are typically derived from the planned dose distribution. The planned dose, however, often deviates from the delivered dose due to both daily setup and anatomical variations. The purpose of this study was to investigate