ESTRO 2020 Abstract Book

S852 ESTRO 2020

risk 2) loco-regional recurrence of disease 3) acute and late toxicity. Material and Methods In this preliminary analysis, the IVIM MRI parameters (mean, median, skewness, kurtosis and others) of the pharynx constrictor muscles were evaluated before treatment (RM0) and during treatment (RM1). The parameters of the apparent diffusion coefficient (ADC), diffusion (D) and pseudo-diffusion (D*) coefficient maps were correlated with acute and late toxicity data reported both by the physician using the CTCAE scale (dysphagia > G3), and by the patient through patient reported outcomes (PROs), in particular analyzing the responses of the VHNSS- IT questionnaires (Vanderbilt Head and Neck Symptom Survey Italian version) of the subscales related to dysphagia (“nutrition”, “swallow-solid” and “swallow- liquid”). The dysphagia assessment was performed at baseline and at 3, 6 and 12 months of follow-up. Results The RM0 and RM1 data of 19 patients enrolled in the protocol were analyzed (Figure 1).

swallow-solid and nutrition subscale, both in RM0 and this seems to reflect an intrinsic predisposition of the patient to develop major dysphagia, and in RM1 and this could to be correlated to the irradiation of the constrictor muscles. The data provided by the PROs were more accurate in describing dysphagia than the CTCAE toxicity scale. This is justified by the fact that by CTCAE scale assessment only those patients who require enteral or intensive parenteral nutrient supports are considered to have severe dysphagia. In the specific case of our study, only one patient at 6 and 12 months needed this suport. In such a small sample it is therefore difficult to discriminate only those patients with severe toxicity. Such analyzes should therefore be extended to a larger sample. PO-1573 First clinical experience with model-based selection for breast cancer proton therapy K. Verhoeven 1 , G. Vilches-Freixas 1 , K. Limpens 1 , J. Mannens 1 , K. Van der Klugt 1 , M. Velders 1 , F. Visser 1 , S. Peeters 1 , G. Bosmans 1 , L. Boersma 1 1 Maastro, Department of Radiation Oncology- GROW School for Oncology and Developmental Biology- Maastricht University Medical Centre, Maastricht, The Netherlands Purpose or Objective Proton therapy (PrTh) for breast cancer (BC) patients has a potential advantage over standard photon therapy (PhTh) in a selection of patients. In the Netherlands, PrTh is reimbursed for BC patients, if the delta Normal Tissue Complication Probability (NTCP) for an acute coronary event (ACE) between a PrTh-plan and PhTh-plan is ≥ 2%. The NTCP has to be calculated based upon the Darby model (1), applied to the lifetime cardiac risk of Dutch people, specified for age, gender and the presence of cardiac risk factors (RF), as described in the approved national indication protocol. The aim of this study is to determine the percentage of BC patients that qualifies for PrTh using this model-based selection. Material and Methods In the period between March 2019 and October 2019, a plan comparison was made for BC patients with an indication for adjuvant radiotherapy (RT), whose mean heart dose (MHD) with the standard PhTh resulted in a NTCP for ACE of > 2% compared to a MHD of 0 Gy (no RT). Patients were only eligible for PrTh, if optical surface scanning showed < 5 mm motion of the chest wall. The PhTh plan was made using a hybrid technique calculated in ECLIPSE, using the Acuros algorithm on a breath hold CT for left sided BC, and on a free breathing CT for right sided BC. Coverage of the planning target volume (PTV) was evaluated according to ICRU guidelines, i.e. Volume(V)95% of PTV ≥ 98%. For PrTh robust intensity modulated proton plans (IMPT) were made around the Clinical Target Volume (CTV) using Raystation with a Monte Carlo algorithm for the Mevion machine on a free breathing CT. Two to 4 beam directions were used with a set-up uncertainty of 0.5 cm and a range uncertainty of 3% (2). Coverage of the CTV was evaluated in the voxel wise minimum plan, based on 28 scenario’s, and should fulfill V95% (CTV) ≥ 98%. Results 443 BC patients had an indication for adjuvant RT in a single center of which for 18 patients a plan comparison was made. Fifteen patients had a delta NTCP of ≥ 2% (mean delta NTCP of 2.9% with a range 2% - 4.6%) of whom 14 agreed with a PrTh. In total 3.2% of the BC patients (14/443) were selected for PrTh. These patients had a mean age of 48 years (range 30–59) and most of them (12/14) had a cardiac RF. The indication for adjuvant RT in the selection group was variable. Although it was expected that mainly patients with an indication for left sided parasternal RT would be selected, only 5/14 had an indication for parasternal RT, of which 2 had right sided BC. The target volumes in the other 9 patients varied, and included even 3 patients with breast only. The average

The analysis showed that the mean and median values of the ADC maps of the constrictor muscles of RM0 and RM1 were correlated with worse dysphagia scores (scores> 7) in VHNSS-IT. The values considered most significant were described using ROC curves (example in Figure 2). The dysphagia reported by the doctor according to the CTCAE scale was instead significant only at baseline and 3 months after the end of therapy. D and D* maps have shown to be less effective in predicting dysphagia. Conclusion The mean and median values of the ADC and D* maps were found to be higher in patients with worse scores of the