ESTRO 2020 Abstract Book

S55 ESTRO 2020

1 Holland Proton Therapy Center, Radiotherapy, Delft, The Netherlands Purpose or Objective In the Netherlands, proton therapy (PT) is standard of care for patients with head & neck cancer (HNC) if the risk of xerostomia, dysphagia, and/or tube feeding dependence can be lowered compared to photon radiotherapy with 10%, 10%, and 5%, respectively. It is currently unknown whether the pretreatment predicted Normal Tissue Complication Probability (NTCP) remains stable throughout the PT course if anatomy changes due to tumor regression or weight loss occur. In this study we investigated the stability of NTCP during adaptive PT in HNC. Material and Methods All HNC patients who completed PT for HNC between January and October 2019 were selected for this study. Patients were treated with a simultaneous integrated boost of 70 Gy RBE on the macroscopic tumor and 54.25 Gy RBE on the elective neck areas in 35 fractions, using scenario- based robust treatment planning and subsequent evaluation. Repeat CTs were acquired weekly during the treatment course. The dose distribution was recalculated on each weekly CT to evaluate target coverage and organs at risk (OARs) dose. If clinical constraints were exceeded in the dose recalculation, the treatment plan was adapted. For every individual patient the average OAR dose and NTCP on all repeated CTs were compared with the OAR dose and NTCP on the planning-CT. Then the average OAR doses and NTCPs for all patients were calculated. Results Sixteen patients completed PT. Seventy repeat CTs were acquired and used for dose recalculation and NTCP assessment. For 8 patients, in total 11 plan adaptations were performed. In the next table the mean difference in OAR and NTCP dose with respect to baseline for the whole population is presented. During the course of PT, 4 of 16 (25%) patients had an increase of at least 3% in the risk of xerostomia, 0 of 16 (0%) patients of dysphagia, and 0 of 16 (0%) patients of tube feed dependence. The maximum dose variation in the contralateral parotid gland was 3,2 Gy.

B, van der Laan HP, van 't Veld AA, Langendijk JA, Korevaar EW. PLoS One. 2016 Mar 31;11(3):e0152477. OC-0111 Patient-reported acute diarrhea in a cervical cancer patient cohort correlates with dose to rectum D. Reijtenbagh 1 , J. Godart 1 , J. Mens 1 , S. Heijkoop 1 , W. Heemsbergen 1 , M. Hoogeman 1,2 1 Erasmus MC, Radiation Oncology, Rotterdam, The Netherlands ; 2 HollandPTC, Medical Physics & Informatics, Delft, The Netherlands Purpose or Objective Patient-Reported Outcomes Measures (PROMs) are an increasingly important tool to record the patient’s individual perception of toxicity. Understanding the relations between dosimetric parameters and PROMs could help to optimize the treatment. However, these relations are poorly described for cervical cancer patients and the spatial information of dose distributions is usually not included in the analysis. In this study patient-reported acute diarrhea symptoms are evaluated specifically, and their relationship with dosimetric parameters of the bowelbag and rectum, as well as spatial information, is investigated. Material and Methods In this prospective study, 103 locally-advanced cervical cancer (LACC) patients treated according to an IMRT/VMAT plan-of-the-day protocol were included. Radiotherapy was combined either with concurrent chemotherapy, or with neo-adjuvant chemotherapy and concomitant hyperthermia. Selected patients had filled in a combined EORTC QLQ-C30 and QLQ-CX24 questionnaire at baseline, and at least one combined questionnaire in the acute phase of the treatment (week 4, week 5, 1 week after RT, 4 weeks after RT or 3 months after RT). Maximum deterioration from baseline was recorded for the question regarding diarrhea symptoms, and was scored as an event when a deterioration of two points or more was registered. Dose-volume parameters were collected for the rectum and bowelbag. Dose Surface Maps (DSMs) were constructed for the rectum, which is a technique to visualize a dose distribution on the surface of a tubular organ in a standardized way. Clinical variables and dose-volume parameters were included in the analysis. Permutation testing was performed for the DSMs to find areas of significance ( p < 0.05). Results Within the entire cohort, a diarrhea incidence of 59% was found. The use of concomitant chemotherapy resulted in a significant increase in reported diarrhea with an odds ratio of 2.5 (CI: 1.1-5.9). The dose-volume parameters V 5Gy -V 25Gy of the rectum were significantly associated with diarrhea, but no significant relation was found for the bowelbag. Conversely, the PTV volume was significantly related with diarrhea with an odds ratio of 1.17 (CI: 1.01- 1.37) per 100 cc increase in PTV volume. The results of the rectal DSMs are shown in Figure 1, where a significantly higher dose to the inferior part of the rectum is visible.

Conclusion The average difference of OAR dose and NTCP on the repeated CTs versus the planning-CT was small (<1 Gy and <1.5% respectively). On an individual patient basis larger variations occured, especially for xerostomia. Therefore regular PT dose recalculation and adaptation with special focus on contralateral parotid gland dose is necessary to maintain NTCP stability for all patients during the course of PT. [1] Robust Intensity Modulated Proton Therapy (IMPT) Increases Estimated Clinical Benefit in Head and Neck Cancer Patients. van Dijk LV, Steenbakkers RJ, ten Haken