ESTRO 36 Abstract Book

S848 ESTRO 36 2017 _______________________________________________________________________________________________

EP-1595 NTCP models for early toxicities in patients with prostate or brain tumours receiving proton therapy A. Dutz 1,2 , L. Agolli 1,3 , E.G.C. Troost 1,2,3,4,5 , M. Krause 1,2,3,4,5 , M. Baumann 1,2,3,4,5 , A. Lühr 1,2,3,4 , S. Löck 1,3,5 1 OncoRay - Center for Radiation Research in Oncology, Faculty of Medicine and University Hospital Carl Gustav Carus- Technische Universität Dresden, Dresden, Germany 2 Helmholtz-Zentrum Dresden-Rossendorf, Institute of Radiooncology, Dresden, Germany 3 Department of Radiation Oncology, Faculty of Medicine and University Hospital Carl Gustav Carus- Technische Universität Dresden, Dresden, Germany 4 German Cancer Research Center DKFZ, Heidelberg and German Cancer Consortium DKTK partner site Dresden, Dresden, Germany 5 National Center for Tumor Diseases, partner site Dresden, Dresden, Germany Purpose or Objective To identify patients who are likely to benefit most from proton therapy, based on the potential reduction in normal tissue complication probability (NTCP) compared to photon therapy. The NTCP models required for this comparison were developed using clinical data on early side effects for patients with brain or prostate cancer having received proton therapy. Material and Methods Eighty patients with primary brain tumours and 30 patients with adenocarcinoma of the prostate who received proton therapy were included in this study. For the brain tumour patients, the radiation-induced early toxicities alopecia, erythema, pain and fatigue were considered, while for prostate cancer proctitis, diarrhoea, urinary frequency, urgency and incontinence, obstructive symptoms and radiation-induced cystitis were investigated. The occurrence of these side effects was correlated with different dose-volume parameters of associated organs at risk. NTCP models were created using logistic regression. A retrospective comparative treatment planning study was conducted to predict the potential reduction in NTCP of proton therapy compared to volumetric modulated arc therapy using the created models. For patients with brain tumours different subgroups were defined to identify patient groups which show a particularly high reduction in the considered toxicities. Results For patients with primary brain tumours significant correlations between the occurrence of alopecia grade 2 as well as erythema grade ≥ 2 and the dose-volume parameters D5% and V25Gy of the skin were found. Plan comparison showed an average reduction in NTCP for alopecia grade 2 of more than 5 % (see figure) and for erythema grade ≥ 2 of about 5 % using proton therapy. For patients with a brain tumour located in the skull base, with a clinical target volume less than 115 cm³ or with a prescribed dose less than 60 Gy, a potential reduction in NTCP for alopecia grade 2 of about 10 % could be achieved. For patients with prostate cancer significant correlations between obstructive symptoms grade ≥ 1 and the dose parameter D30% of the bladder as well as radiation- induced cystitis grade ≥ 1 and D20% of the bladder were found. Plan comparison showed an average reduction in NTCP for obstructive symptoms ≥ grade 1 of about 25 % and for radiation-induced cystitis about 15 % using proton

Conclusion OC resulted to be a parallel organ for mean G≥1.5 OM, while severe OM was associated to a synergic effect between PG mean dose and high doses received by small OC volumes, with BMI acting as a dose-modifying factor. On the other hand, OC resulted as a fairly serial organ for G≥2 SD; this could be due to the inclusion of minor salivary glands in OC contour, that the target often overlaps. Older age and smoking history were also associated to a SD increased risk. We did not find a strictly significant association between G3 dysphagia and dosimetric features, but the step trend resulting from our model calibration suggests that the ML model may not describe well the dose-response relationship in this case, with a step function possibly being more suitable. Validation of these models in a larger pts cohort is ongoing.