ESTRO 36 Abstract Book

S870 ESTRO 36 _______________________________________________________________________________________________

RT since the QUANTEC publications. There is an increased recognition of intra- and inter-structure specific doses, and even though rectal bleeding remains the most studied symptom, there is also a trend towards other non- aggregated symptoms. EP-1610 Predictors for morbidity from planned vs. delivered rectal dose maps in RT of prostate cancer J. Trane 1 , O. Casares Magaz 1 , L. Bentzen 2 , K. Busch 1 , M. Thor 3 , L.P. Muren 1 1 Aarhus University Hospital - Aarhus University, Medical Physics, Aarhus, Denmark 2 Aarhus University Hospital, Oncology, Aarhus, Denmark 3 Memorial Sloan-Kettering Cancer Center, Medical Physics, New York, USA Purpose or Objective Patient-reported gastro-intestinal (GI) symptoms following radiotherapy (RT) for prostate cancer have recently been associated with metrics derived from rectal dose surface maps. In a recent study we developed rectum dose map based normal tissue complication probability (NCTP) models for three common late GI symptoms (at least 20% prevalence in the cohort used for modelling). In the present study we used such dose maps and connected NTCP models to compare the planned, daily and summed rectal dose distributions for patients with repeat volumetric imaging acquired during the course of RT. Material and Methods The patients included in this study were treated according to a national clinical trial for patients with locally advanced prostate cancer, irradiating concomitantly the pelvic lymph nodes and seminal vesicles to 55 Gy and the prostate to 78 Gy using volumetric modulated arc therapy. The treatment plans were recalculated on weekly repeat cone-beam (CB) CTs (6-8 CBCTs per patient) following Hounsfield Unit override to bone and water. Rectal dose maps were created for the planned dose distribution as well as for the dose distributions re-calculated on weekly CBCTs using a method recently developed by our group. The weekly CBCTs were averaged to provide a measure for the summed/accumulated dose across the course of RT. NTCPs were calculated for the planned, weekly and averaged rectal dose maps using three spatially based response models (based on areas and extents from the rectal dose maps) for three patient-reported GI symptoms: faecal leakage, obstruction and defecation urgency. The study included four prostate cancer patients, one with and three free from late Grade 2+ GI symptoms after RT. Results Dose differences exceeding +/- 10 Gy (scaled to the full treatment course) were seen in the dose maps for all patients and in all scans (Fig. 1). The largest systematic dose increase in the maps during the course of therapy was seen in the patient that experienced Grade 2+ GI symptoms after treatment. This patient also had higher NTCPs for all three spatial dose metric based models for the average map across treatment compared to the planned dose distribution (e.g. 10% vs 6% for faecal leakage), while smaller differences were seen for the three other patients (Table 1).

Conclusion The rectum dose maps and the connected NTCP models used in this study identified clinically relevant changes in rectum dose distributions caused by organ motion during the course of therapy. This model validation study showed that these maps and models are useful tools to evaluate the risk of normal tissue reactions in the rectum. EP-1611 Dose-response relationships for radiation- induced urgency syndrome after gynecological radiotherapy E. Alevronta 1 , V. Skokic 1 , U. Wilderäng 1 , G. Dunberger 2 , F. Sjöberg 1 , C. Bull 1 , K. Bergmark 1,3,4 , G. Steineck 1,4 1 Institute of Clinical Sciences- Sahlgrenska Academy at the University of Gothenburg, Department of Oncology- Division of Clinical Cancer Epidemiology, Gothenburg, Sweden 2 Ersta Sköndal College University, Department of Health Care Sciences, Stockholm, Sweden 3 Sahlgrenska University Hospital, Department of Oncology, Gothenburg, Sweden 4 Karolinska Institutet, Department of Oncology and Pathology- Division of Clinical Cancer Epidemiology, Stockholm, Sweden Purpose or Objective To find out what organ and doses are most relevant for ‘radiation-induced urgency syndrome’ in order to derive the corresponding dose-response relationships as an aid for avoiding the syndrome in the future. Material and Methods Of the 99 survivors treated with radiation therapy for gynecological cancer, 24 developed ‘radiation-induced urgency syndrome’. The survivors included in the study had not received brachytherapy, but other treatment combinations of external radiation therapy, surgery, and chemotherapy at the Karolinska University Hospital, Stockholm or the Sahlgrenska University Hospital, Gothenburg during the period 1991 to 2003. The rectum, the sigmoid and small intestines were delineated and the dose-volume histograms were exported for each patient.