ESTRO 36 Abstract Book

S858 ESTRO 36 2017 _______________________________________________________________________________________________

1 Agency for Healthcare research and quality, US department of Health in Human services; https://effectivehealthcare.ahrq.gov/ehc/products/322/ 998/MethodsGuideforCERs_Viswanathan_IndividualStudie s.pdf Results The search strategy resulted in 1095 abstracts ( Figure 1 ) of which 44 studies fulfilled the inclusion criteria and were available in full text (one study was excluded based on study quality criteria). The review is expected to include syntheses of dose-response relationships for seven gastro- intestinal symptoms (bleeding/diarrhea/frequency/incontinence/pain/proctit is/urgency: n=18/3/4/10/3/4/4), three genitourinary symptoms (hematuria/incontinence/obstruction: n=4/4/3), and one sexual dysfunction symptom (erectile dysfunction: n=3) following EBRT. The corresponding figures for BT±EBRT will be two (bleeding/urgency: n=6/2), four (incontinence/obstruction/pain/stricture: n=4/2/3/2), and one symptom (erectile dysfunction: n=2). Results for diarrhea, stool frequency, and defecation urgency following EBRT are presented in Figure 2 . Dose cut points for the rectum and anal canal/sphincter region generally followed the same linear slope for diarrhea/defecation urgency across studies; for stool frequency they were less consistent.

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 Our review demonstrates continuous and innovative activity in the field of late toxicity after prostate cancer 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