ESTRO 38 Abstract book

S837 ESTRO 38

The analysis will be extended to early and late toxicity (1- year minimum follow-up) and the systematic analysis of DVHs will be used to infer exploratory constraints for ultra-hypofractionated PCa radiotherapy. EP-1551 Interpretation of T3 found after MRI in low- intermediate risk patients with prostate adenocarcinoma. C. García Torres 1 , L.A. Glaría Enríquez 1 , A. Escribano Uzcudum 1 , R. Morera López 1 1 Hospital Universitario La Paz, Radiotherapy Oncology, Madrid, Spain Purpose or Objective Multiparametric magnetic resonance (mpMR) is the best imaging technique to achieve precise delimitation of disease-affected areas in PCa. Urological guidelines do not systematically recommend MRI at low risk stage of prostate cancer, because of its low sensitivity to detect focal extra-prostatic extension (EPE) disease and cost. After mpMR of low risk PCa patients, EPE data could be found (rmT3). The finding of rmT3 upgrades the patient to high- risk group, which should change the type of treatment. This change of decision also implies exposing the patient to a greater risk of adverse events (AEs) related to the treatment. However in this group 85-95% is free of biochemical recurrence at 10 years, with any standard treatment. Objetive: 1- Determinate the percentage of low-risk patients studied with mpMR have EPE disease (T3) 2- Determinate factors associated to upstage of T after mpMR. Material and Methods We have retrospectively analyzed 58 patients diagnosed of low-risk prostate adenocarcinoma or favorable- intermediate risk, with PSA <20ng/ml and studied with mpMR. We use risk group classification according to NCCN 2018. Those with very low risk (VLR), low risk (LR) and favorable- intermediate (FI) were grouped as low risk. The definition of grade group (GG) according to AJCC 2010 was used. Related to stage, we will group patients in 3mm of EPE) and T3b when seminal vesicle invasion is found. Results The mean age was 68.9 years (+/- 8.47 years). Among these patients, 36 (62.1%) belonged to GG1 and 22 (37.9%) corresponded to GG2. After mpMR was performed 25% of patients presented some grade of EPE, becoming a locally advanced stage (T3a-T3b).

intraprostatic lesion (DIL). The treatment scheme was 36.25-37.5 Gy/5 fractions to the whole prostate gland and to the DIL, respectively. Univariate and multivariate logistic regression models were applied to identify independent factors associated with toxicity adjusting for confounding factors. The systematic analysis of urinary bladder and rectum dose- volume histograms (DVHs) was performed with Fisher exact test. The correlation between the relative volume receiving a given absolute dose and acute toxicity was investigated, with cutoffs at grade G>0 and G>1. Results Sixty-five patients fulfilled the inclusion criteria. At 1- month after the end of radiotherapy, 42 patients (65%) had no GU toxicity, whereas 18 (28%), 4 (6%) and 1 (1%) patients had G1, G2 and G3 GU toxicity. As concerning GI toxicity, 54 (83%) patients had no toxicity, whereas 11 (17%) reported G1 toxicity. IPSS changed significantly (P<0.05) from baseline to 1 month and changes during time are significantly different by symptomatic score at first month (mild, moderate and severe, P<0.0001). At univariate analysis (Table 1), no relationship was found between the prostate volume and acute GU (P=0.97) or GI toxicity (P=0.83), nor between the urinary bladder volume and any grade of acute GU toxicity (P=0.60) whereas the correlation between the rectum volume and any grade of acute GI toxicity was at the boundary of statistical significance (P=0.05). At multivariate analysis adjusting for age and the presence of severe concomitant disease, IPSS at 1 month is positively significantly associated with acute GU G≥1 toxicity (P=0.03), whereas rectum volume is only borderline significantly associated with GI G≥1 toxicity (P=0.06). Prostate volume is not significantly associated with any toxicity (P=0.78). For urinary bladder, the area of DVH between 17-24 Gy vs. volume 12-22% was correlated with any acute GU toxicity and the region between 25-35 Gy vs. volume 2-6% was correlated with acute G>1 GU toxicity. As concerning the relation between rectum DVH and GI toxicity, the portion between 18-34 Gy was correlated with any acute GI toxicity (Figure 1).

Patients were analyzed according to Risk Group: very low, low or intermediate. None very-low-risk patients (5 in total) increased their stage after mpMR. In low- risk

Conclusion The acute toxicity profile after ultra-hypofractionated radiotherapy for PCa is satisfactory. Prostate volume does not affect the toxicity probability.