Parikh, Parag J.

162 Background: In preclinical studies, the tryptophan-metabolizing enzyme, Indoleamine 2, 3-dioxygenase 1 (IDO1), promotes resistance to radiation in colorectal cancer, and IDO1 inhibition with epacadostat improves tumor radiosensitivity. In a phase 1 trial, patients (pts) with locally advanced rectal cancer (LARC, n=17) were treated with epacadostat in combination with short course radiotherapy (SCRT) and CAPOX which was well-tolerated, and the recommended phase 2 dose (RP2D) of epacadostat was 400mg BID. Here we report efficacy and safety from the NCI-supported phase 2 (NCT03516708) multicenter, open-label trial (treatment cohort). Methods: Pts were treated with standard of care (SOC) SCRT, then SOC neoadjuvant chemotherapy (NAC) followed by SOC surgical resection or non-operative management (NOM). Epacadostat (400 mg BID) started with SCRT and continued until starting NAC (a minimum of 21 days). Eight pts were enrolled prior to discontinuation of epacadostat by Incyte in February/March 2025. The primary endpoint is Neoadjuvant Rectal (NAR) score. In patients who underwent NOM, we calculated MRI-based tumor regression grade (MR-TRG). Secondary endpoints are pathologic complete response (pCR) rate, complete clinical response (cCR) rate, and progression free survival (PFS). Results: Table 1 summarizes baseline characteristics and endpoints (primary and secondary). NOM was pursued in 7/8 (87.5%) pts, surgical resection was performed on 1/8 (12.5%). At median follow-up of 12 months, local tumor regrowth occurred in 1 pt initially treated with NOM (after tumor regrowth, he underwent salvage surgery and is currently on surveillance). Treatment with epacadostat was interrupted in 2/8 (25%) patients due to grade 3 maculopapular skin rash. Most frequent treatment-emergent adverse events (TEAEs) regardless of causality (all grades (G)/G3-4) were bloating (75%/0%), increased ALT (62.5%/0%), increased AST (50%/0%), nausea (50%/0%), and constipation (50%/0%). Grade 3 diarrhea and grade 3 non-cardiac chest pain occurred in 1 patient each (both unrelated to epacadostat). Conclusions: Epacadostat demonstrates promising efficacy and manageable safety in patients with LARC. The biomarker cohort and correlative studies are ongoing. Clinical trial information: NCT03516708 . Baseline characteristics and endpoints. Patient Age Sex Ethnicity Stage Surgery/NOM NAR MR-TRG cCR/pCR Recurrence 1 52 M Caucasian T3N1 NOM NA 2 Y/NA No 2 45 M Caucasian T3N1 NOM NA 3 Y/NA Local tumor regrowth 3 48 M Caucasian T3N2 NOM NA 3 Y/NA No 4 61 M Caucasian T4N2 NOM NA 3 Y/NA No 5 57 M Caucasian T3N0 NOM NA 1 Y/NA No 6 60 F Caucasian T3N1 Surgery 15 3 N/ypT3N0 No 7 67 M Caucasian T4N0 NOM NA 3 Y/NA No 8 51 M Caucasian T4N2 NOM NA P NA No

On‐board magnetic resonance (MR) image guidance during radiation therapy offers the potential for more accurate treatment delivery. To utilize the real‐time image information, a crucial prerequisite is the ability to successfully segment and track regions of interest (ROI). The purpose of this work is to evaluate the performance of different segmentation algorithms using motion images (4 frames per second) acquired using a MR image‐guided radiotherapy (MR‐IGRT) system. Manual contours of the kidney, bladder, duodenum, and a liver tumor by an experienced radiation oncologist were used as the ground truth for performance evaluation. Besides the manual segmentation, images were automatically segmented using thresholding, fuzzy k‐means (FKM), k‐harmonic means (KHM), and reaction‐diffusion level set evolution (RD‐LSE) algorithms, as well as the tissue tracking algorithm provided by the ViewRay treatment planning and delivery system (VR‐TPDS). The performance of the five algorithms was evaluated quantitatively by comparing with the manual segmentation using the Dice coefficient and target registration error (TRE) measured as the distance between the centroid of the manual ROI and the centroid of the automatically segmented ROI. All methods were able to successfully segment the bladder and the kidney, but only FKM, KHM, and VR‐TPDS were able to segment the liver tumor and the duodenum. The performance of the thresholding, FKM, KHM, and RD‐LSE algorithms degraded as the local image contrast decreased, whereas the performance of the VP‐TPDS method was nearly independent of local image contrast due to the reference registration algorithm. For segmenting high‐contrast images (i.e., kidney), the thresholding method provided the best speed ( ) with a satisfying accuracy ( ). When the image contrast was low, the VR‐TPDS method had the best automatic contour. Results suggest an image quality determination procedure before segmentation and a combination of different methods for optimal segmentation with the on‐board MR‐IGRT system. PACS number(s): 87.57.nm, 87.57.N‐, 87.61.Tg

368 Background: 5-Fluorouracil (5FU) and mitomycin-C (MMC) chemoradiation for anal cancer is associated with high rates of acute morbidity. We have previously shown that dose-painted IMRT (DP-IMRT) significantly reduces grade 3+ GI and dermatologic acute toxicity, as compared to the RTOG 9811 5FU/MMC arm, which used non-conformal radiation techniques. We now report on the two-year outcomes of this DP-IMRT approach. Methods: T2-4N0-3M0 anal canal cancers received 5FU (1,000 mg/m2/day 96 hour infusion) and MMC (10 mg/m2 bolus) days 1 and 29 of DP-IMRT prescribed as follows - T2N0: 42 Gy elective nodal and 50.4 Gy anal tumor planning target volumes (PTVs), 28 fractions; T3-4N0-3: 45 Gy elective nodal, 50.4 Gy ≤ 3 cm and 54 Gy > 3 cm metastatic nodal and 54 Gy anal tumor PTVs, 30 fractions. The following two-year outcomes were assessed: local-regional (LRF) and colostomy failures (CF) using the cumulative incidence method, and disease-free (DFS), overall (OS) and colostomy-free survivals (CFS) using the Kaplan-Meier method. Results: Of 63 accrued patients, 52 were analyzable. Median age was 58 years; 81% female; 54% stage II; 25% IIIA; 21% IIIB. Median follow-up was 23.2 months (0.2-33). Two-year LRF, CF, DFS and 95% confidence intervals are 20% (9%, 31%), 8% (0.4%, 15%) and 77% (62%, 86%), respectively. The causes of death for the 7 patients that died are: anal cancer in 5, morbidity in one and second primary outside the radiation field in one. Two-year comparison data from the RTOG 9811 5FU/MMC arm are shown in the table below. Conclusions: DP-IMRT with 5FU/MMC for the treatment of anal canal cancer yields similar two-year outcomes as the RTOG 9811 conventional radiation, 5FU/MMC arm. Because of the associated acute toxicity sparing, DP-IMRT will be used as the platform, and may allow for radiation dose escalation, in future RTOG anal canal trials. Supported by RTOG U10 CA21661, CCOP U10 CA3742 and ATC U24 CA 81647 NCI grants. [Table: see text] No significant financial relationships to disclose.