External Validation of Early Regression Index (ERITCP) as Predictor of Pathologic Complete Response in Rectal Cancer Using Magnetic Resonance-Guided Radiation Therapy

• DOI: 10.1016/j.ijrobp.2020.07.2323
• Publication Date: 2020-08-04T00:27:29Z
• Authors: Davide Cusumano; Luca Boldrini; Poonam Yadav; Gao Yu; Bindu Musurunu; Giuditta Chiloiro; Antonio Piras; Jacopo Lenkowicz; Lorenzo Placidi; Sara Broggi; Angela Romano; Martina Mori; Brunella Barbaro; Luigi Azario; Maria Antonietta ...; Marco De Spirito; Michael F. Bassetti; Yingli Yang; Claudio Fiorino; Vincenzo Valentini
• Keywords: Adult; Male; Magnetic Resonance Imaging, Interventional; Sensitivity and Specificity; 03 medical and health sciences; 0302 clinical medicine; Predictive Value of Tests; Humans; Aged; Probability; Retrospective Studies; Aged, 80 and over; Rectal Neoplasms; Chemoradiotherapy, Adjuvant; Middle Aged; Neoadjuvant Therapy; 3. Good health; Treatment Outcome; ROC Curve; Area Under Curve; Female; Algorithms; Relative Biological Effectiveness; Radiotherapy, Image-Guided

Tumor control probability (TCP)-based early regression index (ERITCP) is a radiobiological parameter that showed promising results in predicting pathologic complete response (pCR) on T2-weighted 1.5 T magnetic resonance (MR) images of patients with locally advanced rectal cancer. This study aims to validate the ERITCP in the context of low-tesla MR-guided radiation therapy, using images acquired with different magnetic field strength (0.35 T) and image contrast (T2/T1). Furthermore, the optimal timing for pCR prediction was estimated, calculating the ERI index at different biologically effective dose (BED) levels.Fifty-two patients with locally advanced rectal cancer treated with neoadjuvant chemoradiation therapy were enrolled in this multi-institutional retrospective study. For each patient, a 0.35 T T2/T1-weighted MR image was acquired during simulation and on each treatment day. Gross tumor volume was contoured according to International Commission on Radiation Units Report 83 guidelines. According to the original definition, ERITCP was calculated considering the residual tumor volume at BED = 25 Gy. ERI was also calculated in correspondence with several BED levels: 13, 21, 32, 40, 46, 54, 59, and 67. The predictive performance of the different ERI indices were evaluated in terms of receiver operating characteristic curve. The robustness of ERITCP with respect to the interobserver variability was also evaluated considering 2 operators and calculating the intraclass correlation index.Fourteen patients showed pCR. ERITCP correctly 47 of 52 cases (accuracy = 90%), showing good results in terms of sensitivity (86%), specificity (92%), negative predictive value (95%), and positive predictive value (80%). The analysis at different BED levels shows that the best predictive performance is obtained when this parameter is calculated at BED = 25 Gy (area under the curve = 0.93). ERITCP results are robust with respect to interobserver variability (intraclass correlation index = 0.99).This study confirmed the validity and the robustness of ERITCP as a pCR predictor in the context of low-tesla MR-guided radiation therapy and indicate 25 Gy as the best BED level to perform predictions.