BACKGROUND The high rate of malignancy of peritoneal carcinomatosis makes its accurate detection vital in treatment planning. The combination of anatomical and functional information, provided by T2-weighted imaging (T2WI) and diffusion-weighted imaging (DWI), respectively, could potentially aid in the segmentation of tumors. The aim of this study was to compare the functional tumor volume estimated by a T2WI- and DWI-derived apparent diffusion coefficient (ADC) map, with metabolic tumor volume measured by 18F-fluoro-deoxyglucose positron emission tomography with computed tomography (FDG PET/CT). MATERIAL AND METHODS In 108 lesions from 108 patients with peritoneal carcinomatosis, gross tumor volume (GTV) was manually delineated on DWI. Within each region of interest, k-means clustering was used to exclude non-tumorous tissue from tumorous tissue. The high-cellularity tumor volume estimated from ADC (HCTVADC) and combined high-cellularity tumor volume (HCTVC), which was estimated by combining anatomical information from T2WI and functional information from ADC, were generated. Taking metabolic tumor volume (MTV) in PET/CT as a reference, GTV, HCTVADC, and HCTVC were compared with MTV. RESULTS GTV (P=0.017) and HCTVADC (P=0.048) differed significantly from MTV. However, the HCTVC measured by combining DWI and T2WI showed high concordance (ICC=0.99) with the MTV measured by FDG PET/CT in differentiating tumorous tissues with high cellularity from non-tumorous tissues. CONCLUSIONS In conclusion, our results suggested the potential value of this semiautomatic method serving as an alternative to PET/CT in radiotherapy tumor contouring.