Definition of the CTV prostate in CT and MRI by using CT-MRI image fusion in IMRT planning for prostate cancer

Abstract

Purpose: To determine the prostate volumes defined by using MRI and CT scans, as well as the difference between prostate delineation in MRI and CT in three dimensions (3D). A further goal was to use MRI to identify subgroups of patients in whom seminal vesicle irradiation can be avoided.

Methods and materials: A total of 294 patients with biopsy-proven prostate cancer (MRI stages: T(1), 16 [5%]; T(2), 84 [29%]; T(3), 191 [65%]; T(4), 3 [1%]) underwent pelvic CT and MRI scans before intensity-modulated radiation therapy (IMRT) planning. 3D images were used to compare the prostate volumes defined by superimposed MR and CT images. Prostate volumes were calculated in cm(3).

Results: The mean prostate volume defined by MRI (44.3 cm(3) [range, 8.8-182.8 cm(3)]) was 35% smaller than that defined by CT (68.5 cm(3) [range, 15.2-241.3 cm(3)]). The areas of nonagreement were observed predominantly in the most superior and inferior portions of the prostate. The incidence of seminal vesicle invasion (SVI) identified by MRI was 63% (n = 182 of 290). The median length of SVI was 2.6 cm (range, 1.1-4.7 cm; 62% of the median SV length). The low-risk patients (59%, n = 171 of 290) calculated by applying the Roach and Diaz formula had a SVI rate of 57% (n = 97 of 171), the high-risk patients (41%, n = 119 of 290) of 71% (n = 85 of 119).

Conclusions: Compared with MRI, CT scans overestimate prostate volume by 35%. CT-MRI image fusion-based treatment planning allows more accurate prediction of the correct staging and more precise target volume identification in prostate cancer patients.