Interobserver comparison of CT and MRI-based prostate apex definition: Clinical relevance for conformal radiotherapy treatment planning

CT is widely used for conformal radiotherapy treatment planning of prostate carcinoma. Its limitations are especially at the prostatic apex which cannot be separated from the urogenital diaphragm. The aim of this study was to compare the localization of the prostatic apex in CT and axial MRI to the sagittal MRI in an interobserver analysis. 22 patients with pathologically proven prostatic carcinoma were included in the analysis. In all patients sagittal and axial T2-weighted MRI and conventional CT were performed. The position of the MRI and CT apices were localized independently by three observers in relation to the intertrochanteric line. Additional subjective judgment of the ability to define the apical border of the prostatic gland was performed by a five-scaled score. The apex of the prostate could be discriminated statistically significant (p < 0.001) better in the MRI as compared to CT with best judgement for the sagittal MRI. The interobserver variation for the definition of the prostatic apex was statistically significant (p = 0.009) smaller for the sagittal MRI compared to axial MRI and CT. On the average the apex as determined by sagittal MRI, axial MRI and CT was located 29 mm, 27 mm and 24 mm above the intertrochanteric line. The apex defined by CT would have led to an additional treatment of 6-13 mm in 10/22 patients compared to the sagittal MRI, defined by axial MRI only in five patients. Additional MRI provides a superior anatomic information especially in the apical portion of the prostate. It should be recommended for every single patient in the treatment planning process. It helps to avoid an unnecessary irradiation of healthy tissue and could lead to a decrease of anal side effects and radiation-induced impotency due to a reduction of the extent of irradiated penile structures.