Definition of Fields Margins for Optimized 2D Radiotherapy of Prostate Carcinoma

Prostate cancer (CAP) is one of the most common malignancies in men, both in Western countries and developing countries. Radiation therapy (RT) is an important therapeutic option. New technologies (3D, IMRT, IGRT, VMAT) have been introduced in the last decades, with a progressive improvement of clinical outcome. However, in many countries the only treatment option is the traditional 2D technique based on standard simulation. The indications for field definition in this treatment are still based on expert's opinions. The aim of this analysis is to propose new indications for 2D fields definition based on three-dimensional simulation in a population of patients with CAP. Twenty patients with CAP consecutively treated with RT in our center were identified. Patients underwent CT-simulation in supine position. Pelvic MRI images were fused with CT-simulation images. In this way, delineation of the prostate and seminal vesicles was performed on MRI images. Clinical Target Volume definition (CTV) was performed according to EORTC guidelines simulating 4 different categories: low-risk CAP, intermediate-risk CAP, high-risk CAP without involvement of the seminal vesicles, and high-risk CAP with involvement of seminal vesicles. The Planning Target Volume (PTV) was defined by adding a margin of 10 mm to the CTV in all directions. For each patient, 8 treatment plans were calculated. In particular, for each of the 4 categories of risk, 2 treatment plans were calculated by using a cobalt source or 10 MV photons. Treatment plans were calculated using the box technique. Progressive optimization was realized with an iterative procedure by evaluating the three-dimensional dose distribution. Once the final plan was achieved (respecting the PTV constraint: D98 > 95%), distances of the fields edges from a set of reference points were measured.