Conformal static field irradiation of localized prostatic cancer without 3D planning system

Since 1994 we have introduced at our department an isocentric four fields conformal technique to irradiate localized prostate cancer. All patients were simulated supine and underwent a retrograde urethrogram and a rectal contrast; then, they are CT scanned, obtaining 1 cm thick cuts from the superior up to the inferior edge of the fields; after identifying the CTV on axial CT slices, each field has manual BEV reconstruction of CTV and customized blocks are defined with appropriate margins (minimum margin of 2 cm) in order to allow for daily treatment inaccuracies, internal organ movement and patient motion (no immobilization customs). The homogeneity and CTV coverage are evaluated on each axial cut using a 2D treatment planning system. Each patient is planned with Linear Accelerator (2100C Varian) with 18 MV photons. Target volumes (prostate +/- sv) have been differentiated according to stage, grade and PSA values. A total of 104 films are reviewed for 12 patients, and the deviation of each portal from the simulation film (simulation-to-treatment variability) was determined in both the Superior-Inferior (SI) and Anterior-Posterior (AP) dimensions. The average AP portal deviation was 4.5 mm (σ=±2.9 mm) and the corresponding deviation in SI dimension was 5mm (σ=±2.8 mm). The above values translate into an average difference of the isocenter of 7.3 mm (σ=±2.9 mm). An analysis of relationship between the margin used and the percentage of dose at the inferior edge of the prostate showed that 95% of the dose has delivered at 1 cm below the prostate with margin of 2 cm. The curves is quite steep below a margin < 1.5 cm and becomes relatively flat at a margin > 2cm. In order to evaluate dosimetric accuracy of treatment, measurements in humanoid phantom (Alderson-Rando) were performed by using TL dosimeters (TLD-100 rods). Then, we have determined the target dose (ICRU 50), by checking the entrance and exit dose (in vivo dosimetry)