Forward Planning Intensity Modulated Radiotherapy Technique for Prostate Cancer

Abstract

In this study an intensity modulated radiotherapy technique based on forward planning dose calculation (FIMRT) is presented to provide the concave dose distribution to prostate and seminal vesicles. Dynamic arcs (350°) fit prostate and seminal vesicles while shielding the rectum combined with two laterals oblique conformal fields (15o with respect to laterals) fit prostate only were applied to deliver doses 78 Gy and 61.23 Gy in 39 fractions to prostate and seminal vesicles respectively. Dynamic wedges (45° of thick part anteriorly oriented) were employed with conformal beams to adjust the dose homogeneity to prostate however, in some cases, static wedges (30° of thick part inferiorly oriented) were used with arcs to adjust the dose coverage to seminal vesicles. FIMRT was applied to 10 patients in supine and 10 others in prone positioning aiming to have the proper patient positioning for optimum protection to the rectum. FIMRT was compared with the simplified intensity modulated arc therapy (SIMAT) technique composed of three phases bilateral dynamic arcs. The mean rectal dose in FIMRT for prone patients was (22.5 ± 5.1) Gy while it was (30.2 ± 5.1) Gy and (39.4 ± 6.0) Gy in FIMRT and SIMAT for supine patients, respectively. The doses to 15%, 25%, 35% and 50% of rectum volume in FIMRT for prone patients were (44.5 ± 10.2) Gy, (33.0 ± 8.2) Gy, (25.3 ± 6.4) Gy and (16.3 ± 5.6) Gy, respectively. These values were lower than that in FIMRT and SIMAT for supine patients by (7.7 %, 19.2 %, 21.8 % and 28.5 %) and (25.0 %, 32.1 %, 34.9 % and 41.9 %) of the prescribed dose (78 Gy), respectively. Ion chamber and film dosimetery verified good agreement of the calculated and measured isocentric dose (maximum deviation of 3.5%) and isodose line position (maximum isodose shift of 4 mm). In conclusion, the single phase FIMRT technique with patients in prone positioning was found to provide the intended coverage of the prescribed doses to prostate and seminal vesicles with improved rectum protection. Accordingly, FIMRT has replaced non-modulated conformal radiotherapy or SIMAT as the standard treatment for prostate cancer in our department.