Comparison of Intensity Modulated Radiotherapy and Forward Planning Dynamic Arc Therapy Techniques For Prostate Cancer.

Abstract

In the this study, an inverse planning intensity modulated radiotherapy (IMRT) technique is compared with three, previously published, forward planning dynamic arc therapy techniques and a new implemented one for treatment of prostate only. The IMRT technique was composed of seven beams of angles 0^o, 51^o, 102^o, 153^o, 207^o, 258^o, and 309^o. The three previously published dynamic arc techniques were: (i) the dynamic arc therapy (DAT), composed of bilateral arcs of ranges from 36^o to 136^o and 226^o to 326^o, (ii) the two-axis dynamic arc therapy (2A-DAT), composed of two dynamic arcs with half rotation range (180^o), and (iii) the modified dynamic arc therapy (M-DAT) technique, composed of two full arcs combined by two wedged (45° of thick end anteriorly oriented) laterals oblique (15^o with respect to laterals) conformal fields. The new implemented technique is composed of bilateral wedged dynamic arcs (BW-DAT) of half rotation range (180^o) with hard wedge 45^o of thin end oriented to the side of rotation of each arc at zero gantry and collimator angles. In all dynamic arcs the multileaf collimator (MLC) is moving during rotation to fit the prostate, except for in 2A-DAT it is fitting two separate symmetrical rhombi including the prostate. The rectum is shielded during rotation only in cases of M-DAT and BW-DAT. The obtained results indicate that the BW-DAT, M-DAT and DAT techniques provide the intended coverage of the prescribed dose to the prostate while IMRT provided a slightly better coverage. Compared with the IMRT, BW-DAT produced slightly lower protection to a 3 cm margin health tissue surrounds the planning target volume, which is better than the others. The dose gradient in the rest of the health body in case of BW-DAT is steeper than that in the others, and of maximum dose 33.15 Gy ± 2.18 Gy. This dose covers percentage health body volumes of 8 % ± 3.2 % with IMRT, 4 % ± 1.5 % with DAT and 6 % ± 1.2 % with both 2A-DAT and M-DAT. Also, this dose is much lower than the accepted maximum dose (52 Gy) to the femoral heads and necks according to ICRU Report 62. Accordingly, it would be possible to neglect the delineation of the femoral heads and necks as organs at risk in case of BW-DAT. The rectum dose in cases of BW-DAT and M-DAT are comparable, but they are better than those in the others in most of rectal volume. The doses to the 15 %, 25 %, 35 % and 50 % (D_15%, D_25%, D_35%, and D_50%) of the rectum volume in case of BW-DAT were 43.5 Gy ± 8.6 Gy, 24.2 Gy ± 8.7 Gy, 13.2 Gy ± 4.2 Gy, and 5.7 Gy ± 2.1 Gy respectively. The D_15% of the rectum in case of IMRT is lower than those in BW-DAT, M-DAT, 2A-DAT and DAT by 7.3 %, 10.2 %, 33.0 %, and 17.6 % of the prescribed dose (78 Gy in 39 fractions) respectively. The D_25%, D_35% and D_50% of rectum volume in case of IMRT are comparable to those in DAT, but they are higher than those in both M-DAT and BW-DAT by 8.9 %, 12.8 %, and 5.7 % of the prescribed dose respectively. These doses are also higher than those in case of 2A-DAT by 41.6 % of the prescribed dose in average. The D_15%, D_25%, D_35% and D_50% of the bladder volume in case of BW-DAT are 33.2 Gy ± 10.9 Gy, 25.1 Gy ± 7.9 Gy, 6.5 Gy ± 4.3 Gy, and 4.2 Gy ± 3.5 Gy, respectively. The low doses to the bladder in cases of IMRT, M-DAT, and BW-DAT are comparable, but the high doses are higher, which covers small volumes. The D_15% and D_25% of bladder in case of DAT are lower than those in IMRT, M-DAT, and BW-DAT by 14.3 % and 21.9 % of the prescribed dose in average respectively. Also, the D_15%, D_25% and D_35% in case of DAT are lower than those in 2A-DAT by 20.1 %, 27.0 %, and 16.0 % of the prescribed dose respectively, but the difference in D_50% is insignificant. Ion chamber measurements showed good agreement of the calculated and measured isocentric dose (maximum deviation of 3.2 %). Accuracy of the dose distribution distributions calculation of BW-DAT was evaluated by film dosimetry using a gamma index, allowing 3 % dose variation and 3 mm distance to agreement as the individual acceptance criteria. We found that fewer than 6.5 % of the pixels in the dose distributions of scanned and calculated area of 10 x 10 cm² failed the acceptance criteria. It is concluded that, in addition to the simplicity of the dose calculation of the BW-DAT technique, it is not only a comparable or better treatment quality than IMRT but also, the most favorable forward planning dynamic arc technique for treatment of prostate cancer.