The impact of the number of fields on a SBRT-IMRT plan for prostate cancer. An evaluation with different objective criteria

Introduction For the stereotactic irradiation it is highly recommended to use a great number of fields to improve the conformity of the dose to the target and to reduce the entrance dose to the patient. The purpose of this study was to evaluate the impact of the number of fields on a SBRT delivery for prostate cancer (5 × 6.5 Gy) with a simultaneous integrated boost (SIB) into the tumor (5 × 8Gy). Materials and Methods The SBRT-IMRT simulations were made for nine patients with three different non-coplanar-beam conformations: 7-fields (Plan1), 9-fields (Plan2) and 11-fields (Plan3). For all three plans there were used the same optimization values and a number of 300 iterations. The comparison between plans was made using three conformity indexes (CI): RTOG, Lomax and SALT, the gradient index (dose fall-off) and the homogeneity index, as well as the dose-volume histogram (DVH) values for rectum and bladder. The gradient index was calculated only for the PTVprostate and the homogeneity index only for the PTVtumor, as a consequence of the influence of the SIB dose gradient. Results With similar coverage of the prescription isodose the dose conformity was comparable between the three plans. For example, for the PTVprostate, the CIRTOG was 1.28 [1.17–1.4], 1.26 [1.17– 1.38] and 1.27 [1.16–1.39] for Plan 1, 2 and 3, respectively. For the PTVtumor, the CIRTOG was 1.57 [1.33–1.81], 1.52 [1.33–1.7] and 1.51 [1.28–1.7] for the same plans. The average gradient index was 3.8 [3.5–4.4], 3.7 [3.6–4] and 3.8 [3.5–4.1] for Plans 1, 2 and 3, respectively. The average doses received by 2 cc, 5 cc and 10 cc of the rectum-wall were 33.7 Gy, 25.8 Gy and 16.7 Gy for Plan1, 33.2 Gy, 24.7 Gy and 15.8 Gy for Plan2 and 33.1 Gy, 24.5 Gy and 15.1 Gy for Plan3. Similarly, the average doses received by the bladder-wall were 28.9 Gy, 21.5 Gy and 13.9 Gy for Plan1, 28.8 Gy, 21.3 Gy and 13.6 Gy for Plan2 and 28.7 Gy, 21.1 Gy and 13.5 Gy for Plan3. Conclusion There wasn't established any impact of the number of beams on the dosimetric parameters, conformity, gradient, or homogeneity indexes of the present prostate SBRT schema with a SIB.