Dosimetric analysis of intensity modulated radiotherapy plans having one or more pairs of parallel opposed beams among the set of beams in some special cases

摘要

The purpose of this study was to analyze the dosimetric parameters of intensity modulated radiotherapy (IMRT) plans with a set of beams having one or more pairs of parallel opposed beams for the pelvis, head-and-neck (H and N) and brain cases having large secondary nodes. We selected three pelvis (carcinoma of prostate with pelvic bony mets), three H and N (two cases of carcinoma of the parotid with large secondary nodes and one case of carcinoma of the base of the tongue with large secondary nodes), and three brain (glioblastoma multiform) IMRT patients. IMRT plans were done with a set of 6-9 beams having one or more pairs of parallel opposed beams. Each plan was done using 6 MV photon energy. Each plan was analyzed on the basis of planning target volume (PTV) coverage with 93%, 95%, 100%, 107%, and 110% of the prescribed dose (PD), organs at risk (OAR) doses, homogeneity index (HI), conformity index (CI), and normal tissue integral dose (NTID). The doses to OARs were well within tolerance limits and the PTV coverage for 93%, 95%, and 100% of PD was obtained very well (followed the radiation therapy oncology group criteria "95% and 99% volume of PTV should receive 95% and 93% of the PD, respectively"), and values of HI, CI, and NTID were also satisfactory. In summary, very good OAR sparing and PTV coverage were observed in all plans. Hence, it can be concluded that use of one or more pairs of parallel opposed beams in IMRT plans in some special cases as selected for this study offers the benefit in terms of critical target volume irradiation, while maintaining the OAR within tolerable limits.