Ninety-degree angled collimator: a dosimetric study related to dynamic intensity-modulated radiotherapy in patients with endometrial carcinoma

BackgroundOur purpose was to ensure that the dose constraints of the organs at risk (OARs) were not exceeded while increasing the prescription dose to the planning target volume (PTV) from 45 to 50.4 Gray (Gy) with the dynamic intensity-modulated radiotherapy (IMRT) technique. While trying for this purpose, a new dynamic IMRT technique named 90 degrees angled collimated dynamic IMRT (A-IMRT) planning was developed by us.MethodsThis study was based on the computed tomography data sets of 20 patients with postoperatively diagnosed International Federation of Gynecology and Obstetrics stage 2 endometrial carcinoma. For each patient, conventional dynamic IMRT (C-IMRT, collimator angle of 0 degrees at all gantry angles), A-IMRT (collimator angle of 90 degrees at gantry angles of 110 degrees, 180 degrees, 215 degrees, and 285 degrees), and volumetric modulated arc therapy (VMAT) were planned. Planning techniques were compared with parameters used to evaluate PTV and OARs via dose-volume-histogram analysis using the paired two-tailed Wilcoxon's signed-rank test; p ResultsAll plans achieved adequate dose coverage for PTV. Although the technique with the lowest mean conformality index was A-IMRT (0.76 +/- 0.05) compared to both C-IMRT (0.79 +/- 0.04, p = 0.000) and VMAT (0.83 +/- 0.03, p = 0.000), it protected the OARs especially the bladder (V45 = 32.84 +/- 2.03 vs. 44.21 +/- 6.67, p = 0.000), rectum (V30 = 56.18 +/- 2.05 vs. 73.80 +/- 4.75, p = 0.000) and both femoral heads (V30 for right = 12.19 +/- 1.34 vs. 21.42 +/- 4.03, p = 0.000 and V30 for left = 12.58 +/- 1.48 vs. 21.35 +/- 4.16, p = 0.000) better than C-IMRT. While the dose constraints of the bladder, rectum and bilateral femoral heads were not exceeded in any patient with A-IMRT or VMAT, they were exceeded in 19 (95%), 20 (100%) and 20 (100%) patients with C-IMRT, respectively.ConclusionsOARs are better protected when external beam radiotherapy is applied to the pelvis at a dose of 50.4 Gy by turning the collimator angle to 90 degrees at some gantry angles with the dynamic IMRT technique in the absence of VMAT.