Comparison between Midline and Mid-Breast Isocenter Placement for Prone Breast Irradiation Using an O-Ring Jawless Linac

Abstract

For prone breast radiation therapy using Halcyon O-ring jawless linac, mid-line isocenter placement as advantage over mid-breast placement due to before clearance from the bore cover. This study evaluated whether or not isocenter placement would have an impact in plan quality of the prone breast cases using an automated electronic compensation (ECOMP) technique. 15 patients previously treated prone to whole breast were restrospecitvely studied. Physician first defined tangential treatment fields using MLC shaping. Dosimetrist then developed treatment plans using 6 MV FFF beam on the machine based on the given fields and irradiated volume defined by 50% isodose line generated by MD-defined open fields. For each case two treatment plans were set on the same longitudinal direction, but different lateral direction; one with isocenter at midline, and another with isocenter at mid-breast. Both plans were calculated using ECOMP technique using treatment planning system plug-in (EZFluence) to achieve homogeneous dose within the irradiated breast volume. All plans are normalized to 95% of the breast PTV volume to receive 95% of the prescription (Rx) dose. Plan quality was compared using global maximal dose, homogeneity index within breast PTV (D5% - D95%); delivery efficiency was evaluated by total MU per plan. Wilcoxon signed rank test was used to determine statistical significance. All planning objectives were met. Global maximal dose (Dmax), planning efficiency were not different, primarily due to the use of automatic ECOMP technique. All cases had global Dmax < 105% Rx. Difference in homogeneity index was statistically significant (p=0.004) but clinically insignificant (0.3±0.4%). There was a decrease (p=0.0002) of 0.21±0.23Gy in heart mean dose when isocenter was placed at midline. For midline isocenter plans the average MU of the medial field was 28% higher than the average MU of lateral field to overcome 15.8±4.2cm difference in SSD, while the average difference of the mid breast plans were about 4%. However total MU were not different between midline and mid-breast isocenter placement (p=0.56). Placing isocenter at mid-breast requires additional 6.9±1.1cm lateral shift of the patient, which could increase the chance of collision with the linac bore cover. For prone breast irradiation with ring-gantry linear accelerator, midline isocenter placement produces clinically similar plan quality as compared to mid-breast isocenter. For midline Isocenters there was a large difference in MUs between the treatment fields; however, there is no difference in total MU and hence delivery efficiency. Given its simplicity for setup and improve clearance, midline isocenter is a preferred setup position for prone breast treatment on the ring-gantry linear accelerator.