The Effect Of Collimator Rotation On Scatter Contribution To Organs At Risk In Breast Cancer Tangent Fields

Reducing organ at risk (OAR) dose to as low as reasonably achievable (ALARA), without sacrificing planning target volume (PTV) coverage, is a tenet of radiation therapy planning. Indeed, a wide array of techniques and technology have been developed in this regard. We sought to evaluate the impact of a simple 90-degree collimator rotation on OAR dose. Rotating the collimator 90 degrees results in the “X” jaws being closer to the patient, resulting in reduced head scatter. Sixty patients were selected for this study and were planned using an inverse-planned 3D conformal technique. Plans were compared with and without the additional collimator rotation by evaluating PTV coverage and dose to critical structures. Dosimetric data was evaluated for normality, and medians were compared using the Wilcoxon Signed Rank test. Effect size was evaluated per Cohen’s classification. All statistical analyses were performed using RStudio. Collimator rotation resulted in a statistically significant reduction in the median dose maximum (Dmax) (7.4 vs. 6.9 Gy, P < 0.001) and mean heart (1 vs. 0.77 Gy, P < 0.001) dose. The median percent reduction in mean heart dose was 22.7% (IQR 8.9%, Range 4.6-46.5%). Likewise, there was a statistically significant reduction in the median dose maximum (Dmax) (4.6 vs. 4.4 Gy, P < 0.001) and mean left anterior descending artery (LAD) dose (2.4 vs. 2.2 Gy, P < 0.001). Additionally, collimator rotation resulted in a statistically significant reduction in the mean lung (4.4 vs 4.1 Gy, P < 0.001) dose, without affecting the volume of PTV receiving 95% (99.2 vs. 99%, P = 0.1) or 90% (100 vs. 100%, P = 0.43) prescription dose. Rotating the collimator in certain linear accelerators with can reduce scatter contribution and reduce OAR dose without sacrificing PTV coverage. This simple modification in treatment planning results in clinically significant reductions in dose to multiple organs at risk.