Comparison of Three Setup Methods of Skin Marker Alignment Using Diamond-Shaped Light Field for Prone Breast EBRT
H Xu*, S Cheston, A Gopal, S Chen, B Zhang, S Lee, S Dudley, University of Maryland School of Medicine, Baltimore, MD
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PO-GePV-M-99 (Sunday, 7/25/2021) [Eastern Time (GMT-4)]
Purpose: For the positioning of prone breast patients treated with tangential fields, a diamond-shaped light field(DSLF) is used daily at our institution to align with a corresponding skin marker besides IGRT. This study compares three different DSLF setups used in several hospitals to help standardize/optimize the clinical workflow.
Methods: Sixty-one prone breast patients with daily kV tangential IGRT following DSLF were retrospectively categorized into three groups according to their DSLF setups in different hospitals: 1) field size(FS)=10x10cm2 at central axis with direct lateral gantry(=90 or 270˚), 2)Same as 1) except FS= 4x4cm2, and 3) FS=4x4-6x8cm2 at off-axis with tangential gantry as treatment beam (so that DSLF is approximately at the breast center). All the kV-based couch shifts in vertical and longitudinal directions were analyzed. The DSLF-only dose distribution was simulated by reversely shift the couch (to exclude IGRT) from the CT plan, which was assumed equivalent to the delivered dose when both DSLF and IGRT are used for patient alignment. The simulated DSLF-only dose to the target and OAR structures throughout the treatment course were calculated.
Results: The mean daily couch shifts following DSLF No.1, 2, 3 are 0.64, 0.68, 0.8cm with standard deviation of 0.4, 0.3, 0.6cm respectively. The average target dose ratios of DSLF-only to DSLF-and-IGRT are 0.99, 0.99, 1 for three setups. Most planning objectives are still achieved for DSLF-only scenarios. The DSLF-only dose values to heart, chest wall and lung increases from setup No.1 to 2 to 3. Generally, Setup No.1 appears to spare OARs the best, whereas setup No.3 results in greater target coverage, and setup No.2 is in between No.1 and No.3.
Conclusion: In general, the three DSLF setups are similar but yield different tradeoffs between target coverage and OAR dose sparing when daily IGRT is lacking. Our finding may help select DSLF setup.
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