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Case Study of the Impact of Incorrectly Assigned Densities in Monaco for Step-And-Shoot Planning of a Patient with a High Density Breast Expander

N Cao

N Cao1,2*, M Lavilla2 , A Kalet1,2 , L Young1,2 , J Kim1,2 , J Meyer1,2 , (1) University of Washington Medical Center, Seattle, WA, (2) Seattle Cancer Care Alliance, Seattle, WA


SU-I-GPD-T-438 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To investigate the dosimetric impact of incorrect electron densities, automatically assigned by the Monaco treatment planning system, for patients with high density breast expanders.

Methods: One of the implementation characteristics, often unknown to the users of Monaco, is that the collapsed-cone algorithm (CC) automatically sets any density above that of aluminum (2.46) to that of iron (6.6) and this value cannot be overridden. With the Monte Carlo (MC) algorithm, densities up to 15 are correctly taken into account. For forward planned step-and-shoot plans with CC, the segment generation is therefore based on incorrect isodose lines. To investigate the impact, the segments of a 50.4 Gy step-and-shoot breast plan with expander (max density 3.4) were generated with CC and then recalculated with MC (CC-MC). An additional plan was generated with each segment generated based on MC.

Results: To generate segments based on the 107% isodose line, for each iteration MC took approximately 1 minute for recalculation, while CC only took about 10 seconds. Similar breast coverage of 97.64 +/- 0.1% was achieved for all three plans requiring the same number of MUs. The mean lung dose was 12.64, 13.31, 13.19 Gy, while mean heart dose was 1.70, 1.84, and 1.85 Gy for CC, CC-MC, and MC plans, respectively. Generally, the doses to the lung and heart were slightly underestimated by CC. The CC-MC plan had only marginally higher maximum dose compared to CC plan.

Conclusion: The impact of the incorrect density override in Monaco for CC was minimal for a step-and-shoot breast plan with a high density tissue expander. Based on this case study it is warranted to say that it should be sufficient to use CC for the segment generation to speed up the planning process but we recommend performing final dose calculations with MC.

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