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Investigation of Tumor Bed Delineation Accuracy for Targeted Breast Radiation Therapy Following Oncoplastic Surgery

G Aldosary1*, J Caudrelier2, T Tse3, A Arnaout3, C Foottit4, J Belec4, E Vandervoort1,4, (1) Department of Physics, Carleton University, Ottawa, ON, Canada, (2) Department of Radiation Medicine, The Ottawa Hospital Cancer Centre, Ottawa, ON, Canada, (3) Department of General Surgery, The Ottawa Hospital, Ottawa, ON, Canada, (4) Department of Medical Physics, The Ottawa Hospital Cancer Centre, Ottawa, ON, Canada


(Thursday, 7/16/2020) 10:30 AM - 11:30 AM [Eastern Time (GMT-4)]

Room: Track 5

Purpose: Radiation oncologists (ROs) use surgical clips (SCs) for tumor bed (TB) delineation in boost and partial breast irradiation (PBI). However, SCs may not provide a reliable radiographic surrogate of TB locations post-oncoplastic breast surgery (OBS). The accuracy of RO-contoured TBs (TB_RO) post-OBS was investigated.

Methods: Six representative OBS techniques were performed on realistic breast phantoms. Eight SCs were used to mark the TB following tumor excision. Additional radio-opaque markers were used to track TB deformation post-OBS closure (TB_True). Each phantom was CT imaged at different surgery phases. Pre- and post-OBS closure SC displacements were recorded. Two ROs were asked to delineate TBs on CTs with homogenous breast parenchyma density and SCs. Inter- and intra-RO contour agreements were calculated using the dice similarity coefficient (DSC) and Hausdorff distance (HD). Clinical-target-volume (CTV) was defined by applying expansions from 5-15 mm to TB_RO contours. The DSC, HD, undercontoured volumes (UCV) and overcontoured volumes (OCV) were measured for RO contours compared to TB_True. Results were reported as an average for all cases.

Results: The SC displacement was 9.2 ± 8.7 mm. DSC and HD were 0.79 ± 0.06 and 12.7 ± 3.6 mm (intra-RO), and 0.77 ± 0.05 and 12.4 ± 2.8 mm (inter-RO). ROsAvg and TB_RO+15mm DSC and HD were 0.54 ± 0.12 and 25.2 ± 9.2 mm, 0.57 ± 0.08 and 23.8 ± 6.5 mm, respectively. ROsAvg and TB_RO+15mm UCVs and OCVs were 28.0 ± 16.7 cc and 35.8 ± 28.1 cc, 5.4 ± 7.6 cc and 154.4 ± 82.0 cc, respectively.

Conclusion: Intra- and inter-RO contours agreed but were systematically off from the True_TB. Defining CTV margins did not improve contour agreement and may cause significant over-irradiation of normal tissue. Following OBS-procedures, the accuracy of TB delineation by ROs is challenging and makes indication of dose boost or PBI questionable.


Breast, Radiation Therapy, Treatment Planning


TH- External Beam- Photons: treatment planning/virtual clinical studies

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