SU-F-J-19: Robust Region-Of-Interest (ROI) for Consistent Registration On Deteriorated Surface Images.

PURPOSE: For African-American patients receiving breast radiotherapy with a bolus, skin darkening can affect the surface visualization when using optical imaging for daily positioning and gating at deep-inspiration breath holds (DIBH). Our goal is to identify a region-of-interest (ROI) that is robust against deteriorating surface image quality due to skin darkening.

METHODS: We study four patients whose post-mastectomy surfaces are imaged daily with AlignRT (VisionRT, UK) for DIBH radiotherapy and whose surface image quality is degraded toward the end of treatment. To simulate the effects of skin darkening, surfaces from the first ten fractions of each patient are systematically degraded by 25-35%, 40-50% and 65-75% of the total area of the clinically used ROI_ipsilateral-chestwall. The degraded surfaces are registered to the reference surface in six degrees-of-freedom. To identify a robust ROI, three additional reference ROIs - ROI_chest+abdomen, ROI_bilateral-chest and ROI_extended-ipsilateral-chestwall are created and registered to the degraded surfaces. Differences in registration using these ROIs are compared to that using ROI_ipsilateral-chestwall.

RESULTS: For three patients, the deviations in the registrations to ROI_ipsilateral-chestwall are > 2.0, 3.1 and 7.9mm on average for 25-35%, 40-50% and 65-75% degraded surfaces, respectively. Rotational deviations reach 11.1° in pitch. For the last patient, registration is consistent to within 2.6mm even on the 65-75% degraded surfaces, possibly because the surface topography has more distinct features. For ROI_bilateral-chest and ROI_extended-ipsilateral-chest registrations deviate in a similar pattern. However, registration on ROI_chest+abdomen is robust to deteriorating image qualities to within 4.2mm for all four patients.

CONCLUSION: Registration deviations using ROI_ipsilateral-chestwall can reach 9.8mm on the 40-50% degraded surfaces. Caution is required when using AlignRT for patients experiencing skin darkening since the accuracy of AlignRT registration deteriorates. To avoid this inaccuracy, we recommend use of ROI_chest+abdomen, on which registration is consistent within 4.2mm even for highly degraded surfaces.