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Evaluation of Conventional Setup for Prone-Breast External Bream Radiation Therapy Boosts

N Piechowska*, A Simon , J Mayadev , L Cervino , UC San Diego, La Jolla, CA


(Sunday, 7/14/2019)  

Room: ePoster Forums

Purpose: Breast radiotherapy using tangents is delivered in the supine or prone position, with prone positioning often decreasing heart and lung dose. Tumor bed boosts, however, are typically performed in the supine position with electrons, requiring an additional simulation CT scan for patients initially treated prone. Boost delivery in the prone position with mini-tangent photon beams is an alternative; however, the reliability of daily set-up is less certain. Here we evaluate the reliability of our daily set-up protocol for a prone mini-tangent boost, using daily CBCT to verify the adequacy of daily target coverage

Methods: A retrospective study of 7 patients treated with prone mini-tangent boost with a prescription dose of 10Gy in 5 fractions. The patients’ daily setup protocol consisted of standard visual verification of SSDs and lateral light fields with weekly port films. A daily post-setup CBCT was acquired for the purpose of this study, and was not used to further adjust setup. A physician contoured the tumor bed on all CBCTs and an automated set of contours generated by deformable image registration of the simulation CT to the CBCT using MIM Maestro. Digitally reconstructed radiographs from the contoured CBCTs were then evaluated to determine how often the tumor bed was not fully contained within the radiation beam.

Results: Characterization of the contoured tumor bed location indicated that the target was within the boost setup field parameters 100% of the time. In 14% of the cases the tumor was near the edge of the boost field.

Conclusion: Our results suggest that daily clinical setup for prone breast boost delivery is reliable, with low probability of missing the target. Future work will include detailed dosimetric analysis of the tumor bed and ipsilateral lung to verify adequate target coverage and normal tissue sparing with the prone boost approach.


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