Room: Exhibit Hall | Forum 3
Purpose: Post-mastectomy breast patients with tissue expanders have been treated with spot-scanning proton beams by appropriately placing spots around or through the metallic ports. In this study, the dosimetric robustness of both techniques was evaluated considering the range and setup uncertainties.
Methods: Five sequential patients with identical tissue expanders were planned and treated using both techniques. Two chest wall fields and 1 supraclavicular field were used in the around technique, while 2 uniform-dose fields covering the whole treatment volume, treated on alternating days, were used in the through technique. A range uncertainty of 3.5% and setup uncertainties of 2 mm and 5 mm were used to evaluate the dosimetric robustness including the dose coverage of the CTV around the supraclavicular lymph nodes (CTV_SC), CTV surrounding the tissue expander (CTV_s/TE), and the dose sparing of the ipsilateral lung and heart.
Results: Dose coverage was comparable in both techniques (CTV V95%>95% in all nominal plans). The CTV_s/TE of only one patient was found to have V95% = 92.4% in the under-range scenario using the around technique with a larger than normal hinge angle between the 2 chest wall fields. The through technique provided lower mean dose sparing of the ipsilateral lung (V20Gy(RBE): through: 11.2% vs. around: 13.8%) with negligible range uncertainty sensitivity. The heart mean doses were slightly higher in the through plans (through: 0.82 Gy(RBE) vs. around: 0.69 Gy(RBE)). The supraclavicular field with optimized gantry angle used in the around technique are more forgivable to setup variations for CTV_SC, while the through technique provides more robust dose coverage of CTV_s/TE.
Conclusion: A daily single-field through the metallic port could be delivered per fraction to reduce the treatment time (<15 mins) significantly compared with the around technique, which minimizes the intra-fraction motion of patients without compromising dosimetric toxicity and plan robustness.