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Authors: Maija Rossi, PhD ∗, †, ∗∗, Tuomas Virén, PhD ‡, Janne Heikkilä, PhD ‡, Jan Seppälä, PhD ‡, §, Eeva Boman, PhD ∗, †
* Department of Medical Physics, Tampere University Hospital, 33521 Tampere, Finland
† Department of Oncology, Tampere University Hospital, 33521 Tampere, Finland
‡ Center of Oncology, Kuopio University Hospital, 70210 Kuopio, Finland
§ Department of Applied Physics, University of Eastern Finland, 70211 Kuopio, Finland
To investigate the near-surface doses and target coverage in modulated arc radiotherapy (RT) of the breast or chest wall in two treatment planning systems (TPS) in the presence of soft-tissue deformations. This retrospective study consisted of 10 breast cancer patients with axillary lymph node inclusion. For each case, ﬁve RT plans were created: (1) tangential 3D conformal ﬁeld-in-ﬁeld (FinF) technique; (2) 200° to 240° arcs with optimization bolus (OB) in Eclipse (EB); (3) 243° to 250° arcs with an 8-mm OB in Monaco (MB); (4) 243° to 250° arcs with automatic skin ﬂash tool (ASF) in Monaco TPS (MA); (5) 243° to 250° arcs with both ASF and OB in Monaco (MAB). Soft tissue deformation was simulated by editing CT-images with 4-, 8-, and 12-mm swelling and recalculating the dose. The increasing swelling from 0 to 12 mm caused the coverage (V95%) in clinical target volume to decrease from 96% ± 2% to 90% ± 6% for the FinF plans. For volumetric-modulated arc therapy (VMAT), the coverage decreased from 99% ± 1% to 92% ± 4% in the EB plans, and from 97% ± 1% to 68% ± 8%, 85% ± 6%, and 86% ± 5% for MA, MB, and MAB, respectively. The mean dose in the surface extending from 0 to 3 mm from the skin decreased on average 5%, 17%, 20%, 15%, and 8% in FinF, EB, MA, MB, and MAB, respectively. In the Monaco plans, the use of an OB( + ASF) provided better target coverage and lower dose maxima despite of tissue swelling than the ASF alone. With modulated arc therapy, we recommend the use of an OB instead of or in addition to the ASF. The use of 8 mm OB with VMAT plans is robust to account deformations extending outside up to 8mm. If soft tissue deformation is larger than 8 mm, the need for replanning should be evaluated.