Program Information
The Effect of Breast Bolus On Skin Dose for Hypofractionated Partial Breast Irradiation in An MRI-LINAC
A Soliman1,2*, A Kim2,3 , S Al-Ward2 , A Sahgal1,2,3 , J Lee2,3 , B Keller2,3 , (1) Sunnybrook Research Institute, Toronto, ON (2) Odette Cancer Centre, Sunnybrook Health Science Centre, Toronto, ON (3) Radiation Oncology, University of Toronto, Toronto, ON
Presentations
SU-F-205-7 (Sunday, July 30, 2017) 2:05 PM - 3:00 PM Room: 205
Purpose: Patient irradiation in a transverse magnetic field can lead to an increase in dose at a tissue-air interface due to the electron return effect. This work explores the effects on skin dose when adding bolus for Hypofractionated Partial Breast Irradiation (HPBI) patients treated in an MRI-LINAC with a 1.5T transverse magnetic field (B0). The consequences of introducing unintentional air gaps between the skin and the bolus are also investigated.
Methods: The planning was performed on two patients who previously received HPBI. The Monaco TPS (Research version 5.19.02, Elekta AB, Stockholm) with an MRI-LINAC beam model was used. For each patient, ten plans were created: bolus of 1 cm in presence and absence of B0; No bolus in presence and absence of B0; bolus of 1 cm with 0.1, 0.3 and 0.6cm air gaps in presence of B0; bolus of 0.5, 1.5 and 2cm in presence of B0. The prescription dose was 40Gy in 5 fractions using a five beam IMRT beam arrangement. Plans were either optimized such that the PTV coverage was V₉₅ >= 99% or recalculated in the case of the creation of unintentional air gaps.
Results: Whether in the presence or absence of B0, plans optimized with bolus exhibited higher skin dose than those optimized without bolus. When air gaps are introduced, PTV-V95 was 99, 97.9, 90.8 and 84.5% for 0, 0.1, 0.3 and 0.6cm air gaps, respectively, although skin V₃₀= 23.4, 13.0, 9.7 and 1.6cc, respectively. No notable difference was observed in the PTV coverage or skin dose for different bolus thicknesses. In all plans, OAR dose constraints were met.
Conclusion: The presence of bolus increased the skin dose, irrespective of B0. Air gaps render the PTV coverage suboptimal for PTV’s close to the skin, while notably reducing the skin dose for >0.5cm air gap.
Funding Support, Disclosures, and Conflict of Interest: This research was funded by Elekta AB, Stockholm, Sweden.
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