ESTRO 2024 - Abstract Book

S3591

Physics - Dose prediction, optimisation and applications of photon and electron planning

ESTRO 2024

uncertainty parameters" tool. The aim was to find an optimal optimization skin flash bolus Hounsfield Units (HU) and develop a robust planning technique.

Material/Methods:

On the present study ten breast radiotherapy patients with axillary lymph node were planned with VMAT. The plans were optimized with auxiliary structures, the planning target volume (PTV) extending outside the skin contour by 8 mm [2] (labeled zz_PTVexpanded); and with optimization boluses of 10 mm. The bolus was assigned 0HU, -100HU and -300HU. An additional structure (labeled zz_skin) was created to avoid loss coverage near de surface, the region of interest (ROI) was approximately 8mm thick, see Figure 1. In order to reduce the plan complexity, the aperture shape controller (ASC) [3] was set to high and the monitor units (MU) were limited to 650MU for a 40Gy in 15fraction prescription. The final dose was calculated without the bolus. The doses were recalculated using "the plan uncertainty parameters tool" in eclipse, where all fields move together in the X/Y/Z directions, a 5mm shift was entered. The plans were compared in terms of skin dose coverage by assessing the dose to zz_Skin structure and the PTV, D95% and D2% were recorded. The doses to organs at risk (OARs) were also evaluated, mean dose (Gy) and V16Gy(%) to the ipsilateral lung and heart

Results:

The plans optimised with three different HU on the bolus the PTV and surface doses D95% and D2% were higher with a HU=0 to HU=-300, and very similar to HU=-100, see Table 1. The doses to the OARs are very similar on the three plans. Similarly, the plans recalculated using the plan uncertainty parameters the surface dose coverage is slightly higher (+1%) with HU=0 compared to HU=-100, and lower with HU=-300 (-2.5%), see Table 2.