Robustness of the pseudo-skin flash in VMAT breast radiotherapy with Acuros XB algorithm
Ari-Pekka Honkanen,
Finland
PO-1737
Abstract
Robustness of the pseudo-skin flash in VMAT breast radiotherapy with Acuros XB algorithm
Authors: Ari-Pekka Honkanen1, Liisa Porra1, Anna Rintala1, Vappu Reijonen1, Tiina Seppälä1, Mikko Tenhunen1
1Helsinki University Hospital, Comprehensive Cancer Center, Helsinki, Finland
Show Affiliations
Hide Affiliations
Purpose or Objective
Deformation of the surface of the breast due to e.g., swelling and patient positioning is often observed during the external radiotherapy. The robustness of the treatment plan can be increased by expanding the field apertures to include an air margin. One approach to perform this in VMAT planning is to expand the original PTV outside the skin and fill it with an expanded body structure or virtual bolus; this method is referred to as pseudo skin flash.
The optimal features of the bolus and expanded PTV has been investigated earlier [1,2]. However, dose calculations in these studies were done with the AAA algorithm which is known to underperform in the regions with large inhomogeneities in density such as the skin-air interface, compared to the Acuros XB (AXB) algorithm [3]. The purpose of our study was to investigate the optimization of the bolus thickness and HU value selection to produce pseudo skin flash in VMAT breast plans using AXB dose calculation algorithm.
Material and Methods
A test patient with a left breast PTV (no lymph nodes) was used to study the effect of pseudo skin flash: two back-and-forth VMAT field pairs for 6 MV photons were set at 282°-355° and 95°-175° with collimator rotations at 10° and 350°. The virtual boli used in the study were 5, 10, and 15 mm in thickness with the CT numbers of -1000, -500, 0, 100, and 250 HU. The VMAT plans were optimized to an extended PTV (PTV + bolus - 5 mm margin) with criteria for the extended PTV and NTO only, and doses were calculated using AXB in Eclipse treatment planning system (v.16.1, Varian Medical Systems, CA, USA). The robustness of the plans was examined by a) calculating the doses with ±7 mm isocenter shifts along the main axes and b) using a 7 mm thick bolus of -100 HU to simulate swelling.
Results
The use of virtual bolus leads to a decrease between 1-5 percentage points in PTV D98% and D2% statistics (renormalized PTV V50% = 100% prescription) implying a need for extra effort to optimize a virtual bolus plan compared to the bolusless control case. The effect was the stronger the thicker the bolus was and the further away its HU value was from -500. More radiopaque boli were associated with a deficiency of the dose in the lateral breast. The robustness analyses revealed that in terms of the minimum dose, any virtual bolus is significantly better (D2% = 80 – 90 %) than no bolus at all (D2% < 70 %) but the best results were obtained with 10 mm thick boli in range -100 – 200 HU (D2% ≈ 90 %).
Conclusion
In terms of the dose homogeneity and robustness, using virtual boli with thicknesses around 10 mm and radiometric densities in the approximate range from - 100HU to 100HU appears to produce VMAT breast plans with the highest clinical quality when AXB is used for evaluation.
[1] Lizondo et al. (2019) Phys. Med. 63:56-62
[2] Rossi et al. (2019) Med. Dos. 44:266-73
[3] Han et al (2011) Med. Phys. 38:2651-64