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Computational Evaluation of Different Dose Engines and In-Vivo Measurements of a VMAT Chest Wall Irradiation Case with Brass Bolus

M. M. Zaini1,2*, S. S. Honeywell2, M. A. Whiton2, A. P. Mazza1,2, R. Poortvliet2, C. Kahnke2 (1) Northwest Medical Physics Center, Lynnwood, WA (2) Skagit Valley Hospital, Mount Vernon, WA

Presentations

(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: To evaluate two TPS algorithms and their agreement with in-vivo measurements of a brass bolus VMAT treatment case.

Methods: VMAT treatment was prescribed for a young post-mastectomy right breast patient with curative intent. Surgery had resulted in a large uneven skin surface, yet full skin dose was desired. Brass mesh bolus was employed to escalate the skin dose. In-vivo measurements using MOSFETs in conjunction with treatment plan-based evaluations guided the radiation delivery setup schemes. Both convolution-superposition and Monte Carlo techniques were used in this comparison. Modelling the brass bolus in RayStation had to be conducted differently for the two photon algorithms because of their underlying physics.

Results: In-vivo measurements using a buildup cap on the MOSFETs, which is a good surrogate of d??? dose of the underlying tissue, revealed that the prescribed dose was delivered accurately to within the precision range of such measurements. However, MOSFET measurements with no buildup cap indicated that employing no bolus on the treatment field resulted in about 45% underdosage of skin. Two layers of brass bolus provided adequate skin dose values. One layer enhanced the skin dose by 20% and two layers by about 35% from the no bolus case. Medical judgements determined the duration of employing the bolus to reach the desired skin dose, while avoiding skin toxicity. Appending VMAT treatment with electron beams was also considered based on clinical evaluations.

Conclusion: It is possible to achieve clinically acceptable skin dose levels in VMAT treatment of chest wall patients using brass bolus. Combination of different treatment techniques can help reach this goal. Modelling brass bolus using the convolution-superposition algorithm was not as adherent to measurements as that achieved by employing careful Monte Carlo computational systems.

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