Author: Awens Alphonse, Nebi Demez, Michael Kasper, Noufal Manthala Padannayil, Shyam Pokharel, Suresh Rana, Lauren A. Rigsby, Nishan Shrestha 👨🔬
Affiliation: Lynn Cancer Institute, Boca Raton Regional Hospital, Baptist Health South Florida 🌍
Purpose: This study aims to quantify the dose discrepancy associated with improper placement of the Leipzig applicator on the patient's skin.
Methods: Monte Carlo simulations were performed using the TOPAS system to assess the dose distribution. A simplified geometry was created, consisting of a 1 cm diameter skin lesion embedded in a water medium, for dose calculations. A 3 cm diameter Leipzig applicator was modeled within the TOPAS framework. Two common misplacement scenarios were simulated: skin gaps of 1 mm and 2 mm, and applicator tilts of 3° and 5°. For each case, one billion histories were run to evaluate the dose differences through dose volume histograms (DVH) and 2D depth dose distributions. Ideal coverage is defined as 95% of the lesion volume being covered by 95% of the prescription dose and recommended depth of prescription is at 3mm which is defined as 100% isodose line at central axis.
Results: As the skin gap increases, the prescription isodose line moves to shallower depths, which can be observed in the 2D depth dose distribution. Additionally, hot spots on the skin decrease from 140% to 135% and then to 120% with increasing skin gaps. The DVH results indicate a decline in ideal dose coverage as the skin gap increases, dropping from 95% to 65% and 40% with 1 mm and 2 mm gaps, respectively. Similarly, the applicator tilt also affects dose distribution, with dose coverage decreasing from 95% to 60% as the tilt increases. However, the dose reduction due to increasing tilt is not as significant as that caused by the increasing skin gap.
Conclusion: Our study shows the importance of proper placement of skin applicators. While both scenarios result in a decreased dose, an increased skin gap has a more significant impact on dose coverage compared to an increased applicator tilt