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Updated: Jul 14, 2026

Irradiator Commissioning and Dosimetry for Assessment of LQ α and β Parameters, Radiation Dosing Schema, and in vivo Dose Deposition
Published on: March 11, 2021
Dose modification factors for 192Ir high-dose-rate irradiation using Monte Carlo simulation.
Bassel Kassas1, Firas Mourtada, John L Horton
1Radiation Oncology Department, Greater Baltimore Medical Center, Baltimore, Maryland 21204, USA. bkassas@gbmc.org
Accurate partial breast irradiation dosimetry with the MammoSite system requires accounting for reduced scatter in thin tissue. Monte Carlo simulations show that insufficient surrounding tissue can lead to underdosing, impacting treatment effectiveness.
Area of Science:
- Medical Physics
- Radiation Oncology
- Dosimetry
Background:
- The MammoSite system is used for partial breast irradiation, delivering dose 1 cm from the balloon surface.
- Current brachytherapy planning systems often use water-based dosimetry, assuming full scatter regardless of surrounding tissue thickness.
- This assumption may lead to inaccurate dose delivery when tissue beyond the prescription depth is limited.
Purpose of the Study:
- To investigate the impact of surrounding tissue thickness on dose delivery for MammoSite brachytherapy.
- To quantify the dose modification factor due to scatter variations in different tissue depths.
Main Methods:
- Monte Carlo simulations were performed using spherical water phantoms with varying balloon applicator sizes (4-6 cm).
- Dose rates were calculated at 1 cm from the balloon surface with varying tissue thicknesses (0-10 cm) beyond the prescription depth.
- The dose modification factor was determined by comparing dose rates in a full scatter phantom to those with finite tissue.
Main Results:
- The dose modification factor varied with balloon diameter and surrounding tissue thickness.
- At the breast-skin interface, factors ranged from 1.098 (4-cm balloon) to 1.132 (6-cm balloon) with no tissue beyond the prescription depth.
- At the breast-lung interface, factors ranged from 1.067 (4-cm balloon) to 1.096 (6-cm balloon).
- Even 5 cm of tissue did not fully restore scatter, indicating its significant impact.
Conclusions:
- Accurate dosimetry for MammoSite brachytherapy necessitates considering the effects of diminished scatter.
- Failure to account for the dose modification factor can result in underdosing patients.
- These findings highlight the importance of patient-specific dosimetry adjustments based on surrounding tissue characteristics.
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