Interstitial Brachytherapy Basal Dose Calculations Utilizing Paris System
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PO-GePV-T-46 (Sunday, 7/10/2022) [Eastern Time (GMT-4)]
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Purpose: The Paris system is traditionally a complete dosimetric system facilitating brachytherapy using Iridium-192 with low, high and pulsed dose rates. The aim of this study has been to calculate basal dose using Task Group 43 formalism for Paris system dedicated to interstitial brachytherapy. The principles of the Paris system with its dose specification modes within the implanted volume and the value of the fixed reference isodose at 85% of the basal dose rate (BD), defined as the average of the minimum dose between sources are addressed.
Methods: An orange has been considered in place of a patient to represent as a tumor bed for needle insertions. The insertions were done according to the Paris system (ICRU-58) at a depth of 5cm. Five 70mm long stainless steel radioactive needles (Product no: 11-00324, GM 11006070) and template with holes with 15 mm separation in equilateral triangle were used. The needles were parallel, equidistant and arranged such that their center was in the same plane perpendicular to the long direction of the needles. The BD rates were determined according to the BD points identified relatively to the implant. Once BD points were located, the rates are merely the summation of dose rate contributions from all sources. All TG-43 parameters, geometry, radial dose and anisotropy functions were calculated utilizing HEBD report-229.
Results: The deviation of doses calculated from Varian BrachyVision Treatment Planning system and manual calculations were approximately 2.1%, which is acceptable according to ICRU-58 protocol. This study showed that BD rates were less than expected, affecting on isodose curve of the target. However, each elementary BD rate was within ±10% of the average BD rate.
Conclusion: The study has a clear support of Paris system in interstitial Brachytherapy and will certainly be a viable option in addition to other techniques in brachytherapy.
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