Commissioning COMS Eye Plaques In Varian Brachyvision: Are Point Type Calculations Sufficient?
Abstract
Purpose
To quantify the impact of LDR seed line vs point calculation types in Varian BrachyVision for COMS eye plaque geometry.
Methods
IsoAid I-125 line seed parameters were configured in Varian Aria RT-Admin utilizing the manufacturer’s data and characteristics from AAPM TG43U1. A duplicate seed with the same parameters but point type calculation was also configured. The COMS seed pair coordinates from AAPM TG129 were then entered into BrachyVision for the COMS 10mm and 22mm eye plaques. Sk units were unchanged between the line and point-type calculations and were selected to represent a standard COMS treatment: 120-hour implant, 85Gy to 5mm depth. Reference points moving out along the axis perpendicular to the plaque at 1mm intervals were generated.
Results
For the COMS 10mm geometry: the line type calculation reports a dose 5.72% higher than the point type calculation. Moving out axially, the line type calculation reports lower doses of 3.52% at 1mm, 2.18% at 2mm, 1.26% at 3mm, 0.62% at 4mm, and 0.18% at 5mm. For the COMS 22mm geometry: line type calculation reports lower doses of 4.71% at the origin, 2.21% at 1mm, 1.18% at 2mm, 0.65% at 3mm, 0.32% at 4mm, and 0.1% at 5mm. Additionally, BrachyVision can only accept coordinate pairs of the seed’s active length. As TG129 selected 4.5mm physical length, when entering the coordinate pairs, BrachyVision non-symmetrically fits the coordinates to the active length. Coordinates must be manually remapped for your seed’s active length to ensure accuracy.
Conclusion
As expected per AAPM TG43U1, the agreement gets better with distance as the line source collapses to a point approximation. The larger plaque showed better agreement axially. At the COMS standard 5mm, point type calculations would be acceptable, but for tumors below 5mm apex distance, point type calculations are not recommended.