(Sunday, 7/14/2019)
Room: ePoster Forums
Purpose: The purpose of this work was to arrive at comparable average doses over points A1_A3 as defined in PORTEC-4 protocol for Varisource IX and Elekta V3 afterloaders.
Methods: Treatment plans for standard cylinder diameters of 2.5 cm, 3.0 cm and 3.5 cm were created for a typical active length of 3.5 cm. Optimization was performed such that the average dose to points A1 and A3 was 100%. Prescription was 21 Gy in 3 fractions. Doses were recorded parallel to central axis and 0.5 cm away from surface of cylinder at A2 (half active length), A4 (at first dwell position), A5 (between A2 and A4) and A6 (1 cm inferior from A2). Maximum dose at 5 mm from dome, at surface of dome, and at surface along length of the cylinder were also recorded. EQD2 for alpha/beta of 10 and 3 were computed for maximum dose at 5 mm from dome and at its surface.
Results: For the same mean dose of 100% over points A1 and A3, doses to points A2, A5 and A6 were comparable (within 0.2-1.5 Gy) between the two systems as was average dose to cylinder surface along its length. Doses to A4 showed a slightly higher difference (3.5 Gy) since that is the level at which the isodose follows the curvature of the cylinder surface. An exception was noted in the 2.5 cm diameter cylinder where the difference in dose to point A4 was noted to be 9 Gy. The dosimetry at the dome noted that a/b 10 and a/b 3 EQD2 was on average 48% and 59% greater with Varisource. And, almost twice as much at the cylinder dome surface.
Conclusion: Although comparable mean prescription A1_A3 was achieved there remains remarkable difference in the hot spot. The clinical significance of this warrants further study.
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