Three-dimensional lookup tables for Henschke applicator cervix treatment by HDR ¹⁹²IR remote afterloading

Purpose: We have generated three-dimensional (3D) lookup tables for dosimetric analysis and optimization of high-dose rate (HDR) gynecological treatments using the Henschke applicator. The new dosimetry data have been compared with two-dimensional (2D) data currently in use. The 3D dosimetry tables have been implemented in an existing cervix treatment-planning system and have been evaluated through analysis of clinical cases. Methods and Materials: A general Monte Carlo N-Particles (MCNP) transport code was used to compute absorbed dose distributions around the intrauterine tandem and tungsten-shielded ovoid separately. The dosimetry data are represented in the x-y coordinate system for the intrauterine tandem table. The 3D table for the ovoid contains a radial dose function and an anisotropy function, as formulated in the Spherical coordinate system. Absorbed dose at a spatial point is calculated by applying bilinear interpolation for the anisotropy function and linear interpolation for the radial dose function. The geometry factor for a finite line source is used. 3D dose calculations and optimization were performed for 20 treatments of 10 patients. The absorbed dose to critical structures, bladder and rectum, was compared by applying both the 2D table currently in use and the new tables. Results: The new 2D table for the intrauterine tandem yields doses different by less than 10% from those with the current table, The 3D table for the shielded ovoids shows as large as a factor of 4 reduction of dose behind the shield compared with the present 2D table. This shielding effect leads to 21.6 ± 9.3% and 20.0 ± 6.6% dose reduction at rectum and bladder, respectively, for actual treatments. Conclusion: Our analysis indicates a need for patient-specific 3D dosimetry to permit more accurate dosimetric evaluation of HDR cervix treatments using shielded applicators. We have also shown that a Monte Carlo simulation code enabled us to derive the lookup tables necessary for 3D planning.