PubMed · 3950594
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A R Harwood, K T Noell. 1986. New perspectives.. https://pubmed.ncbi.nlm.nih.gov/3950594/
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PURPOSE: To characterize the magnitude and duration of post-implant edema following the implantation of I-125 or Pd-103 seeds into the prostate and to investigate its effect on the CT-based calculation of the total dose delivered by the implant. MATERIALS AND METHODS: A pre-implant CT scan and 3 to 5 serial post-implant CT scans were obtained on 10 patients who received either I-125 or Pd-103 seed implants. None of the patients received hormone therapy. The magnitude and duration of edema were determined from the change in the spatial distribution of the implanted seeds as the edema resolves. Dose volume histograms were compiled to determine the percentage of the prostate volume that received a dose equal to, or greater than, the prescribed dose. RESULTS: The magnitude of the edema, expressed as the ratio of the post- to pre-implant volume on the day of the procedure, ranged from 1.33 to 1.96 (mean 1.52). The edema decreased exponentially with time; however, the edema half-life (time for the edema to decrease by 1/2) varied from 4 to 25 days (mean 9.3 days). As the edema resolved, the percentage of the prostate that received a dose equal to or greater than the prescribed dose increased by at least 7% in 7 of the 10 patients and increased by more than 15% in 2. In those patients in whom dose coverage was unaffected by the resolution of edema, more than 90% of the prostate was covered by the prescribed dose in the initial CT scan. CONCLUSION: Post-implant edema increased the prostate volume by factors which ranged from 1.33 to 1.96 (mean: 1.52). The edema resolved exponentially with an edema half-life which varied from 4 to 25 days (mean: 9.3 days). Edema had a significant effect on the post-implant dosimetry in 7 of 10 cases. Factors that affect the impact of edema on the dosimetry are the magnitude of the edema and the planned margin between the prescribed isodose line and the periphery of the prostate.
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.