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Dosimetry of template-guided stereotactic implants: A comparison of I-125 and IR-192
234
Radiation Oncology, Biology, Physics
Volume 21, Supplement 1
1083 DOSIMETRY OF TEMPLATE-GUIDED
STEREOTACTIC IMPLANTS: A COMPARISON
OF I-125 AND IR-192
Lulu, B. Dept. of Radiation
Oncology,
University
of Arizona Health Sciences Center, Tucson, AZ 85724
Purpose: We analyze three representative template-guided stereotactic treatments of brain tumors using iridium-192 seeds. Comparable treatments using iodine-125 are compared and contrasted with the actual iridium treatments. Materials and Methods: A Phase I study involving the treatment of 29 patients with brain tumors by interstitial Ir192 seed radiation and ferromagnetic seed hyperthermia was completed at our institution in late 1990 (see paper by B. Stea et al). Parallel catheters were placed with hexagonal pattern templates, with an inter-catheter spacing of 12 mm. Inter-seed spacing was 10 mm center-to-center. Three representative cases (7 catheter, 15 seed, 15 cc; 15 catheter, 49 seed, 60 cc; 22 catheter, 78 seed, 85 cc) were selected for analysis. The volume of tissue treated to the prescription dose rate line actually used in the patient’s treatment was computed. The iodine seed strength was selected so that the prescription dose rate line would treat the same volume as the iridium treatment. Anisotropic dose distributions were used for each type of seed. The dose-volume calculations were done on 1.5 mm grids for the two larger volume cases and on a 1.0 mm grid for the small volume case. Results: The data are presented in many ways: dose-volume histogram, volume difference, percent volume difference, volume ratio, equivalent radius difference, percent equivalent radius difference. We find volume ratio and equivalent radius shift (“isodose line shift”) most useful. For dose values equal to 50% of the prescription dose, i.e., in “normal” tissue, the iodine isoline would contract (compared to iridium) by 0.6, 1.9, and 2.2 mm for the small, medium and large cases respectively. In the high dose region, approximately 150% of prescription dose, the ratio of iodine volume to iridium volume is similarly 1.08, 1.13, and 1.11. Conclusion: Compared to iridium, iodine spares tissue in regions outside the prescription isodose line, but overtreats the tissue inside the prescription line. In any case, the differences are not readily noticed on an isodose plot, and must be determined from dose-volume histogram analysis.
1084 IMPACT OF BONE DENSITY CORRECTIONS S.
ON DOSE DELIVERED
TO TIIE PROSTATE WITH
4-MV,6-m’, lo-MV,AND
18-MV PHOTONS
Nemeth, M.S.‘, K Ekstrand, Ph.D.‘, K. Greven, M.D.‘, M.E. Randall, M.D.’ and J. Hendrix, C.M.D’, A. McCunniff, M.D.‘, L. Evans, M.D.’
1 Bowman Gray School of Medicine/Wake Forest University/Winston-Salem,
NC. 2 High Point Regional Hospital/High Point, NC.
Purpose: Doses for definitive prostate irradiation have been empirically derived without availability of bone density corrections using low energy megavoltage equipment. With their increased availability, higher energy photons are more Although inhomogeneity corrections lead to greater frequently used because of their improved depth of penetration. accuracy of dose delivery, the clinical utility of corrections in the pelvis is unclear. This study evaluates the effect of bone density on the dose delivered with respect to the photon energy employed. Materials & Methods: Contours and volumes for 10 patients were taken from pelvic computed tomography scans at the center of the prostate. Treatment plans for bilateral prostate arc fields were run on the Capintec Treatment Planning System for 4-, 6-, lo-, and 18-MV photon energies. Treatment plans were calculated both with and without bone correction based on the monitor units needed to deliver 6500 cGy to the isocenter without bone correction using the equivalent pathlength algorithm. Results: The median dose to the isocenter was 6500 cGy for all energies without bone correction. The median doses using the uncorrected monitor units for the 4-, 6-, lo-, and 18-MV photon beams correct for bone density were 6033, 6062, 6166, and 6228 cGy, respectively. The variance in isocenter doses observed in our patient sample was: 2.3%, +2.2%, + 1.7% and 2 1.4%, respectively, for the 4-, 6-, lo- and 18-MV beams without bone correction. Conclusion: The increased density of bone in the pelvis does alter the actual dose to the prostate from external beam treatments. A slightly higher dose is absorbed by the prostate and surrounding normal tissue as photon energy increases. These observations have clinical implications regarding determination of dose-response data for both control of prostate cancer and rate of complications.
Radiation Oncology, Biology, Physics
Volume 21, Supplement 1
1083 DOSIMETRY OF TEMPLATE-GUIDED
STEREOTACTIC IMPLANTS: A COMPARISON
OF I-125 AND IR-192
Lulu, B. Dept. of Radiation
Oncology,
University
of Arizona Health Sciences Center, Tucson, AZ 85724
Purpose: We analyze three representative template-guided stereotactic treatments of brain tumors using iridium-192 seeds. Comparable treatments using iodine-125 are compared and contrasted with the actual iridium treatments. Materials and Methods: A Phase I study involving the treatment of 29 patients with brain tumors by interstitial Ir192 seed radiation and ferromagnetic seed hyperthermia was completed at our institution in late 1990 (see paper by B. Stea et al). Parallel catheters were placed with hexagonal pattern templates, with an inter-catheter spacing of 12 mm. Inter-seed spacing was 10 mm center-to-center. Three representative cases (7 catheter, 15 seed, 15 cc; 15 catheter, 49 seed, 60 cc; 22 catheter, 78 seed, 85 cc) were selected for analysis. The volume of tissue treated to the prescription dose rate line actually used in the patient’s treatment was computed. The iodine seed strength was selected so that the prescription dose rate line would treat the same volume as the iridium treatment. Anisotropic dose distributions were used for each type of seed. The dose-volume calculations were done on 1.5 mm grids for the two larger volume cases and on a 1.0 mm grid for the small volume case. Results: The data are presented in many ways: dose-volume histogram, volume difference, percent volume difference, volume ratio, equivalent radius difference, percent equivalent radius difference. We find volume ratio and equivalent radius shift (“isodose line shift”) most useful. For dose values equal to 50% of the prescription dose, i.e., in “normal” tissue, the iodine isoline would contract (compared to iridium) by 0.6, 1.9, and 2.2 mm for the small, medium and large cases respectively. In the high dose region, approximately 150% of prescription dose, the ratio of iodine volume to iridium volume is similarly 1.08, 1.13, and 1.11. Conclusion: Compared to iridium, iodine spares tissue in regions outside the prescription isodose line, but overtreats the tissue inside the prescription line. In any case, the differences are not readily noticed on an isodose plot, and must be determined from dose-volume histogram analysis.
1084 IMPACT OF BONE DENSITY CORRECTIONS S.
ON DOSE DELIVERED
TO TIIE PROSTATE WITH
4-MV,6-m’, lo-MV,AND
18-MV PHOTONS
Nemeth, M.S.‘, K Ekstrand, Ph.D.‘, K. Greven, M.D.‘, M.E. Randall, M.D.’ and J. Hendrix, C.M.D’, A. McCunniff, M.D.‘, L. Evans, M.D.’
1 Bowman Gray School of Medicine/Wake Forest University/Winston-Salem,
NC. 2 High Point Regional Hospital/High Point, NC.
Purpose: Doses for definitive prostate irradiation have been empirically derived without availability of bone density corrections using low energy megavoltage equipment. With their increased availability, higher energy photons are more Although inhomogeneity corrections lead to greater frequently used because of their improved depth of penetration. accuracy of dose delivery, the clinical utility of corrections in the pelvis is unclear. This study evaluates the effect of bone density on the dose delivered with respect to the photon energy employed. Materials & Methods: Contours and volumes for 10 patients were taken from pelvic computed tomography scans at the center of the prostate. Treatment plans for bilateral prostate arc fields were run on the Capintec Treatment Planning System for 4-, 6-, lo-, and 18-MV photon energies. Treatment plans were calculated both with and without bone correction based on the monitor units needed to deliver 6500 cGy to the isocenter without bone correction using the equivalent pathlength algorithm. Results: The median dose to the isocenter was 6500 cGy for all energies without bone correction. The median doses using the uncorrected monitor units for the 4-, 6-, lo-, and 18-MV photon beams correct for bone density were 6033, 6062, 6166, and 6228 cGy, respectively. The variance in isocenter doses observed in our patient sample was: 2.3%, +2.2%, + 1.7% and 2 1.4%, respectively, for the 4-, 6-, lo- and 18-MV beams without bone correction. Conclusion: The increased density of bone in the pelvis does alter the actual dose to the prostate from external beam treatments. A slightly higher dose is absorbed by the prostate and surrounding normal tissue as photon energy increases. These observations have clinical implications regarding determination of dose-response data for both control of prostate cancer and rate of complications.