Prostate brachytherapy: comparison of dose distribution with different 125I source designs.

PURPOSE: To evaluate the interchangeability of various commercially available iodine 125 ((125)I) sources and to assess the dosimetric effect of a change in source. MATERIALS AND METHODS: A modified peripherally loading prostate brachytherapy plan to deliver 145 Gy was devised by using a model (125)I source, which until recently was the only available (125)I source. A dose-volume histogram was generated. By using the available radial dose functions and anisotropy distributions for eight other currently commercially available sources, the same implant placement was planned and dose-volume histogram distributions tabulated. This exercise was performed for 15-, 45-, and 60-cm(3) glands. No implants were placed, and no physical radiation measurements were made. Dose calculations were theoretic: They were generated by using a widely available treatment planning system. RESULTS: There was little difference in dose distribution to the volume receiving 100% of the prescribed dose (<6%); only one source showed a difference greater than 2%. Large differences, up to -40% to +60%, were seen in the volume of tissue encompassed within internal high-dose regions receiving 150% or 200% of the prescribed dose. These findings held true, irrespective of gland size, within a clinically relevant range (15-60 cm(3)) and for a uniformly loaded radionuclide distribution. CONCLUSION: Reviewing only peripheral dose at or near the prescription dose of 145 Gy revealed little difference in doses for various source designs. Marked differences in high-dose regions were seen and may affect the dose received by internal sites. Effects of these changes on cure and/or complications remain speculative.

Isoseparation curves: a mechanism for optimizing off-axis dose homogeneity of intact breast irradiation.

The purpose of this paper is to determine whether using off-axis isoseparation curves to optimize the collimator rotation angle improves dose homogeneity. Eleven intact breast irradiation patients underwent computerized tomography (CT) treatment planning with 1 cm abutting slices. Central plane treatment planning, using 6 MV photons, tissue inhomogeneity corrections, and isocentric opposed tangent treatment fields, was performed. Collimators were rotated to match chest wall slope through the use of a beam's-eye-view setting. Patient separations were measured from the apex of the breast to the posterior field border on each axial CT slice. Sagittal-plane isoseparation curves were constructed from these measurements. Using these curves, the collimator rotation that minimized off-axis separation differences was determined. A comparison of off-axis dose inhomogeneity was performed for patients with a > or =10 degrees difference between this optimized collimator angle and the angle determined by chest wall slope. These comparative treatment plans differed only with respect to collimator angle rotation. The mean optimal collimator rotation angle differed significantly from the mean rotation angle which matched the chest wall slope (5.4 degrees vs. 11 degrees, respectively, P < 0.001). Four of the 11 patients had rotation angle differences of 10 degrees. In these patients, the optimization of collimator angle reduced the percentage of breast volume to "that" received > or =110% of the prescribed dose. For the patient with the largest breast size to the patient with the smallest breast size the decreases were, respectively, 5% (15% to 10%), 3% (24% to 21%), 1% (4% to 3%), and 1% (0.9% to 0%). The mean reduction in dose inhomogeneity was greatest in the inferior breast quadrants. At 6 cm and 4 cm off axis, the mean reductions in the percentages of the breast tissue to "that" received 110% of the prescribed dose were respectively 15.1% and 5.3 %. Optimizing the collimator angle through the use of isoseparation curves decreases dose inhomogeneity. The greatest improvements are in the inferior quadrants of the intact breast. The improved dose homogeneity may have clinical relevance in the treatment of patients with large breast sizes.

Dosimetric analysis of intact breast irradiation in off-axis planes.

PURPOSE/OBJECTIVE: The purpose of this investigation is to quantify dose inhomogeneity of intact breast irradiation in off-axis planes, and determine how dose inhomogeneity varies according to patient breast size and anatomical region of the breast. METHODS AND MATERIALS: Eleven patients treated with intact breast radiation underwent a treatment-planning computer tomography (CT) scan with 1-cm slices through the entire breast. The area of breast tissue was defined on each CT slice. Treatment planning with lung correction factors was performed using a two-dimensional treatment-planning system that calculates off-axis dose distributions on a slice-by-slice basis. Each plan utilized tangential beams with matched nondivergent posterior borders and with collimator rotation to match the chest wall slope. Dose inhomogeneity within the central plane was minimized during treatment planning by the use of a wedge on the lateral tangent field and by the differential weighting of fields. Dose was normalized at the breast and pectoralis major interface at midseparation in the central plane. Off-axis dose inhomogeneity was not considered in the optimization of the treatment plan. Dose distributions were plotted for each 1-cm slice, and the area of each isodose curve within the breast on each CT slice was calculated. The results of each slice were summed to give an approximation of dose-volume relationships. RESULTS: For the entire population, an average of 10% of the breast volume (range 1-40%) received 110% or greater of the prescribed dose. Increasing dose inhomogeneity was positively correlated with increasing breast sizes (r = 0.72, p = 0.01--Spearmen rank test). Analysis of dose as a function of location within the breast, revealed that the greatest dose inhomogeneity occurred in the lower anatomical quadrants of the breast (p = 0.003-Kruskal-Wallis test). For the group, the mean breast volume that received a 110% or greater dose was: 30% at 6 cm below central axis, 14% at 4 cm below central axis, 6% at central axis, 5% at 4 cm above central axis, and 7% at 6 cm above central axis. CONCLUSION: Our study demonstrates that a significant volume of breast tissue receives 110% or greater of the prescribed dose. This inhomogeneity is greatest in women with larger breast sizes, providing a possible explanation for the poorer cosmetic result seen in this subset of patients compared to women with small breast sizes. In addition, our results show the greatest dose inhomogeneity in the lower quadrants of the breast. Off-axis dose inhomogeneity should be considered in the planning of tumor bed boosts in women with lower quadrant tumors.