Proceedings of the 35th Annual ASTRO Meeting 195 RESULTS: CONTRALATERAL BREAST DOSE (as 96 of dose to treated breast) CONCLUSIONS: 1. As expected, dose to the contralateral breast increases with the use of a block or wedge. 2. Lowest contralateral breast dose is with the Asy Jaw with no medial wedge. 3. ,4sy Jaw with block is equivalent to JCRT technique in contralateral dose. 111 ISON OF HIGH POSES Purpose/Objective: The Burnett applicators, used for vaginal cuff irradiation, are available in diameters of 2.0 to 3.5 cm. They are designed to hold low activity sources along the central axis but can be modified to accommodate crossing sources at the apex. Alternatively, vaginal cylinders of 2.0 to 4.0 cm in diameter are available for HDR systems. A high activity Ir-192 source moves through a central catheter stopping at select dwell positions for variable dwell times determined by computer optimization designed to give a uniform dose over defined points. There is not a crossing source option. Dose distributions for LDR cylinders with variable loading patterns and HDR cylinders with variable dose specification points and calculation algorithms were compared. Materials/Methods: Dose distributions were evaluated for 2.0, 3.0, and 3.5 cm cylinders for HDR and LDR systems. Point dose calculations were performed at various locations on the applicator surface. For the LDR system, calculations were performed with and without a crossing source at the apex, using a line source Sievert-type calculational algorithm. For the HDR system, calculations were performed using dose points along the surface of the cylinders, with and without an apex point. Isotropic and anisotropic calculations were performed. Results: A crossing Source at the apex of the LDR cylinder increases the apical dose and improves dose uniformity. The HDR dose distributions are a function of the diameter of the cylinder, points chosen for optimization and type of calculation. The apical dose is higher when an apex point is not used for optimization. Anisotropic calculations indicate a significantly lower dose at the apex than isotropic calculations. Conclusion: The vaginal apex dose with HDR varies substantially depending on the diameter of the cylinder, optimization points chosen, and type of calculation and for LDR on the presence of a crossing source. Attention to these details is required as most recurrences are at the apex.

Comparison of high dose rate (hdr) and low dose rate (ldr) dose distributions for vaginal cylinders

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Page 1: Comparison of high dose rate (hdr) and low dose rate (ldr) dose distributions for vaginal cylinders

Proceedings of the 35th Annual ASTRO Meeting 195

RESULTS: CONTRALATERAL BREAST DOSE (as 96 of dose to treated breast)

CONCLUSIONS: 1. As expected, dose to the contralateral breast increases with the use of a block or wedge. 2. Lowest contralateral breast dose is with the Asy Jaw with no medial wedge. 3. ,4sy Jaw with block is equivalent to JCRT technique in contralateral dose.

111 ISON OF HIGH POSES

Purpose/Objective: The Burnett applicators, used for vaginal cuff irradiation, are available in diameters of 2.0 to 3.5 cm. They are designed to hold low activity sources along the central axis but can be modified to accommodate crossing sources at the apex. Alternatively, vaginal cylinders of 2.0 to 4.0 cm in diameter are available for HDR systems. A high activity Ir-192 source moves through a central catheter stopping at select dwell positions for variable dwell times determined by computer optimization designed to give a uniform dose over defined points. There is not a crossing source option. Dose distributions for LDR cylinders with variable loading patterns and HDR cylinders with variable dose specification points and calculation algorithms were compared.

Materials/Methods: Dose distributions were evaluated for 2.0, 3.0, and 3.5 cm cylinders for HDR and LDR systems. Point dose calculations were performed at various locations on the applicator surface. For the LDR system, calculations were performed with and without a crossing source at the apex, using a line source Sievert-type calculational algorithm. For the HDR system, calculations were performed using dose points along the surface of the cylinders, with and without an apex point. Isotropic and anisotropic calculations were performed.

Results: A crossing Source at the apex of the LDR cylinder increases the apical dose and improves dose uniformity. The HDR dose distributions are a function of the diameter of the cylinder, points chosen for optimization and type of calculation. The apical dose is higher when an apex point is not used for optimization. Anisotropic calculations indicate a significantly lower dose at the apex than isotropic calculations.

Conclusion: The vaginal apex dose with HDR varies substantially depending on the diameter of the cylinder, optimization points chosen, and type of calculation and for LDR on the presence of a crossing source. Attention to these details is required as most recurrences are at the apex.