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Use of a lucite beam spoiler for high energy breast irradiation
Proceedings of the 36th Annual ASTRO Meeting
327
1152 EVALUATION
OF A COMMERCIAL
SHIELDING BLOCK CUTTING SYSTEM
Salhani, D., Ash, K., Soubra. M., Laewen, A. Ottawa Regional Cancer Centre,Medical
Physics Dept., 190 Melrose Ave., Ottawa, Ontario, Canada, KlY 4K7
This report describes the evaluation and commissioning of the PAR Scientific Purpose/Objective: Automated Block Cutting System that is currently in clinical use. In this evaluation we were mainly concerned with the accuracy of the constructed blocks and the ease of use and convenience of the system, including simplicity of setup and daily operation, existence of documentation and ease of record keeping. Materials & Method:The block cutting system consists of an IBM compatible 386 personal computer with associated software, a light box/digitizer for input directly into the computer of block shapes from simulator films and a computer driven hot wire cutter (device ACD-4). Two Blocks of various sizes and shapes were constructed to test the accuracy of the system. independent methods were devised to compare the block shapes as drawn on the simulator film with the blocked areas of the radiation field on linacs and cobalt units.In both methods accuracy was evaluated by measuring the relative displacements between all radiation field block edges on a check film taken under the mounted cerrobend blocks and the corresponding edges the original simulator films. Results: Results disclose a maximum displacement error with respect to the original block drawings and electron cutouts of less than 1.7mm at 1OOcm SAD in all cases tested. This compared favourably with results involving a manual Styrofoam cutter previously used in our clinic, which in some circumstances showed displacement errors of up to 5mm at 100cm. The automated block cutter as implemtnted by us displays an improvement in accuracy over Conclusions: The ease of use of this the manual Styrofoam hot wire cutter previously employed at this clinic. system manifests itself in increased daily productivity of shielding blocks and/or electron cutouts. The main drawbacks of this system are the inefficient path tracing algorithm used to drive the cutter and the patient library facility which is unusable.
1153 USE OF A LUCITE BEAM SPOILER
FOR HIGH ENERGY BREAST IRRADIATION
Michaletz-Lorenz, Martha, Klein, Eric E. and Taylor, Marie E. Washington University School of Medicine, Mallinckrodt Institute of Radiology, St. Louis, MO 63110 Purpose/Objective:Treatmentplanningforbreastcancerposesa challengewhen bridge separations exceed 24 cm. Protocols require dose homogeneity to the entire breast parenchyma to within 10% while maintaining good cosmesis (skin sparing). Facilities with low energy beams (16 MV) may not be able to achieve protocol specifications due to large lateral or medial hot spots for patients with large bridge separations. Facilities with high energy beams available may treat these patients with some fractions using high energy (i.e., 18 MV). The build-up dose region for the high energy beams may lead to underdosing in the superficial regions of the treatment volume. Some facilities will simply bolus the skin (2.0cm) to pull the idosodes back to surface, losing skin sparing. Use of a beam spoiler will increase the contribution of medium energy scattered electrons and photons which will increase the superficial dosing. At the same time, skin sparing is maintained due to the displacement of the spoiler from the skin. Methods and Materials: We chose Lucite as the spoiler material for our 18 MV beam. Build-up measurements were performed with a parallel-plate ionization chamber to find the optimal spoiler thickness using a maintained skin to spoiler distance of 25 cm. Proper corrections were made to account for the finite chamber size. A breast phantom was designed allowing thermoluminescint dosimeter (TLD) measurements to be performed with and without the spoiler in place. The phantom allowed data accumulation at depths ranging from surface to 3.0 cm in 0.5 cm steps, at multiple locations. Dose verification was performed at the mid-bridge to confirm the attenuation of the spoiler. Results: We found an optimized spoiler thickness of 18 mm. The spoiler has increased the PDD from the depths of 0.5 cm to the d msx depth of 3.0 cm, while the relative skin dose was held to 50%. The build-up effect was less at further spoiler to skin distances (apex of breast), which is the region inherently receiving higher doses with the uncompensated 18 MV beam. The same is true in off-axis transverse planes. Our treatment schema has typically utilized 6 MV photons with customized 2D compensation (filters) for half or more of the treatments, and 18 MV photons (without compensation) and the spoiler for the remaining fractions.
ConcIuaIona:Thespoilerhasalloweduseof18MV photonsforbreasttreatmentbyyieldingadosebuild-upregionsimilarto6 MV photons,whilemaintainingmcellent skin sparing. The spoiler may be used in coqjunction with thin bolus (l.Ocm) if there is any concern for therapeutic dosing to the skin (scars, inflammatory disease, etc.). The treatment plans have maintained dose homogeneity for Bargepatients, without the consequence of skin reaction.
327
1152 EVALUATION
OF A COMMERCIAL
SHIELDING BLOCK CUTTING SYSTEM
Salhani, D., Ash, K., Soubra. M., Laewen, A. Ottawa Regional Cancer Centre,Medical
Physics Dept., 190 Melrose Ave., Ottawa, Ontario, Canada, KlY 4K7
This report describes the evaluation and commissioning of the PAR Scientific Purpose/Objective: Automated Block Cutting System that is currently in clinical use. In this evaluation we were mainly concerned with the accuracy of the constructed blocks and the ease of use and convenience of the system, including simplicity of setup and daily operation, existence of documentation and ease of record keeping. Materials & Method:The block cutting system consists of an IBM compatible 386 personal computer with associated software, a light box/digitizer for input directly into the computer of block shapes from simulator films and a computer driven hot wire cutter (device ACD-4). Two Blocks of various sizes and shapes were constructed to test the accuracy of the system. independent methods were devised to compare the block shapes as drawn on the simulator film with the blocked areas of the radiation field on linacs and cobalt units.In both methods accuracy was evaluated by measuring the relative displacements between all radiation field block edges on a check film taken under the mounted cerrobend blocks and the corresponding edges the original simulator films. Results: Results disclose a maximum displacement error with respect to the original block drawings and electron cutouts of less than 1.7mm at 1OOcm SAD in all cases tested. This compared favourably with results involving a manual Styrofoam cutter previously used in our clinic, which in some circumstances showed displacement errors of up to 5mm at 100cm. The automated block cutter as implemtnted by us displays an improvement in accuracy over Conclusions: The ease of use of this the manual Styrofoam hot wire cutter previously employed at this clinic. system manifests itself in increased daily productivity of shielding blocks and/or electron cutouts. The main drawbacks of this system are the inefficient path tracing algorithm used to drive the cutter and the patient library facility which is unusable.
1153 USE OF A LUCITE BEAM SPOILER
FOR HIGH ENERGY BREAST IRRADIATION
Michaletz-Lorenz, Martha, Klein, Eric E. and Taylor, Marie E. Washington University School of Medicine, Mallinckrodt Institute of Radiology, St. Louis, MO 63110 Purpose/Objective:Treatmentplanningforbreastcancerposesa challengewhen bridge separations exceed 24 cm. Protocols require dose homogeneity to the entire breast parenchyma to within 10% while maintaining good cosmesis (skin sparing). Facilities with low energy beams (16 MV) may not be able to achieve protocol specifications due to large lateral or medial hot spots for patients with large bridge separations. Facilities with high energy beams available may treat these patients with some fractions using high energy (i.e., 18 MV). The build-up dose region for the high energy beams may lead to underdosing in the superficial regions of the treatment volume. Some facilities will simply bolus the skin (2.0cm) to pull the idosodes back to surface, losing skin sparing. Use of a beam spoiler will increase the contribution of medium energy scattered electrons and photons which will increase the superficial dosing. At the same time, skin sparing is maintained due to the displacement of the spoiler from the skin. Methods and Materials: We chose Lucite as the spoiler material for our 18 MV beam. Build-up measurements were performed with a parallel-plate ionization chamber to find the optimal spoiler thickness using a maintained skin to spoiler distance of 25 cm. Proper corrections were made to account for the finite chamber size. A breast phantom was designed allowing thermoluminescint dosimeter (TLD) measurements to be performed with and without the spoiler in place. The phantom allowed data accumulation at depths ranging from surface to 3.0 cm in 0.5 cm steps, at multiple locations. Dose verification was performed at the mid-bridge to confirm the attenuation of the spoiler. Results: We found an optimized spoiler thickness of 18 mm. The spoiler has increased the PDD from the depths of 0.5 cm to the d msx depth of 3.0 cm, while the relative skin dose was held to 50%. The build-up effect was less at further spoiler to skin distances (apex of breast), which is the region inherently receiving higher doses with the uncompensated 18 MV beam. The same is true in off-axis transverse planes. Our treatment schema has typically utilized 6 MV photons with customized 2D compensation (filters) for half or more of the treatments, and 18 MV photons (without compensation) and the spoiler for the remaining fractions.
ConcIuaIona:Thespoilerhasalloweduseof18MV photonsforbreasttreatmentbyyieldingadosebuild-upregionsimilarto6 MV photons,whilemaintainingmcellent skin sparing. The spoiler may be used in coqjunction with thin bolus (l.Ocm) if there is any concern for therapeutic dosing to the skin (scars, inflammatory disease, etc.). The treatment plans have maintained dose homogeneity for Bargepatients, without the consequence of skin reaction.