VMAT couch attenuation model testing for prostate plans
Darryl Kaurin,1,2 Edward Marshall,1 M. M. Zaini,1,2 Larry Sweeney,1,2 Saikanth Mahendra1,2
1Northwest Medical Physics Center, Lynnwood, WA; 2Seattle Cancer Care Alliance, Seattle, WA
ASTRO Annual Scientific Meeting, Miami Beach, FL (2011).
Purpose/Objective(s): Our institution has two accelerators that are beam matched, one having a kevlar couch and the other carbon-fiber. We were concerned about couch attenuation for volumetric modulated arc therapy (VMAT), as well as being able to switch patients to either unit without replanning. The couches were characterized dosimetrically to determine if the couches needed to be included in the treatment plan, and if patients could be treated on either couch without additional planning.
Materials/Methods: Attenuation measurements of both couches were made by positioning an ion chamber with buildup cap on top of the couch and measuring transmission of a 5cmx5cm beam at ten different posterior gantry angles (PA and then every 10 degees up to 10 degrees below the horizontal) for 6MV and 10MV energies, relative to an unattenuated beam. Both couches were CT’d and imported into the treatment planning system (TPS).
Attenuation measurements were simulated in the TPS. The couches were contoured as shells of varying thickness and density, with air in the middle of the couch, as the recommended TPS air threshold density for the entire couch attenuated the beam too much. We had good measurement agreement for a kevlar couch model shell of 3-mmthickness, and the carbon-fiber model shell of 4-mm-thickness, both with 0.65g/cc density. Both couches were stored in the TPS organ model library.
To validate the couch models, five prostate patients (two 6MV, three 10MV cases) with test VMAT plans were copied onto a cylindrical phantom with a central 0.13cc ion chamber and also a rectangular phantom with a 2-demonsional diode array in a sagittal orientation. Doses and fluences were calculated for both phantoms, both couches and no couch (30 plans); and measured (20 measurements).
Results: Ion chamber results comparing the phantom plan with and without the couch gave a 1% improvement in measured-to-calculated values. Planar fluences were evaluated using absolute dose with 3% in 3mm gamma criteria, with comparisons between the phantom plan with and without the couch showing no improvement in measured-to-calculated values (agreement range/average: 93.7-100/98.51% with couch, 97.2-100/99.1% no couch). To further investigate the planar doses, calculated planar doses between the kevlar and carbon-fiber couches were compared to each other; with differences only noticeable using a criteria of 1% in 1mm gamma (agreement range/average: 92.1- 100/97.1%).
Conclusions: Prostate patient treatment plans may benefit about 1% with inclusion of the treatment couch. It will be fine clinically for patients to be treated using the couch they were not planned with for several fractions. These results may not be applicable for other disease-sites that have more of a posterior-beam component.