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Dose Differences Between the Three Dose Calculation Algorithms in Leksell GammaPlan

Y Xu

Y Xu1*, J P Bhatnagar1, G Bednarz1, A Niranjan2, J Flickinger1,2, L D Lunsford1,2, M S Huq1, (1) Dept of Radiation Oncology, Univ of Pittsburgh Cancer Institute, Pittsburgh, PA, 15213 (2) Dept of Neurological Surgery, Univ of Pittsburgh Medical Center, Pittsburgh, PA, 15213

SU-E-T-578 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: To evaluate the dose differences introduced by the TMR10 and the convolution dose calculation algorithms in GammaPlan version 10.

Methods: A target with a prescription dose of 20Gy was defined on a human head CT image set and the treatment times for single collimator, single shot placement were calculated using the 3 dose calculation algorithms in GammaPlan. Three comparative studies were conducted: first, the matrix position is varied every 20mm in the X and the Y directions on the central slice (Z = 100mm) and the shot times were compared on each matrix for all collimators of a Perfexion unit. A total of 55 matrix positions were identified; second, the study was repeated for all the 4 collimators of a 4C unit; third, the comparison was made for the 8mm collimator of the Perfexion unit on the transverse slices with Z = 20,40,60,80,100,120,140,160. A total of 312 matrix positions were included.

Results: The treatment times from TMR10 and TMR classic agree within ±2.5% for all the treatment shots using all the 7 collimators from both machines. The time differences between the convolution and the TMR classic are similar on each matrix for all the 7 collimators but depend on the location of the treatment shots. We identified a maximum decrease in delivered dose of 11.5% for treatment in the superior frontal/parietal vertex region for older calculations lacking inhomogeneity correction to account for the greater percentage of the average beam path occupied by bone. The differences in the inferior temporal lobe and the cerebellum/neck regions are significantly less, owing to the counter-balancing effects of both bone and the air-cavity inhomogeneities.

Conclusion: Dose prescriptions based on the experiences with the TMR classic may need to be adjusted to accommodate the up to 11.5% difference between the convolution and the TMR classic.

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