Verification of dose profiles generated by the convolution algorithm of the gamma knife® radiosurgery planning system

Abstract

Purpose: A convolution algorithm that takes into account electron-density inhomogeneity was recently introduced to calculate dose distributions for the Gamma Knife (GK) Perfexion™ treatment planning program. The accuracies of the dose distributions computed using the convolution method were assessed using an anthropomorphic phantom and film dosimetry.

Methods: Absorbed-dose distributions inside a phantom (CIRS Radiosurgery Head Phantom, Model 605) were calculated using the convolution method of the GK treatment-planning software (Leksell Gamma Plan^® version 10.1; LGP) for various combinations of collimator size, location, direction of calculation plane, and number of shots. Computed tomography (CT) images of the phantom and a data set of CT number versus electron density were provided to the LGP. Calculated distributions were exported as digital-image communications in medicine-radiation therapy (DICOM-RT) files. Three types of radiochromic film (GafChromic^® MD-V2-55, MD-V3, and EBT2) were irradiated inside the phantom using GK Perfexion™. Scanned images of the measured films were processed following standard radiochromic film-handling procedures. For a two-dimensional quantitative evaluation, gamma index pass rates (GIPRs) and normalized agreement-test indices (NATIs) were obtained. Image handling and index calculations were performed using a commercial software package (DoseLab Pro version 6.80).

Results: The film-dose calibration data were well fitted with third-order polynomials (R² ≥ 0.9993). The mean GIPR and NATI of the 93 analyzed films were 99.3 ± 1.1% and 0.8 ± 1.3, respectively, using 3%/1.0 mm criteria. The calculated maximum doses were 4.3 ± 1.7% higher than the measured values for the 4 mm single shots and 1.8 ± 0.7% greater than those for the 8 mm single shots, whereas differences of only 0.3 ± 0.9% were observed for the 16 mm single shots. The accuracy of the calculated distribution was not statistically related to the collimator size, number of shots, or centrality of location (P > 0.05, independent-sample t-test). The plans in the axial planes exhibited poorer agreement with the measured distributions than the plans in the coronal or sagittal planes; however, their GIPR values (≥ 96.9%) were clinically acceptable. The plans for an arbitrary virtual target of volume 1.6 cm³ at an axial plane close to the top of the phantom showed the worst agreement and the greatest fluctuation (GIPR = 96.9 ± 1.2%, NATI = 3.9 ± 1.7).

Conclusions: The measured accuracies of the dose distributions calculated by the convolution algorithm of the LGP were within the clinically acceptable range (GIPR ≥ 96.9%) for various configurations of collimator size, location, direction of calculation plane, and number of shots. Due to the intrinsic asymmetry in the dose distribution along the z-axis, the treatment plan should also be verified in coronal or sagittal plane.

Keywords: convolution algorithm; gamma index pass rate; gamma knife perfexion; normalized agreement test index; radiosurgery.