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Verification of dose profiles generated by the convolution algorithm of the gamma knife ® radiosurgery planning system
Author(s) -
Chung HyunTai,
Park JeongHoon,
Chun Kook Jin
Publication year - 2017
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1002/mp.12347
Subject(s) - imaging phantom , radiosurgery , convolution (computer science) , dosimetry , dicom , collimator , radiation treatment planning , nuclear medicine , calibration , medical imaging , physics , algorithm , computer science , mathematics , radiation therapy , optics , artificial intelligence , medicine , statistics , radiology , artificial neural network
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 ( R 2 ≥ 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 cm 3 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.