Gamma Knife 3‐D dose distribution near the area of tissue inhomogeneities by normoxic gel dosimetry

Abstract

The accuracy of the Leksell GammaPlan, the dose planning system of the Gamma Knife Model-B, was evaluated near tissue inhomogeneities, using the gel dosimetry method. The lack of electronic equilibrium around the small-diameter gamma beams can cause dose calculation errors in the neighborhood of an air-tissue interface. An experiment was designed to investigate the effects of inhomogeneity near the paranosal sinuses cavities. The homogeneous phantom was a spherical glass balloon of 16 cm diameter, filled with MAGIC gel; i.e., the normoxic polymer gel. Two hollow PVC balls of 2 cm radius, filled with N2 gas, represented the air cavities inside the inhomogeneous phantom. For dose calibration purposes, 100 ml gel-containing vials were irradiated at predefined doses, and then scanned in a MR unit. Linearity was observed between the delivered dose and the reciprocal of the T2 relaxation time constant of the gel. Dose distributions are the results of a single shot of irradiation, obtained by collimating all 201 cobalt sources to a known target in the phantom. Both phantoms were irradiated at the same dose level at the same coordinates. Stereotactic frames and fiducial markers were attached to the phantoms prior to MR scanning. The dose distribution predicted by the Gamma Knife planning system was compared with that of the gel dosimetry. As expected, for the homogeneous phantom the isodose diameters measured by the gel dosimetry and the GammaPlan differed by 5% at most. However, with the inhomogeneous phantom, the dose maps in the axial, coronal and sagittal planes were spatially different. The diameters of the 50% isodose curves differed 43% in the X axis and 32% in the Y axis for the Z =90 mm axial plane; by 44% in the X axis and 24% in the Z axis for the Y=90 mm coronal plane; and by 32% in the Z axis and 42% in the Y axis for the X=92 mm sagittal plane. The lack of ability of the GammaPlan to predict the rapid dose fall off, due to the air cavities behind or near the lesion led to an overestimation of the dose that was actually delivered. Clinically, this can result in underdosing of lesions near tissue inhomogeneities in patients under treatment.

MAGAT gel and EBT2 film‐based dosimetry for evaluating source plugging‐based treatment plan in Gamma Knife stereotactic radiosurgery

“…This new feature overcomes the limitation of older LGP versions, which lack the ability to predict the rapid dose fall off due to air cavities behind or near lesion that leads to overestimation of dose. Such overestimation was demonstrated previously through gel measurement by calculating 3D dose distribution in inhomogeneous phantom (3) . Dose maps in the inhomogeneous phantom were found to be spatially different.…”

“…In a plug plan, the plugged beams are excluded from the overall dose calculations and hence the plug modifies the radiation distribution. The LGP system (LGP Version 8.0) does not take into account tissue heterogeneities, leading to larger dose uncertainty if the treatment volume is near an air–tissue interface or the skull surface 2 , 3 . Additionally, the external head contour is defined using 33 discrete points, leading to possible uncertainties for patients with irregular skull shapes.…”

“…MAGIC gel was used for 3D dose verification in radiosurgery previously by several authors 3 , 27 , 28 . MAGAT gel, which is a modified version of MAGIC gel, was not tested for radiosurgery verification.…”

“…A 9% methacrylic‐based gel was found to give linear doses response up to 40 Gy and is ideal for performing radiosurgery verification (31) . Several studies exist for Gamma Knife verification with polymer gel dosimetry 3 , 13 , 31 , 32 , 33 , 34 , 35 , 36 . Interestingly, 3D measurement based on gel dosimetry has not been used so far for plug planning verification which is the goal of this study.…”

Technical Note: Construction of heterogeneous head phantom for quality control in stereotactic radiosurgery

“…The treatment planning system of the Leksell Gamma Knife (LGP) assumes the human head as a homogeneous media (water equivalent), and tissue heterogeneities such as air and bone are not taken into account . However, some studies were performed with one or two instances of heterogeneity in the head phantom to measure the dose delivered to the target, and to calculate the difference between the treatment planning system and the measured dose . All the studies that consider heterogeneity in the head phantom assume the head as a sphere.…”