3-D gamma knife distribution by normoxic gel dosimetry near tissue inhomogeneties

Abstract

The primary goal in this study was to investigate the three dimensional dose distributions, near the areas of tissue inhomogeneities, in Gamma Knife Radiosurgery with the normoxic gel dosimetry. Following irradiation, when scanned in MR and post processing the MR images, dose imparted to any particular point in the gel phantom can be calculated via the true T2 relaxation time at that point. In the neighborhood of air-tissue inhomogeneity in the head, electronic disequilibrium can lead to errors in dose calculated with the treatment planning algorithms that presume the head as a homogeneous media. Two experiments were designed to investigate the inhomogeneity effects in the Gamma Knife radiosurgery: one experiment simulating the volume near the auditory canal cavity and, the other simulating the volume near the paranosal sinuses cavity. In the auditory canal cavity experiment, an identical balloon of a diameter of 16 cm with two corks placed on each side to represent the air cavities constitutes the inhomogeneous phantom. In the paranosal sinuses cavity experiment, a cylindrical cork is placed to represent the maximal sinuses. In both experiments, the homo geneous phantom is a spherical glass balloon filled with normoxic polymer gel. For dose calibrations, 100 ml vials filled with the same gel are irradiated at predefined doses, and the R2-dose calibration curve is extracted. Dose distributions are the results of a single shot, by using all 201 Cobalt sources, delivered to a known point in the phantoms. In the aspect of dosimetrical quality control, the Gamma Knife planning system predicted dose distribution is compared with the experimental results. In the homogeneous phantoms, the gel dosimetry calculated dose distribution is in good agreement with the GammaPlan predicted dose distribution. However, in the case of inhomogeneous phantoms, the dose distribution is spatially different and significant differences in dose levels are observed. The dose decrease near the air-tissue interface causes the overshooting of the dose by the GammaPlan. This underdosing effect can be essential for the lesions near tissue inhomogeneities. In the auditory canal cavity experiment, the diameter of the 50% isodose curves differ by 35% in the X axis and 1% in the Y axis for Z=105 mm axial plane; and by 39% in the X axis and 5% in the Z axis for Y=105 mm coronal plane in the inhomogeneous phantom as compared to the homogeneous phantom. In the paranosal sinuses cavity experiment, the diameter of the 50% isodose curves differ by 42% in the X axis and 47 %in the Y axis for Z=71mm axial plane; 42% in the X axis and 60%in the Y axis for Z=75 mm axial plane; 52% in the X axis and 70% in the Y axis for Z=79 mm axial plane respectively in the inhomogeneous phantom when compared to the homogeneous phantom. The dose decrease near the airtissue interface causes the Gamma Plan’s predicted dosage to be higher than that actually delivered. The resulting underdosing effect can be critical for the control of the lesions near tissue inhomogeneities.