Development of a Monte Carlo model for treatment planning dose verification of the Leksell Gamma Knife Perfexion radiosurgery system

Abstract

Detailed Monte Carlo (MC) modeling of the Leksell Gamma Knife (GK) Perfexion (PFX) collimator system is the only accurate ab initio approach appearing in the literature. As a different approach, in this work, we present a MC model based on film measurement. By adjusting the model parameters and fine‐tuning the derived fluence map for each individual source to match the manufacturer's ring output factors, we created a reasonable virtual source model for MC simulations to verify treatment planning dose for the GK PFX radiosurgery system. The MC simulation model was commissioned by simple single shots. Dose profiles and both ring and collimator output factors were compared with the treatment planning system (TPS). Good agreement was achieved for dose profiles especially for the region of plateau (<2%), while larger difference (<5%) came from the penumbra region. The maximum difference of the calculated output factor was within 0.7%. The model was further validated by a clinical test case. Good agreement was obtained. The DVHs for brainstem and the skull were almost identical and, for the target, the volume covered by the prescription (12.5 Gy to 50% isodose line) was 95.6% from MC calculation versus 100% from the TPS.PACS number(s): 87.55.dk

A Monte Carlo model and its commissioning for the Leksell Gamma Knife Perfexion radiosurgery system

“…We used the same apparatus for the individual source film measurements as in our previous work . A number of aspects including the postfilm analysis have been improved since then.…”

“…We used the same apparatus for the individual source film measurements as in our previous work. 19 A number of aspects including the postfilm analysis have been improved since then. Unlike in previous work that used all 24 sources in a sector for each collimator, we averaged the fluence map for each ring.…”

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

“…Thus, when we compared the measured images with the computed dose distributions, the choice of slice in the axial images of a computed distribution had more profound effects. Second, the dose distribution of GK Perfexion is steep and asymmetric along the z‐axis of a GK around the radiological center, whereas it is flat and symmetric along the other axes . Unlike along the x ‐axis or y ‐axis, where a small shift causes a slight difference, a small error in the z‐coordinate can cause a substantially larger error due to the higher dose gradient and asymmetry.…”

“…The relative dose distribution in the plane perpendicular to the beam path is obtained using predetermined Monte‐Carlo (MC) simulation results. The relative dose distributions of a GK have been verified in many reports using film dosimetry or independent MC simulations . However, these verifications only employed relative comparison methods in one dimension or qualitative verification with two‐dimensional lines instead of analytic quantitative verification, with the exception of Park et al.…”

“…The relative dose distributions of a GK have been verified in many reports using film dosimetry or independent MC simulations. [2][3][4][5][6][7] However, these verifications only employed relative comparison methods in one dimension or qualitative verification with two-dimensional lines instead of analytic quantitative verification, with the exception of Park et al's study analyzing TMR dose calculations using single-shot dose distributions at the center of a solid-water phantom. 4 Absolute measurement of the absorbed dose rate of a GK is not a simple task because of the small field dosimetry.…”