Abstract
In treatment planning of Leksell Gamma Knife (LGK) radiosurgery, the skull geometry defined by generally dedicated scalar measurement has a crucial effect on dose calculation. The LGK Perfexion (PFX) unit is equipped with a cone‐shaped collimator divided into eight sectors, and its configuration is entirely different from previous model C. Beam delivery on the PFX is made by a combination of eight sectors, but it is also mechanically available from one sector with the remaining seven blocked. Hence the treatment time using one sector is more likely to be affected by discrepancies in the skull shape than that of all sectors. In addition, the latest version (Ver. 10.1.1) of the treatment planning system Leksell GammaPlan (LGP) includes a new function to directly generate head surface contouring from computed tomography (CT) images in conjunction with the Leksell skull frame. This paper evaluates change of treatment time induced by different skull models. A simple simulation using a uniform skull radius of 80 mm and anthropomorphic phantom was implemented in LGP to find the trend between dose and skull measuring error. To evaluate the clinical effect, we performed an interobserver comparison of ruler measurement for 41 patients, and compared instrumental and CT‐based contours for 23 patients. In the phantom simulation, treatment time errors were less than 2% when the difference was within 3 mm. In the clinical cases, the variability of treatment time induced by the differences in interobserver measurements was less than 0. 91%, on average. Additionally the difference between measured and CT‐based contours was good, with a difference of −0.16%±0.66% (mean ±1 standard deviation) on average and a maximum of 3.4%. Although the skull model created from CT images reduced the dosimetric uncertainty caused by different measurers, these results showed that even manual skull measurement could reproduce the skull shape close to that of a patient's head within an acceptable range.PACS number: 87.53.Ly
Highlights
29 Nakazawa et al.: Effect of skull contours on dose calculations surrounding normal brain.[1,2] In the planning of Leksell Gamma Knife (LGK), treatment time is calculated by composite of multiple beams emitted from the 60Co source and by exponential attenuation allowed for the transit distance within the skull shape.[3]
29 Nakazawa et al.: Effect of skull contours on dose calculations surrounding normal brain.[1,2] In the planning of LGK, treatment time is calculated by composite of multiple beams emitted from the 60Co source and by exponential attenuation allowed for the transit distance within the skull shape.[3]. Because the skull shape is usually provided by manual measurement based on limited measuring points using a skull measurement sphere and a dedicated ruler that measures the distance to the scalp, it differs from the actual patient contour (Fig. 1(a))
A computer tomography (CT)-based skull contour function has recently been implemented in Leksell GammaPlan (LGP) Version 10.1.1 for GK treatment planning
Summary
29 Nakazawa et al.: Effect of skull contours on dose calculations surrounding normal brain.[1,2] In the planning of LGK, treatment time (i.e., the beam-on time to deliver the prescribed dose to the target) is calculated by composite of multiple beams emitted from the 60Co source and by exponential attenuation allowed for the transit distance within the skull shape.[3]. The change of treatment time caused by different skull models was previously evaluated for LGK model C,(4) but this impact with the latest model LGK Perfexion (PFX) (Elekta, Stockholm, Sweden) has not been investigated. The treatment time using a single sector is more likely to be affected by discrepancies between the skull shape and actual head contour than all sectors. A computer tomography (CT)-based skull contour function has recently been implemented in Leksell GammaPlan (LGP) Version 10.1.1 for GK treatment planning. This function allows drawing along the outline of the patient’s head without effect of artifacts induced by the metallic stereotactic skull frame, posts, and fixation screws (Fig. 1(b)), and reduces differences in skull model caused by different measurers. The aims of the current work were to investigate the impact induced by expansion of the skull model on treatment time in phantom simulations, and in clinical cases, the change of treatment time by differences of skull delineation produced by manual scaling compared with CT-based scaling and by variations in intermeasurer scale reading in PFX treatment
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