Abstract
The present study is aimed at determination of accuracy of relocation of Gill‐Thomas‐Cosman frame during fractionated stereotactic radiotherapy. The study aims to quantitatively determine the magnitudes of error in anteroposterior, mediolateral and craniocaudal directions, and determine the margin between clinical target volume to planning target volume based on systematic and random errors. Daily relocation error was measured using depth helmet and measuring probe. Based on the measurements, translational displacements in anteroposterior (z), mediolateral (x), and craniocaudal (y) directions were calculated. Based on the displacements in x, y and z directions, systematic and random error were calculated and three‐dimensional radial displacement vector was determined. Systematic and random errors were used to derive CTV to PTV margin. The errors were within ± 2 mm in 99.2% cases in anteroposterior direction (AP), in 99.6% cases in mediolateral direction (ML), and in 97.6% cases in craniocaudal direction (CC). In AP, ML and CC directions, systematic errors were 0.56, 0.38, 0.42 mm and random errors were 1.86, 1.36 and 0.73 mm, respectively. Mean radial displacement was 1.03 mm ± 0.34. CTV to PTV margins calculated by ICRU formula were 1.86, 1.45 and 0.93 mm; by Stroom's formula they were 2.42, 1.74 and 1.35 mm; by van Herk's formula they were 2.7, 1.93 and 1.56 mm (AP, ML and CC directions). Depth helmet with measuring probe provides a clinically viable way for assessing the relocation accuracy of GTC frame. The errors were within ± 2 mm in all directions. Systematic and random errors were more along the anteroposterior axes. According to the ICRU formula, a margin of 2 mm around the tumor seems to be adequate.PACS number: 87.55.‐x
Highlights
IntroductionVarious immobilization devices like Gill–Thomas–Cosman relocatable stereotactic head frame (GTC frame), BrainLAB device and thermoplastic ray casts have been used for fractionated stereotactic radiotherapy
Das et al.: Errors and margin in GTC frame sessions including intra- and interobserver variation of readings, and linear accelerator, laser and treatment setup related inaccuracies.Various immobilization devices like Gill–Thomas–Cosman relocatable stereotactic head frame (GTC frame), BrainLAB device and thermoplastic ray casts have been used for fractionated stereotactic radiotherapy
The present study shows that the depth helmet with measuring probe is a clinically viable method for assessing the relocatable accuracy of GTC frame in fractionated stereotactic radiotherapy
Summary
Various immobilization devices like Gill–Thomas–Cosman relocatable stereotactic head frame (GTC frame), BrainLAB device and thermoplastic ray casts have been used for fractionated stereotactic radiotherapy. These are highly reproducible and allow keeping a narrow margin between the CTV and the PTV, minimizing the volume of normal tissue irradiated around the target.(1) In addition to these devices, various other immobilization methods have been reported in literature. The Heidelberg group developed a relocatable stereotactic radiotherapy system using a mask substantially more rigid than a conventional thermoplastic mask, together with a stereotactic frame and localizer.(2) Using this immobilization device along with a photogrammetric apparatus to detect displacements, they reported standard deviations of reproducibility in anteroposterior (0.9 mm), mediolateral (0.6 mm) and craniocaudal (1.3 mm) directions. In 92% of the patients in this study the displacement vector was less than 2 mm and only in 3% of patients was it more than 2.5 mm
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