Accuracy of relocation, evaluation of geometric uncertainties and clinical target volume (CTV) to planning target volume (PTV) margin in fractionated stereotactic radiotherapy for intracranial tumors using relocatable Gill-Thomas-Cosman (GTC) frame

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.

Figure 1

GTC frame with depth helmet fitted on a patient with dental occlusion and occipital pad. Radial measurements are taken with the help of the measuring probe at the depth helmet portal.

Figure 2

Computed radiographic film of the head phantom fitted with GTC frame and the angiographic localizer box. Small lead target is placed at the center Lateral (left) and AP (right) images are taken and mismatch of the target compared to the fiducials are found. The direction of x, y and z axes are also illustrated.

Figure 3

Distribution of random errors in anteroposterior, mediolateral and craniocaudal directions is normally distributed. As is seen from the figure, random error is more in anteroposterior axis.

Figure 4

Mean radial displacements calculated for 10 patients from the displacements in AP, ML and CC axes. The values are well encompassed in a 2 mm margin and mean value is 1.03 mm $\pm \hspace{0.17em}0.34\hspace{0.17em}\text{mm}$.