ESTRO 2024 - Abstract Book

S3300

Physics - Detectors, dose measurement and phantoms

ESTRO 2024

from RIT113 software. Displacements found with this software were compared to those of the standard End-to-End test performed during standard QA of the Cyberknife system.

SRS Mapcheck measurements were analyzed with SNC Patient software (Sun Nuclear, USA), this software calculates gamma index and the most suitable displacement between measured and planned dose distribution.

For each method, two different planes were measured: coronal and sagittal with SRS MapCheck, axial and sagittal with film. Displacements of the measured against planned dose distribution and γ2%/1mm were obtained for each plane, then 3-dimensional displacements were obtained by merging information from both planes. Statistical Process Control (SPC) was employed with both methods to obtain capability indices and control limits of all parameters evaluated. The aim was to study the ability of both methods to assure that treatment accuracy was within ±0.75mm in any direction and in absolute distance.

Results:

SRS Mapcheck

Film

LR

SI

AP

Total

LR

SI

AP

Total

Lower Limit Central Limit Upper Limit

-0.60

-0.58

-0.31

0

-0.59

-0.51

-0.34

0

Control Limits

0.09

0.02

0.15

0.33

0.09

0.01

0.15

0.33

0.77

0.62

0.60

0.76

0.77

0.55

0.64

0.74

Cp

1.09

1.24

1.63

0.81

1.11

1.41

1.53

0.84

Capability (Tolerance ±0.75mm)

Cpl

1.22

1.27

1.95

-

1.24

1.44

1.83

-

Cpu

0.96

1.21

1.3

0.90

0.97

1.37

1.23

0.95

Cpk

0.96

1.21

1.3

0.90

0.97

1.37

1.23

0.95

Mean value of γ2%/1mm passing rates for SRSMapcheck were 99.3% (SD=1.5%) and 97.7% (SD=5.0%) for coronal and sagittal plane respectively. In case of film, passing rates were 96.3% (SD=3.8%) and 95.8% (SD=6.6%) in axial and sagittal planes. Lower passing rates in film is explained by plans failing in areas of low isodose values (20%-30%). Film displacement values (averaging for both planes with each method) had a mean (SD) values of -0.1mm (0.2mm), 0.015mm(0.16mm) and 0.15mm (0.22mm) in Lateral, SI and AP directions respectively, while SRS Mapchek displacement values were -0.2mm (0.2mm), 0.07mm(0.2mm) and -0.3mm (0.15mm) in Lateral, SI and AP directions. Comparison between E2E geometrical error and that found with our in-house registration method showed differences of 0.0mm, -0.15mm and 0.12mm in Lateral, SI and AP directions, with an absolute difference of 0.2mm. Geometrical accuracy measurement with SRS Mapcheck is not reliable as it depends on the accurate localization of its central detector in the planning CT. Thus, SRS Mapcheck geometrical errors were corrected by the mean difference between both methods. With this data, SPC was employed to analyse both methods to obtain appropriate control limits and capability indices. Results are shown in Table 1.