Dosimetric Systems in Pre-Treatment QA for Stereotactic Treatments: Correlation Agreements and Target Volume Dependency

Abstract

Aim: This study comprehensively investigated pre-treatment quality assurance (QA) for 100 cancer patients undergoing stereotactic treatments (SRS/SRT) using various detectors.

Methods: The study conducted QA for SRS/SRT treatments planned with a 6MV SRS beam at a dose rate of 1,000 MU/min, utilizing Eclipse v13.6 Treatment Planning System (TPS). Point dose measurements employed 0.01cm3 and 0.13cm3 cylindrical ionization chambers, while planar dose verification utilized Gafchromic EBT-XD Film and Portal Imager (aS1000). Plans were categorized by target volume, and a thorough analysis compared point dose agreements, planar dose gamma pass rates, and their correlations with chamber volume mean dose, detector type, and point dose agreement. Additionally, the consistency between different ionization chambers was assessed.

Results: Point dose agreement generally improved with increasing target volume, except for volumes over 10cm3 with 0.01cm3 chambers, showing a contrary trend. Significant differences (p<0.05) were observed between TPS and measured doses for both chambers. Gamma pass rate improved with increasing target volume in EBT XD and aS1000 analyses, except for the >10cm3 group in EBT XD. EBT XD demonstrated better agreement with TPS for target volumes up to 10cm3 compared to aS1000, with a statistically significant difference (p<0.05) between the detectors. Strong correlations were found between chamber point dose and chamber volume mean dose agreement, as well as between the two gamma criteria analyses of the same detector type in the planar dose correlation analysis. However, weak correlations were discovered for other analyses.

Conclusion: This study found weak correlation between different detector types in pre-treatment QA for point dose and planar dose evaluation. However, within a specific detector type, strong correlation was observed for different point dose evaluation methods and gamma criteria. This highlights the importance of cautious interpretation of QA results, particularly for SRS QA, due to the lack of correlation between detector types.

Keywords: Film; Ion chamber; Portal dosimetry; SRS PSQA; Small fields.

Figure 1

The Contoured Chamber Volume for Mean Dose Analysis: (a) 0.13 cm3 ionization chamber and (b) 0.01 cm³ ionization chamber

Figure 2

(a), Setup for film dosimetry; (b), Film Calibration curve from 0.5 Gy to 40 Gy

Figure 3

Correlation Analyses: (a), Point Dose % Dose Difference vs. Chamber Volume Mean % Dose Difference; (b), 0.13 cm³ Point & Volume Mean % Dose Difference vs. 0.01 cm³ Point & Volume Mean % Dose Difference; (c), Target Volume vs. 0.13 cm³ Chamber (Point & Chamber Volume Mean % Dose Difference); (d), Target Volume vs. 0.01 cm³ Chamber (Point & Chamber Volume Mean % Dose Difference)

Figure 4

Correlation Analyses: (a), 0.13 cm³ Absolute Point % Dose Difference vs. Gamma Pass % (2%/1mm); (b), 0.13 cm³ Absolute Volume Mean % Dose Difference vs. Gamma Pass % (2%/1mm); (c), 0.01 cm³ Absolute Point % Dose Difference vs. Gamma Pass % (2%/1mm); (d), 0.01 cm³ Absolute Volume Mean % Dose Difference vs. Gamma Pass % (2%/1mm)

Figure 5

Correlation Analyses: (a), 0.13 cm3 Absolute Point % Dose Difference vs. Gamma Pass % (3%/1mm); (b), 0.13 cm³ Absolute Volume Mean % Dose Difference vs. Gamma Pass % (3%/1mm); (c), 0.01 cm³ Absolute Point % Dose Difference vs. Gamma Pass % (3%/1mm); (d), 0.01 cm³ Absolute Volume Mean % Dose Difference vs. Gamma Pass % (3%/1mm)

Figure 6

Correlation Analyses: (a), Target Volume vs. Film Gamma Pass % (2%/1mm & 3%/1mm); (b), Target Volume vs. PD Gamma Pass % (2%/1mm & 3%/1mm); (c), Film Gamma Pass % (2%/1mm) & PD Gamma Pass % (3%/1mm) vs. Film Gamma Pass % (2%/1mm) & PD Gamma Pass % (3%/1mm); (d), Film & PD Gamma Pass % (2%/1mm) vs. Film & PD Gamma Pass % (3%/1mm)