Two years experience with quality assurance protocol for patient related Rapid Arc treatment plan verification using a two dimensional ionization chamber array

Abstract

Purpose: To verify the dose distribution and number of monitor units (MU) for dynamic treatment techniques like volumetric modulated single arc radiation therapy-Rapid Arc-each patient treatment plan has to be verified prior to the first treatment. The purpose of this study was to develop a patient related treatment plan verification protocol using a two dimensional ionization chamber array (MatriXX, IBA, Schwarzenbruck, Germany).

Method: Measurements were done to determine the dependence between response of 2D ionization chamber array, beam direction, and field size. Also the reproducibility of the measurements was checked. For the patient related verifications the original patient Rapid Arc treatment plan was projected on CT dataset of the MatriXX and the dose distribution was calculated. After irradiation of the Rapid Arc verification plans measured and calculated 2D dose distributions were compared using the gamma evaluation method implemented in the measuring software OmniPro (version 1.5, IBA, Schwarzenbruck, Germany).

Results: The dependence between response of 2D ionization chamber array, field size and beam direction has shown a passing rate of 99% for field sizes between 7 cm×7 cm and 24 cm×24 cm for measurements of single arc. For smaller and larger field sizes than 7 cm×7 cm and 24 cm×24 cm the passing rate was less than 99%. The reproducibility was within a passing rate of 99% and 100%. The accuracy of the whole process including the uncertainty of the measuring system, treatment planning system, linear accelerator and isocentric laser system in the treatment room was acceptable for treatment plan verification using gamma criteria of 3% and 3 mm, 2D global gamma index.

Conclusion: It was possible to verify the 2D dose distribution and MU of Rapid Arc treatment plans using the MatriXX. The use of the MatriXX for Rapid Arc treatment plan verification in clinical routine is reasonable. The passing rate should be 99% than the verification protocol is able to detect clinically significant errors.

Figure 1

a) Example of measured 2D dose distribution of a head and neck case. The MLC positions were changed in the region of the spinal cord to got higher dose to the spinal cord which could not be clinically tolerated. b) Passing rate against the maximum dose to the spinal cord of the same head and neck case. 4 Rapid Arc treatment plans were generated by changing the MLC positions. The changed Rapid Arc treatment plans were compared to the original Rapid Arc treatment plan. The original Rapid Arc treatment plan showed a passing rate of 99.3%. The lines indicate the in our clinic tolerated limits: 45 Gy maximum dose to the spinal cord and 99% passing rate.