Dosimetry for pretreatment quality assurance of volumetric modulated arc therapy

Abstract:

Purpose: This study compares the results of dosimetry for pre-treatment QA of VMAT plans by using two dosimetry systems: MatriXX and Portal Dosimetry.

Materials and Methods: Analyze 15 cases of head and neck and pelvis VMAT plans from Jan 2015 to Aug 2015 at Radiation Oncology Center - Vinmec International Hospital. These plans were created on treatment planning system Eclipse v13.0. After that, convert them to VMAT QA plans according to 2 ways: VMAT QA plan on MatriXX and Portal Dosimetry. Then, perform QA of these VMAT QA plans on the Clinac iX. Compare measured results on MatriXX and Portal Dosimetry with calculated results on the VMAT plan by using Gamma Index evaluation method. Acceptable standard for VMAT plan: pass rate ≥ 95% with ∆D/∆d = 3%/3mm. Sometimes, allow the pass rate of ≥ 90% for several complex plans.

Results: All of 15 cases VMAT plans are accepted to treat with the pass rate of ≥ 95% for both MatriXX and Portal Dosimetry systems. The pass rate of Portal Dosimetry (98 ÷ 99.9%) is independent on the complex of VMAT plans and usually more than pass rate of MatriXX (95 ÷ 97%).

Conclusion: This study demonstrates that both systems are effective in the accurate pretreatment QA of VMAT. MatriXX is able to compare directly measured dose with plan calculated dose. Portal Dosimetry only compares measured fluence with plan calculated fluence map.