Dosimetric and radiobiological comparison for quality assurance of IMRT and VMAT plans

Introduction: The gamma analysis used for quality assurance of a complex radiotherapy plan examines the dosimetric equivalence between planned and measured dose distributions within some tolerance. This study explores whether the dosimetric difference is correlated with any radiobiological difference between delivered and planned dose. Methods: VMAT or IMRT plans optimized for 14 cancer patients were calculated and delivered to a QA device. Measured dose was compared against planned dose using 2-D gamma analysis. Dose volume histograms (for various patient structures) obtained by interpolating measured data were compared against the planned ones using a 3-D gamma analysis. Dose volume histograms were used in the Poisson model to calculate tumor control probability for the treatment targets and in the Sigmoid dose-response model to calculate normal tissue complication probability for the organs at risk. Results: Differences in measured and planned dosimetric data for the patient plans passing at 94.9% rate at 3%/3 mm criteria are not statistically significant. Average standard deviation tumor control probabilities based on measured and planned data are 65.8 +/- 4.0% and 67.8 +/- 4.1% for head and neck, and 71.9 +/- 2.7% and 73.3 +/- 3.1% for lung plans, respectively. The differences in tumor control probabilities obtained from measured and planned dose are statistically insignificant. However, the differences in normal tissue complication probabilities for larynx, lungs-GTV, heart, and cord are statistically significant for the patient plans meeting 94.9% passing criterion at 3%/3 mm. Conclusion: A 90% gamma passing criterion at 3%/3 mm cannot assure the radiobiological equivalence between planned and delivered dose. These results agree with the published literature demonstrating the inadequacy of the criterion for dosimetric QA and suggest for a tighter tolerance.