Pretreatment patient-specific IMRT quality assurance: a correlation study between gamma index and patient clinical dose volume histogram.

PURPOSE: The aim of this work is to investigate the predictive power of a common conventional intensity modulated radiation therapy (IMRT) quality assurance (QA) performance metric, the gamma passing rate (%GP), through the analysis of the sensitivity and of the correlation between %GP and different dose discrepancies between planned dose-volume histogram (DVH) and perturbed DVH. The perturbed DVH is calculated by using a dedicated software, 3DVH (Sun Nuclear Corporation, Melbourne, FL), which is able to modify the dose distribution calculated by the treatment planning system (TPS) according to the dose discrepancies detected with planar measurements in order to predict the delivered 3D dose distribution in the patient.
METHODS: Twenty-seven high-risk prostate cancer (PP) patients and 15 head and neck (HN) cancer patients, treated with IMRT technique, were analyzed. Pretreatment verifications were performed for all patients' plans by acquiring planar dose distributions of each treatment field with 2D-diode array. Measured dose distributions were compared to the calculated ones using the gamma index (GI) method applying both global (Van Dyk) and local normalization, and %GP were generated for each pair of planar doses using the following acceptance criteria: 1%∕1, 2%∕2, and 3%∕3 mm. Planar dose distributions acquired during pretreatment verifications, together with patient's DICOM RT plan, RT structure set, and RT dose files from TPS were loaded into the 3DVH software. Percentage dose differences (%DE) between DVHs, obtained by TPS and by 3DVH, were calculated; statistical correlation between %DE and %GP was studied by using Pearson's correlation coefficient (r). This analysis was performed, for each patient, on planning target volumes and on some typical organs at risk of the prostatic and head and neck anatomical district. The sensitivity was calculated to correctly identify the pretreatment plans with high dose errors and to quantify the incidence of false negatives, on varying the gamma index method.
RESULTS: Analysis of %DE vs %GP showed that there were only weak correlations (Pearson's r-values < 0.8). The results also showed numerous instances of false negatives (cases where high IMRT QA passing rates did not imply good agreement in anatomy dose metrics) and the reverse, mainly for the 3%∕3 mm global gamma passing rate.
CONCLUSIONS: The lack of correlation between conventional IMRT QA performance metrics gamma passing rates and dose errors in DVHs values and the low sensitivity of 3%∕3 mm global gamma method show that the most common published acceptance criteria have disputable predictive power for per-patient IMRT QA.