Background:The DVH-based and Gamma Index patient-specific QA measures commonly applied in radiotherapy planning are unable to simultaneously deliver detailed locations and magnitudes of discrepancy between isodoses of planned and delivered dose distributions. By exploiting the receiver operating characteristic (ROC) statistical classification tool, compliance between a planned and delivered isodose may be locally evaluated, both for OAR and PTV, at any given isodose level. Thus, a patient-specific QA tool may be developed to supplement those presently available in clinical radiotherapy.Materials and Methods:A method to locally establish and report dose delivery errors in 3-D isodoses of planned (reference) and delivered (evaluated) dose distributions as a function of both the dose level and spatial location was developed. At any given isodose level, the total volume of dose delivery containing the reference and the evaluated isodoses is locally decomposed into four subregions: true positive – subregions within both reference and evaluated isodoses, true negative – outside of both of these isodoses, false positive – inside the evaluated isodose occurs but not the reference one, and false negatives – inside the reference isodose but not the evaluated one, as established over the total volume of dose delivery. From this decomposition a confusion matrix is derived and various indices calculated to quantify the discrepancies between the given planned and delivered isodose distributions.Results:Examples of clinical photon radiotherapy plans underwent analysis using the method developed. At some isodose levels, at anatomically significant locations, dose delivery errors were found which would not be highlighted either by dose volume histogram (DVH)-based QA metrics or by gamma analysis.Conclusions:The proposed method which generalizes the DVH-based QA method approach and is able to spatially locate delivery errors at selected isodose levels may supplement the presently applied gamma analysis and DVH-based QA measures in patient-specific radiotherapy planning.