Purpose: Our department has a long-established comprehensive quality assurance (QA) planning clinic for patients undergoing radiation therapy (RT) for head and neck cancer. Our aim is to assess the impact of a real-time peer review QA process on the quantitative and qualitative radiation therapy plan changes in the era of intensity modulated RT (IMRT). Methods and Materials: Prospective data for 85 patients undergoing head and neck IMRT who presented at a biweekly QA clinic after simulation and contouring were collected. A standard data collection form was used to document alterations made during this process. The original pre-QA clinical target volumes (CTVs) approved by the treating-attending physicians were saved before QA and compared with post-QA consensus CTVs. Qualitative assessment was done according to predefined criteria. Dice similarity coefficients (DSC) and other volume overlap metrics were calculated for each CTV level and were used for quantitative comparison. Changes are categorized as major, minor, and trivial according to the degree of overlap. Patterns of failure were analyzed and correlated to plan changes. Results: All 85 patients were examined by at least 1 head and neck subspecialist radiation oncologist who was not the treating-attending physician; 80 (94%) were examined by ≥3 faculty members. New clinical findings on physical examination were found in 12 patients (14%) leading to major plan changes. Quantitative DSC analysis revealed significantly better agreement in CTV1 (0.94 ± 0.10) contours than in CTV2 (0.82 ± 0.25) and CTV3 (0.86 ± 0.2) contours (P=.0002 and P=.03, respectively; matched-pair Wilcoxon test). The experience of the treating-attending radiation oncologist significantly affected DSC values when all CTV levels were considered (P=.012; matched-pair Wilcoxon text). After a median follow-up time of 38 months, only 10 patients (12%) had local recurrence, regional recurrence, or both, mostly in central high-dose areas. Conclusions: Comprehensive peer review planning clinic is an essential component of IMRT QA that led to major changes in one-third of the study population. This process ensured safety related to target definition and led to favorable disease control profiles, with no identifiable recurrences attributable to geometric misses or delineation errors.