Patient-specific point dose measurement for IMRT monitor unit verification

Purpose: To review intensity-modulated radiation therapy (IMRT) monitor unit verification in a phantom for 751 clinical cases. Methods and Materials: A custom water-filled phantom was used to measure the integral dose with an ion chamber for patient-specific quality assurance. The Corvus IMRT planning system was used for all cases reviewed. The 751 clinical cases were classified into 9 treatment sites: central nervous system (27 cases), gastrointestinal (24 cases), genitourinary (447 cases), gynecologic (18 cases), head and neck (200 cases), hematology (12 cases), pediatric (3 cases), sarcoma (8 cases), and thoracic (12 cases). Between December 1998 and January 2002, 1591 measurements were made for these 751 IMRT quality assurance plans. Results: The mean difference (MD) in percent between the measurements and the calculations was +0.37% (with the measurement being slightly higher). The standard deviation (SD) was 1.7%, and the range of error was from -4.5% to 9.5%. The MD and SD were +0.49% and 1.4% for MIMiC treatments delivered in 2-cm mode (261 cases) and -0.33% and 2.7% for those delivered in 1-cm mode (36 cases). Most treatments (420) were delivered using the step-and-shoot multileaf collimator with a 6-MV photon beam; the MD and SD were +0.31% and 1.8%, respectively. Among the 9 treatment sites, the prostate IMRT (in genitourinary site) was most consistent with the smallest SD (1.5%). There were 23 cases (3.1% of all cases) in which the measurement difference was greater than 3.5%; of those, 6 cases used the MIMiC in 1-cm mode, and 14 of the cases were from the head-and-neck treatment site. Conclusion: IMRT monitor unit calculations from the Corvus planning system agreed within 3.5% with the point-dose ion chamber measurement in 97% of 751 cases representing 9 different treatment sites. A good consistency was observed across sites.