Quantification of the role of lead foil in flattening filter free beam reference dosimetry.

PURPOSE: To quantify the potential error in outputs for flattening filter free (FFF) beams associated with use of a lead foil in beam quality determination per the addendum protocol for TG-51, we examined differences in measurements of the beam quality conversion factor kQ when using or not using lead foil. METHODS: Two FFF beams, a 6 MV FFF and a 10 MV FFF, were calibrated on eight Varian TrueBeams and two Elekta Versa HD linear accelerators (linacs) according to the TG-51 addendum protocol by using Farmer ionization chambers [TN 30013 (PTW) and SNC600c (Sun Nuclear)] with traceable absorbed dose-to-water calibrations. In determining kQ , the percentage depth-dose at 10 cm [PDD(10)] was measured with 10×10 cm2 field size at 100 cm source-to-surface distance (SSD). PDD(10) values were measured either with a 1 mm lead foil positioned in the path of the beam [%dd(10)Pb ] or with omission of a lead foil [%dd(10)]. The %dd(10)x values were then calculated and the kQ factors determined by using the empirical fit equation in the TG-51 addendum for the PTW 30013 chambers. A similar equation was used to calculate kQ for the SNC600c chamber, with the fitting parameters taken from a very recent Monte Carlo study. The differences in kQ factors were compared for with lead foil vs. without lead foil. RESULTS: Differences in %dd(10)x with lead foil and with omission of lead foil were 0.9 ± 0.2% for the 6 MV FFF beam and 0.6 ± 0.1% for the 10 MV FFF beam. Differences in kQ values with lead foil and with omission of lead foil were -0.1 ± 0.02% for the 6 MV FFF and -0.1 ± 0.01% for the 10 MV FFF beams. CONCLUSION: With evaluation of the lead foil role in determination of the kQ factor for FFF beams. Our results suggest that the omission of lead foil introduces approximately 0.1% of error for reference dosimetry of FFF beams on both TrueBeam and Versa platforms.

The impact of peak-kilovoltage settings on heterogeneity-corrected photon-beam treatment plans.

Differences were evaluated in external-beam treatment plan dose calculations that result from the use of different Hounsfield-unit to electron-density conversion curves with CT images acquired with various tube potentials. These differences were found to be clinically insignificant and it was concluded that the impact of CT tube potential on treatment planning is negligible.

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.