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.

Monitor unit checking in heterogeneous stereotactic body radiotherapy treatment planning.

Treatment of lung cancer using very-high-dose fractionation in small fields requires well-tested dose modeling, a method for density-averaging compound targets constructed from different parts of the breathing cycle, and monitor unit verification of the heterogeneity-corrected treatment plans. The quality and safety of each procedure are dependent on these factors. We have evaluated the dosimetry of our first 26 stereotactic body radiotherapy (SBRT) patients, including 260 treatment fields, planned with the Pinnacle treatment planning system. All targets were combined from full expiration and inspiration computed tomography scans and planned on the normal respiration scan with 6-MV photons. Combined GTVs (cGTVs) have been density-averaged in different ways for comparison of the effect on total monitor units. In addition, we have compared planned monitor units against hand calculations using 2 classic 1D correction methods: (1) effective attenuation and (2) ratio of Tissue-Maximum Ratios (TMRs) to determine the range of efficacy of simple verification methods over difficult-to-perform measurements. Different methods of density averaging for combined targets have been found to have minimal impact on total dose as evidenced by the range of total monitor units generated for each method. Nondensity-corrected treatment plans for the same fields were found to require about 8% more monitor units on average. Hand calculations, using the effective attenuation method were found to agree with Pinnacle calculations for nonproblematic fields to within ±10% for >95% of the fields tested. The ratio of TMRs method was found to be unacceptable. Reasonable choices for density-averaging of cGTVs using full inspiration/expiration scans should not strongly affect the planning dose. Verification of planned monitor units, as a check for problematic fields, can be done for 6-MV fields with simple 1D effective attenuation-corrected hand calculations.

Measurement and evaluation of inhomogeneity corrections and monitor unit verification for treatment planning.

Heterogeneous lung and bone phantoms have been constructed for the purpose of testing monitor unit calculations at or near interfaces for different planning systems. Data have been acquired for 2 linear accelerators: a Varian 2300cd (6 and 25 MV) and an Elekta Synergy (6, 10, and 18 MV). We have reviewed Pinnacle and the correction-based, pre-AAA version of Eclipse planning systems with the intent of exploring the limits of these systems with energy and field size. Data were acquired from 2 x 2 to 10 x 10 cm(2) field sizes over the available range of energies. Our measurements confirm that Pinnacle predicts doses mostly to within +/- 5%, even near lung-tissue interfaces over the full range of energies and field sizes tested. The Eclipse-modified Batho and equivalent TMR algorithms overpredicted doses by 10% or more in the lung and near the lung-tissue interfaces if the field size was less than 10 x 10 cm(2) when the energy was 18 MV or higher. At lower energies, the field size had to be at least 6 x 6 cm(2) for calculated doses to be within 10% of measurement. For bone-tissue interfaces, doses were generally underestimated by 5% to 10% or more by all calculation methods over the range of field sizes and energies reviewed. A second goal of this study was to review methods for hand-checking monitor units when heterogeneities are included. We evaluated the range of applicability of 2, one-dimensional (1D) inhomogeneity correction factors: the effective attenuation method and the TMR ratio method. The effective attenuation method for monitor unit checking was within +/- 5% to as small as 6 x 6 cm(2) fields for 6 to 10 MV, usable for 4 x 4 cm(2) fields (within 7%) for 6 MV and close to +/- 5% for 10 x 10 cm(2) fields in the 18- to 25-MV range. The TMR ratio method was not as good, being within about +/- 5% to 7% of measurements only for 6 x 6 to 10 x 10 cm(2) fields at 6 MV and 10 x 10 cm(2) fields at the higher energies. Both simple 1D correction methods performed almost as well as Pinnacle for the bone-soft tissue cases. We recommend that if direct measurement of dose for heterogeneous treatment plans is not practiced, then one of these simple cross checks be performed to assure patient safety.