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.