Dose Coverage Beyond the Gross Tumor Volume for Various Stereotactic Body Radiotherapy Planning Techniques Reporting Similar Control Rates for Stage I Non-Small-Cell Lung Cancer

Purpose: To investigate the dose falloff region for various stereotactic body radiotherapy (SBRT) planning techniques used in the treatment of Stage I non-small-cell lung cancer reporting similar control rates. Methods and Materials: The SBRT plans were constructed on five patient data sets using seven different planning regimens. These regimens varied in the number of beams, number of fractions, prescription target, and prescribed dose used. For each case all regimens were planned using a common gross tumor volume (GTV). To compare dose falloff for the various regimens, resulting physical dose grids were converted into normalized total dose (NTD) grids. Furthermore, to determine the potential coverage of microscopic extension of the various regimens minimal peripheral NTD (NTD-MP₁₀₀) were calculated and plotted as a function of incremental volume expansions of the GTV. Results: Average values for NTD-MP₁₀₀ varied over a range of 174 Gy at the GTV periphery, but this range fell to 10 Gy at a distance of 14 mm from the GTV. Of 35 plans, 23 resulted in potential microscopic extension coverage of 78% to 95%. Averages for five of seven regimens fell within the range of 80% to 85%. Results were negligibly affected when intrafraction motion effects were accounted for. Conclusions: Although average NTD-MP₁₀₀ varied dramatically at the GTV, periphery values became similar at a distance of 14 mm from the GTV. With the exception of two, potential coverage of microscopic extension was similar for all planning techniques, with averages falling within a 5% range.