Abstract
Linacs equipped with flattening filter‐free (FFF) megavoltage photon beams are now commercially available. However, the commissioning of FFF beams poses challenges that are not shared with traditional flattened megavoltage X‐ray beams. The planning system must model a beam that is peaked in the center and has an energy spectrum that is softer than the flattened beam. Removing the flattening filter also increases the maximum possible dose rates from 600 MU/min up to 2400 MU/min in some cases; this increase in dose rate affects the recombination correction factor, Pion, used during absolute dose calibration with ionization chambers. We present the first‐reported experience of commissioning, verification, and clinical use of the collapsed cone convolution superposition (CCCS) dose calculation algorithm for commercially available flattening filter‐free beams. Our commissioning data are compared to previously reported measurements and Monte Carlo studies of FFF beams. Commissioning was verified by making point‐dose measurement of test plans, irradiating the RPC lung phantom, and performing patient‐specific QA. The average point‐dose difference between calculations and measurements of all test plans and all patient specific QA measurements is 0.80%, and the RPC phantom absolute dose differences for the two thermoluminescent dosimeters (TLDs) in the phantom planning target volume (PTV) were 1% and 2%, respectively. One hundred percent (100%) of points in the RPC phantom films passed the RPC gamma criteria of 5% and 5 mm. Our results show that the CCCS algorithm can accurately model FFF beams and calculate SBRT dose distributions using those beams.PACS number: 87.55.kh
Highlights
40 Foster et al.: Commissioning of flattening filter-free (FFF) beams data exist on modeling these beams with the collapsed cone convolution superposition (CCCS) algorithm as implemented in the widely used Pinnacle3 treatment planning system (Philips Radiation Oncology Systems, Fitchburg, WI)
We report on the model parameters determined for 6 and 10 MV flattening filter-free (FFF) beams for the CCCS algorithm and compare those parameters to the flattened beams
The difference in beam quality between the flattened and FFF beams is readily apparent from the plotted spectra; the FFF beams are shifted towards lower energy bins
Summary
40 Foster et al.: Commissioning of FFF beams data exist on modeling these beams with the collapsed cone convolution superposition (CCCS) algorithm as implemented in the widely used Pinnacle treatment planning system (Philips Radiation Oncology Systems, Fitchburg, WI). Huang et al[9] recently published results from an equivalent quality unflattened beam obtained by removing the flattening filter and tuning the electron energy in a Siemens Oncor linac (Siemens Medical Solutions, Concord, CA). They modeled the beam in Pinnacle and treated patients using this beam, which was matched in quality to a flattened 6 MV beam by tuning the electron energy until the %dd[10]x of the unflattened beam matched the flattened beam. By tuning the electron energy to achieve an equivalent quality unflattened beam as reported by Huang et al, many of the energy spectrum-related model parameters would be similar to those of a flattened 6 MV beam. We provide results of commissioning measurements and patient-specific QA results as validation of the accuracy of our model
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