Evaluation of Flattening Filter-free Lung/Tissue Heterogeneity Dose Calculations for 2 Commercial Planning Systems

Abstract

To evaluate the accuracy of two commercial treatment planning systems (TPS) to calculate dose distributions produced by a flattening filter free (FFF) beam incident on a heterogeneous lung/tissue phantom. Two TPS were evaluated using phantom geometries constructed from Solid Water and lung equivalent material (0.28g/cm3). The phantom consisted of 3cm Solid Water, 11cm lung equivalent material, and 4 cm of Solid Water, with a 2cm x 2cm cylinder of Solid Water located at 10cm depth along the central axis to represent a lung tumor. Planar dose measurements with film for a 10MV FFF beam were taken at four lung/tissue interfaces (3cm, 9cm, 11cm, and 14cm depth) and at mid-tumor (10cm depth) for two field sizes (MLC shaped 2x2cm2, and a jaw collimated 4x4cm2). A CT scan of the phantom was transferred to the TPSs for dose calculations. Planar dose distributions corresponding to the measurement planes were exported from the TPSs and compared to measurements in RIT113 software. Ionization chamber point dose measurements at mid-tumor depth along the central axis confirmed the film measurements. Penumbra width (90%-10%) and gamma analysis (3%, 3mm) were used to evaluate the accuracy of the calculated dose distributions which were normalized relative to the central axis for each depth. The effect of the dose grid was evaluated by calculating both TPSs at 3mm, 2mm, and 1mm resolution. The largest discrepancies existed in the 50%-10% penumbra region at mid-tumor depth and at the tissue/lung interface distal to the tumor, for the 2x2cm2 field, with the calculated dose for both planning systems exceeding the measured dose by more than the gamma criteria (Table). Other calculated planes had less than 3% of pixels exceeding the gamma criteria, with the TPS1 calculated dose marginally more accurate than TPS2 dose calculations. One notable exception was the Eclipse calculated dose exceeding measured dose by 7% along the central axis for the 2x2cm2 field at the plane distal to the lung tissue. The 1mm resolution was used for comparison to film measurements for best accuracy. Current clinical TPSs over-estimate the penumbral width for small FFF fields incident on tissue equivalent material located within lung, resulting in incorrect dose calculations to the neighboring healthy lung tissue.Poster Viewing Abstract 3741; TablePenumbra size and percent of pixels failing gamma criteria for mid and distal tumor planesMeasurement 2x2cm2 Field SizeTPS1 2x2cm2 Field SizeTPS2 2x2cm2 Field SizeLocationPenumbra 90%-10% (cm)Penumbra 90%-10% (cm)Gamma(% Fail)Penumbra 90%-10%(cm)Gamma(% Fail)Mid tumor0.811.124.881.24.37Distal to tumor0.881.1715.221.227.12 Open table in a new tab