SU-G-BRC-15: The Potential Clinical Significance of Dose Mapping Error for Intra- Fraction Dose Mapping for Lung Cancer Patients

Abstract

Purpose: To evaluate the dose‐mapping error (DME) inherent to conventional dose‐mapping algorithms as a function of dose‐matrix resolution. Methods: As DME has been reported to be greatest where dose‐gradients overlap tissue‐density gradients, non‐clinical 66 Gy IMRT plans were generated for 11 lung patients with the target edge defined as the maximum 3D density gradient on the 0% (end of inhale) breathing phase. Post‐optimization, Beams were copied to 9 breathing phases. Monte Carlo dose computed (with 2*2*2 mm3 resolution) on all 10 breathing phases was deformably mapped to phase 0% using the Monte Carlo energy‐transfer method with congruent mass‐mapping (EMCM); an externally implemented tri‐linear interpolation method with voxel sub‐division; Pinnacle's internal (tri‐linear) method; and a post‐processing energy‐mass voxel‐warping method (dTransform). All methods used the same base displacement‐vector‐field (or it's pseudo‐inverse as appropriate) for the dose mapping. Mapping was also performed at 4*4*4 mm3 by merging adjacent dose voxels. Results: Using EMCM as the reference standard, no clinically significant (>1 Gy) DMEs were found for the mean lung dose (MLD), lung V20Gy, or esophagus dose‐volume indices, although MLD and V20Gy were statistically different (2*2*2 mm3). Pinnacle‐to‐EMCM target D98% DMEs of 4.4 and 1.2 Gy were observed (2*2*2 mm3). However dTransform, which like EMCM conserves integral dose, had DME >1 Gy for one case. The root mean square RMS of the DME for the tri‐linear‐to‐ EMCM methods was lower for the smaller voxel volume for the tumor 4D‐D98%, lung V20Gy, and cord D1%. Conclusion: When tissue gradients overlap with dose gradients, organs‐at‐risk DME was statistically significant but not clinically significant. Target‐D98%‐DME was deemed clinically significant for 2/11 patients (2*2*2 mm3). Since tri‐linear RMS‐DME between EMCM and tri‐linear was reduced at 2*2*2 mm3, use of this resolution is recommended for dose mapping. Interpolative dose methods are sufficiently accurate for the majority of cases. J.V. Siebers receives funding support from Varian Medical Systems.