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Physics Contribution
Dosimetric quantification of dose fall-off in liver SBRT planning using dual photon energy IMRT
J. Henry, C. Moreno, R.L. Crownover, D. Baacke, N. Papanikolaou and A.N. Gutierrez

Introduction: The purpose of this study was to dosimetrically compare 6 MV, 10 MV and a hybrid of 6 and 10 MV photon beam energies in liver stereotactic body radiotherapy (SBRT) patients using a fixed-field IMRT technique. The objectives of the study were to evaluate dosimetric differences in the target volume and investigate if dose fall-off could be improved with 10MV beam energy.

Methods and materials: Sixteen (n=16) liver SBRT patients previously treated using a non-coplanar, fixed-field IMRT technique with 6 MV were replanned using 10 MV and dual photon energy (DE). Plans were generated in Pinnacle3 using a Novalis Tx with HD120 MLC. For each patient, three plans with the same beam geometry were created using 6 MV, 10 MV and DE. For DE plans, the central axis effective depth from each beam was calculated and the values averaged. Beams with an effective depth greater than the average were assigned to 10 MV. All patients were optimized with the same planning objectives and normalized such that 98% of the target received 100% of prescription dose. Metrics used for comparison were the homogeneity index, conformity indices, and dose fall-off parameters at various isodose levels.

Results: The three techniques showed comparable PTV conformity and inhomogeneity for all patients—differences in the median values <0.6%. With regard to dose fall-off, no statistically significant differences were noted among the techniques for R80, R60 and R50; however, 10 MV showed statistical significance in the lowest median values of R40, R30, and R20. Ten MV and DE plans also demonstrated a statistically significant reduction in the total number of monitor units (MU) of 14.9% (p <0.01) and 12.0% (p <0.01) as compared to 6 MV, respectively.

Conclusion: Both dual energy and 10 MV photon beams had similar PTV dosimetric characteristics to 6 MV for liver SBRT but findings show faster dose fall-off for 10 MV and DE plans at the 40%, 30%, and 20% prescription isodose levels.

Keywords: SBRT, photon energy, liver cancer, IMRT

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