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Evaluation of the Anisotropic Analytical Algorithm (AAA) Heterogeneity Correction Dose Calculation in Flattened and Flattening-Filter-Free (FFF) Beams for High Energy X-Ray Beams Using the Radiological Physics Center (RPC) Lung Phantom

D Followill

R Repchak1, A Molineu2, R Popple3, S Kry4, R Howell5, D Followill6*, (1) MD Anderson Cancer Center, Houston, TX, (2) UT MD Anderson Cancer Center, HOUSTON, TX, (3) Univ Alabama Birmingham, Birmingham, AL, (4) MD Anderson Cancer Ctr., Houston, TX, (5) UT MD Anderson Cancer Center, Houston, TX, (6) UT MD Anderson Cancer Center, HOUSTON, TX

SU-E-T-447 Sunday 3:00:00 PM - 6:00:00 PM Room: Exhibit Hall

Purpose: Compare the accuracy of AAA heterogeneity corrected dose calculation algorithm for high energy x-ray beams (>10 MV) for flattened and FFF beams using RPC anthropomorphic thorax phantom.

Methods: Six static beam SBRT treatment plans were created using the Varian Eclipse treatment planning system (TPS) AAA v.8.9.08 heterogeneity correction algorithm. Two flattened beam plans (6 MV and 18 MV) and four other plans (6 MV, 6 MV FFF, 10 MV FFF and 15 MV) were delivered using a Clinac 21EX and TrueBeam STx, respectively. Prescription dose/coverage, 6 Gy to 95% PTV, and constraints were the same for all plans. The phantom contained radiochromic films in the 3 major planes and TLDs in the heart, spine, and tumor. Point doses and 2D dose distributions were exported from the Eclipse TPS and compared with the measured doses. The gamma index analysis evaluation criteria of ±5% dose to agreement and 3 mm distance to agreement was used.

Results: TLD to TPS tumor point dose ratios were 0.971±0.006(6MV) and 0.957±0.002(6MV), 0.995±0.005(15MV), 1.114±0.006(18MV), and 0.957±0.003(6MV FFF), 0.974±0.011(10MV FFF) for the six plans. Using ±5%/3mm gamma analysis criteria, the average passing rates for all three films were 96.3% and 95.5%, 97.4%, 66.1%, 93.7%, and 96.3% for the 6 MV, 6 MV, 15 MV, 18 MV, 6 MV FFF, and 10 MV FFF plans, respectively. Dose profiles were also evaluated.

Conclusions: The current RPC credentialing criteria are: RPC/Inst. tumor dose ratio of 0.97±0.05 and 85% of the pixels in each film plane must pass a ±5%/5mm gamma index analysis. These data demonstrate that the AAA heterogeneity correction dose calculation algorithm is accurate for photon energies in 6-15 MV range for flattened and FFF beams. Heterogeneity corrected dose calculations for photon energies >15 MV were not accurate.

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