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Maximizing the Biological Effect Via Minimizing Radiation Protraction with Flattening Filter Free (FFF) Beams for Single-Isocenter/Multi-Lesions Lung SBRT

D Pokhrel1 , L Sanford2 , M Bernard3*, R McGarry4 , M Randall5 , J Molloy6 , (1) University of Kentucky, Lexington, KY, (2) University of Kentucky, Lexington, KY, (3) University of Kentucky, Lexington, KY, (4) ,Lexington, KY, (5) ,Lexington, KY, (6) Univ Kentucky, Lexington, KY


(Sunday, 7/14/2019)  

Room: ePoster Forums

Purpose: Prolonged SBRT treatment for multiple primary or metastasis lung lesions is undesirable due to decreased tumor cell kill, intrafraction motion error, and patient discomfort. Shortening SBRT treatment time with FFF-beams could improve outcomes. Maximizing tumor cell kill with FFF-beams when treating multiple lung lesions synchronously using single-isocenter VMAT SBRT was investigated.

Methods: Fourteen patients with multiple (2-5) peripherally located lung cancer lesions (total 32 tumors) were treated synchronously using single-isocenter non-coplanar VMAT (n-VMAT) SBRT plans. A single-isocenter was placed in between/among all lesions with an average distance to tumor of 5.6 cm. Mean tumor volume from 4D-CT scans was 15.9±15.3cc. Doses were 54Gy/50Gy in 3/5 fractions using 6X-FFF beam and Acuros-based algorithm. Utilizing identical planning parameters, dosimetrically equivalent plans were generated with flattened-6X and 10X-FFF beam. The radiobiological effectiveness of sub-lethal damage was calculated using linear-quadratic model with protraction factor, G, described by Lea and Catcheside: BED = nd [1.0 + Gd/(α/β)]. Here, n is fraction number, d=mean dose/fraction, α/β=10.0 Gy, G=(2/µT)[1.0-(1.0-exp(-µT)/µT))] and µ=ln 2/T1/2=18.0 min. For n-VMAT SBRT plan, fraction time was calculated as T=MU/Dr+Ti, Dr = constant dose-rate, and Ti = dose-independent term of fraction time. Influence of fraction time on radiobiological effectiveness of tumors and normal lung were quantified.

Results: Fraction time was reduced by 50% (6X-FFF) and 80% (10X-FFF), on average, compared to flattened-6X. Therefore, the corresponding BED10 was improved by 3% (up to 16%) with 6X-FFF and 7% (up to 17%) with 10X-FFF, on average, compared to flattened-6X. Additionally, fraction time was further reduced by 20% with 10X-FFF compared to 6X-FFF. Minimal increased in normal lung BED3 (<2 Gy) was obtained.

Conclusion: Single-isocenter n-VMAT plan with FFF-beam resulted in faster SBRT treatment of multiple lung lesions and enhanced radiobiological effectiveness. Additionally, FFF-beams can reduce intrafraction tumor motion and improve patient comfort without added toxicity.


Bioeffect Dose, Linear Quadratic Model, Stereotactic Radiosurgery


TH- Radiobiology(RBio)/Biology(Bio): RBio- LQ/TCP/NTCP/outcome modeling

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