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Comparative Analysis of Pencil Beam and Monte Carlo Based Algorithms for Thoracic Spine Stereotactic Body Radiation Therapy (SBRT)

R Badkul

R Badkul*, H Jiang , H Saleh , F Wang , A Chen , C Lominska , University of Kansas Medical Center, Kansas City, KS


SU-I-GPD-T-590 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Stereotactic body radiation therapy(SBRT) is increasingly used for management of spinal metastases.Pencil-beam algorithm performs suboptimal dose calculations in heterogeneous medium such as thorax. We evaluated the impact of Monte-Carlo(MC) based calculation algorithm for thoracic spine SBRT and compared with conventionally used Pencil-Beam(PB)algorithm.

Methods: Nine thoracic spine patients who were treated with SBRT using hybrid plans (2-3 conformal-dynamic arcs and 5-7 IMRT beams)with 6MV photon and HD-MLC on Novalis-Tx. Dose prescriptions varied from 20 to 30 Gy in 5 fractions encompassing 90% of target volume. Plans were done on Brainlab-Iplan treatment planning system using pencil-beam algorithm.Monte-carlo based calculations were retrospectively applied to all 9 plans while keeping all beam-parameters and monitor-units(MUs) constant.From Dose-volume-histograms(DVH) Various dose parameters for PTV and Organs-at-risk(OAR) volumes: lung, esophagus and spinal cord were analyzed and compared between the two algorithms.For PTV, minimum dose(PTVmin),maximum dose(PTVmax), mean dose(PTVmean),dose to 99% (PTV99),dose to 90%(PTV90), for lung V20, V10, V5 and dose to 1000cc volume(D1000cc), for esophagus maximum dose(Dmax)and 5cc(D5cc),for spinal cord Dmax, dose to 0.035cc(D0.03cc), dose to 0.35cc(D0.35cc) were analyzed.

Results: For Monte-Carlo plans,percent difference of average of PTVmin,PTVmax,PTVmean,PTV99 and PTV90 were 2.7±5.9%(range:-9.1%-8.8%),-0.3±2.6%(range:-6.7%-2.4%),-0.8±2.1%(range:-5.3%-1.5%),0.1 ± 3.1%(range:-7.8%-2.3%) and -0.8±2.1%(range:-5.3%-1.8%)respectively. For lung, V20 volumes were negligible due to lower prescription dose and were not compared.For Lung, MC plans percent difference of average V10, V5 and D1000cc were 3.7±21%(range:-21.1%-56%),-6.5±12%(range:-32.9%-7.2%) and 20.7±18%(range:-3.1%-48.6%).For esophagus,percent difference of average Dmax and D5cc were 2.6±3.9%(range:-0.4%-12.3%) and 4.5±4%(range:-2.1%-10%). For spinal cord,average Dmax, D0.03cc and D0.35cc were 7.1±4.4%(range:1.7±13.5%), 6.9 ± 4.3%(range:1.7%-12.2%)and 9.1±5.3% (range: 2.3%-18.1%)respectively.

Conclusion: Thoracic spine SBRT treated with hybrid-plans using pencil-beam calculation,PTVmin,PTV99% and PTV90% can diminish as much as by 9%. Lung and esophagus doses can also vary upto 49% and 12% respectively.Spinal cord doses are underestimated using pencil-beam upto 14%,this is particularly crucial in re-irradiation scenario.Therefore,it is recommended to consider monte-carlo based calculations for thoracic spine SBRT.

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