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Effects of Hinge Angle On Dose, Biological Dose and LET in Critical Organs in Proton Therapy

F Pirlepesov

F Pirlepesov*, M Gargone , V Moskvin , C Tinkle , J Farr , T Merchant , St. Jude Children's Research Hospital, Memphis, TN


SU-I-GPD-T-110 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Using a pediatric posterior fossa tumor scenario, quantify the effect of hinge angle on dose (D), linear energy transfer (LET), and biological dose (BD) in critical normal tissues at the distal edge of the spread out Bragg peak (SOBP) for proton treatment plan optimization.

Methods: Single-field optimized treatment plans were created using a commercial planning system (Varian Medical Systems, Inc., Palo Alto, CA; Eclipse v13.7.15) to meet clinical coverage goals and dose constraints. Brainstem and cochleae were delineated as avoidance structures. Dose and LET were computed with the Monte Carlo transport code TOPAS. Six hinge angles (30°-180°) were defined by couch rotation for 90° gantry angle. The clinical target volume was asymmetric and partially overlapped the brainstem contour and approximated the right cochlea.

Results: Brainstem D50% and BD50% averaged over all angles were, respectively, 5091.7 CcGE and 5608.2 CcGE for two beam plans and 5078.9 CcGE and 5624.7 CcGE for three beam plans. The brainstem BD50% was reduced for hinge angles 30°, 140°, and 160° and were, respectively, 5614.1 CcGE, 5591.7 CcGE and 5584.8 CcGE for three beam plans. The cochlear BD50% and D50% were lower for larger hinge angles and higher for smaller hinge angles when comparing three beam plans to two beam plans. Considering the 140° hinge angle plan used in patient treatment, D50% was reduced in three beam plan compared to the two beam plan by 0.2 CcGE, 388.4 CcGE and 188.0 CcGE, respectively, for brainstem and left and right cochlea.

Conclusion: BD increases in brainstem and cochleae at the distal edge of the SOBP due to high LET effects. Reflecting critical structure location relative to beam arrangement, the D50% and BD50% in the brainstem and cochleae changed nonlinearly with hinge angle. The dependence on hinge angle should be taken into consideration in proton therapy planning.

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