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Robust Evaluation of Treatment Planning Techniques in Pencil Beam Scanning Proton Therapy for Cranio-Spinal Irradiation

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S Rana

S Rana*, T Romaguera , A Gutierrez , J Bennouna , Miami Cancer Institute, Miami, FL


SU-I-GPD-T-174 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: The purpose of this study is to compare the dosimetric quality of treatment planning techniques in pencil beam scanning proton therapy for cranio-spinal irradiation (CSI).

Methods: A CSI case with brain clinical target volume (CTV) and spinal CTV was selected for this retrospective study. Brain CTV and Spinal CTV were combined to generate combined CTV (CTVcombined), which was then used to generate two treatment techniques:(1)abutting target volume (ATV) and (2)junction target volume (JTV). For ATV technique, CTVcombined was divided into 3 sections:ATV-CTVupper,ATV-CTVmiddle, and ATV-CTVLower. For JTV technique, CTVcombined was divided into 5 sections:JTV-CTVupper,Upper-Junction(UJ),JTV-CTVmiddle,Lower-Junction(LJ), and JTV-CTVLower. A total of 3 posterior-anterior beams were used to cover the CTVcombined. For each technique, nominal treatment plans (ATV and JTV) were generated using robust optimization (3% range uncertainty and 3mm setup error) in RayStation (version 5.0.2). Treatment plans were optimized such that at least 99% of the CTVcombined received the prescription dose of 36 Gy(RBE). For each nominal plan, additional 8 plans were generated to evaluate the robustness by introducing isocenter shift (±3mm in x,y, and z), and density change(±3%). Plans were then evaluated for conformity number (CN) and homogeneity index (HI).

Results: The HI was better in nominal ATV plan (1.03) compared to nominal JTV plan (1.08), whereas the CN was better in nominal JTV plan (0.84) compared to nominal ATV plan (0.70). The average difference in HI between nominal and re-calculated plans (ATV and JTV) was less than 0.1%. The average reduction in CN was higher for JTV technique (1.6%) compared to ATV technique (0.1%).

Conclusion: The robustness for HI was comparable using both ATV and JTV techniques. The ATV technique produced more robustness for CN when compared to JTV technique. Treatment plans on additional CSI cases are currently being generated to confirm the preliminary findings of this study.

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