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  4. Potential Dose Escalation for Pancreas SBRT Patients as a Result of Simulated Tumor Tracking and Margin Reduction in a MR-Linac

Potential Dose Escalation for Pancreas SBRT Patients as a Result of Simulated Tumor Tracking and Margin Reduction in a MR-Linac

S Al-Ward1*, M Ruschin1,2 , A Kim1,2 , W Chu1,2 , S Myrehaug1,2 , A Sahgal1,2 , B Keller1,2 , (1) Sunnybrook Odette Cancer Centre, Toronto, Canada, (2) University of Toronto, Toronto, Canada,

Presentations

(Tuesday, 7/31/2018) 3:45 PM - 4:15 PM

Room: Exhibit Hall | Forum 7

Purpose: To investigate the potential for dose escalation to pancreas tumors through tumor tracking and margin reduction in a MR-linac.

Methods: Two SBRT treatment planning methods were compared using the Monaco TPS (Elekta, research version 5.19.03) with 4DCT data sets: 1) the conventional ITV method using a 6MV Elekta Agility beam with a 5mm PTV margin, and 2) a MLC tracking method with a 7MV Elekta MR-linac beam model containing a 1.5 T transverse magnetic field. For method 2, the GTV was tracked with either a 2mm (method 2a) or 0mm (method 2b) PTV margin. For both methods 2a and 2b, dose escalation was performed relative to method 1. The dose to 1cc of the duodenum (D1cc) was maintained to within 5%. Dose escalation was evaluated on 3 patients who clinically received 24Gy, 27Gy, and 27Gy in 3 fractions using the ITV approach. All plans were normalized such that the prescribed dose covered ≥98% of the PTV.

Results: For method 2a (2mm PTV), dose escalation was possible by an average of 2.3Gy±0.47. For method 2b (0mm PTV), dose escalation was possible by an average of 7.7 Gy±0.94. The average increase in BED (Biological Effective Dose) was 6.6Gy ± 1.0 (method 2a), and 25.3Gy ± 0.6 (method 2b). Eventhough the D1cc was maintained for the duodenum, the mean dose to the duodenum was decreased by 2.8Gy, 1.2Gy and 4.1Gy for patients 1, 2 and 3 respectively using method 2a. When method 2b was used, duodenal mean dose was decreased by 5.3Gy, 2.9Gy and 6.2Gy for the same patients.

Conclusion: This work demonstrates the potential benefit for a MR-linac tracking system to escalate dose in SBRT pancreas patients. Dose escalation was possible as a result of tumor tracking and margin reduction by up to 9Gy, with a BED improvement of 26.1Gy.

Funding Support, Disclosures, and Conflict of Interest: The senior author has received funding support from Elekta AB, Stockholm, Sweden

Keywords

Modeling

Taxonomy

TH- External beam- photons: Motion management (intrafraction)

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