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Removal of Motion in Liver SABR Facilitates Reduction in Liver Dose and Increase in Tumour Dose

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M Gargett

M Gargett1,2*, C Haddad1 , J Booth1,3 , A Kneebone1 , N Hardcastle1,2 , (1) Northern Sydney Cancer Centre, Royal North Shore Hospital, Sydney, Australia (2) CMRP, University of Wollongong, Wollongong, Australia (3) School of Physics, University of Sydney, Sydney, Australia


SU-I-GPD-T-517 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Liver Stereotactic Ablative Body Radiotherapy (SABR) is typically treated to an isotoxic prescription structure to limit probability of radiation induced liver and gastrointestinal toxicity. Liver tumours are subject to motion with respiration and the inclusion of motion in planning target volume (PTV) margins increases the treatment volume. A retrospective planning study was performed to determine the potential clinical gains of removal of respiratory motion from liver SABR treatments, which may be achieved with gating or tumour tracking.

Methods: Conventionally, motion is accounted for using an internal target volume (ITV) approach. For 20 liver SABR patients elimination of the motion component of the margin was modelled by contouring the gross tumour volume (GTV) on the exhale phase of the 4D-CT. A uniform 5mm GTV-PTV expansion was applied to account for setup uncertainty. All patients were replanned with the no-motion PTV and tumour dose was escalated to the highest prescription level where feasible given organ-at-risk constraints.

Results: Correlation between PTV size and prescribed dose exists; PTVs encompassing <10% of the liver could receive the highest prescription level. A monotonically increasing (Spearman’s rho 0.771, p=0.002) correlation between the degree of PTV size reduction and motion vector magnitude was observed for GTV sizes <100cm³. For 11/13 patients initially treated to a decreased prescription, tumour dose escalation was possible (5.4Gy₁₀ - 21.4Gy₁₀ BED) using the no-motion PTV. Dose escalation was not achievable in one case due to a large GTV (40% of liver volume) with small motion, and in the other case due to persisting PTV-chest wall overlap.

Conclusion: A reduction in PTV size by removal of tumour motion was shown to directly correlate to the ability to escalate dose to the PTV for liver SABR treatments in cases where dose to the surrounding liver volume was the prescription-limiting factor.

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