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Minimum Monitor Unit Effects On Plan Quality for Multi-Field Optimized Spot Scanning Proton Therapy

M Howard

M Howard*, C Beltran , M Herman , Mayo Clinic, Rochester, MN


SU-E-T-552 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: To investigate the influence of the minimum monitor unit (MU) on the quality of clinical treatment plans for scanned proton therapy.

Methods: Delivery system characteristics limit the minimum number of protons that can be delivered per spot, resulting in a min-MU limit. Plan quality can be impacted by the min-MU limit. Two sites were used to investigate the impact of min-MU on treatment plans: pediatric brain tumor at a depth of 5-10 cm; a head and neck tumor at a depth of 1-20 cm. Three field intensity modulated spot scanning proton plans were created for each site with the following parameter variations: min-MU limit range of 0.0000-0.0060; and spot spacing range of 0.5-2.0σ of the nominal spot size at isocenter in water (σ=4mm in this work). Comparisons were based on target homogeneity and normal tissue sparing.

Results: The increase of the min-MU with a fixed spot spacing decreases plan quality both in homogeneous target coverage and in the avoidance of critical structures. Both head and neck and pediatric brain plans show a 20% increase in relative dose for the hot spot in the CTV and 10% increase in key critical structures when comparing min-MU limits of 0.0000 and 0.0060 with a fixed spot spacing of 1σ. The DVHs of CTVs show min-MU limits of 0.0000 and 0.0010 produce similar plan quality and quality decreases as the min-MU limit increases beyond 0.0020. As spot spacing approaches 2σ, degradation in plan quality is observed when no min-MU limit is imposed.

Conclusion: Given a fixed spot spacing of ≤ 1σ of the spot size in water, plan quality decreases as min-MU increases greater than 0.0020. The effect of min-MU should be taken into consideration while planning spot scanning proton therapy treatments to realize its full potential.

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