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Dosimetric Trade-Off Between Dose Calculation Precision and Speed for Head and Neck Plans with Large Superior and Inferior Tumor Extent

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M Sands

M Sands*, T Cui , E Murray , S Koyfman , N Joshi , N Woody , P Xia , The Cleveland Clinic Foundation, Cleveland, OH


SU-I-GPD-T-444 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Volumetric modulated arc therapy (VMAT) plans require longer dose calculation time than previous step and shoot techniques. This study analyzes the trade-off between the dose calculation precision and treatment planning speed when using finer dose grid resolution for highly modulated head and neck (HN) plans.

Methods: Forty-one plans for patients with HN cancer treated from 2014 to 2016 were restored and selected for this study. Plans selected were initially calculated with the adaptive convolution algorithm and a dose grid resolution of 0.4 cm. These plans were recalculated using the collapsed cone convolution algorithm and a dose grid resolution of 0.3 cm. Three methods were used to assess the effect: (1) prescription dose coverage to the planned tumor volume (PTV) while keeping the monitor units (MU) constant, (2) required adjustment of MUs to meet the tumor volume planning goals, (3) potential dose increase to organs at risk (OAR).

Results: Among the 41 plans, 5% of the recalculated plans did not meet the PTV prescription dose coverage goal of 95%. Due to the steep dose gradient present in the dose volume histogram, 98% of the prescription dose covered at least 98.8% of those PTV’s and required a maximum adjustment of +3.5 MU per fraction to meet planning goals. Two percent of recalculated plans resulted in a maximum dose increase greater than 2 Gy in the mandible and cochlea. An average dose increase of 1 Gy or higher was present in the lips and parotid for about 2% and 5% of the recalculated plans, respectively. These dose increases did not exceed OAR dose limits.

Conclusion: The resulting dosimetric changes for the recalculated plans were clinically insignificant. An adaptive convolution dose calculation algorithm in combination with 0.4 cm dose grid resolution is an appropriate dose calculation method for VMAT HN planning.

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