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Monitor Unit Correction of Homogeneous Treatment Planning in Lung SBRT Patients with Very-Low Lung Density

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M Petronek

M Petronek*, M Sands , T Zhuang , A Magnelli , P Qi , S Balik , The Cleveland Clinic Foundation, Cleveland, OH


SU-I-GPD-T-588 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Some SBRT RTOG protocols require homogeneous dose calculation regardless of anatomical variations exist among patients. One of these variations is the lung density which is assumed to have 0.3 g/cc density in general. This assumption can cause some tumors to be underdosed if patient had significantly lower average total lung density. We compared the effect of lung density variations for previously treated patients with much lower and higher than the normal lung density.

Methods: 24 patients previously received lung SBRT with 60 Gy in 3 fractions (12 with lung density below 0.25 g/cc and 12 above 0.46 g/cc) were selected out of a pool of 120 patients. The PTV was generated after 5 mm expansion of the ITV. VMAT plans were made for each patient where each met the following goals: PTV V60Gy = 95%, V54Gy = 99%, ITV V60Gy = 100% with a homogenous dose calculation and 2 half arcs. The monitor units were held constant and then dose was recalculated with a heterogeneity correction. Then the monitor units required to achieve the planning goals were evaluated.

Results: After heterogeneous dose calculation, the mean PTV V60Gy, V54Gy, and ITV V60Gy were 76.9% ± 17.2%, 95.5 ± 5.7% and 99.4 ± 1.1% for the low density patients; and 96.1% ± 3.0%, 99.9 ± 0.1%, and 100.0 ± 0.02% for the high density patients respectively. For the low density patients, the average additional MU needed to achieve planning goals were 322 ± 266 (9.4 ± 8.0% of original MU).

Conclusion: Planning goals were not met for patients with low lung densities when employing heterogeneous dose calculations, and in order for the goals to be met the prescribed MU would have to be adjusted to account for tissue heterogeneities.

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