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On the Validity of Target Density Overrides for RapidArc Lung SBRT Treatment Planning

D Wiant

D Wiant*, C Yount, J Pursley, J Terrell, J Maurer, B Sintay, Cone Health Cancer Center, Greensboro, NC

TH-A-137-8 Thursday 8:00AM - 9:55AM Room: 137

Purpose: It is common practice to calculate lung SBRT plans on free breathing CT scans. Recently, literature has suggested using time average scans or ITV density overridden scans to more accurately calculate dose in lung. We evaluate each of these techniques.
Methods: A free breathing (FB) and 4D-CT were acquired for a Quasar Motion Phantom with lung insert and 3 cm spherical target (Modus Medical Devices Inc, Canada). Target motion was set to ±1cm parallel to CT table travel at 16 cycles per minute. An ITV was defined on the 4D-CT with a PTV defined as a 5 mm expansion. RapidArc plans giving 1000 cGy to 96-97% of the PTV in two arcs were created on four image sets: the time average, FB, FB with the ITV set to tissue density, and FB with the PTV set to tissue density. Plans were delivered on a Varian TrueBeam STx (Varian Medical Systems, Palo Alto, CA) and measured on EBT3 film placed in the phantom target. Reference dose distributions that accounted for film motion were created for each plan by shifting the dose in 2 mm steps over the range of motion and convolving these shifted doses. The film was analyzed in with a gamma metric of 1mm/1%.
Results: The passing gamma percentages were 77.9%, 83.6%, 89.1%, and 92.7% for the FB, time average, ITV, and PTV plans, respectively. Profiles along the direction of travel showed hot spots of 5-10% above the predicted dose just outside the target for the FB and average plans. The ITV plan showed similar hot spots ~2% above predicted dose. The PTV profile was in good agreement with predicted dose.
Conclusion: Though this study is a greatly simplified case, it shows that planning to a tissue density target in lung may produce the most accurate dose predictions.

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