(Sunday, 7/12/2020) [Eastern Time (GMT-4)]
Room: AAPM ePoster Library
Purpose: To comparatively evaluate plans based on Acuros and AAA
Methods: Eight patients with a lung tumor were selected, each with a 2-4 cm lung planning target volume (PTV) surrounded by lung parenchyma. During simulation, they were imaged using a slow scan protocol, followed by four-dimensional (4D) imaging limited to the disease sites. On each phase-specific image of each patient, gross tumor volume (GTV) was contoured. Summed over all phases, integrated GTV (ITV) was generated, and copied to the slow scan. A treatment plan was created using a dynamic conformal arc technique with AAA to prescribe 60 Gy to 95% of PTV (ITV+0.5cm) and to meet constraints: chest wall: V30Gy <30 mL; lung: V20 <10%. In order to determine the delivered dose to the PTV a 4D dose was produced by copying and running the plan to each phase-specific images, after overriding the density of PTV-GTV with that of GTV (to generate a realistic delivered dose accounting for potential tumor movement within the PTV margin); calculating with Acuros (for greater accuracy); summing the phase-specific plans through organ/dose registration. Proposing to use a more accurate algorithm, Acuros, each AAA-based plan was altered by changing AAA with Acuros and overriding the density in PTV-ITV. Considering colder dose provided by Acuros, the density override with the density of ITV was considered in PTV-ITV. The 4D dose was similarly produced.
Results: The Acuros-based plan and the AAA-based plan delivered the followings: D95%=64.2 and 65.2Gy, respectively, averaged over 8 patients; V60Gy=98% of PTV for both plans, greater than the prescribed 95%. The density override for the former required less MUs than the latter, thereby delivering smaller 4D dose.
Conclusion: The Acuros plan provided closer 4D dose to the planned dose. With breathing irregularity not considered, it is uncertain how sufficient the 4D dose will be.
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