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Can PTV Density Be Overridden for SBRT Lung Planning?

 Fu

Fu*, H Perera, S Rudoler, Y Yu, Thomas Jefferson University, Philadelphia, PA

SU-E-T-480 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: To demonstrate a PTV density overridden method that may reduce planned MU while still delivering sufficient dose to the tumor for SBRT lung planning.

Methods: The tumor motion inside the planning target volume (PTV) during treatment changes the density distribution inside the PTV. The worst case scenario occurs when CTV fills the PTV at the same moment, so the PTV density becomes the same as the CTV density. Our hypothesis is that the MU calculated with appropriate PTV density should be sufficient for delivering the prescription dose to the tumor. To find support for this, the plan MU with the PTV density assigned as 0.8g/cm3 were recalculated using Pinnacle planning software for twelve patients. Three of those patients had four CTV structures copied and manually placed at the border of the PTV. The new CTV density was overridden to 1g/cm3 and the density of the PTV, excluding new CTV, was overridden to 0.2 g/cm3. The plans were recalculated using the MU from the PTV density overridden plan. Each new plan with the CTV coverage and dose distribution was compared to the plan with the original density.

Results: MU was reduced by more than 10% for three patients whose CTV sizes were smaller than 1.2cc. For the remaining patients, MU was reduced by an average of 3%. Without considering density redistribution due to tumor relocation, the new CTVs of those three patients only had 58%, 74%, and 47% of their volumes receiving the prescription dose. However, the coverage of the CTVs increased to 95% in average if density redistribution was applied. The prescription isodose line changed with the CTV location.

Conclusion: This work finds support for a significant reduction in MU for very small CTV lung SBRT plan, while still delivering sufficient dose to the tumor.

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