SU‐E‐J‐180: Imaging‐Based ITV May Provide Insufficient Internal Margin for Lung SBRT Patients with Tumor Misalignments Between 3D and 4D Planning CTs

Abstract

Purpose: To evaluate the target positional differences between 3D/4D planning CTs (pCTs) and daily CBCTs, and assess ITV coverage of daily GTVs, for lung SBRT patients with misaligned tumor positions between the 3D and 4D pCTs. Methods: Simulation 3D and 4D pCTs, both under free breathing, were acquired on a Sensation Open (Siemens) with RPM (Varian). ITV_All is the union of the GTVs delineated on the 3D pCT (GTV_3D) and on all phases of the 4D pCT (ITV_4D). While for most patients GTV_3D is included in ITV_4D, for some patients GTV_3D shows a substantial misalignment in the axial plane from ITV_4D with spine‐based rigid registration. Under IRB approval, pCTs of 51 patients were retrospectively reviewed to assess the frequency of such misalignments. For one patient with daily kV CBCTs available, all planning and treatment CTs were rigidly registered. GTV_3D, ITV_4D, GTV_F1 through GTV_F5 (on daily CBCTs of Fractions 1–5) were delineated. ITV_All and ITV_5mm (5mm uniform expansion of GTV_3D) were generated. Volumes and centers‐of‐mass (COMs) of all structures were analyzed. The inclusion relations (percentage of Structure‐A included in Structure‐B) were studied between GTV_F1 to GTV_F5 (Structure‐As) and GTV_3D, ITV_4D, ITV_All, ITV_5mm (Structure‐Bs). Results: Three patients showed substantial target misalignments between the 3D and 4D pCTs. For the analyzed patient, daily GTV positions agreed better with GTV_3D than with ITV_4D, with a median (range) COM vector difference of 2.45mm (1.00–4.58) and 16.06mm (15.39–17.44), respectively. The average inclusion of daily GTVs was 99.47% and 69.53% in ITV_5mm and ITV_All, respectively; although ITV_5mm was ∼35% larger than ITV_All for the ∼2cc tumor (GTV_3D). Conclusion: Substantial lung tumor misalignments were observed between 3D and 4D pCTs for some patients. For one such patient, daily tumor positions agreed better with the 3D pCT. Using spinebased localization for SBRT, ITV_All may provide insufficient internal margin.