Geographic miss of lung tumours due to respiratory motion: a comparison of 3D vs 4D PET/CT defined target volumes

Abstract

BackgroundPET/CT scans acquired in the radiotherapy treatment position are typically performed without compensating for respiratory motion. The purpose of this study was to investigate geographic miss of lung tumours due to respiratory motion for target volumes defined on a standard 3D-PET/CT.Methods29 patients staged for pulmonary malignancy who completed both a 3D-PET/CT and 4D-PET/CT were included. A 3D-Gross Tumour Volume (GTV) was defined on the standard whole body PET/CT scan. Subsequently a 4D-GTV was defined on a 4D-PET/CT MIP. A 5 mm, 10 mm, 15 mm symmetrical and 15×10 mm asymmetrical Planning Target Volume (PTV) was created by expanding the 3D-GTV and 4D-GTV’s. A 3D conformal plan was generated and calculated to cover the 3D-PTV. The 3D plan was transferred to the 4D-PTV and analysed for geographic miss. Three types of miss were measured. Type 1: any part of the 4D-GTV outside the 3D-PTV. Type 2: any part of the 4D-PTV outside the 3D-PTV. Type 3: any part of the 4D-PTV receiving less than 95% of the prescribed dose. The lesion motion was measured to look at the association between lesion motion and geographic miss.ResultsWhen a standard 15 mm or asymmetrical PTV margin was used there were 1/29 (3%) Type 1 misses. This increased 7/29 (24%) for the 10 mm margin and 23/29 (79%) for a 5 mm margin. All patients for all margins had a Type 2 geographic miss. There was a Type 3 miss in 25 out of 29 cases in the 5, 10, and 15 mm PTV margin groups. The asymmetrical margin had one additional Type 3 miss. Pearson analysis showed a correlation (p < 0.01) between lesion motion and the severity of the different types of geographic miss.ConclusionWithout any form of motion suppression, the current standard of a 3D- PET/CT and 15 mm PTV margin employed for lung lesions has an increasing risk of significant geographic miss when tumour motion increases. Use of smaller asymmetric margins in the cranio-caudal direction does not comprise tumour coverage. Reducing PTV margins for volumes defined on 3D-PET/CT will greatly increase the chance and severity of a geometric miss due to respiratory motion. 4D-imaging reduces the risk of geographic miss across the population of tumour sizes and magnitude of motion investigated in the study.

Four‐dimensional computed tomography (4DCT): A review of the current status and applications

“…Similar benefits have been demonstrated in using SABR to treat liver tumours . Integrating 4DCT technology into PET planning for lung cancer has also been shown to reduce the geographic miss of lung tumours …”

Australia and New Zealand Faculty of Radiation Oncology Lung Interest Cooperative: 2015 consensus guidelines for the use of advanced technologies in the radiation therapy treatment of locally advanced non‐small cell lung cancer

“…If 4DCT is not available, other methods should be employed such as breath hold 3DCTs acquired at inhale and exhale for motion management purposes. The use of 3D PET alone for motion estimation has been shown to underestimate tumour motion and PTV margin reduction below 1.5 cm in this situation is discouraged . Unless the respiratory motion is otherwise managed, the use of a single fast 3D CT simulation scan is usually inappropriate for curative RT.…”

“…The use of 3D PET alone for motion estimation has been shown to underestimate tumour motion and PTV margin reduction below 1.5 cm in this situation is discouraged. 12 Unless the respiratory motion is otherwise managed, the use of a single fast 3D CT simulation scan is usually inappropriate for curative RT. The entire lung volume should be scanned at 2-3 mm slices for the purposes of calculating dose-volume histograms.…”

Respiratory motion correction in 4D-PET by simultaneous motion estimation and image reconstruction (SMEIR)

“…Respiratory motion in the upper abdomen and chest areas during PET imaging leads to organ displacement (primarily lung and liver) and a reduction in quantitative and qualitative image accuracy, affecting subsequent diagnosis and treatment (Callahan et al ., 2014). The spatial resolution of current PET scanners is approximately equal to 3–5 mm full width half-maximum (FWHM).…”