![]() ![]() This allows optimal definition of the tumour with good reproducibility while sparing delivery of dose to normal organs. When defining target volumes for lung cancer the information obtained from the two imaging modalities provides complementary information about both tumour morphology (CT) and physiology (PET). The use of integrated PET/CT in establishing radiotherapy target volumes is becoming increasingly accepted as the optimal approach for many malignancies, in particular lung cancer. ![]() 4D-imaging reduces the risk of geographic miss across the population of tumour sizes and magnitude of motion investigated in the study. 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. Use of smaller asymmetric margins in the cranio-caudal direction does not comprise tumour coverage. Without 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. Pearson analysis showed a correlation (p < 0.01) between lesion motion and the severity of the different types of geographic miss. The asymmetrical margin had one additional Type 3 miss. There was a Type 3 miss in 25 out of 29 cases in the 5, 10, and 15 mm PTV margin groups. All patients for all margins had a Type 2 geographic miss. This increased 7/29 (24%) for the 10 mm margin and 23/29 (79%) for a 5 mm margin. When a standard 15 mm or asymmetrical PTV margin was used there were 1/29 (3%) Type 1 misses. The lesion motion was measured to look at the association between lesion motion and geographic miss. Type 3: any part of the 4D-PTV receiving less than 95% of the prescribed dose. ![]() Type 2: any part of the 4D-PTV outside the 3D-PTV. Type 1: any part of the 4D-GTV outside the 3D-PTV. The 3D plan was transferred to the 4D-PTV and analysed for geographic miss. A 3D conformal plan was generated and calculated to cover the 3D-PTV. 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. Subsequently a 4D-GTV was defined on a 4D-PET/CT MIP. A 3D-Gross Tumour Volume (GTV) was defined on the standard whole body PET/CT scan. MethodsĢ9 patients staged for pulmonary malignancy who completed both a 3D-PET/CT and 4D-PET/CT were included. 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. PET/CT scans acquired in the radiotherapy treatment position are typically performed without compensating for respiratory motion. ![]()
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