Setting up 4D-CT based image guided radiotherapy (IGRT) for locally advanced lung cancer: Is it safe to reduce PTV margin for dosimetric benefit?

4D-CT based IGRT is currently standard in many developed countries; for radical lung cancer radiotherapy. Here, we aim to assess the volumetric and dosimetric differences of 4D-CT based and Helical free breathing CT (FBCT) based radiotherapy planning for lung cancer patients. We would also evaluate the safety of reduced PTV margin for 4D-CT planning. 46 patients decided for radical radiotherapy was planned based on 4D-CT based contouring with 7 mm PTV margin (PTV_4D). FBCT based volume was also generated using PTV margin of 1 cm all around and 1.3 cm cranio-caudally (PTV_3D). Patients had two IMRT plans for PTV_3D and PTV_4D; using similar planning parameters. Patients were treated with plans generated on 4D-CT based volumes. The FBCT based plans were extrapolated onto the 4D-CT volume and vice versa. Two plans were compared for PTV, PTV coverage by 95% of prescribed dose and organ at risk (OAR) dose. Dice Similarity Coefficient (DIC) was calculated to examine the overlap between PTV_3D and PTV_4D. Set-up data of 890 fractions were also analysed. We calculated the systematic error (Σ) and random error (δ) in each of the three axes, i.e. x, y, z and PTV margin using van Herk formula. We also calculated the translational vector of each fraction of individual patients. PTV was significantly low with 4D-CT based planning (mean PTV 509 cc vs 739 cc); so as OAR doses (Mean heart dose 11.6 Gy vs 14.5 Gy, Mean lung dose 13.5 Gy vs 15.5 Gy, and Spinal cord max dose 35.7 Gy vs 37.5 Gy). Mean DIC of PTV_3D and PTV_4D was 0.8 (80%). Our calculated PTV margin was 0.7 cm, 1.1 cm and 0.5 cm in X, Y and Z axis. PTV margin calculated for translational vector was 0.4 cm. Percentage of shifts >5 mm was 8.3%, 31.4% and 1.1% in X, Y and Z axis. Percentage of shifts >7 mm was 3.8%, 17.7% and 0.2% in X, Y and Z axis. Translational vector shift >5 mm and >7 mm was 23.1% and 0.2% respectively. 4D-CT based radiotherapy planning for locally advanced lung cancer with reduced PTV margin of 7 mm, can significantly reduce the PTV, OAR doses. However, while using 7 mm PTV daily imaging should be recommended to ensure PTV coverage particularly Y-axis where shifts more than 7 mm can be higher. Further PTV margin reduction to 5 mm was not found to be safe in our set up.