Investigation of the effect of respiratory motion on internal target volume in lung cancer

The aim of this study was to investigate the use of alternative target delineation strategies to reduce the workload of GTV contouring on 10 4D-CT phases in lung cancer SBRT. A cylindrical phantom containing tumor (1, 4 cm diameter) and moving with ±1.5 cm amplitude was placed in CIRS lung phantom. 4D-CT scans of the phantom were acquired. The gold standard ITV (internal target volume) was created by combining of GTVs contoured in 10 phases. Alternative ITVs were generated on 2-phase (0%, 50%), 4-phase (0%, 20%, 50%, 70%), maximum intensity projection (MIP)-BT and average intensity projection (AIP)-BT images. PTVs were created with a 5 mm margin. The conformity number (CN) of the alternative volumes was calculated. Dose distributions of SBRT plans measured by film were analyzed. ITVs and PTVs were generated on 4D-CT images of 15 lung cancer patients using alternative target delineation strategies used in the phantom. The CN of the alternative volumes were calculated. In phantom, CN of PTVs generated on 2-phase, 4-phase, MIP, AIP images of small and large tumors were 0.813, 0.971, 0.846, 0.817 and 0.936, 0.953, 0.946, 0.901, respectively. The passage rate for the 3%/3mm criterion, for the 4-phase plan of the small tumor and all plans of the large tumor were above 90%. In patients; CN of PTVs generated on 2-phase, 4-phase, MIP, AIP images of tumors with motion >1 cm and <1 cm were 0.842±0.054, 0.920±0.028, 0.798±0.076, 0.730±0.088 and 0.931±0.025, 0.959±0.011, 0.827±0.043, 0.816±0.059, respectively. 4-phase alternative volume may be useful in lung cancer SBRT.