Difference in target definition using three different methods to include respiratory motion in radiotherapy of lung cancer

Introduction: Minimizing the planning target volume (PTV) while ensuring sufficient target coverage during the entire respiratory cycle is essential for free-breathing radiotherapy of lung cancer. Different methods are used to incorporate the respiratory motion into the PTV. Material and methods: Fifteen patients were analyzed. Respiration can be included in the target delineation process creating a respiratory GTV, denoted iGTV. Alternatively, the respiratory amplitude (A) can be measured based on the 4D-CT and A can be incorporated in the margin expansion. The GTV expanded by A yielded GTV+resp, which was compared to iGTV in terms of overlap. Three methods for PTV generation were compared. PTVdel (delineated iGTV expanded to CTV plus PTV margin), PTV sigma (GTV expanded to CTV and A was included as a random uncertainty in the CTV to PTV margin) and PTV Sigma (GTV expanded to CTV, succeeded by CTV linear expansion by A to CTV+resp, which was finally expanded to PTV Sigma). Results: Deformation of tumor and lymph nodes during respiration resulted in volume changes between the respiratory phases. The overlap between iGTV and GTV+resp showed that on average 7% of iGTV was outside the GTV+resp implying that GTV+resp did not capture the tumor during the full deformable respiration cycle. A comparison of the PTV volumes showed that PTV sigma was smallest and PTV sigma largest for all patients. PTV sigma was in mean 14% (31 cm3) smaller than PTVdel, while PTVdel was 7% (20 cm3) smaller than PTV sigma. Conclusions: PTV sigma yields the smallest volumes but does not ensure coverage of tumor during the full respiratory motion due to tumor deformation. Incorporating the respiratory motion in the delineation (PTVdel) takes into account the entire respiratory cycle including deformation, but at the cost, however, of larger treatment volumes. PTV sigma should not be used, since it incorporates the disadvantages of both PTVdel and PTV sigma.