Abstract
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.
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. PTV
(delineated iGTV expanded to CTV plus PTV margin), PTV
(GTV expanded to CTV and A was included as a random uncertainty in the CTV to PTV margin) and PTV
(GTV expanded to CTV, succeeded by CTV linear expansion by A to CTV + resp, which was finally expanded to PTV
).
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
was smallest and PTV
largest for all patients. PTV
was in mean 14% (31 cm
) smaller than PTV
, while PTV
was 7% (20 cm
) smaller than PTV
.
PTV
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 (PTV
) takes into account the entire respiratory cycle including deformation, but at the cost, however, of larger treatment volumes. PTV
should not be used, since it incorporates the disadvantages of both PTV
and PTV
.