Tracking, gating, free-breathing, which technique to use for lung stereotactic treatments? A dosimetric comparison

The management of breath-induced tumor motion is a major challenge for lung stereotactic body radiation therapy (SBRT). Three techniques are currently available for these treatments: tracking (T), gating (G) and free-breathing (FB).

To evaluate the dosimetric differences between these three treatment techniques for lung SBRT.

Pretreatment 4DCT data were acquired for 10 patients and sorted into 10 phases of a breathing cycle, such as 0% and 50% phases defined respectively as the inhalation and exhalation maximum. GTV_ph, PTV_ph (=GTV_ph + 3 mm) and the ipsilateral lung were contoured on each phase.For the tracking technique, 9 fixed fields were adjusted to each PTV_ph for the 10 phases. The gating technique was studied with 3 exhalation phases (40%, 50% and 60%). For the free-breathing technique, ITV_FB was created from a sum of all GTV_ph and a 3 mm margin was added to define a PTV_FB. Fields were adjusted to PTV_FB and dose distributions were calculated on the average intensity projection (AIP) CT. Then, the beam arrangement with the same monitor units was planned on each CT phase.The 3 modalities were evaluated using DVHs of each GTV_ph, the homogeneity index and the volume of the ipsilateral lung receiving 20 Gy (V _20Gy).

The FB system improved the target coverage by increasing D _mean (75.87_(T)-76.08_(G)-77.49_(FB)Gy). Target coverage was slightly more homogeneous, too (HI: 0.17_{(T and G)}-0.15_(FB)). But the lung was better protected with the tracking system (V _20Gy: 3.82_(T)-4.96_(G)-6.34_(FB)%).

Every technique provides plans with a good target coverage and lung protection. While irradiation with free-breathing increases doses to GTV, irradiation with the tracking technique spares better the lung but can dramatically increase the treatment complexity.