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

Abstract

Background

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).

Aim

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

Materials and methods

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+3mm) 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 3mm 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 20Gy (V_20Gy).

Results

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)%).

Conclusions

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.