Tradeoffs for assuming rigid target motion in Mlc-based real time target tracking radiotherapy: a dosimetric and radiobiological analysis.

We report on our assessment of two types of real time target tracking modalities for lung cancer radiotherapy namely (1) single phase propagation (SPP) where motion compensation assumes a rigid target and (2) multi-phase propagation (MPP) where motion compensation considers a deformable target. In a retrospective study involving 4DCT volumes from six (n=6) previously treated lung cancer patients, four-dimensional treatment plans representative of the delivery scenarios were generated per modality and the corresponding dose distributions were derived. The modalities were then evaluated (a) Dosimetrically for target coverage adequacy and normal tissue sparing by computing the mean GTV dose, relative conformity gradient index (CGI), mean lung dose (MLD) and lung V(2)0; (b) Radiobiologically by calculating the biological effective uniform dose (D) for the target and organs at risk (OAR) and the complication free tumor control probability (P(+)). As a reference for the comparative study, we included a 4D Static modality, which was a conventional approach to account for organ motion and involved the use of individualized motion margins. With reference to the 4D Static modality, the average percent decrease in lung V(20) and MLD were respectively (13.1-/+6.9) % and (11.4-/+ 5.6)% for the MPP modality, whereas for the SPP modality they were (9.4-/+6.2) % and (7.2-/+4.7) %. On the other hand, the CGI was observed to improve by 15.3-/+13.2 and 9.6-/+10.0 points for the MPP and SPP modalities, respectively while the mean GTV dose agreed to better than 3% difference across all the modalities. A similar trend was observed in the radiobiological analysis where the P(+) improved on average by (6.7-/+4.9) % and (4.1-/+3.6) % for the MPP and SPP modalities, respectively while the D computed for the OAR decreased on average by (6.2-/+3.6) % and (3.8-/+3.5) % for the MPP and SPP tracking modalities, respectively. The D calculated for the GTV for all the modalities was in agreement to better than 2% difference. In general, respiratory motion induces target displacement and deformation and therefore the complex MPP real time target tracking modality is the preferred. On the other hand, the SPP approach affords simplicity in implementation at the expense of failing to account for target deformation. Radiobiological and dosimetric analyses enabled us to investigate the consequences of failing to compensate for deformation and assess the impact if any on the clinical outcome. While it is not possible to draw any general conclusions on a small patient cohort, our study suggests that the two tracking modalities can lead to comparable clinical outcomes and as expected are advantageous when compared with the static conventional modality.