Robustness of IMPT treatment plans with respect to inter-fractional set-up uncertainties: Impact of various beam arrangements for cranial targets

In the current study IMPT plan robustness was evaluated with respect to inter-fractional patient positioning for various beam arrangements and two tumor indications in the cranial region. Material and methods. For 14 patients suffering from tumors in the cranial region [skull base (SB; n = 7) and paranasal sinus (PS; n = 7)] the CTV and OARs were delineated. A safety margin of 3 mm was applied to the CTV. A prescribed dose of 2 GyE was planned via three beam arrangements (α, β, γ). Beam arrangement α consisted of lateral opposed fields for both tumor groups while beam arrangement β was optimized according to respective tumor and OAR locations, using two beams only. Beam arrangement γ applied four beams in the SB group and three beams in the PS group. Dose distributions were recalculated subjected to virtual patient translations along the major anatomical axes. The following dosimetric indices were evaluated and compared to original plans: target coverage (TC), target dose homogeneity (HI), CTV median and average dose (Dmedian, Dmean). For OARs near maximum dose and average dose (D2%, Dmean) were evaluated. Results. Dose distributions were distorted after introducing shifts. In the SB group, TC and HI were significantly different for caudal, cranial and anterior shifts for all beam arrangements. For PS patients, all but right shifts differed significantly from the original plans for all beam arrangements, although clinical relevance was not reached for arrangement γ (ΔTC < 1.5%). In general, beam arrangement γ exhibited the least spread of data regarding target indices and was consequently considered the most robust. Dosimetric parameters regarding the brainstem were mostly influenced by shifts along the anterio-posterior axis. Conclusion. For cranial IMPT, set-up uncertainties may lead to pronounced deterioration of dose distributions. According to our investigations, multi-beam arrangements were dosimetrically more robust and hence preferable over two beam arrangements.