Impact of varying the number of irradiation fields on dose distribution: A four-dimensional scanned carbon-ion lung radiotherapy

To shorten the treatment procedure and increase the treatment efficiency, we performed planning studies to evaluate the 4D dose (4DD) distributions of different number of irradiation fields with the fixed treatment ports of scanning carbon-ion beam delivery. We enrolled 10 lung cancer patients treated with respiratory-gated scanning carbon-ion therapy. 4-field, 3-field, and 2-field plans were generated with fixed treatment ports by rotating the beam angle with a 20-40-degree difference. For each field, the range-adapted internal target volume (raITV) was calculated. The prescription dose of 50 Gy (RBE)-weighted dose with 1 fraction was optimized on the reference phase (T50). The 4DD distributions were calculated for each patient, and dose metrics of CTV dose coverage and OARs sparing were evaluated. p < 0.05 was considered to be statistically significant by Wilcoxon signed-rank test. The sufficient CTV-D95 > 97.8% could be achieved for all the three types of treatment plan. The Dmax of skin was increased (= 19.1 & PLUSMN; 5.7 Gy (RBE)) for the 2-field plans as compared to 4-field plans (= 14.2 & PLUSMN; 6.6 Gy (RBE), p = 0.006), which is still acceptable for clinical use (<35 Gy (RBE)). And there were no differences (p > 0.076) of V20 of lung (= 15.5 & PLUSMN; 6.6%, 16.2 & PLUSMN; 7.0%, 15.6 & PLUSMN; 6.3%) and Dmax of spinal cord (= 3.0 & PLUSMN; 2.9, 3.4 & PLUSMN; 3.9, 3.3 & PLUSMN; 4.3 Gy (RBE)) among the three types of plans. Lung treatment plans under the fixed orthogonal beam ports with zero-degree couch pitch angle is feasible for clinical use to further increase the treatment efficiency and throughput.