Proof-of-Concept Comparative Dosimetric Analysis of Coplaner Horizontal-Port Carbon-Ion Radiotherapy in the Head and Neck

Abstract

Carbon-ion radiotherapy (CIRT) may offer improved treatment of radioresistant disease in complex anatomy, such as in head and neck (H&N) cancer. Nonetheless, center cost remains prohibitive. A compact center implementing only horizontal ports has been theorized; coplaner irradiation may improve target coverage. As proof of concept, horizontal+coplaner (H+C) treatment was dosimetrically compared in variously located, complex H&N cases, to evaluate feasibility of this approach. 3 previously-treated patients with adenoid cystic carcinoma (ACC) (L. maxillary sinus, R. parotid, upper R. sinus), 1 with R. nasal angiosarcoma, and 1 with R. parotid adenocarcinoma, were selected. A H+C plan was developed on identical CIRT software per standard. Prescription dose was maintained (64-70.4 Gy (RBE)). Bed roll of +/-30 degrees and off-medial transcranial irradiation were employed to improve targeting and organ at risk (OAR) protection. OAR limits included a Dmax 40 Gy (RBE) with V40 20% for optic nerves and chiasm and Dmax 30 Gy (RBE) for brain stem. Individual dose-volume histograms (DVH) were qualitatively compared. PTV and GTV coverage varied <1% in all cases, with comparable DVHs. Stem dose was below Dmax 30 Gy (RBE) in all patients, with reduced H+C dose noted in the upper R. sinus and L maxillary sinus ACC patients (Dmax 6.8 vs 14.7 Gy (RBE) and 1.3 vs 17.8 Gy (RBE), respectively). H+C unilateral optic nerve dose was above the Dmax limit of 40 Gy (RBE) in the upper R sinus and L maxillary sinus ACC cases as well (40.4 vs 37.0 Gy (RBE) and 45.7 vs 46.2 Gy (RBE), respectively). In the latter case, left H+C optic nerve V40 dose exceeded the 20% limit, at 21.6% vs 3.6% standard. In the former, the upper sinus location increased bilateral eye irradiation: V10 rose from 57% and 0% to 100% each, with Dmax increased to 51.7 from 47 Gy (RBE) in the right, and 30.9 from 8 Gy (RBE) in the left. The sarcoma case experienced increased H+C chiasm Dmax to 28.7 from 1.8 Gy (RBE), below the 40 Gy (RBE) limit. H+C irradiation raised all V5, V10, and V20 brain dosages, with averages of 16.2 vs 2.2%, 10.9 vs 1.5%, and 2.8 vs 0.8%, respectively. Maximums of 25%, 20%, and 7% were noted in the right parotid ACC. Adequate CIRT dose delivery with protection of key OARs in complex H&N cancer appears feasible using only a horizontal port with coplaner approach. Dose increase could be seen in optic nerves and eyes when near or obstructing tumor from the port, highlighting disease location and anatomical limitations of treating solely within the horizontal plane. Increased brain dosage was noted in all cases. The impact of low-dose high-LET CIRT on neural tissue requires further exploration prior to deployment of a coplaner approach. Potentials such as sitting-up immobilization for anteroposterior delivery may mitigate the limitations seen here. A larger cohort analysis is in progress.