Simulation study of comparative dosimetric analysis of coplanar horizontal- port scanned carbon- ion beam therapy in the head and neck

Objective: Carbon - ion radiotherapy (CIRT) has demonstrated success in treating radioresistant disease within the head and neck, owing to its unique physical and radiobiological properties. Construction cost remains prohibitive; a center offering only a horizontal port may bridge this difficulty, but removal of the vertical port may prohibit treatment of disease near critical organs - at - risk. Building a center only containing a horizontal treatment port has been proposed as one method for cost savings. Methods: 20 complex cases of head and neck cancer previously treated with conventional CIRT were retrospectively planned using horizontal- port - only treatment incorporating noncoplanar treatment angles to achieve greater degrees of freedom. These were dosimetrically compared with the previous plans. Results: Comparable D95 coverage of both planning target volume and gross tumor volume with ability to meet organ - at - risk constraints were feasible with horizontal- port - only treatment. Collectively differences were noted in PTV D95, brain stem Dmax, contralateral eye Dmax and V10 Gy (RBE); further qualitative differences were noted on a plan - by - plan basis dependent on disease location. Conclusion: Horizontal -port -only treatment employing noncoplanar angles was feasible for complicated head and neck disease typically treated with CIRT, though careful consideration is necessary on a plan - by - plan basis. Advances in knowledge: It is worth noting that noncoplanar approaches are not typically used with the current treatment gantry and may extend further the difference between horizontal port planning and a gantry -based gold -standard.