Dosimetric Comparison Between Carbon-ion Uniform Scanning Mode Radiotherapy and Photon IMRT in the Malignant Skull Base Tumors

Comparative analysis of the dosimetric differences between Chinese-produced heavy ion accelerator uniform scanning and photon intensity-modulated radiotherapy(IMRT) treatment plans in the treatment of skull base malignancies. A retrospective analysis of the treatment plans for 8 patients with skull base malignancies treated with the domestic heavy ion equipment (Heavy Ion Medical Machine, HIMM) using uniform scanning, including 7 chordoma and 1 chondrosarcoma. Carbon ion radiotherapy(CIRT) plans were designed using the ciPlan planning system (V1.0) from Lanzhou Kejintaiji Company, with a prescription dose of 57.6~60.8 Gy (RBE) in the planning target volume, delivered in 16 fractions of 3.6~3.8 Gy (RBE) per fraction. The treatment used a multi-leaf collimator(MLC) to adjust beam conformity, a ridge filter(RF) to spread-out the Bragg peak, and a bolus to adjust beam conformity at the distal end. A fixed horizontal nozzle was used at a couch angle of 0°, and the couch rotated 90° or 180° to achieve two-field intersecting or opposing fields. The photon IMRT plan used the Eclipse planning system (V13.5) from Varian company to design a fixed 5-field IMRT plan, with the same prescription dose as the CIRT plan. All plans were evaluated for dosimetric parameters based on organ-at-risk(OAR) dose limits, including target coverage, conformity index(CI), homogeneity index(HI), and OAR dose. There was no statistically significant difference in PTV V95 between the two plans (P=0.106), but D_mean, CI, and HI were statistically different, with the IMRT plan superior to the CIRT plan (P values of 0.048, 0.031, and 0.024, respectively). Regarding OAR dose, most OAR doses in the CIRT plan were smaller than those in the IMRT plan, but there was no statistically significant difference. However, the maximum dose to the optic chiasm, left and right lenses, and the mean dose to the brainstem and right optic nerve differed significantly between the two plans (P values of 0.034, 0.000, 0.047, 0.008, and 0.030, respectively). The distance between OAR and target volume was described using the minimum Hausdorff distance(HD_min), and when HD_min was greater than 7.1 mm, the CIRT plan was significantly superior to the IMRT plan. The uniform scanning CIRT was inferior to the photon IMRT plan in terms of target uniformity and conformity, but superior to the IMRT plan in terms of OAR dose. Whether the dosimetric advantages can be translated into clinical benefits remains to be further validated by clinical studies.