Presenter(s)
Y. Zhao, D. Zhang, T. Qiu, J. Xie, M. Zhou, X. Xin, X. Wang, J. Li, B. Tang, Y. Huang, J. Lang, and L. C. Orlandini; Sichuan Cancer Hospital and Research Institute, University of Electronic Science and Technology of China, Chengdu, China
Purpose/Objective(s): Proton and CyberKnife stereotactic radiotherapy exhibit distinct dosimetric profiles. In patients with single brain metastases, we evaluated uninvolved brain exposure to determine whether proton therapy reduces normal brain dose compared with CyberKnife SRT.
Materials/Methods:
Fifteen patients with a single brain metastasis were retrospectively analyzed. Proton plans (TPPT) were generated on a Proteus Plus system delivering 30 Gy (RBE) in 5 fractions, following international SRT guidelines and OAR constraints, using robust optimization (2 mm setup and 3.5% range uncertainty). For each case, a corresponding CyberKnife SRT plan (TPCK) was generated using identical imaging datasets and prescription. Target dosimetry (GTV D99%, Dmean, and homogeneity index [HI = (D2% - D98%)/Dmean]) and OAR metrics were compared between modalities. OAR evaluation included brainstem and bilateral cochlear Dmax (0.035 cc), whole-brain V24–V2 Gy, and uninvolved brain (brain–GTV) V30–V12 Gy. Paired comparisons were performed using the Wilcoxon signed-rank test.Results: Median metastasis volume was 2.98 cm³ (range, 0.38–8.93). Target coverage was comparable between modalities, with no significant differences in GTV D99% (median 30.0 Gy [range 29.6–30.1] vs 30.0 Gy [30.0–33.5]), Dmean (31.9 Gy [31.1–36.0] vs 32.7 Gy [31.1–35.1]), or HI (0.09 [0.06–0.40] vs 0.12 [0.04–0.27]) for proton therapy and CyberKnife, respectively (all p > 0.05). Brainstem and cochlear constraints were met in all patients. Whole-brain high-dose volumes (V24 Gy and V18 Gy) did not differ significantly between modalities (both p = 0.06). In contrast, proton therapy significantly reduced low-dose whole-brain exposure, with marked decreases in V12–V2 Gy (all p < 0.05). Uninvolved brain (brain–GTV) volumes receiving 30–12 Gy were consistently lower with proton therapy, confirming a significant reduction in dose bath (Table 1).
Conclusion:
Both proton therapy and CyberKnife SRT achieved excellent target coverage while respecting organ-at-risk constraints for single brain metastases. Although high-dose whole-brain exposure was similar, proton therapy consistently reduced low-dose irradiation of normal brain tissue. These findings support consideration of proton therapy when prioritizing normal brain sparing in SRT. Table 1. Brain dose–volume comparison between hypofractionated proton therapy (TPPT) and CyberKnife (TPCK) for single brain metastases; data are reported as median (range)| TPPT Volume (cm3) Median (range) | TPCK Volume (cm3) Median (range) | p | ||
| Whole brain | V24Gy | 6.3 (2.1-19.2) | 7.27 (2.4-21.6) | 0.06 |
| V18Gy | 9.5 (4.4-29.0) | 10.3 (3.8-31.1) | 0.06 | |
| V12Gy | 15.2 (6.3-46.6) | 16.06 (6.63-52.3) | < 0.01 | |
| V4Gy | 33.2 (13.09-186.0) | 103.0 (31.5-377.8) | < 0.01 | |
| V2Gy | 44.6 (16.9-233.7) | 227.1 (103.1-869.5) | < 0.01 | |
| Brain-GTV | V30Gy | 0.69 (0.1-3.56) | 1.79 (0.8-5.7) | < 0.01 |
| V24Gy | 3.8 (1.4-12.6) | 4.5 (1.9-15.7) | < 0.01 | |
| V18Gy | 7.4 (3.7-22.3) | 7.8 (3.4-25.2) | 0.01 | |
| V12Gy | 11.7 (6.8-36.7) | 13.3 (6.2-46.4) | < 0.01 |