Image-guided hypofractionated simultaneous integrated boost and adaptive radiotherapy for spine metastasis.

Background: (I) Dosimetric comparison between Helical tomotherapy (HT) and intensity-modulated radiation therapy (IMRT) with hypofractionated simultaneously integrated boost (HF-SIB) technique for spine metastasis. (II) Mega-voltage CT (MVCT) imaging can be used to monitor changes and calculate the true spinal cord dose over multiple fractions, and compare it with the planned dose by deformable image registration. Methods: We retrospectively analyzed 57 patients with spine metastasis receiving HT at our institution between March 2016 and March 2019. (I) The dose/fraction to planning gross tumor volume (PGTV) and planning target volume (PTV) were 40 or 50 Gy/10 fractions and 30 or 40 Gy/10 fractions. Plans were compared according to PTV coverage and OARs sparing. (II) The daily MVCT were strictly registered with plan CT according to the rigid structures. Contours of the target and critical organs were then deformed from plan CT to MVCT using MIM deformable registration algorithm. After rigid imaging registration, the planning dose matrix was mapped to the MVCT images. The total doses of 10 fractions to the deformed structures were compared with the planned doses. Results: Comparing HT and IMRT plan averaged over all patients, differences were observed for both homogeneity index (HI) (P<0.001) and conformity index (CI) (P=0.032). The maximum dose of the spinal cord was significantly lower in the HT group than the IMRT group (P=0.037). The actual dose of the spinal cord obtained based on the MVCT modified adaptive radiation therapy (ART) was slightly higher than the initially planned dose, while no significant difference was observed (P=0.083). Conclusions: HF-SIB with HT was able to achieve lower spinal cord doses, which was dosimetrically superior to IMRT, especially in those cases where strict control of the spinal cord dose is required. With deformable contours from the planning CT, daily MVCT along with rigid dose transformation can provide quantitative dose guidance for replanning.