Purpose: To investigate the appropriate timing of adaptive radiotherapy (ART) for high-grade glioma.
Methods: Ten patients with high-grade gliomas were selected and underwent CT/MRI (CT₁/MRI₁, CT₂/MRI₂, CT₃/MRI₃, and CT₄/MRI₄) scans before RT and during 10-, 20- and 30-fraction RT, and the corresponding RT plans (plan₁, plan₂, plan₃ and plan₄) were made. The dose of the initial plan (plan₁) was projected to CT2 and CT3 using the image registration technique to obtain the projection plans (plan_{1– 2} and plan_{1– 3}) and by superimposing the doses to obtain the ART plans (plan₁₀₊₂₀ and plan₂₀₊₁₀), respectively. The dosimetric differences in the target volume and organs at risk (OARs) were compared between the projection and adaptive plans. The tumor control probability (TCP) for the planning target volume (PTV) and normal tissue complication probability (NTCP) for the OARs were compared between the two adaptive plans.
Results: Compared with the projection plan, the D₂ to the PTV of ART decreased, the conformity index (CI) to the PTV increased, and the D₂/D_mean to the brainstem, optic chiasm and pituitary, as well as the V₂₀, V₃₀, V₄₀ and V₅₀ to the normal brain decreased. The D₂ to the pituitary and optic chiasm as well as the V₂₀, V₃₀, V₄₀ and V₅₀ to the normal brain in plan₁₀₊₂₀ were lower than those in plan₂₀₊₁₀, while the CI to the PTV was higher than that in plan₂₀₊₁₀. The TCP of the PTV in plan₁₀₊₂₀ was higher than that in plan₂₀₊₁₀.
Conclusion: ART can improve the precision of target volume irradiation and reduce the irradiation dose to the OARs in high-grade glioma. The time point after 10 fractions of RT is appropriate for ART.
Keywords: adaptive radiotherapy, high-grade glioma, dosimetry, biophysical model