The standard treatment for glioblastoma is surgery followed by radiation therapy (RT) and temozolomide (TMZ) chemotherapy. A total dose of 60 Gy given in 2 Gy fractions (fr) with concurrent and adjuvant TMZ has been recommended; however, local recurrences are frequent and the prognosis remains very poor. In this study, the equivalent dose in 2Gy fr (EQD2) at the recurrent site of glioblastoma was assessed to evaluate the biological effect of RT on glioblastoma considering that α/β ratios might vary from 1 to 10 Gy.
Recurrences were found in gross tumor volume (GTV) areas in all 11 patients, and 8 of them also showed recurrence in clinical target volume (CTV). Differences in EQD2 according to α/β ratios were relatively small in high-dose areas around 60 Gy; however, low-dose areas often showed significant differences of EQD2 according to the α/β ratios. In patients that received 60 Gy in 2 Gy fr, EQD2 was less than the original physical dose and became smaller as the α/β ratio became smaller. The comparison of the dose distribution of EQD2 and dose volume histogram (DVH) of EQD2 between α/β ratios 1 and 10 suggested that little difference was found in relatively high-dose areas but a significant difference was found in low-dose areas. In contrast, if the fraction size was larger than 2 Gy, EQD2 was greater than the original physical dose and it became larger as the α/β ratio became smaller. In conclusion, this study showed that the standard RT 60 Gy in 2 Gy fr is insufficient for glioblastoma, and it suggested that biological effects might differ significantly according to each fraction size of radiation and α/β ratio of the linear quadratic (LQ) model
