Purpose: The objective of this study is to investigate the variations in diffusion tensor imaging (DTI) parameters at different distances surrounding the operative cavity, with a specific focus on exploring the potential utility of DTI in accurately delineating radiotherapy clinical target volume for glioblastoma patients. Methods: A retrospective study was conducted on 41 patients with glioblastoma, in which apparent diffusion coefficient (ADC) and fractional anisotropy (FA) values were measured at various distances beyond the surgical cavity. Recurrent patients were prospectively followed up according to the RANO criteria, aiming to investigate discrepancies between ADC and FA values in recurrent regions compared to normal control tissues prior to recurrence. Results: The rADC and rFA ratio approach 1 at a distance of 3 cm beyond the cavity. At the edge of the operative cavity and 2 cm beyond, the subtotal resection (STR) group exhibited higher ADC and rADC values compared to the gross total resection (GTR) group (p < 0.05). Similarly, FA and rFA values in the STR group were lower than those in the total resection group both at 1 cm beyond and 2 cm beyond (p < 0.05). Conventional MRI did not reveal any abnormalities prior to marginal or distant recurrence; however, the ADC value within this region was higher than that of control normal tissues (p = 0.023). Conclusions: The margins of GBM tumour invasion are typically not isotropic and could be > 2 cm and sometimes up to 3 cm. We recommend appropriately larger expansion of the target volume for patients with subtotal tumour resection. The utilisation of DTI in delineating the boundary of GBM's radiotherapy clinical target volume represents a promising avenue that holds potential to enhance precision and accuracy.