Background The optimal treatment volume for Glioblastoma multiforme (GBM) is still a subject of debate worldwide. The current study was aimed to determine the distances between recurring tumors and the edge of primary lesions, and thereby provide evidence for accurate target area delineation. Methods Between October 2007 and March 2019, 68 recurrent patients with GBM were included in our study. We measured the distance from the initial tumor to the recurrent lesion of GBM patients by expanding

more »

... he initial gross tumor volume (GTV) to overlap the center of recurrent lesion, with the help of the Pinnacle Treatment Planning System. Results Recurrences were local in 47(69.1%) patients, distant in 12(17.7%) patients, and both in 9(13.2%) patients. Factors significantly influencing local recurrence were age (P = 0.049), sex (P = 0.049), and the size of peritumoral edema (P = 0.00). A total number of 91 recurrent tumors were analyzed. All local recurrences occurred within 2 cm and 94.8% (55/58) occurred within 1 cm of the original GTV based on T1 enhanced imaging. All local recurrences occurred within 1.5 cm and 98.3%(57/58) occurred within 0.5 cm of the original GTV based on T2-FLAIR imaging. 90.9% (30/33) and 81.8% (27/33) distant recurrences occurred >3 cm of T1 enhanced and T2-Flair primary tumor margins, respectively. Conclusions The 1 cm margin from T1 enhanced lesions and 0.5 cm margin from T2-Flair abnormal lesions could cover 94.8 and 98.3% local recurrences respectively, which deserves further prospective study as a limited but effective target area.