Session Item

Local failure remains the major concern in glioblastoma (GBM) despite aggressive treatment. Pilot studies have shown that escalated  radiotherapy (RT) dose beyond 60 Gy improves outcome in GBM, although the conclusive data is lacking. Here we present our preliminary data of RT dose escalation using pentixafor PET scan. High ⁶⁸Ga-pentixafor uptake is seen in glioma patients expressing chemokine receptor-4 which helps in sharp demarcation between normal brain and glioma cells.

The prospective study was conducted over a period of 18 months from March 2018 to December 2019. Post-operatively, all GBM patients underwent MRI brain and pentixafor PET scan. A delayed contrast enhanced planning CT scan was co-registered with MRI and PET. RT was planned in 2 phases. Phase 1 GTV (GTV1) comprised of T2/flair abnormality, PET avid disease and post-op cavity. A margin of 2 cm was given to GTV1 for phase 1 CTV (CTV1), which was expanded for 0.5 cm to generate phase 1 PTV (PTV1). RT dose of 46Gy/23# was prescribed to PTV1. Phase 2 GTV (GTV2) consisted of CT/MRI contrast enhancing lesion, PET avid disease and post op cavity. A margin of 2 cm was given to GTV2 to create phase 2 CTV (CTV2) which was expanded 0.5 cm to create phase 2 PTV (PTV2).  RT dose of 14 Gy/7# was prescribed to PTV2. PET avid disease was demarcated as GTV PET and a margin of 3mm was given to generate PTV PET. PTV PET received escalated RT dose 21 Gy/7# using simultaneous integrated boost in phase 2 (Total dose 67 Gy/30 #). All patients received RT by VMAT along with concurrent and adjuvant TMZ. Following RT, patients who were lost to follow up, were contacted telephonically. The data was prospectively maintained in Microsoft Excel sheet. SPSS v 23 was used for statistical analysis. 

Figure 1 is showing (A)   Pentixafor PET showing metabolically active disease; (B) Target volume delineation ; (C) Phase 1 dose colour wash; (D) Phase 2 dose colour wash including SIB

A total of 20 patients were analysed. Patient and treatment characteristics have been shown in table 1. Median follow up was 23 months (20.8 ± 7.97). Fourteen patients had documented recurrence and succumbed to the disease. A median overall survival (OS) of 23 months (95% CI, 17.15-28.8) was observed with estimated 1, 2 and 3 years OS of 80%, 43.7% and 0% respectively. A significant association of OS was seen with extent of surgery (0.04). A non-significant trend towards increased OS was seen in those with KPS >70, age <40 years and >6 cycles of TMZ. No patient developed symptomatic radiation necrosis. 

Although radiotherapy dose escalation remains challenging in GBM, latest technologies can help in targeting the metabolically active tumor with higher dose without increasing the risk of radiation necrosis. The index study shows slightly higher median OS as compared to the historical data indicating that radiotherapy dose escalation is feasible and well tolerable in GBM patients and should be further explored in the randomised trials.