Volumetric discordance between ¹⁸F-FET-PET/MRI and MRI-based radiotherapy target volumes in high-grade glioma: a prospective pilot study (FETSMaRT).

In 30 seconds

This prospective pilot study enrolled 30 patients with WHO grade 3 or 4 high-grade glioma receiving postoperative radiotherapy. FET-PET/MRI was obtained before treatment, while the radiation oncologist was blinded to PET, allowing comparison of PET/MRI-derived and MRI-based target volumes. FET-PET/MRI volumes were significantly larger for gross tumor volume (43.6 vs. 38.6 cc; p < 0.0001), clinical target volume (119.2 vs. 110.8 cc; p = 0.0045), and planning target volume (171.1 vs. 159.6 cc; p = 0.0003). In the glioblastoma subgroup, volume discordance was not significantly associated with time to progression.

Key findings

• GTV discordance occurred in 17/30 patients (56.7%), and CTV discordance in 15/30 (50.0%).
• FET-PET/MRI-derived GTV was larger than MRI-based GTV (43.6 vs. 38.6 cc; p < 0.0001).
• FET-PET/MRI-derived CTV and PTV were also larger: 119.2 vs. 110.8 cc (p = 0.0045) and 171.1 vs. 159.6 cc (p = 0.0003), respectively.
• Greater GTV and CTV discordance showed a directional association with shorter median time to progression, but neither association was statistically significant.

Why it matters

Radiotherapy target delineation depends on identifying the extent of tumor to be treated. These findings show that metabolic imaging can identify target volumes larger than those defined by anatomical MRI, highlighting potential differences in treatment coverage. However, the study did not establish that incorporating PET improves progression outcomes, so the clinical significance of the additional volume remains uncertain.

What to keep in mind

This was a pilot study of 30 patients. The radiation oncologist was blinded to PET, so the study assessed volume discordance rather than outcomes after PET-guided planning; progression associations were not statistically significant.