Is there any additional benefit of 68Ga-PSMA PET on radiotherapy target volume definition in patients with glioblastoma?

Abstract

Objective: To investigate the contribution of ⁶⁸Gallium (⁶⁸Ga)-PSMA (prostate-specific membrane antigen) positron emission tomography (PET) in defining radiotherapy (RT) target volume for glioblastoma and to compare the target volumes defined by Magnetic Resonance Imaging (MRI).

Methods: RT planning Computed Tomography (CT) images were fused separately with pre-operative MRI and PET/MRI images of 10 glioblastoma patients, retrospectively. The contrast-enhanced area in T₁ weighted MRI was contoured as gross tumor volume (GTV) and clinical target volume (CTV1) was obtained by including the cavity and T2/FLAIR hyperintense areas after giving a margin of 2 cm to the GTV. ⁶⁸Ga-PSMA uptake area was contoured as biological tumor volume (BTV) and CTV2 was obtained with a margin of 2 cm to BTV. Planning target volumes (PTVs) were created with the 3 mm added to the CTVs. Conformity index (CI), dice similarity coefficient (DSC) and overlap volume (OV) were calculated by obtaining the intersection and union volumes. Volumetric comparison, similarity and overlap analyzes were performed statistically by Wilcoxon signed rank and One sample t-test.

Results: The median GTV was 21,96 cc (1,04 - 82,04) and BTV was 25,58 cc (2,43 - 99,47). BTV was on average 47% larger than GTV which was statistically significant (p = 0.03). For GTV-BTV, CTV1-CTV2 and PTV1-PTV2; mean values of CI were 0,56, 0,76 and 0,76; DSC were 0,70, 0,86 and 0,86; OV were 0,88, 0,94 and 0,94, respectively. There was no significant difference on size and spatial similarity between CTV1 and CTV2, PTV1 and PTV2.

Conclusion: Altough BTV was larger than GTV, this significance was lost while we gave the same CTV margin including the peripheral edema. It seems that it may help to improve defining non-enhancing tumor part and also recurrent tumor volume.

Advances in knowledge: Recent studies have focused on the role of ⁶⁸Ga-PSMA PET in imaging of glial tumors. It has been observed that ⁶⁸Ga-PSMA PET can clearly define the tumor borders and it can be beneficial in target volume delineation, especially in reirradiation of recurrent tumors.

Figure 1.

Axial images of Case 3 (A, B). In simulation CT section; green line is GTV, orange line is BTV, pink line is CTV1, blue line is CTV2. CTV1 is larger than CTV2 due to inclusion of the cavity. (C) Contrast enhancements that did not give a clear mass image in contrast-enhanced T ₁ weighted MRI. (D) The cross-sectional images of mass in ⁶⁸Ga PSMA PET/MRI. BTV, biological tumor volume; CTV, clinical target volume; PET, positron emission tomography; PSMA, prostate-specific membrane antigen; PTV, planning target volume

Figure 2.

Axial images of Case 8 (A, B). In simulation CT section; green line is GTV, orange line is BTV, pink line is CTV1, blue line is CTV2. CTV1 is larger due to the inclusion of T2/FLAIR hyperintense areas. (C, D) Small increased uptake areas in ⁶⁸Ga-PSMA PET/MRI in different regions than in contrast-enhanced T ₁ weighted MRI. BTV, biological tumor volume; CTV, clinical target volume; FLAIR, fluid attenuated inversion recovery; PET, positron emission tomography; PSMA, prostate-specific membrane antigen; PTV, planning target volume

Figure 3.

Axial images of patient number 9 (A, B). In simulation CT section; green line is GTV, orange line is BTV, pink line is CTV1, blue line is CTV2. CTV1 is larger due to inclusion of the cavity (C, D). The cross-sectional images of mass in contrast-enhanced T ₁ weighted MRI and ⁶⁸Ga-PSMA PET/MRI, respectively. BTV, biological tumor volume; CTV, clinical target volume; PET, positron emission tomography; PSMA, prostate-specific membrane antigen; PTV, planning target volume.