ESTRO 2024 - Abstract Book

S3948

Physics - Image acquisition and processing

ESTRO 2024

Glioblastoma is notorious for its extensive tumor infiltration. As microscopic infiltration cannot be depicted on conventional MRI, the clinical target volume (CTV) is typically defined by the gross tumor volume (GTV) plus a 15-mm safety margin. 1 This commonly results in large target areas, inevitably encompassing considerable amounts of healthy tissue and increasing the risk of radiation-induced side-effects. 2 Amide proton transfer-weighted chemical exchange saturation transfer (APTw-CEST) MRI – an advanced imaging technique that reflects local levels of mobile proteins and peptides – has been associated with increased Ki-67 expression and cellular proliferation, and can aid in indirectly visualizing tumor infiltration. 3-5 This offers an opportunity to more accurately define the GTV and, potentially, re evaluate the general 15-mm CTV-margin. 6 In this study, the potential of integrating APTw-CEST MRI for more accurate definition of the GTV is explored.

Material/Methods:

In a prospective cohort study, patients diagnosed with glioblastoma and scheduled for treatment with radiotherapy underwent an extended planning MRI-scan. This extended MRI-protocol included the clinical brain tumor MRI protocol and APTw-CEST MRI. The APTw-image was generated according to the acquisition and post-processing methods described by Wu et al . 7 The construction of the GTV, CTV and radiotherapy treatment plan adhered to standard clinical guidelines, using the post-contrast T1-weighted, T2-weighted and T2-weighted-FLAIR MRI-scans. 1 For each patient, a GTV that included information derived from the APTw-image (GTV APT ), was constructed by expanding the conventional GTV to encompass areas that exhibited hyperintense APTw signal. After segmentation of the contralateral normal-appearing white matter (cNAWM), these areas were identified by establishing a patient-specific threshold (S APT,thr ) calculated through S APT,thr > µ APT,cNAWM + 2*σ APT,cNAWM . 8 Areas with hyperintense APTw signal within a 15-mm range of the GTV were delineated using this threshold and, after correction for anatomical barriers and a smoothing process, incorporated into the GTV to construct the GTV APT . A Wilcoxon signed-rank test was performed to compare the size of the GTV and CTV with the size of the GTV APT . Additionally, the dice similarity coefficient (DSC) between the GTV and the GTV APT was computed to examine the similarity between the two volumes.

Results:

Between June 2023 and October 2023, a total of nine patients with glioblastoma were included; one patient was excluded from the analysis as the patient did not proceed with treatment due to clinical deterioration. Seven out of the remaining eight patients were male and received a total dose of 60 Gy; the one female patient received a total dose of 40.05 Gy. The median threshold to define regions with hyperintense APTw signal was 0.085 (IQR 0.080-0.087). The absolute volumes of the GTV, GTV APT and CTV for each patient are shown in Fig. 1 .