ESTRO 2024 - Abstract Book

S756

Clinical - CNS

ESTRO 2024

519

Poster Discussion

Inter-fractional glioblastoma dynamic changes observed during chemoradiotherapy with MR Linac

Alonso La Rosa 1 , Kathryn E. Mittauer 1,2 , Amy E. Rzepczynski 1 , Michael D. Chuong 1,2 , Sreenija Yarlagadda 1 , Tugce Kutuk 1 , Nema Bassiri-Gharb 1 , Nicole C. McAllister 1 , Matthew D. Hall 1,2 , Diane Alvarez 1 , Alonso N. Gutierrez 1,2 , Ranjini Tolakanahalli 1,2 , Minesh P. Mehta 1,2 , Rupesh Kotecha 1,2,3 1 Miami Cancer Institute. Baptist Health South Florida, Radiation Oncology, Miami, USA. 2 Herbert Wertheim College of Medicine, Florida International University,, Radiation Oncology, Miami, USA. 3 Herbert Wertheim College of Medicine, Florida International University,, Translational Medicine, Miami, USA

Purpose/Objective:

Following maximal safe resection, chemoradiation (CRT) has an essential role in the management of glioblastoma (GBM). Current guidelines recommend a post-surgical or post-biopsy MRI for target volume delineation although longitudinal imaging to account for interfractional changes of the surgical cavities/tumors is not routine. Magnetic resonance (MR)-guided radiation therapy (MRgRT) facilitates the evaluation of target volumes and cavity/tumor migration, yet continuous interfraction changes to guide treatment decisions have not been described.

Material/Methods:

We evaluated 5 consecutive GBM patients treated with our institutional 0.35 T MR Linac BrainTx imaging workflow. The GTV_46Gy was defined as the T2/FLAIR volume, while the GTV_60Gy (boost) was the MR T1 Gadolinium enhanced cavity/tumor. CTVs were defined as expansions of each of the GTVs by 1.5 cm, respecting natural anatomic barriers to tumor spread. PTVs were defined as the CTVs plus uniform 3 mm expansions. During simulation and RT delivery, T1-post contrast and T2/FLAIR MR sequences were acquired at baseline, and on weekly fractions #1, 6, 11, 16, 21, and 26 (Figure 1). Also, creatinine and glomerular filtration rate were obtained weekly during CRT on the contrast injection days to verify renal function.

Results:

Thirty-five MR scans (enhanced T1 and T2/FLAIR) were evaluated. No renal function impairment was observed in any patient during RT. Most tumors were situated within the right hemisphere (n=4), and by brain lobe location: frontal (n=3), parietal (n=1), and temporo-occipital (n=1). The median times from surgery to RT and from simulation to RT were 32 days (R: 28-40) and 13 days (R: 12-14), respectively. The median number of beams and segments used were 36 (R: 22-37) and 129 (R: 114-141) for the first phase of RT (PTV_46Gy), and 35 (R: 22-37) and 121 (R: 107-128) for the sequential boost (PTV_60Gy), respectively. The median treatment beam-on and complete fraction times were 14 min (Interquartile range [IQR]: 12-16 min) and 34 min (IQR: 31-37 min), respectively. The median GTV_46Gy and GTV_60Gy volumes were 68.43 cc (R: 52.14-110.84) and 25.08 cc (R: 6.15-61.86), respectively; the median PTV_46Gy and PTV_60Gy volumes were 294 cc (R: 248.34-441.74) and 164.42 cc (R: 98.92-291.53), respectively. The overall median relative volume reductions for GTV_46Gy and GTV_60Gy for each individual MR, in comparison to baseline, were 40.0% (R: 8.3 to 86.5%) and 37.1% (R: 15 to 67.5%), respectively. GTV changes compared to baseline were