ESTRO 2024 - Abstract Book

S750

Clinical - CNS

ESTRO 2024

Glioblastoma (GB) is the most common primary malignant brain tumor in adults with low survival prognosis despite advances in treatment, usually composed of surgical resection followed by radiotherapy (RT)/ chemotherapy.(1, 2)The optimal treatment volume is open for debate among experts and can only be settled by performing more studies that trace GB recurrences.(3) The introduction of a magnetic resonance (MR)-only RT workflow opens the possibility to achieve greater accuracy in target definition and treatment due to the elimination of uncertainties intrinsic to the registration process necessary for CT-based MR-assisted (CT/MR) RT workflow.(4) Karolinska University Hospital recently implemented an MR-only workflow for GB patients in two phases: commissioning using CT and MR for dose delivery, and validation using MR-only RT with CT as a backup. This work is a study on the GB recurrences of the commissioning cohort with the aim to set a benchmark of treatment quality against which the MR-only outcome will be compared to. The study included 51 patients diagnosed with de novo Glioblastoma (glioma grade III without IDH-1 mutation; glioma grade IV) or Anaplastic glioma grade III with IDH-1 mutation, recruited between February to December 2021. CT/MR RT were given either 60 Gy in 30 fractions (n=27), 40.05 Gy in 15 fractions (n=12) or 34 Gy in10 fractions (n=8). Dosing and scheduling of RT were determined by diagnostic and prognostic factors, which include target size, age, and performance (PS) status according to WHO. Routine follow-up after RT includes performing an MR every 3 months. The date of the MR in which a relapse was first identified is defined as the relapse date. The relapse volume (GTV_rel) was then contoured on the follow-up MR and transferred to the RT planning CT through rigid registration. The intersecting volume between the GTV_rel and the 95% isodose line was classified as being in-field (> 80% is inside of isodose), marginal (20-80% is inside of isodose) or distant (< 20% is inside of isodose).(3) Overall survival (OS) and progression free survival (PFS) were calculated from intervention date. Material/Methods:

Results:

Four patients did not receive RT due to immediate tumor progression after surgical intervention and rapid clinical deterioration and therefore could not be included in this study. Of the 47-patient cohort, 32 underwent surgical resection, 13 were biopsied, and 2 received Laser Interstitial Thermal Therapy (LITT). Of the 32 resected patients, 10 were found to have residual disease, 8 suspected residual disease, and 14 complete resection. All 13 biopsied patients were considered to have residual disease. Of those receiving LITT, one had residual disease and the other was considered as having complete resection.

The median age was 62 years (range 33-78), and median target size was 222 ml (range 53-609). The median PFS was 12,7 months (range 2,8-34,2) and the median OS was 22,8 months (range 5,2-34,2).

With a median follow up time of 28,5 months, 36 of the 47 patients relapsed with a median PFS of 7,1 months. Five of the patients with residual disease before RT have no confirmed relapse (median 32 months) whereas 3 patients without residual disease before RT have remained relapse free (median 25 months) until September 2023. The PS of the 36 patients with confirmed relapse was found to be: 27 with PS 0-1, 8 with PS 2, and 1 with PS 2-3. Finally, GTV_rel was found to be in-field for 28 patients, marginal for 3 and distant for 4 patients.

Conclusion:

The PSF and OS found in this work are in line with published literature.(2, 5, 6) Moreover, the recurrence location analysis performed in this work supports other studies reporting that GB recurrences after RT are usually found