Help your rad onc-do thin-slice, isotropic 3D acquisitions for RT planning if you see a brain tumor

Gross target volume contouring in canine extra-axial brain tumors: Effects of magnetic resonance image slice thickness and time between subsequent image sets

Valerie J. Poirier, Tracy Gieger, Monica Jensen, Samuel Hocker, Christopher J. Pinard, Fiona M. K. James, Stephanie Nykamp

Background
Accurate gross target volume (GTV) contouring is crucial for radiation therapy (RT) planning in dogs with brain tumors to avoid underdosing tumors or overdosing nearby critical organs. This study evaluated the effects of MRI slice thickness and the interval between diagnostic (MRI-1) and RT planning (MRI-2) MRI acquisitions on GTV measurements in dogs with presumed meningiomas. The hypothesis was that GTV would increase over time due to tumor growth.

Methods
This retrospective observational study included 46 dogs with presumed extra-axial brain tumors based on imaging from two veterinary centers. Inclusion required paired high-field MRIs (1.5T or 3T) with T1-weighted post-contrast sequences acquired within 12 weeks of each other. MRI-1 was used for diagnosis, while MRI-2 was performed for RT planning. GTV was measured and compared between the two MRIs. Factors such as slice thickness, volumetric acquisition, and steroid use between MRIs were analyzed for their impact on GTV changes.

Results
Clinical Data:
-Median time between MRI-1 and MRI-2: 22 days (range: 8–74 days).
-Most tumors (98%) were classified as meningiomas, with the supratentorial region (74%) being the most common location.
-78% of dogs received steroids between MRIs.

MRI Findings:
-MRI-1 had a median slice thickness of 3.9 mm (range: 0.8–6 mm), while MRI-2 had 0.9 mm (range: 0.6–4.5 mm; P < .001).
-MRI-2 frequently used volumetric (3D) acquisition (52%) compared to MRI-1 (7%; P < .00001).

GTV Measurements:
-GTV-1 (mean: 1.78 cm³) was significantly larger than GTV-2 (mean: 1.62 cm³; P < .0001).
-65% of cases showed a smaller GTV on MRI-2, with 15% showing reductions >30%.
-Changes were attributed primarily to differences in MRI slice thickness and acquisition techniques rather than actual tumor size changes.
-Steroid use did not significantly influence GTV changes (P = .697).

Limitations
Retrospective design with non-standardized imaging protocols across institutions.
Small sample size limited the ability to evaluate subtle effects.
Lack of histopathological confirmation for all cases; some tumors could have been non-meningiomas.
GTV was contoured by a single observer, which may limit generalizability.

Conclusions
Differences in MRI slice thickness and acquisition methods significantly affect GTV measurements, often resulting in smaller GTVs on RT planning MRIs compared to diagnostic MRIs. This highlights the need for standardized imaging protocols with thin-slice, isotropic 3D acquisitions for RT planning. A single MRI protocol used for both diagnostic and RT planning purposes may improve accuracy and efficiency, provided the imaging-to-treatment interval is minimized.

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