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Published on: December 28, 2017
Volumetric and dosimetric impact of post-surgical MRI-guided radiotherapy for glioblastoma: A pilot study
Marcus Tyyger1, Suchandana Bhaumik1, Michael Nix1
1Leeds Cancer Centre, Leeds Teaching Hospitals NHS Trust, Leeds, UK.
Objectives:
Glioblastoma (GBM) radiotherapy (RT) target delineation requires MRI, ideally concurrent with CT simulation (pre-RT MRI). Due to limited MRI availability, <72 h post-surgery MRI is commonly used instead. Whilst previous investigations assessed volumetric differences between post-surgical and pre-RT delineations, dosimetric impact remains unknown. We quantify volumetric and dosimetric impact of using post-surgical MRI for GBM target delineation.
Methods:
Gross tumour volumes (GTVs) for five GBM patients receiving chemo-RT with post-surgical and pre-RT MRIs were delineated by three independent observers. Planning target volumes (PTVs) and RT plans were generated for each GTV. Volumetric and dosimetric differences were assessed through: absolute volumes, volume-distance histograms and dose-volume histogram statistics.
Results:
Post-surgical MRI delineations had significantly (p < 0.05) larger GTV and PTV volumes (median 16.7 and 64.4 cm3, respectively). Post-surgical RT plans, applied to pre-RT delineations, had significantly decreased (p < 0.01) median PTV doses (ΔD99% = -8.1 Gy and ΔD95% = -2.0 Gy). Median organ-at-risk (OAR) dose increases (brainstem ΔD5% =+0.8, normal brain mean dose =+2.9 and normal brain ΔD10% = 5.3 Gy) were observed.
Conclusion:
Post-surgical MRI delineation significantly impacted RT planning, with larger normal-appearing tissue volumes irradiated and increased OAR doses, despite a reduced coverage of the pre-RT defined target.
Advances In Knowledge:
We believe this is the first investigation assessing the dosimetric impact of using post-surgical MRI for GBM target delineation. It highlights the potential of significantly degraded RT plans, showing the clinical need for dedicated MRI for GBM RT.
Insights
Using post-surgery MRI for glioblastoma radiotherapy planning leads to larger irradiated volumes and increased organ-at-risk doses. This study highlights the clinical need for dedicated MRI scans for accurate radiotherapy planning in glioblastoma patients.
Area of Science:
- Radiation Oncology
- Neuro-oncology
- Medical Imaging
Background:
- Radiotherapy (RT) for glioblastoma (GBM) requires accurate target delineation using MRI.
- Limited MRI availability often necessitates using post-surgical MRI (<72h) instead of pre-RT MRI.
- Previous studies focused on volumetric differences, leaving dosimetric impacts unknown.
Purpose of the Study:
- To quantify the volumetric and dosimetric impact of using post-surgical MRI for GBM target delineation.
- To assess changes in Gross Tumour Volume (GTV) and Planning Target Volume (PTV) delineation.
- To evaluate the consequences on radiotherapy plan dosimetry and organ-at-risk (OAR) doses.
Main Methods:
- Delineation of GTVs from post-surgical and pre-RT MRIs in five GBM patients by three observers.
- Generation of PTVs and RT plans based on delineated GTVs.
- Assessment of volumetric differences (absolute volumes, VDH) and dosimetric differences (DVH statistics).
Main Results:
- Post-surgical MRI delineations resulted in significantly larger GTV and PTV volumes (p < 0.05).
- RT plans using post-surgical MRIs showed significantly decreased PTV doses (ΔD99% = -8.1 Gy, ΔD95% = -2.0 Gy; p < 0.01).
- Median OAR dose increases were observed for the brainstem and normal brain tissue.
Conclusions:
- Post-surgical MRI delineation significantly impacts GBM RT planning, leading to larger irradiated normal-appearing tissue volumes.
- Increased OAR doses and reduced target coverage were observed despite larger delineated volumes.
- Dedicated MRI for GBM RT is clinically necessary to avoid degraded treatment plans.
