Volumetric and dosimetric impact of post-surgical MRI-guided radiotherapy for glioblastoma: A pilot study

Abstract

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.

Adoption of Hybrid MRI-Linac Systems for the Treatment of Brain Tumors: A Systematic Review of the Current Literature Regarding Clinical and Technical Features

“… 93 Indeed, several studies performed in patients that underwent adjuvant RT on conventional linacs demonstrated relevant changes of target volume between post-operative MRIs and planning MRIs 94 , 95 or MRIs performed at the first day of RT 96 – 98 and this could affect CTV definition. 94 Moreover, repeated MRIs performed during RT course invariably resulted in volumetric and/or geometric variations of disease and target volumes 98 – 102 and allowed to improve doses to the OARs 99 , 100 , 102 or to improve target coverage. 99 , 102 Volumetric variation of target volumes was heterogeneous, but in most series the majority of lesions progressively shrinked during radiotherapy 98 – 100 although expansion was consistently reported in a fraction of the cases.…”

“… 94 Moreover, repeated MRIs performed during RT course invariably resulted in volumetric and/or geometric variations of disease and target volumes 98 – 102 and allowed to improve doses to the OARs 99 , 100 , 102 or to improve target coverage. 99 , 102 Volumetric variation of target volumes was heterogeneous, but in most series the majority of lesions progressively shrinked during radiotherapy 98 – 100 although expansion was consistently reported in a fraction of the cases. 98 , 99 , 101 , 102 Adaptive radiotherapy based on serial MRI findings could improve doses to the OARs particularly in case of volume reduction 99 , 101 , 103 and guarantee adequate coverage in case of volumetric increase.…”

“…99,102 Volumetric variation of target volumes was heterogeneous, but in most series the majority of lesions progressively shrinked during radiotherapy [98][99][100] although expansion was consistently reported in a fraction of the cases. 98,99,101,102 Adaptive radiotherapy based on serial MRI findings could improve doses to the OARs particularly in case of volume reduction 99,101,103 and guarantee adequate coverage in case of volumetric increase. 103 An example of volumetric changes during radiotherapy in a glioblastoma patient are represented in Figure 2.…”