Dosimetric analysis of CT/MRI-based target delineation differences in radiotherapy for brain metastases close to the skull

To quantify the dosimetric differences associated with CT/MRI-based target delineation in radiotherapy for brain metastases close to the skull (BMCS), and to provide dosimetric evidence for target delineation and treatment-plan evaluation. This retrospective study included 245 BMCS lesions from 132 patients who underwent radiotherapy, including 88 lesions treated with whole-brain radiation therapy (WBRT), 67 treated with local radiation therapy (LRT), and 90 treated with simultaneous integrated boost (SIB). The gross tumor volumes (GTVs) from the original clinical treatment plans, which had been delineated on contrast-enhanced brain-window CT images, were designated as GTV Brain . After rigid registration of the contrast-enhanced MRI to the planning CT, GTV Skull and GTV MRI were delineated on the fused CT/MRI images based on contrast-enhanced bone-window CT images and contrast-enhanced T1-weighted MRI, respectively. A uniform 3-mm expansion was applied to all three GTVs to generate the corresponding planning target volumes (PTVs), designated as PTV Brain , PTV Skull , and PTV MRI , respectively. MRI-only regions, defined as the portions of PTV MRI extending beyond PTV Brain or PTV Skull , were generated using Boolean operations. With each patient’s original clinical treatment plan and the dose distribution calculated on the planning CT kept unchanged, dose-volume parameters for all evaluation structures were extracted directly from the original radiotherapy dose file (RTDOSE), and dosimetric differences among the delineation approaches were compared across the WBRT, LRT, and SIB groups. The primary dosimetric endpoint was D 98% , defined as the dose received by at least 98% of the target volume. Normalized D 98% was calculated as nD 98% = D 98% /prescription dose. An nD 98% value < 1.00 was defined as a below-prescription-dose event, whereas nD 98% <0.95 was defined as a marked undercoverage event. Across all three treatment modalities, PTV volumes consistently followed the order PTV MRI > PTV Skull > PTV Brain ( P < 0.001). D 98% was highest in PTV Brain , intermediate for PTV Skull , and lowest for PTV MRI . Compared with PTV Brain and PTV Skull , PTV MRI showed significantly lower D 98% values, with reductions of 0.68 Gy and 0.58 Gy in the WBRT group, 1.96 Gy and 1.90 Gy in the LRT group, and 3.16 Gy and 2.82 Gy in the SIB group, respectively ( P < 0.001). D 98% values for PTV MRI−Brain and PTV MRI−Skull were lower than those for the overall PTV MRI . The incidence of below-prescription-dose events for PTV MRI was 22.73%, 35.82%, and 62.22% in the WBRT, LRT, and SIB groups, respectively. The corresponding incidence of marked undercoverage for PTV MRI was 6.82%, 17.91%, and 31.11%, respectively, while marked undercoverage in the MRI-only regions reached 41.11% and 42.22% in the SIB group. Brain-window and bone-window CT-based GTV delineation underestimated the contrast-enhanced MRI-referenced target extent in BMCS, resulting in lower D 98% and inadequate prescription-dose coverage. MRI-only regions tended to lie within low-dose regions and were associated with a higher risk of inadequate prescription-dose coverage, particularly in LRT and SIB. For BMCS treated with LRT or SIB, routine fusion of contrast-enhanced MRI with planning CT is recommended for GTV delineation to reduce the potential risk of target geographic miss.