Imaging of malignant brain tumors has played an important role in radiation treatment planning. Within years of the landmark discovery by Roentgen , the use of radiographs to diagnose cerebral tumors became routine . However, the relative limited resolution and accuracy of plain radiographs and other early imaging modalities such as ventriculography and angiography supported the use of whole brain treatment [17–22]. It was not until the early 1970's that partial brain treatment became a viable option with the introduction of CT which heralded a dramatic change in the diagnostic evaluation and treatment principles of gliomas. The correlation of CT imaging and histological data in conjunction with clinical data demonstrating 80% of local recurrence arising within 2 cm of the original tumor as defined by CT, paved the foundation for successful partial brain treatment [2, 3, 5, 14, 23–28].
Nearly concurrent with introduction of CT imaging, MRI was developed and quickly became an important tool for radiation treatment planning. Biopsy evaluation identified tumor cells in the area of MRI T2 abnormality outside the contrast enhancing CT abnormality  and was subsequently incorporated into the target volume for radiation treatment planning [8, 9]. While T2 MRI improved delineation of the extent of microscopic disease, several limitations became apparent. Specifically, T2 weighting causes CSF to be brighter than the brain and can be degraded by volume averaging and fluid motion artifacts secondary to normal cardiopulmonary cycles. These disadvantages led to the development of the FLAIR sequence . By nullifying the CSF signal and decreasing the contrast between gray and white matter, the conspicuity of lesions in the periventricular and peripheral subcortical areas was improved . Current Radiation Therapy Oncology Group protocols advise using CT and either FLAIR or T2 images to identify tumor volumes . However, the differences in T2 and FLAIR MRI sequences to delineate clinically significant tumor burden have not been clearly defined in radiation treatment planning for high grade gliomas.
The results of this study demonstrate both a qualitative and quantitative difference between the tumor target volumes as defined by T2 and FLAIR. The volumes of both the CTV and PTV delineated using FLAIR were significantly larger than those obtained using T2. Despite this increase in size, there was not a significant difference in the overlap with critical structures suggesting that incorporating the FLAIR abnormality does not necessarily increase toxicity. The discordance index between these techniques was substantial, indicating geographic differences in the visualized abnormality. The majority of the target composite volumes were seen on both the T2 and FLAIR images. However, both sequences contributed unique and equally valid data to the composite volume. With regards to the patterns of failure, most lesions were encompassed by both T2 and FLAIR but several patients' lesions only correlated with one sequence. It is known from the underlying physics that the FLAIR technique nullifies CSF, but it is unclear if other factors may account for the differences between FLAIR and T2.
Other investigators have evaluated the utility of incorporating additional imaging techniques into glioma planning but to our knowledge there is no data regarding the differences using T2 versus FLAIR to delineate high grade gliomas for radiation treatment. Functional imaging such as IMP-SPECT, MRSI, and PET have shown promise in guiding treatment planning as well as predicting response. Similar to our results, studies of these techniques have shown extension beyond the T2 abnormality suggesting that traditional targeting may be inadequate [32–34]. However, the incorporation of these novel advances may be limited by availability and cost while FLAIR is readily accessible.
We recognize several limitations in our study. This a retrospective review of a small cohort. As such, the time between diagnostic MRI and simulation CT as well as the use or dosing of steroids was not controlled and may have influenced our results. In assessing differences between the FLAIR and T2 volumes we did not correct for image registration errors. However, based on our comparison of T2 and FLAIR imaging for radiation treatment planning, both techniques are important and not interchangeable. Each technique can help distinguish normal parenchyma from edema and abnormal tissue. FLAIR is inherently more complex as it includes some T1 weighted effects. Our results do not show one technique to be superior but suggest such differences should not be ignored in high grade treatment conformal or IMRT planning, especially within a clinical trial where the results may be biased by the preference of one sequence over the other.