Early MRI-defined tumor response could affect dose escalation during radiotherapy

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BOSTON—Using MRI to detect heterogeneous early tumor responses to head and neck radiotherapy could potentially guide dose escalation to refractory high-risk disease during treatment, according to a study presented Tuesday during a scientific session at the 50th annual meeting of the American Society for Therapeutic Radiology and Oncology (ASTRO).

Qing Yuan, and colleagues at the University of Texas M.D. Anderson Cancer Center in Houston, sought to prospectively evaluate dynamic contrast enhanced MRI (DCE-MRI) and diffusion-weighted MRI (DW-MRI) for mid and post treatment radiation response in neck nodes.

The researchers used a CT-on-Rails system to deliver daily volumetric image-guided adaptive radiotherapy to oropharyngeal cancer patients enrolled onto a prospective institutional protocol. Daily setup CT images were used to compute dose distributions offline.

“We used deformable image registration to directly correlate delivered doses on a voxel-by-voxel basis to mid-course (three weeks) and post-treatment DCE-MRI and DW-MRI images,” the authors wrote.

All MRI data was acquired using a 3T GE Excite HD scanner and 8-channel HD neurovascular phased-array coil. Contiguous 5-mm thickness T1-weighted and fat-suppressed T2-weighted images were obtained in the axial plane.

They performed a parallel imaging calibration scan was performed, followed by a fat-suppressed spin-echo echo planar DW-MRI acquisition from 3 mm sections using an acceleration factor of two and five b-values. DCE-MRI data was acquired from identical regions every six seconds before, during, and after bolus injection of 0.1 mmol/kg gadopentetate dimeglumine delivered at 3 ml/s.
After a pilot series of four patients completed all MR scans, the researchers observed dose-response relationships across cases, which were more sensitively detected using deformable mapping versus rigid mapping of dose distributions.

The imaging acquisition protocol is “absolutely critical in Ktrans outcome measures,” according to Yuan. Both DW-MRI and change in Ktrans maps (differences between baseline and repeat MR scans) identified heterogeneous responses in diffusion restriction and Ktrans within gross tumor at three weeks. Values remained homogeneous and unchanged in normal muscle and primary and nodal disease response patterns were distinct, they noted.

In one representative case, the researchers observed 20 percent and 60 percent mean reductions in Ktrans in gross nodal disease at 34.5 Gy (mid-course) and six weeks after completing 69.2 Gy treatment, respectively. Conversely, Ktrans increased within a tonsil primary at three weeks (36 Gy), followed by a post-treatment reduction to normal tissue values. In two patients, MRI identified occult gross tumor, facilitating new adaptive treatment plans.

“We demonstrated quantitative dose-response assessment through deformable registration of DCE/DW-MRI maps with treatment dosimetry,” Yuan and colleagues wrote.