Room: Exhibit Hall | Forum 2
Purpose: We investigated the correlation between volumetric changes in ipsilateral parotids and gross neck nodes in OPC patients treated with adaptive therapy and weekly MRI.
Methods: 60 MRI scans from 15 patients with â‰¥ 3 consecutive weekly T2 fat suppression, non-contrast MRI scans were analyzed. MRIs at weeks 4 and 5 were available for 11 and 5 patients, respectively. All patients received definitive chemoradiation with photon IMRT to 70Gy in 35 fractions. 10 patients had adaptive re-planning for the last 5 to 17 fractions.Custom software was used to automatically propagate parotids from the Week 1 MRI to each subsequent MRI. GTV-Node contouring was performed manually by a single expert. Nodal and parotid volume changes were analyzed using linear regression. For patients without images at weeks 4 and 5, volume change was predicted through extrapolation of the data from preceding weeks and included with the available measured data to estimate the volume change.
Results: At week 3, a mean volume reduction of 23%, 14%, and 15% was observed for the nodes, right and left parotids. The estimated volume changes at week 5 were 52%, 31% and 30%. Cumulative volume decreases for both nodes and parotids was observed throughout the first five weeks of treatment. Volume changes for nodes and parotids were highly correlated for patients where the node and parotid overlapped in the superior/inferior direction (average r2= 0.93, n=4,) but not for those without node/parotid overlap (average r2= 0.35, n=11).
Conclusion: Parotid glands and nodes exhibit a steady volumetric reduction over time. Node/parotid volume changes are highly correlated only for patients with parotid/node overlap. This implies that adaptation triggered by node shrinkage for patients with parotid/node overlap may further enhance parotid sparing whereas parotid dose improvements for those without is likely to be independent of nodal changes.