Hmm.. This analysis could be done in two different ways, but both of them are not currently supported.
1. For surface based analysis (cbm), you can merge left and right hemispheres and do a combined cortical analysis.
2. For volume based analysis (tbm), you can do the whole brain analysis, and then report the results on the contralateral side.
3. Alternately, one can do an asymmetry analysis by subtracting the thickness of the left from the right hemisphere within subject and then do a group analysis and mask out the location of the tumor (you will find a huge effect there obviously).
Options 1 and 3 are not currently supported in bssr. But you can run option 2.
Is that what you had in mind?