Any published papers comparing GK to CK?

[arcteryx]

I've come across many potentially biased comparisons of GK versus CK on the net.  For example,
this from a GammaKnife center claiming GK is better:  http://www.sdgkc.com/faqs/CyberKnife.htm.  Or, this from the CKSociety implying that CK is better:  http://www.cksociety.org/PatientInfo/radiosurgery_stereotactic_technology_comparisons.asp.

I was wondering if anybody has come across any academic or published papers comparing the two systems, especially in regards to outcomes/results.

Thanks!