Benadryl or Decadron as pretreatment?

[heblaj01]

I noticed most posts reporting on pretreatment to Herceptin indicate that Benadryl was the drug chosen by the onc.
The headaches & weakness which appear associated with Benadryl seem to confirm the opinion of a local pharmacist when asked to compare Benadryl to Decadron. He was in favor of Decadron as a better drug to control side effects of Herceptin (such as allergies) & with less side effects of its own ( in doses of 10mg every 3 weeks).

Has anyone been told why Benadryl was selected?

Is it prefered for those on weekly Herceptin (lower dose) as opposed to those on every 3 week higher doses?