Use MongoDB for caching? JSON or long string as primary key

Can I use MongoDB to build a lookup JSON => JSON?

I think about building a (permanent) cache for some calculations performed by a WebAPI method. Request and Response are object structures (transferred by converting into/from JSON).

As MongoDB is a documentDB the value part seems to be a good fit. I care about the key part.

Requests may have JSON representations of maybe 500 bytes, values maybe 1000 bytes.

There can be millions of cache entries.

Thinking in C# I would look for something like a disk based Dictionary. I tend to a more complex solution like MongoDB because of distribution (cluster) and debugging options (analyse content easier).

I'm sure there are faster or more specialized solutions just for disk based dictionary with json key, suggestions welcome.

But I'm interested in MongoDB suitability (e.g. show stoppers).