The top of the foot is the dorsum of the foot.

However dorsum is a synonym for top or back, therefore any body part can have a dorsum, in fact every body has a dorsum - as it is a synonym for back.The word dorsum on its own - without a qualifying "of the foot" would mean a person's back.

There is no single word for the 'top' or 'dorsum of the foot' like there is for the bottom of the foot i.e. sole. The word 'sole' represents the entire meaning "bottom of the foot", whereas if one used dorsum one would have to qualify what dorsum one was referring to - i.e. dorsum of the foot. That is probably due to common usage as one can also use sole to describe the underneath of other body parts, objects. If we had the need to refer to the top of our feet as much as we need to refer to the soles of our feet, then it is very likely that the word dorsum, like the word sole, would have no need of the qualifying "of the foot" as it would be implied and understood.