¡La Esquina de la Gramática! Rule 21

Can you say detrás mío, encima suyo?

No. That is incorrect. You should always avoid using adverbs as cerca, detrás, delante, debajo, dentro, encima, enfrente right before possessive adjectives. Then, you have to say:

Detrás de mí, encima de mí, al lado mío, encima de él, etc.

Source: http://www.rae.es