We propose a modification of a previous model by Stepanova (Biophysics, 24 (1980) 917), which describes the specific immunological response against cancer. The model assumes that normal cells and cancer cells coexist in an environment as two different species which compete for nutrients and space. The immune system and the tumor cells fight against each other.
The modification consists of a sustitution of a power law for the exponential rate used for tumoral growth. This
modification is motivated by the numbers of recents works confirming that some tumour growth support power law behavior.