MR to TRUS guided biopsies can be a cost-effective solution for prostate biopsies. Prostate cancer can be detected on MRI and a biopsy can be directed towards a suspicious region. With the help of an accurate MR-US registration method the tumor can also be targeted under transrectal US guidance. For heterogeneous tumors, the needle should be guided towards the most aggressive part of the tumor. Not the tumor size, but the size of this smaller tumor hotspot determines the required accuracy of the registration. We investigate the percentage of tumors that are heterogeneous and the corresponding hotspot volume. Results show a hotspot in 63% of the tumors, with a median volume of 0.3 cm3. By assuming a spherical shape, the required accuracy can be determined. For a 90% tumor hit-rate, the registration error should be less than 2.87 mm.