With the advent of MR guided radiotherapy the relevance of Monte Carlo radiation transport simulations in the presence of strong magnetic fields (B-fields) is increasing. While new tests are available to benchmark these simulation algorithms for internal consistency, their application to known codes such as EGSnrc, PENELOPE, and GEANT4 is yet to be provided. In this paper a method is provided to apply the Fano cavity test as a benchmark for a generic implementation of B-field effects in PENELOPE. In addition, it is investigated whether violation of the conditions for the Fano test can partially explain the change in the response of ionization chambers in the presence of strong B-fields. In the present paper it is shown that the condition of isotropy of the secondary particle field (Charged Particle Isotropy, CPI) is an essential requirement to apply the Fano test in the presence of B-fields. Simulations in PENELOPE are performed with (B  =  0.0 T) and (B  =  1.5 T) for cylindrical cavity geometry. The secondary particle field consists of electrons generated from a mono-energetic source (E  =  0.5-4.0 MeV) with a uniform source density and different angular distributions; isotropic, mono-directional, and Compton. In realistic photon fields the secondary radiation field has a non-isotropic angular distribution due to the Compton process. Based on the simulations for the Compton angular distribution (non-CPI), the response change of the cavity model in a uniform radiation field in the presence of B-fields is investigated. For the angular distributions that violate the CPI condition and B  =  1.5 T, the deviations from 1 are considerable, which emphasizes the requirement of CPI. For the isotropic angular distributions obeying this requirement, both the results for B  =  0.0 T and B  =  1.5 T shows deviations from the predictions for E  ⩾  1.5 MeV with values up to 1.0% for E  =  4.0 MeV. Nevertheless, due to the high correlation in the deviation for B  =  0.0 T and B  =  1.5 T, the accuracy of the PENELOPE code for the simulation of the change in detector response in the presence of B-fields is within 0.3%. The effect of the B-field on the detector response for non-isotropic angular distributions suggests that violation of CPI is a major contribution to the response change of ionization chambers in the presence of B-fields.