Calculations of photonuclear production in "tissue approximation" and the relative heavy particle dose to photon dose at dose maximum in water have been performed. Different 50 MV spectra based on a scanned beam technique to produce flat photon fields were used. These spectra were calculated with the Monte Carlo method and correspond to different target configurations and scan patterns. Photonuclear cross sections of neutron, proton, and alpha production for oxygen, nitrogen, and carbon are taken from the literature. Activation measurements using a HpGe detector have been used to verify the calculations. The relative heavy particle dose to photon dose was found to be 0.30%-0.42%, depending on the spectra used, and the estimated uncertainty was 20%. A relative biological effect (RBE) of 1.01-1.03 in "tissue approximation" for 50 MV were estimated using a RBE for neutron and protons of 2-5 and 4-10 for alpha particles.