Background: We tried to reveal the relationship between the levels of some important minerals, rare elements and heavy metals by measuring serum cobalt (Co), lead (Pb), zinc (Zn), iron (Fe), copper (Cu), cadmium (Cd), magnesium (Mg), manganese (Mn), catalase (CAT) and carbonic anhydrase (CA) levels in head and neck, CNS, esophagus, stomach and breast cancers receiving radiotherapy. Materials and Methods: Serum Cu, Fe, Pb, Zn, Co, Mg, Mn and Cd levels were measured using atomic absorption spectrophotometry. Carbonic anhydrase activity (CA), CO₂ hydration measurements were made using bromothymol blue method as indicator. Results: Catalase activity, carbonic anhydrase activity and serum copper, lead, zinc, iron, cobalt, cadmium, manganese, magnesium measurements for baseline and post-radiotherapy values differ greatly in cancer patients compared to healthy subjects (p<0.05). Conclusion: As a result, these findings have a significant impact on the pathophysiology of cancer. It can be argued that this paper shows a preliminary study for examining the effect of radiotherapy treatment on the activity of erythrocyte catalase, carbonic anhydrase and serum levels of cobalt, lead (Pb), zinc, iron, copper, cadmium, magnesium and manganese in the head and neck, CNS, stomach, esophagus and breast cancers receiving radiotherapy treatment.