Thanks for your question on HCM. In my opinion you should first consult pulmonologist and get done chest x ray and CT THORAX (if needed) to rule out lung infection and radiation pneumonitis. Radiotherapy makes person immunosuppressive, so chances of lung infection ( pneumonia ) are very high. So chest x ray is needed. Another possibility for your cough is radiation pneumonitis. It is not infection but inflammation produced in lung parenchyma by radiation. So tissue edema develops and this causes coughing. CT THORAX is needed for diagnosis. And steroids are the only treatment for this. So better to consult pulmonologist and discuss all these.