A patient with recurrent rectal cancer was treated with the combination of radiotherapy plus hyperthermia. Intratumoral thermometry probes were introduced within closed-tip catheters, inserted through the buttocks under computed tomography (CT) control. Catheters were fixed to the skin to stay in place during the whole treatment series. At the end of the radiotherapy series, tumour progression was apparent. Seven months following treatment, tumour growth was visible at the insertion site of one of the catheters. This finding indicates that catheters should not be placed outside the treatment volume involved in any locally curative treatment.