The authors below have given an excellent summary of what is called Cutaneous Recall Phenomenon. This is usually seen in cancer patients treated initially with radiotherapy. Subsequent treatment with chemotherapy then produces a inflammatory skin rash, only on the part of the skin exposed to Radiotherapy. This is quite an interesting and intriguing phenomenon, believed to be due to altered local skin immune response at the site of Radiotherapy.
Similar reactions are also seen if say someone has had a bad sunburn on the back years ago. Chemotherapy is such situations can make the sunburn reappear (without any exposure to sunlight). in the past. That is why it is called the ‘Recall Phenomenon’. Isn’t that interesting?