Different cancers respond differently to radiation therapy. The response of a cancer to radiation is described by its radiosensitivity.
Highly radiosensitive cancer cells are rapidly killed by modest doses of radiation. These include leukaemias, most lymphomas and germ cell tumours.
The majority of epithelial cancers are only moderately radiosensitive, and require a significantly higher dose of radiation (60-70Gy) to achieve a radical cure.
Some types of cancer are notably radioresistant, that is, much higher doses are required to produce a radical cure than may be safe in clinical practice. Renal cell cancer and melanoma are generally considered to be radioresistant.
It is important to distinguish the radiosensitivity of a particular tumour, which to some extent is a laboratory measure, from the radiation "curability" of a cancer in actual clinical practice. For example, leukaemias are not generally curable with radiotherapy, because they are disseminated though the body. Lymphoma may be radically curable if it is localised to one area of the body. Similarly, many of the common, moderately radioresponsive tumours are routinely treated with curative doses of radiotherapy if they are at an early stage. For example: non-melanoma skin cancer, head and neck cancer, non-small cell lung cancer, cervical cancer, anal cancer, prostate cancer. Metastatic cancers are generally incurable with radiotherapy because it is not possible to treat the whole body.
Before treatment, a CT scan is often performed to identify the tumor and surrounding normal structures. The patient is then sent for a simulation so that molds can be created to be used during treatment. The patient receives small skin marks to guide the placement of treatment fields.
The response of a tumour to radiotherapy is also related to its size. For complex reasons, very large tumours respond less well to radiation than smaller tumours or microscopic disease. Various strategies are used to overcome this effect. The most common technique is surgical resection prior to radiotherapy. This is most commonly seen in the treatment of breast cancer with wide local excision or mastectomy followed by adjuvant radiotherapy. Another method is to shrink the tumour with neoadjuvant chemotherapy prior to radical radiotherapy. A third technique is to enhance the radiosensitivity of the cancer by giving certain drugs during a course of radiotherapy. Examples of radiosensiting drugs include: Cisplatin, Nimorazole, and Cetuximab.
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