Radiobiological Considerations for Stereotactic Irradiation

Stereotactic radiosurgery (SRS: stereotactic irradiation [STI] delivered in a single high dose) was initially developed by Leksell for non-malignant brain lesions, but there has been increasing interest in using it to treat small primary brain tumors or metastases. In more recent years, stereotactic radiotherapy (SRT: fractionated STI) has been developed, but radiobiological factors have not been sufficiently evaluated in relation to these techniques. Larson classified potential STI targets into four categories according to whether the target tissue is early- or late-responding and whether it is embedded within or only surrounded by normal tissue. We have actually calculated biologically effective doses for these categories to determine the indications for SRS and SRT, and to be able to choose suitable SRT schedules. Based on our calculations, theoretically SRS would be recommended for AVMs and benign tumors having distinct margins separating them from surrounding normal tissue and SRT would be recommended for low or high grade astrocytomas without clearly defined boundaries and metastasis. Recommended SRT schedules would be 49 Gy/7 fractions, 52 Gy/8 fractions or 54.9 Gy/9 fractions completed within 2 weeks. However, clinically, these indications and SRT schedules should be modified according to the many other factors involved in individual cases, such as tumor size, presence of tumor necrosis, the patient's general condition, prognosis, and so on.