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Stereotactic radiotherapy is an advanced form of radiation therapy that allows precise delivery of high radiation doses to a small focused target from multiple different directions. It started in the late 1960s with the Leksell Gamma Knife. During the 1980s, stereotactic irradiation using a linear accelerator (linac) became feasible for the treatment of intracranial lesions. In the 1990s, this technology was applied to lung tumors, which was the start of stereotactic body radiotherapy (SBRT). Since then, numerous encouraging clinical data have been accumulated regarding SBRT for lung tumors, including the authors’ own, and SBRT has now become an indispensable treatment for patients with inoperable stage I non-small cell lung cancer and solitary lung metastasis and a treatment option for operable patients. Based on these encouraging results, the indication of SBRT was then widened to include liver tumors, tumors of the spine, prostatic cancers, and renal tumors. In this special issue, a number of reviews and a dozen original articles are included regarding SBRT for cancers of various organs. These articles support the wide applicability and excellent clinical results of SBRT, with all potentially contributing to the establishment of SBRT as a definitive treatment for patients with various tumors. Thus, the techniques for SBRT may have mostly been established, but some issues regarding physics and biology remain controversial, and so they need to be addressed. Below, we present our opinions and ideas. Several methods of SBRT have been developed. A conventional and the most commonly used one is to use multiple (at least 7) noncoplanar and coplanar fixed beams of a linac. CyberKnife is also useful as it can deliver many more beams serially and automatically with a compact linac unit on a robotic arm. More recently, helical tomotherapy and volumetric modulated arc therapy (VMAT) have also been investigated as SBRT modalities, and especially the use of VMAT seems to be increasing. The use of noncoplanar beams in addition to coplanar beams may be better in terms of the dose distribution in the surrounding normal tissues, but helical tomotherapy and coplanar VMAT may also be useful depending upon the tumor location, provided that the treatment plan achieves a dose distribution comparable to that produced by multiple portal coplanar and noncoplanar irradiation and respiratory motion is explicitly managed. Volumetric modulated arc therapy with a combination of coplanar and noncoplanar beams is of course useful. Hence, if the dose distribution in the planning target volume (PTV) and dose constraints in normal tissues satisfy the criteria that have mostly been established for each organ and tissue, all of these techniques could provide appropriate SBRT. Management of respiratory movements has been a very important issue in SBRT of the lung, liver, and so on, and it has been studied by many investigators in detail. Gating and chasing methods have been investigated extensively, but many efforts are still required to ensure precise treatment delivery and quality assurance. Recently, ultrahigh-dose-rate irradiation at over 8 Gy/min using flattening filter-free beams has become available and is being increasingly used. With such beams, one portal can be treated within 15 seconds during breath-holding. This method appears to be more practical, precise, and useful than the gating and chasing methods, provided that patients can hold their breath for 15 to 20 seconds. It may become the standard approach. A useful device for breath-holding named “Abches” was developed by Onishi’s group and it is becoming increasingly used in Japan.

Stereotactic Body Radiotherapy as an Alternative to Definitive Surgery in Cancers of Various Organs