Abstract
BACKGROUND: Adaptive radiotherapy (ART) is an advanced form of image-guided radiotherapy that involves the re-contouring and re-planning of a patient's treatment plan, either while the patient is on the table (online) or in between fractions (offline). ART allows for the adjustment of a treatment plan to respect a patient's changes in internal anatomy, something that is critical in the treatment of gastrointestinal (GI) malignancies in which the mobile and radiosensitive GI tract plays a key role in driving toxicity. Herein we review the indications for both online and offline ART in GI cancers. MAIN TEXT: Online ART plays a critical role in the treatment of pancreatic cancer when using stereotactic body radiotherapy (SBRT). A variety of ART workflows have demonstrated that ART allows for the safe dose-escalated treatment of locally advanced pancreatic cancer. In addition to pancreatic cancer, there are now a bevy of data demonstrating that ART plays a key role in the treatment of liver cancers and abdominal oligometastases when using SBRT and allows for the safe delivery of single-fraction abdominal SBRT. While lower GI cancers are generally not treated with SBRT-like doses, both online and offline ART workflows have been shown to potentially reduce toxicity in patients with anal and rectal cancers. Improved integration of artificial intelligence and direct-to-unit workflows in ART hold promise that the overall process can become more efficient, allowing for more widespread adoption in GI radiation oncology. CONCLUSIONS: ART is an expanding radiotherapy paradigm in which a patient's treatment plan is adjusted to match observed changes in patient anatomy and has been successfully incorporated into the treatment of a variety of GI cancers. The successful implementation of workflows in pancreatic cancer, liver cancers, and lower GI cancers, amongst others, as well as incorporation into multi-center clinical trials, suggest that ART will continue to play a critical role of GI radiation oncology for years to come. As improvements in efficiency and access allow for increasing use of ART world-wide, we predict that ART will continue to play a critical part in the management of patients with GI malignancies.