Stereotactic Ablative Radiotherapy Uncertainties: Delineation, Setup and Motion

Stereotactic ablative radiotherapy (SAbR), also known as stereotactic body radiation therapy, is biologically distinct from conventionally fractionated radiotherapy in that it is ablative, causing functional incapacitation of targeted tissue including margins added to avoid errors. In striking contrast, small 1.8-2 Gy daily doses associated with historical fractionated radiotherapy cause more modest injury more likely to be repaired by normal tissue than tumor. Strategic plans to deal with uncertainties in tumor location, motion during treatment, dose deposition and heterogeneity effects, and tolerance require a novel approach for SAbR very different from what has been dogma for conventionally fractionated treatments. In essence, the targets must be kept absolutely as small as prudently possible without missing tumor deposits. Given the potency and clinical potential of SAbR, a more sophisticated and rigorous approach to uncertainties and margins is warranted.