The management of imaging dose during image-guided radiotherapy: report of the AAPM Task Group 75.

Radiographic image guidance has emerged as the new paradigm for patient positioning, target localization, and external beam alignment in radiotherapy. Although widely varied in modality and method, all radiographic guidance techniques have one thing in common--they can give a significant radiation dose to the patient. As with all medical uses of ionizing radiation, the general view is that this exposure should be carefully managed. The philosophy for dose management adopted by the diagnostic imaging community is summarized by the acronym ALARA, i.e., as low as reasonably achievable. But unlike the general situation with diagnostic imaging and image-guided surgery, image-guided radiotherapy (IGRT) adds the imaging dose to an already high level of therapeutic radiation. There is furthermore an interplay between increased imaging and improved therapeutic dose conformity that suggests the possibility of optimizing rather than simply minimizing the imaging dose. For this reason, the management of imaging dose during radiotherapy is a different problem than its management during routine diagnostic or image-guided surgical procedures. The imaging dose received as part of a radiotherapy treatment has long been regarded as negligible and thus has been quantified in a fairly loose manner. On the other hand, radiation oncologists examine the therapy dose distribution in minute detail. The introduction of more intensive imaging procedures for IGRT now obligates the clinician to evaluate therapeutic and imaging doses in a more balanced manner. This task group is charged with addressing the issue of radiation dose delivered via image guidance techniques during radiotherapy. The group has developed this charge into three objectives: (1) Compile an overview of image-guidance techniques and their associated radiation dose levels, to provide the clinician using a particular set of image guidance techniques with enough data to estimate the total diagnostic dose for a specific treatment scenario, (2) identify ways to reduce the total imaging dose without sacrificing essential imaging information, and (3) recommend optimization strategies to trade off imaging dose with improvements in therapeutic dose delivery. The end goal is to enable the design of image guidance regimens that are as effective and efficient as possible.

Initial evaluation of Cyberknife technology for extracorporeal renal tissue ablation.

OBJECTIVES: To determine whether Cyberknife technology can be applied to renal tissue safely and effectively. The goal was to achieve the high efficacy of a surgical treatment, with the low morbidity of a noninvasive intervention. METHODS: The Cyberknife is a frameless, image-guided radiosurgical device. This innovative extracorporeal treatment combines a linear accelerator mounted on a highly maneuverable robotic arm. The Cyberknife is unique in that it divides the high-dose radiation necessary to ablate the lesion completely into up to 1200 beams. Each one of these beams of radiation has a significantly reduced dose. Therefore, the individual dose of each beam is essentially benign to the pathway and surrounding tissue. However, at the focal point of these beams, the dose is additive, and the desired ablative dose is attained. Predetermined "lesions" in 16 kidneys were treated in vivo in the porcine model. Complete treatment was accomplished in one session per animal, with no complications. Gross and histologic evaluations were completed at 4, 6, or 8 weeks. RESULTS: The degree of radiation changes correlated with longer treatment intervals. After 8 weeks, the lesions showed complete fibrosis. The zones of complete fibrosis were characterized by dense, paucicellular connective tissue completely devoid of all normal kidney elements, including tubules and glomeruli. CONCLUSIONS: This initial preclinical evaluation of the Cyberknife for extracorporeal renal tissue ablation appears to be very promising and demonstrated its ability to ablate a targeted area precisely and completely with relative sparing of the surrounding tissue. This innovative technology introduces an exciting approach as a potential treatment option of renal masses in the future.