SIRT and Its Unresolved Problems—Is Imaging the Solution? A Review

Abstract

Selective Internal Radiation Therapy (SIRT) is used as a treatment option for unresectable liver tumors. In SIRT, microspheres, which have a radioactive substance as an integral component, are placed via image guided catheters into the hepatic artery. The ionizing radiation is directly delivered to the tumor. Currently used commercially available microspheres are based on Yttrium 90, a β-emitter, which has been shown to be safe and to produce good clinical results. The technical features of Y90, their applications and their limitations are presented. Image guidance and intraoperative depiction of Yttrium 90 microspheres are restricted, which is currently one of the main limitations in SIRT. Therapy planning and control is currently based on pre-and post-operative images to evaluate the placement of the microspheres respectively. Holmium 166, another possible nuclide integrated into the microspheres emits a higher amount of secondary γ-radiation (Bremsstrahlung) than Yttrium 90. This enables an improved depiction of the microspheres inside the patient during and immediately after application, but comes with other shortfalls. Imaging of delivery and verification of the microsphere placement could solve many of the identified problems with SIRT. The different technologies are reviewed and an outlook in future developments is given particularly on image guidance and therapy control.

Development of neutron-activated samarium-153-loaded polystyrene microspheres as a potential theranostic agent for hepatic radioembolization

“…This would greatly improve the current treatment planning practice in 90 Y radioembolization, where a surrogate imaging source, [ 99m Tc]-macroaggregate albumin (MAA), is used to assess lung shunting and nontarget embolization ratio before 90 Y treatment. However, it should be appreciated that [ 99m Tc]-MAA particles are irregular in size and differ from the shape of [ 90 Y]-microspheres, so they do not fully represent the distribution of the [ 90 Y]-microspheres [38]. The post-administration imaging of 90 Y radioembolization also presents a challenge due to the absence of an imaging component.…”

“…One of the critical requirements of a hepatic radioembolic agent is the diameter of the microspheres, which must be in the range of 20-60 µm [38]. This is to ensure that the microspheres will be lodged at the arteriolar network in and around the tumour without crossing over to the venular side through the capillary network (~8-10 µm) [39].…”