Selective Internal Radiation Therapy (SIRT)

Selective internal radiation therapy (SIRT), also known as SIR-Spheres (Work-up and Implant), is a minimally invasive interventional radiology procedure used for the treatment of primary and metastatic liver tumors. It involves the targeted delivery of high-energy radioactive microspheres directly to the liver vasculature.

The procedure involves the injection of tiny radioactive microspheres, typically made of yttrium-90 (Y-90), into the hepatic artery. Yttrium-90 is a beta-emitting radioisotope with a half-life of approximately 64 hours. The microspheres are designed to be selectively trapped within the tumor vasculature due to their size, blocking the blood supply to the tumor cells. As a result, the radiation emitted by the Y-90 microspheres is delivered directly to the tumor, minimizing the impact on healthy liver tissue.

The radioactive decay of Y-90 emits high-energy beta particles, which have a limited penetration range in tissue. This property allows for the targeted destruction of tumor cells while sparing adjacent healthy liver cells. The radiation damages the DNA within the tumor cells, disrupting their ability to divide and grow, ultimately leading to tumor shrinkage and potentially complete eradication.

Before undergoing SIRT, patients typically undergo pre-procedural evaluation, including imaging studies such as angiography and nuclear medicine scans. These evaluations help determine the suitability of SIRT, assess the extent of liver disease, and identify the precise location of the tumors.

SIRT is considered a palliative treatment option for patients who are not suitable candidates for surgery or other local therapies. However, contemporary practices are using SIRT in conjunction with other conventional treatments including chemotherapy and radiation therapy. It can be used to treat both primary liver tumors, such as hepatocellular carcinoma (HCC), as well as liver metastases from various cancers, including colorectal cancer.

Post-procedural care involves monitoring the patient for potential side effects, such as abdominal pain, fatigue, nausea, and radiation-induced liver injury. Radiological imaging is performed to assess the response to treatment and monitor any potential complications.

SIRT is divided into two procedures:

(1) SIRT Work-up procedure to determine the hepatic artery vasculature supplying the liver tumours which involves the injection of Tc-99-MAA (technecium-99 combined with macro-aggregated albumin) to determine the liver-to-lung shunting and the reaction of the tumour/s to the radioactive isotope. A nuclear medicine PET-CT scan is performed following the angiography and interventional procedure.

(2) SIRT Implant procedure is performed within 1-2 weeks from the work-up procedure, and involves the injection of the Y-90 SIR-Spheres using the previously mapped hepatic vascular anatomy. A post-procedure PET-CT scan is then performed.

A&I Protocol

The femoral approach for performing SIRT-Work-up is detailed here:

Femoral Approach:

Seldinger technique is used to gain access into the right common femoral artery.
A 5 French (F) sheath is inserted into the right common femoral artery over an 0.035" guidewire.
A 5 F Hockey-stick catheter is navigated up the abdominal towards the level of the celiac trunk, at approximately the twelfth thoracic vertebrae (T12).
Once positioned in the celiac trunk, a long DSA run is performed with patient expiration to see the arterial blood flow of hepatic system, as well as the portal venous system.
The right and/or left main hepatic arteries are catheterised and accessed with an 0.021" or 0.027' microcatheter.
If the tumours are present in the right lobe, the right hepatic artery is accessed. If the tumours are present in the left lobe, the left hepatic artery is accessed. If both lobes are affected with tumours, the right hepatic artery is accessed, and then a second femoral artery puncture is performed, then using a second catheter and microcatheter system, the left hepatic artery is accessed.
Once the microcatheters are positioned in the desired region of the artery that is supplying the tumours, DSA runs are performed. Further checks with an on-table cone-beam CT (CBCT) scan can also be performed to check for non-target vessels and to confirm catheter position for treatment. This can be done for both lobes of the liver.
For complex hepatic artery anatomy, the CBCT can be used to create a 3D model of the vasculature and tumour, where the angiography system software can detect the vessels and trace them to the tumour/s. This 3D model can be overlayed onto the 2D fluoroscopy image for device guidance. Confirmation using DSA is used to check the position of the microcatheter/s and tumour blushing with contrast.
When microcatheter positions are confirmed, the Nuclear Medicine physician is called upon and prepares the Tc-99-MAA and is injected under fluoroscopic guidance with the use of contrast media into the hepatic arteries.
When completed, all devices are disposed of safely.
A vascular plug device or manual pressure is applied on the groin region post-procedure.

The femoral approach for performing SIRT-Implant is detailed here:

Femoral Approach:

Seldinger technique is used to gain access into the right common femoral artery.
A 5 French (F) sheath is inserted into the right common femoral artery over an 0.035" guidewire.
A 5 F Hockey-stick catheter is navigated up the abdominal towards the level of the celiac trunk, at approximately the twelfth thoracic vertebrae (T12).
Once positioned in the celiac trunk, the catheter and wire are tracked into the common hepatic artery.
Using the knowledge of the vascular anatomy from the SIRT Work-up, the 0.021" or 0.027" microcatheter and wire are navigated to the desired catheter position, similar to where the Tc-99-MAA were injected in the SIRT Work-up procedure.
If the left and right hepatic arteries are involved, then a second CFA puncture and access are required to catheterise the contralateral hepatic artery.
DSA runs are performed to confirm the microcatheter positions.
a long DSA run is performed with patient expiration to see the arterial blood flow of hepatic system, as well as the portal venous system.
The right and/or left main hepatic arteries are catheterised and accessed with an microcatheter.
When microcatheter positions are confirmed, the Nuclear Medicine physician is called upon and prepares the Y-90 and is injected under fluoroscopic guidance with the use of contrast media into the hepatic arteries.
When completed, all devices are disposed of safely.
A vascular plug device or manual pressure is applied on the groin region post-procedure.

[Left] Right hepatic artery angiogram; and [Right] Left hepatic artery angiogram showing bi-lobar tumours. Angiograms to show extent of blood supply to tumours and determine catheter position for Tc99-MAA (SIRT Work-up).

Cone-beam computed tomography (CBCT) of the right liver lobe with contrast highlighting the tumours in each segment.

Double catheter (and microcatheter) technique into the left and right hepatic artery. For SIRT Work-Up, these catheter positions are used to inject the Tc99-MAA. These catheter positions are replicated during the SIRT Implant procedure when injecting Y90 microspheres.

References

Preston E, Shaida N. Selective internal radiation therapy in the management of primary and metastatic disease in the liver. Br J Hosp Med (Lond). 2021 Feb 2;82(2):1–11.
Iñarrairaegui M, Sangro B. Selective Internal Radiation Therapy Approval for Early HCC: What Comes Next? Hepatology. 2021 Nov;74(5):2333–5.
Welsh JS, Kennedy AS, Thomadsen B. Selective Internal Radiation Therapy (SIRT) for liver metastases secondary to colorectal adenocarcinoma. Int J Radiat Oncol Biol Phys. 2006;66(2 Suppl):S62-73.
Prompers L, Bucerius J, Brans B, Temur Y, Berger L, Mottaghy FM. Selective internal radiation therapy (SIRT) in primary or secondary liver cancer. Methods. 2011 Nov;55(3):253–7.
Sundram FX, Buscombe JR. Selective internal radiation therapy for liver tumours. Clin Med (Lond). 2017 Oct;17(5):449–53.