physics

Selective internal radiation therapy

Selective internal radiation therapy is a physics topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Selective internal radiation therapy rather than just read about it. In short: Selective internal radiation therapy (SIRT), also known as transarterial radioembolization (TARE), radioembolization or intra-arterial microbrachytherapy is a form of radionuclide therapy used in interventional radiology to treat cancer. It is generally for selected patients with surgically unresectable cancers, especially hepatocellular carcinoma or metastasis to the liver.

Key takeaways

Selective internal radiation therapy belongs to physics; place it in that map before memorising details.
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Connect Selective internal radiation therapy to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Selective internal radiation therapy from memory before moving on to harder problems.

Reference excerpt

Selective internal radiation therapy (SIRT), also known as transarterial radioembolization (TARE), radioembolization or intra-arterial microbrachytherapy is a form of radionuclide therapy used in interventional radiology to treat cancer. It is generally for selected patients with surgically unresectable cancers, especially hepatocellular carcinoma or metastasis to the liver. The treatment involves injecting tiny microspheres of radioactive material into the arteries that supply the tumor, where the spheres lodge in the small vessels of the tumor. Because this treatment combines radiotherapy with embolization, it is also called radioembolization. The chemotherapeutic analogue (combining chemotherapy with embolization) is called chemoembolization, of which transcatheter arterial chemoembolization (TACE) is the usual form.

Principles Radiation therapy is used to kill cancer cells; however, normal cells are also damaged in the process. Currently, therapeutic doses of radiation can be targeted to tumors with great accuracy using linear accelerators in radiation oncology; however, when irradiating using external beam radiotherapy, the beam will always need to travel through healthy tissue, and the normal liver tissue is very sensitive to radiation. The radiation sensitivity of the liver parenchyma limits the radiation dose that can be delivered via external beam radiotherapy. SIRT, on the other hand, involves the direct insertion of radioactive microspheres to a region, resulting in a local and targeted deposition of radioactive dose. It is therefore well-suited for treatment of liver tumors. Due to the local deposition, SIRT is regarded as a type of locoregional therapy (LRT). The liver has a dual blood supply system; it receives blood from both the hepatic artery and the portal vein. The healthy liver tissue is mainly perfused by the portal vein, while most liver malignancies derive their blood supply from the hepatic artery. Therefore, locoregional therapies such as transarterial chemoembolization or radioembolization, can selectively be administered in the arteries that are supplying the tumors and will preferentially lead to deposition of the particles in the tumor, while sparing the healthy liver tissue from harmful side effects. In addition, malignancies (including primary and many metastatic liver cancers) are often hypervascular; tumor blood supplies are increased compared to those of normal tissue, further leading to preferential deposition of particles in the tumors. SIRT can be performed using several techniques, including whole liver treatment, lobar or segmental approaches. Whole liver SIRT targets the entire liver in one treatment and can be used when the disease is spread throughout the liver. Radiation lobectomy targets one of the two liver lobes and can be a good treatment option when only a single lobe is involved or when treating the whole liver in two separate treatments, one lobe at the time. The segmental approach, also called radiation segmentectomy, is a technique where a high dose of radiation is delivered in one or two Couinaud liver segments only. The high dose results in eradication of the tumor while damage to healthy liver tissue is contained to the targeted segments only. This approach results in effective necrosis of the targeted segments. Segmentectomy is only feasible when the tumor(s) are contained in one or two segments. Which technique is applied is determined by catheter placement. The more distally the catheter is placed, the more localized the technique.

Therapeutic applications Candidates for radioembolization include patients with:

Unresectable liver cancer of primary or secondary origin, such as hepatocellular carcinoma and liver-metastases from a different origin (e.g. colorectal cancer, breast cancer, neuroendocrine cancer, cholangiocarcinoma or soft tissue sarcomas) No response or intolerance to regional or systemic chemotherapy No eligibility for potentially curative options such as radiofrequency ablation. SIRT is currently considered as a salvage therapy. It has been shown to be safe and effective in patients for whom surgery is not possible, and chemotherapy was not effective. Subsequently, several large phase III trials have been started to evaluate the efficacy of SIRT when used earlier in the treatment scheme or in combination treatments with systemic therapy. SIRT, when added to first line therapy for patients with metastases of colorectal cancer, was evaluated in the SIRFLOX, FOXFIRE and FOXFIRE Global studies. For primary liver cancer (HCC), two large trials comparing SIRT with standard of care chemotherapy, Sorafenib, have been completed, namely the SARAH and SIRveNIB trials. Results of these studies, published in 2017 and 2018, reported no superiority of SIRT over chemotherapy in terms of overall survival (SARAH, SIRveNIB, FOXFIRE). In the SIRFLOX study, better progression-free survival was also not observed. These trials did not give direct evidence supporting SIRT as a first-line treatment regime for liver cancer. However, these studies did show that SIRT is generally better tolerated than systemic therapy, with less severe adverse events. Simultaneously, for HCC, data derived from a large retrospective analysis showed promising results for SIRT as an earlier stage treatment, particularly with high dose radiation segmentectomy and lobectomy. More studies and cohort analyses are underway to evaluate subgroups of patients who benefit from SIRT as a first-line or later treatment, or to evaluate the effect of SIRT in combination with chemotherapy (EPOCH, SIR-STEP, SORAMIC, STOP HCC). For HCC patients currently ineligible for liver transplant, SIRT can sometimes be used to decreases tumor size allowing patients to be candidates for curative treatment. This is sometimes called bridging therapy. When comparing SIRT with transarterial chemoembolization (TACE), several studies have shown favorable results for SIRT, such as longer time to progression, higher complete response rates and longer progression-free survival.

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Selective internal radiation therapy

Start with the simplest possible case. Write down what Selective internal radiation therapy claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Selective internal radiation therapy before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Selective internal radiation therapy ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Selective internal radiation therapy

In research: Selective internal radiation therapy appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Selective internal radiation therapy in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Selective internal radiation therapy is common in secondary-school and first-year university syllabi. It links to neighbouring topics Interventional radiology, Nuclear medicine procedures, Radiation therapy procedures, so understanding it makes those chapters shorter.
In everyday life: Look for Selective internal radiation therapy outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Selective internal radiation therapy in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Selective internal radiation therapy means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Selective internal radiation therapy out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Selective internal radiation therapy in simple terms?

Why does Selective internal radiation therapy matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Selective internal radiation therapy?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Selective internal radiation therapy.