Features / Clinical Usefulness / Safety

What is a PET scan?

What kind of examination is a PET scan?

PET is an abbreviation for Positron Emission Tomography. In Japanese, it is referred to as “Yōdenshi Dansō Satsueihō” (positron emission tomography). A PET scan is an imaging technique used to visualize the function of organs such as the heart and brain in cross-sectional images, helping to identify the cause of disease and assess disease conditions. In a PET examination, a radiopharmaceutical suitable for the target of the examination is administered into the body. The PET scanner then images its distribution in the body, allowing visualization of cellular metabolism and function.

PET装置

PET scanner Discovery MI

Why can PET scans detect cancer?

Cancer cells divide more actively than normal cells and therefore require a much larger amount of glucose as an energy source. For this reason, a radiopharmaceutical called FDG—a substance with properties similar to glucose and labeled with a small amount of a radioactive isotope—is administered intravenously. FDG accumulates more abundantly in cancer cells than in normal cells.Using a PET scanner, the distribution of FDG in the body is imaged, allowing the location and size of cancer to be visualized.

Right image: Because cancer cells require more glucose than normal cells, a large amount of FDG accumulates in cancer cells.

How does cancer appear on a PET scan?

The degree of FDG accumulation is displayed as differences in brightness on black-and-white images and as differences in color on color images. Physiological FDG uptake is normally seen in actively functioning tissues such as brain neurons and heart muscle. Because FDG is excreted in the urine, accumulation is also observed in the kidneys, ureters, and bladder. In addition, FDG may accumulate in inflammatory cells and benign tumors. Therefore, not all areas showing FDG uptake indicate cancer.

Features of PET scans

A wide area can be examined in a single session

A PET scan typically images a wide area of the body, from the head to the pelvis, in a single examination. This makes it useful for identifying the primary tumor and determining whether metastasis is present. In some cases, cancer may be detected in unexpected locations that are not usually examined in routine health screenings.

Image provided by Hamamatsu PET Diagnostic Center

Useful for early detection of cancer

PET scans provide information about cellular activity and metabolism. When combined with structural information obtained from conventional imaging methods

such as X‑ray CT or MRI, PET scans can reveal more detailed conditions of the body, making them effective for the diagnosis of early-stage cancer.

A minimally invasive examination

PET examination does not cause pain other than the injection. The scan is performed while wearing clothes, and it does not involve significant discomfort. There is no significant pain or discomfort.

Usefulness of PET examinations

PET examinations do not detect all cancers.

PET examinations are useful for cancer diagnosis and early detection; however, they are not effective for detecting all types of cancer. FDG normally accumulates in the kidneys and urinary bladder even in healthy individuals. For this reason, cancers in these organs, as well as prostate cancer located near the bladder, may be difficult to detect. In addition, conventional screening methods are more effective for certain cancers, such as early-stage gastric cancer and cervical cancer. FDG also accumulates in areas of inflammation and in benign tumors. FDG also accumulates in areas of inflammation and in benign tumors.

When PET is used for health screening purposes, it is important to undergo additional examinations—such as MRI, CT, and endoscopy—in combination, rather than relying solely on PET. As with other screening tests, PET examinations may yield false-positive results (interpreted as suspicious for cancer when no cancer is present) and false-negative results (interpreted as negative even when cancer is present). Furthermore, FDG uptake is affected by blood glucose levels; therefore, in patients with diabetes, images may be more difficult to interpret.

■ Usefulness of FDG-PET in cancer detection

Cancers for which FDG-PET is considered highly useful	Cancers for which FDG-PET Is considered less useful
head and neck cancer lung cancer breast cancer pancreatic cancer colorectal cancer ovarian cancer endometrial cancer malignant lymphoma	esophageal cancer　　　 liver cancer gastric cancer prostate cancer cervical cancer renal cancer bladder cancer * In PET-based cancer screening, it is recommended that PET be used in combination with other diagnostic examinations.

Cancers for which FDG-PET is considered highly useful

Cancers for which FDG-PET Is considered less useful

head and neck cancer
lung cancer
breast cancer
pancreatic cancer
colorectal cancer
ovarian cancer
endometrial cancer
malignant lymphoma

esophageal cancer　　　
liver cancer
gastric cancer
prostate cancer
cervical cancer
renal cancer
bladder cancer

* In PET-based cancer screening, it is recommended that PET be used in combination with other diagnostic examinations.

Source: FDG-PET Cancer Screening Guidelines (3rd Edition),The Japanese Society of Nuclear Medicine / PET and Nuclear Medicine Subcommittee

Which cancer screening tests are considered effective?

Several cancer screening tests listed in the table below have been shown, through scientific studies, to reduce cancer mortality and are therefore considered effective as cancer screening methods. At the time of this report, lung cancer screening using X‑ray CT and prostate cancer screening using PSA testing are categorized as “under evaluation.” The effectiveness of PET cancer screening as a population-based screening test has not yet been confirmed.

■ The kind of cancer screening recommended by the government

Type	Screening Items / Examination Items	Target age	Screening interval
gastric cancer	medical questionnaire + either gastric X‑ray examination or upper gastrointestinal endoscopy	50 years and older	once every 2 years
cervical cancer	medical questionnaire + visual inspection + cervical cytology and pelvic examination	20 years and older	once every 2 years
	medical questionnaire + visual inspection + cervical cytology and pelvic examination	30 years and older	once every 2 years
	medical questionnaire + visual inspection and HPV testing alone	For individuals aged 30 years and older, municipalities select and implement one of the screening methods.	once every 5 years
lung cancer	medical questionnaire and chest X‑ray examination	40 years and older	once a year
breast cancer	medical questionnaire and breast X‑ray examination (mammography)	40 years and older	once every 2 years
colorectal cancer	medical questionnaire and fecal occult blood test	40 years and older	once a year

Source: Ministry of Health, Labour and Welfare (MHLW), Japan, “cancer screening”

Safety of PET examinations

To date, there have been no reports of serious adverse effects caused by FDG. An FDG-PET examination is estimated to involve a radiation exposure of approximately 2.2 mSv (millisieverts) per scan^*1. This is about **half the radiation dose of a gastric X‑ray examination^*2 **, and is considered to be a level of exposure that does not adversely affect the human body. FDG is rapidly excreted in the urine.The amount of radiation emitted by FDG decreases by half approximately every 110 minutes, and continues to decrease further over time. However, pregnant individuals or those who may be pregnant are not eligible to undergo PET examinations.

*1 Sv (sievert): A unit used to express the effect (risk) of radiation on the human body.
*2 Source: PET Examination Q&A, The Japanese Society of Nuclear Medicine / Japan Radioisotope Association

Features / Clinical Usefulness / Safety

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