by Dr. C.H. Weaver M.D. updated 5/2019
A positron emission tomography (PET) scan may be the preferred imaging technique for evaluating cancers of the head, neck and oral cavity. Unlike other techniques that provide anatomical images, such as X-ray, computed tomography (CT) and MRI, a PET scan shows chemical and physiological changes related to cellular metabolism. This is important because these functional changes often occur before structural changes in tissues. PET images may therefore show abnormalities long before they would be revealed by X-ray, CT, or MRI.
Furthermore, PET/CT combines two imaging technologies—CT and PET —into one machine. CT provides information about anatomy and structure, and PET provides information about the function of cells and tissues. When these technologies are combined to produce a single image, doctors are able both to identify abnormal activity within the body and to precisely pinpoint its location. PET/CT scans can be used to identify the extent of the cancer, plan treatment, and assess treatment response.
For example, clinical research in patients with squamous cell cancer of the mouth, imaging with fluorodeoxyglucose positron emission tomography (FDG-PET) detected more hidden lymph node metastases in the neck than computed tomography (CT) or magnetic resonance imaging (MRI).
Because these cancers can spread to lymph nodes in the neck, treatment may involve surgical removal or irradiation of these lymph nodes. While it would be optimal to reserve lymph node treatment for those patients with lymph node metastases, it can be difficult to determine in advance whether lymph node metastases are present. Some lymph node metastases are too small to be felt. They may also be missed by conventional imaging techniques such as computed tomography (CT) or magnetic resonance imaging (MRI).
To determine whether FDG-PET-can be used to identify hidden lymph node metastases in the neck, researchers in conducted a study among 134 patients with oral cancer. None of the patients had lymph node metastases that could be felt by palpating the neck. Before surgical removal of the lymph nodes of the neck, patients underwent imaging with FDG-PET and CT or MRI.
- Surgery detected lymph node metastases in 26% of the patients.
- FDG-PET detected more lymph node metastases than CT or MRI. FDG-PET detected 51% of patients with lymph node metastases, compared to 31% for CT or MRI.
The researchers conclude that FDG-PET was superior to CT or MRI for detecting hidden lymph node metastases in the neck. The performance of FDG-PET was not perfect, however; it produced false-negative results in up to 13% of patients with T1-T3 tumors. In patients with T4 tumors, it produced false-negative results in 25% of patients.(1)
Follow-Up PET Scans Reduce Need for Some Invasive Surgeries in Head and Neck Cancer
Utilizing PET-CT scans following treatment for patients with head and neck cancer that has spread to their lymph nodes also significantly reduces the need for invasive follow-up surgery. Patients diagnosed with head and neck cancer that has spread to nearby lymph nodes are often initially treated with a combination of chemotherapy and radiation therapy (chemoradiation). Following treatment, patients then undergo a type of surgery referred to as neck dissection to remove any remaining cancer cells within the lymph nodes in the neck.
The neck dissection surgical procedure is considered an invasive intervention that requires a follow-up hospital stay, and can result in significant side effects.
Researchers conducted a clinical trial to determine if surveillance using a PET-CT scan performed at 12 weeks following completion of chemoradiation therapy could help discern which patients would benefit from neck dissection.
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The trial was performed from 2007 through 2012 and included 564 patients with cancer that had spread to their local lymph nodes. One group of patients received PET-CT surveillance and only underwent neck dissection if the scan indicated the presence of cancer cells within the lymph nodes. The other group of patients underwent the standard neck dissection following therapy without a prior scan (control group). The median follow-up was 36 months.
- At 2 years, overall survival was 84.9% among patients who underwent PET-CT surveillance, and 81.5% among patients in the control group.
- Only 19% of patients who underwent PET-CT surveillance underwent neck dissection (due to the notable presence of cancer cells on the surveillance scan), compared with 78% in the control group.
- Quality of life was similar between the two groups of patients.
The researchers concluded that surveillance with PET-CT significantly reduces the rates of neck dissection surgery without compromising survival among patients with HNC whose cancer spread to nearby lymph nodes. This approach may provide a promising change in follow-up care for patients with this disease.(2)
PET/CT Combined More Effective Than MRI in Monitoring Head and Neck Cancer Patients’ Response to Treatment
Combining PET/CT to monitor response to treatment in head and neck cancer patients appears more effective than MRI. Using PET/CT may better inform doctors whether treatment for head and neck cancer is working after just one cycle of chemotherapy.
The ability to identify a patients response to chemotherapy early is important because it gives clinicians time to consider reducing treatment to spare side-effects, or may identify ineffective treatment early.
In this study, researchers evaluated 20 head and neck cancer patients as part of the INSIGHT trial based at The Royal Marsden. They assessed patients three months after completion of a combination of chemotherapy and radiotherapy, and determined them to be either responders or non-responders.
The study is the first in head and neck squamous cell carcinoma to compare PET/CT and MRI scans following each cycle of chemotherapy, and the researchers found that, unlike PET/CT, MRI scans were less well able to predict which patients were responding well to treatment.
According to the study author “the study suggests that high-tech PET/CT scans can spot patients whose treatment might not work very rapidly after only one cycle of chemotherapy. That gives patients and clinicians either the confidence to persist with treatment, or early warning that it isn’t working so that it can be urgently switched for an alternative approach.” (3)
- Ng S-H, Yen T-C, Chang T-C et al. Prospective Study of [18F] Fluorodeoxyglucose Positron Emission Tomography and Computed Tomography and Magnetic Resonance Imaging in Oral Cavity Squamous Cell Carcinoma with Palpably Negative Neck. Journal of Clinical Oncology. 2006;24:4371-4376.
- Mehanna H, Wong W-L, McConkey C, et al. PET-CT Surveillance versus neck dissection in advanced head and neck cancer. New England Journal of Medicine. 2016; 374:1444-1454. DOI: 10.1056/NEJMoa1514493.
- Wong K, Panek R, Welsh L, et al. The predictive value of early assessment after one cycle of induction chemotherapy with 18F-FDG-PET/CT and DW-MRI for response to radical chemoradiotherapy in head and neck squamous cell carcinoma. The Journal of Nuclear Medicine. July 14, 2016,doi:10.2967/jnumed.116.174433