Pitfalls in radiation oncology: “Myocardial metastasis” in PET-CT after palliative radiation treatment of the left 5th rib

Response Evaluation Following Radiation Therapy With 18F-FDG PET/CT: Common Variants of Radiation-Induced Changes and Potential Pitfalls

Radiation therapy (RT) is one of the cornerstones in cancer treatment and approximately half of all patients will receive some form of RT during the course of their cancer management. Response evaluation after RT and follow-up imaging with ¹⁸F-Fluorodeoxyglucose (¹⁸F-FDG) positron emission tomography/computed tomography (PET/CT) can be complicated by RT-induced acute, chronic or consequential effects. There is a general consensus that ¹⁸F-FDG PET/CT for response evaluation should be delayed for 12 weeks after completing RT to minimize the risk of false-positive findings. Radiation-induced late side effects in normal tissue can take years to develop and eventually cause symptoms that on imaging can potentially mimic recurrent disease. Imaging findings in radiation induced injuries depend on the normal tissue included in the irradiated volume and the radiation therapy regime including the total dose delivered, dose per fraction and treatment schedule. The intent for radiation therapy should be taken in consideration when evaluating the response on imaging, that is palliative vs curative or neoadjuvant vs adjuvant RT. Imaging findings can further be distorted by altered anatomy and sequelae following surgery within the radiation field. An awareness of common PET/CT-induced changes/injuries is essential when interpreting ¹⁸F-FDG PET/CT as well as obtaining a complete medical history, as patients are occasionally scanned for an unrelated cause to previously RT treated malignancy. In addition, secondary malignancies due to carcinogenic effects of radiation exposure in long-term cancer survivors should not be overlooked. ¹⁸F-FDG PET/CT can be very useful in response evaluation and follow-up in patients treated with RT, however, variants and pitfalls are common and it is important to remember that radiation-induced injury is often a diagnosis of exclusion.

Side Effects of Oncologic Treatment in the Chest: Manifestations at FDG PET/CT

Fluorodeoxyglucose (FDG) PET/CT is a vital imaging technique used for staging, assessing treatment response, and restaging following completion of therapy in patients who are undergoing or have completed oncologic treatment. A variety of adverse effects from chemotherapy, targeted therapy, immunotherapy, and radiation therapy are commonly encountered in oncologic patients. It is important to be aware of the manifestations of these adverse effects seen on FDG PET/CT images to avoid misinterpreting these findings as disease progression. Furthermore, early identification of these complications is important, as it may significantly affect patient management and even lead to a change in treatment strategy. The authors focus on the FDG PET/CT manifestations of a broad spectrum of oncologic therapy-related adverse effects in the thorax, as well as some treatment-related changes that may potentially mimic malignancy. Online supplemental material is available for this article. ^©RSNA, 2021.

Cardiac ¹⁸F-fluorodeoxyglucose uptake on positron emission tomography after thoracic stereotactic body radiation therapy

BACKGROUND AND PURPOSE

Previous studies have shown that increased cardiac uptake of (18)F-fluorodeoxyglucose (FDG) on positron emission tomography (PET) may be an indicator of myocardial injury after radiotherapy. We reviewed patients treated with thoracic stereotactic body radiation therapy (SBRT) and established correlations between SBRT dose and observed changes in cardiac FDG-PET uptake.

MATERIAL AND METHODS

Retrospective analysis identified 39 patients that were treated with SBRT for lung tumors close to the heart. Patients were grouped according to whether or not they had changes in cardiac FDG-PET uptake within the planned SBRT field.

RESULTS

At a median follow-up interval of 39 months (range, 6-81 months), nine patients (23%) showed increased cardiac FDG uptake associated with the heart V₂₀. Of the 19 patients who received 20 Gy to ≥5 cm(3) of the heart, nine (47%) developed increased FDG uptake (vs. 0% for the 20 patients who received 20 Gy to <5 cm(3)) (P=0.0004), all within the 20-Gy isodose line. Patients with hypercholesterolemia prior to SBRT were also more likely to show increased cardiac FDG uptake (P=0.0190).

CONCLUSION

Increased FDG uptake in the heart after SBRT was observed when the 20 Gy isodose line exceeded 5 cm(3) of the heart.