False Positive on PET scan?

Last week I had the followup CT/PET scan. This was technically 5 weeks after the conclusion of the 6th cycle of chemotherapy. (yes, I made it though 6 months of treatment!)

Given the encouraging results from the interim PET scan after 2 cycles, we were expecting a clear scan.  Unfortunately the Dr revealed that there is a "spot" in my chest that is of concern.  He did not say much more about what it may or may not be… of course leaving us fearing the worst.

Aug 19, 2015: PET scan shows bright "spot" in chest

First of all, one of the risks following chemotherapy is a "second cancer," and if this happens the prognosis is much worse than that for the original Hodgkin's lymphoma.

More than 80% of patients with Hodgkin’s disease can be cured with first-line treatment, depending on their risk factor profile. Long-term survivors are at risk for second malignancy, which are often fatal. Solid tumors, leukemia and non-Hodgkin’s lymphoma (NHL) have all been reported.   [1]


However;

PET scans are imperfect with many false-negative and false-positive results. The reasons for false-negative PET scans include technical problems, uncontrolled diabetes, and variability of FDG avidity among lymphoma subtypes. The lengthy list of causes of false positives, which may occur in one quarter of patients, includes inflammation, infection, tumor necrosis, and scanning too soon after chemotherapy or radiotherapy…

Only one third of patients with an inconclusive PET had a suspicious CT scan with a biopsy-proven 33% false-positive rate. For patients with a positive PET and a negative CT, the false-positive rate was 42%. FDG-PET identified unsuspected early relapse in only 10% of patients with Hodgkin's lymphoma, which was assisted by clinical risk and CT findings.  [2]


The CT looked clear, so it would seem that the odds of a "false positive" are there…

Digging further, I learned about;

...a phenomenon called thymic rebound (also called benign thymic hyperplasia), which is defined as thymic regrowth 50% greater than baseline volume. Benign thymic hyperplasia occurs mainly after treatment with chemotherapy, but infrequent case reports describe its occurrence after periods of stress in cancer patients off chemotherapy as well. Chemotherapy causes thymic atrophy and a decrease in average volume of 33%, and regrowth occurs after treatment completion.

The literature in adults is scarce, but it may occur in about 25% of adult chemotherapy patients. Benign thymic hyperplasia in adults has been documented in early stage breast cancer, lymphoma and uterine leiomyosarcoma. It usually occurs within the first year after chemotherapy, but it can present as many as five years later. In one case series, 31 of 134 PET scans (23.1%) for lymphoma showing focal flurodeoxyglucose uptake were diagnosed as nontumoral radiotracer uptake. Five of the 31 false-positive PET scans were related to thymic hyperplasia.   [3]

The "spot" on my scan is in the area of the thymus, slightly above and in front of the heart. Given the published information I feel a lot better about things.

The thymus gland, despite containing glandular tissue and producing several hormones, is much more closely associated with the immune system than with the endocrine system. The thymus serves a vital role in the training and development of T-lymphocytes or T cells, an extremely important type of white blood cell. T cells defend the body from potentially deadly pathogens such as bacteria, viruses, and fungi.

The thymus is a soft, roughly triangular organ located in the mediastinum of the thoracic cavity anterior and superior to the heart and posterior to the sternum…. The function of the thymus is to receive immature T cells that are produced in the red bone marrow and train them into functional, mature T cells that attack only foreign cells. T cells first reside within the cortex of the thymus where they come in contact with epithelial cells presenting various antigens. The immature T cells that respond to the antigens corresponding to foreign cells are selected to survive, mature, and migrate to the medulla while the rest die via apoptosis and are cleaned up by macrophages. This process is known as positive selection.

Several hormones produced by the thymus promote the maturation of the T cells prior to their release into the bloodstream. The now mature T cells circulate through the body where they recognize and kill pathogens, activate B cells to produce antibodies, and store the memory of past infections.

Unlike most organs that grow until the age of maturity, the thymus enlarges throughout childhood but slowly shrinks from the onset of puberty and throughout adulthood. As the thymus shrinks, its tissues are replaced by adipose tissue. The shrinking is due to the reduced role of the thyroid in adulthood – the immune system produces most of its T cells during childhood and requires very few new T cells after puberty. [4]



Is this a case of thymic rebound, thus a "false positive?"

I have been referred to a specialist, so we will learn more…


Update:

On Sept 1 we travelled down to City of Hope in Pasadena.  After much paperwork, bloodwork, and hard work getting a CD of my scans, the Dr more or less confirmed my theory.  In our conversation she mentioned the Thymus and confirmed that this could be what we are seeing.  Multiple spots would be more of a concern.  She also said I probably wouldn't be feeling so good, so while nothing is certain, I consider it a positive prognosis.

Next Steps: port removal and re-scan in 90 days.  Biopsy to follow if needed.


REFERENCES:

1. A positive PET/CT after treatment for Hodgkin’s lymphoma, Carrie Wasserman, MD; Munir Ghesani, MD;  HemOnc Today, July 10, 2008.

2. The Case Against Heavy PETing, Bruce Cheson, American Society of Clinical Oncology, 2009.

3. Enlarging mediastinum on PET/CT after treatment for Hodgkin’s lymphoma,  Carrie Wasserman, MD; Frank Colella, MD; Munir Ghesani, MD;  HemOnc Today, May 10, 2008.

4. Thymus Gland - Anatomy Pictures and Information, Innerbody.com