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Well-differentiated papillary mesothelioma, or WDPM, is a rare mesothelioma subtype. Mesothelioma is a cancer most often associated with asbestos exposure. However, WDPM may not be related to that risk factor at all. There have been only a couple hundred cases of WDPM ever described or studied, limiting our medical understanding of the disease.
What is known is that WDPM is a subtype of the epithelial variety of mesothelioma, the most common and least aggressive form of this rare cancer. WDPM is usually benign but can turn into malignant mesothelioma. Treatments vary and there is no routine treatment strategy due to the rarity of this condition. It is most often seen in the peritoneum of female patients, although there have been reported pleural and testicular forms of WDPM.
Facts about WDPM
The type of mesothelioma can be identified by its location in the body. Most cases are diagnosed as pleural, meaning they formed in the pleura around the lungs. The next most common form attacks the peritoneum, the lining in the abdomen. Rarely, mesothelioma affects the pericardium around the heart or the tunica vaginalis of the testes. Most cases of WDPM have been peritoneal and in women. There have been reported cases of the other forms as well, although these are even rarer.
Relatively few studies have focused on WDPM. These have examined case studies describing individuals diagnosed with the condition. WDPM is mostly benign and comes with good prognosis and survival rates. However, it can turn malignant over time. Therefore, anyone diagnosed with WDPM should receive regular screening for malignancies.
Unlike malignant mesotheliomas, WDPM does not typically cause many symptoms. The disease can cause fluid build-up in the pleura or peritoneum, which in turn can result in swelling or pain. In the tunica vaginalis it may also cause swelling in the scrotum or lumps on the testicles. There is no standard treatment for WDPM because it is so uncommon.
The name of this condition, well-differentiated papillary mesothelioma, is descriptive of its tumor characteristics. Well-differentiated refers to the fact that cells in this kind of tumor resemble healthy cells. Poorly differentiated tumors have cells that are easily distinguished from healthy cells. Well-differentiated tumor cells also grow less rapidly and spread less than those that are poorly differentiated.
The growth pattern of the cells in a WDPM tumor is described as papillary. This means they grow into finger-like structures called papillae. Each structure is lined with a layer of uniform, branching, or coarse mesothelium cells. These structures are only visible under a microscope. To the naked eye, they grow as white to gray nodules between one and three centimeters in diameter. There may be a single tumor or multiple smaller tumors.
Diagnosis of WDPM
WDPM is not always immediately diagnosed because it does not often present symptoms severe enough to send someone to the doctor. However, if a patient complains of swelling or pain in the abdomen or testes, a doctor will begin with a physical examination. Next, an X-ray, CT scan, or MRI will image the body, allowing a doctor to see any tumors or abnormal tissues.
A biopsy is the next likely step. A thin needle is inserted where the imaging scan shows a potential tumor. The needle removes a small tissue or fluid sample to be examined by a pathologist. In cases where the tumor is unreachable by a biopsy needle, a patient may need to undergo a more invasive procedure to obtain a biopsy sample. Because WDPM does not always present symptoms, the condition is often only diagnosed after a patient has had surgery for another reason and a tumor is discovered.
Pathologists examine biopsy samples under a microscope to determine if the sample is benign or malignant. In addition, the pathologist can collect information to identify the type of cancer and its cellular type. Sometimes characteristic structures of WDPM can be seen in a CT scan. However, the best way to diagnose it is with a biopsy. Since malignant and aggressive types of mesothelioma may have papillary structures, a biopsy to distinguish between these cancers and WDPM is important for informing treatment.
Even with a biopsy, it is possible to misdiagnose WDPM. WDPM may be mistaken for an adenomatoid tumor, tuberculosis peritonitis, different types of papillary carcinomas, peritoneal carcinomatosis, or reactive mesothelial hyperplasia. Checking for certain biomarkers in a biopsy sample is another way to make a more accurate diagnosis. Since genetic alterations also drive malignancies in many cancer, checking for cancer specific mutation may also help making rigfht diagnosis. A recent study which include genome-wide sequencing analyses of WDPM tumor showed that all tumors harbored somatic mutations in either the TRAF7 or CDC42 genes.
Because WDPM is so rare, there is no standard treatment plan. Treatment strategy will vary individually. Surgery may remove a WDPM tumor, although some doctors feel this is an overly aggressive treatment strategy for a typically benign cancer.
Chemotherapy and radiation are other treatment options. However, the disease may recur after some time. In one study, patients that received both treatments have a metastasis 13 years after his treatment. This study highlights the malignant potential of WDPM.
In addition, WDPM patients may benefit cytoreductive surgery (CRS) with hyperthermic intraperitoneal chemotherapy (HIPEC). One study suggest that cytoreduction and HIPEC treatment is more effective than debulking surgery in terms of disease recurrence or progression.
Determining a prognosis for WDPM is difficult because it is such a rare disease. Cases that have been studied resulted in varying survival rates, although generally the prognosis is better than for malignant mesothelioma. In one case study, a woman lived 24 years after diagnosis. Another study of 24 patients with pleural WDPM, survival times ranged from three to 15 years. In a case study of WDPM of the tunica vaginalis, the man was healthy and tumor-free four years after diagnosis.
Although WDPM is largely non-malignant, it can become malignant in some cases. It is important that patients understand that the rarity of the condition means definite answers are virtually impossible. Regular follow-up exams and screenings are important to ensure WDPM does not become malignant.
Page Medically Reviewed and Edited by Pinar Kanlikilicer, PhD
Dr. Pinar Kanlikilicer has a PhD in Biomedical Engineering. She completed her 5-years of postdoctoral training in the Department of Experimental Therapeutics at MD Anderson Cancer Center. She is currently working in the field of cancer as a research scientist.