Not All Complex Pelvic Cysts Are Ovarian Tumours

By Dr. Jessie Wai Leng Phoon, MRCOG
Medical Director, GenPrime Fertility Singapore
Last reviewed: January 2026

Being told that imaging has found a “multicystic pelvic mass” can be frightening. Many people immediately worry about ovarian cancer or a serious gynaecological condition—especially when scans show complex or multiloculated cysts.

However, not all complex pelvic cysts are ovarian tumours or malignancies. Some rare but benign conditions can closely mimic cancer on imaging, making diagnosis challenging even for experienced clinicians.

This article explains two such conditions—subserosal adenomyotic cysts and peritoneal inclusion cysts—and why awareness of these diagnoses matters. Drawing from published clinical cases in Singapore and broader specialist experience, it highlights how careful evaluation and specialist insight can help avoid unnecessary anxiety and intervention.

Why some pelvic cysts are misdiagnosed

Large, multiloculated pelvic cysts often raise concern because:

• They may appear complex on ultrasound or MRI
• They are sometimes separate from the ovaries
• Tumour markers may be inconclusive

As a result, these cysts are frequently misinterpreted as ovarian tumours or even malignancy, leading to anxiety and, in some cases, extensive surgery.

Two rare conditions that can present this way are adenomyotic cysts and peritoneal inclusion cysts.

What are subserosal adenomyotic cysts?

Adenomyosis is a common condition where endometrial tissue grows within the muscle of the uterus. However, cystic adenomyosis—especially cysts located on the outer (subserosal) surface of the uterus—is uncommon.

Key features

• Usually affects premenopausal women
• May present with:
  
  • Abnormal uterine bleeding
  • Painful periods
  • Chronic pelvic pain
• Can appear as multiple cysts on the uterus, rather than a single lesion

Because this presentation is rare, it can easily be mistaken for an ovarian or adnexal mass on imaging.

What happened in the reported case of adenomyotic cysts?

In one of the published cases:

• A woman presented with abnormal uterine bleeding and imaging showing a large, complex cystic mass near the uterus
• MRI suggested a possible tubal or ovarian cause
• Surgery revealed multiple cysts on the uterine surface, rather than an ovarian tumour
• Histology confirmed benign adenomyotic cysts

After surgical excision and hormonal treatment, the patient remained well with no recurrence on follow-up imaging.

What are peritoneal inclusion cysts?

Peritoneal inclusion cysts are benign fluid-filled cysts that arise from the lining of the abdominal cavity (the peritoneum).

Important points

• They are rare and often under-recognised
• Many patients have a history of:
  
  • Previous pelvic surgery
  • Inflammation
  • Endometriosis or infection
• They can form multiple cystic pockets that surround pelvic organs

On imaging, these cysts can look alarming and are often mistaken for ovarian cancer or peritoneal spread of disease.

What happened in the reported case of peritoneal inclusion cysts?

In the second case:

• A woman with a history of pelvic surgery was found to have a large, multiloculated pelvic cyst
• Imaging raised concern for possible malignancy
• Surgery revealed multiple cysts over the uterus, bladder lining, and pelvic sidewalls
• Final histology confirmed benign peritoneal inclusion cysts

Follow-up imaging showed no recurrence one year after surgery.

How are these conditions diagnosed?

Unfortunately, imaging alone cannot always give a definitive diagnosis.

• Ultrasound and MRI are helpful for characterisation
• Tumour markers may be normal or mildly raised
• Final diagnosis often requires surgical assessment and histology

This is why these conditions are sometimes only diagnosed after surgery intended to rule out malignancy.

How are adenomyotic cysts treated?

Management depends on symptoms, age, and fertility goals.

Options may include:

• Hormonal treatment, similar to treatment for adenomyosis
• Surgical excision of cysts for symptom relief or diagnostic certainty
• Hysterectomy may be considered for those who do not desire future fertility

Because these cysts can recur, long-term follow-up is often recommended.

How are peritoneal inclusion cysts treated?

Treatment is individualised and may include:

• Surgical excision, often done to exclude malignancy
• Medical management (such as hormonal therapy) in selected cases
• Symptom-focused care, especially given recurrence risk

Recurrence rates can be significant, even after surgery, so management often focuses on relief of symptoms rather than cure.

What this means for fertility and future planning

Both conditions are benign, but:

• They may affect reproductive-age women
• Surgery can involve the uterus or ovaries
• Fertility wishes should be discussed before treatment decisions are made

Where possible, conservative or fertility-preserving approaches may be considered, depending on symptoms and clinical findings.

Questions to ask your doctor

If you’ve been told you have a complex or multicystic pelvic mass, you may want to ask:

• Could this be something other than an ovarian tumour?
• Are adenomyotic cysts or peritoneal inclusion cysts possibilities in my case?
• What additional imaging would help clarify the diagnosis?
• Do I need surgery to confirm what this is?
• How might treatment affect my fertility or hormonal health?
• What follow-up will be needed after treatment?

A simple next step

If imaging shows a complex pelvic cyst, the most important step is specialist evaluation and careful counselling.

While cancer must always be excluded, it’s equally important to recognise that some rare but benign conditions can look alarming on scans. A thoughtful, individualised approach helps avoid unnecessary intervention while ensuring safety.