Risk of Malignancy Index (RMI) for Ovarian Cancer

Why use it

Ovarian cancer is the most lethal gynecological malignancy, with 5-year survival <30% when diagnosed at advanced stage vs. >90% when confined to the ovary (Stage I). The RMI enables stratified triage of adnexal masses: women with RMI ≥200 should be referred to specialist gynecologic oncology centers where comprehensive surgical staging and cytoreductive surgery by trained subspecialists produces the best outcomes. Women with RMI <200 can be safely managed at general gynecology centers with lower operative morbidity. Without systematic risk stratification, women with likely benign masses may be unnecessarily referred to high-volume cancer centers, while women with malignant masses may receive suboptimal surgery at non-specialist centers — both scenarios are harmful. The RMI provides the best single-variable triage tool for this decision and is recommended by NICE guideline NG12 and RCOG Green-top Guideline 62.

About

The Risk of Malignancy Index (RMI) is a validated preoperative scoring tool developed by Jacobs and colleagues and published in the British Journal of Obstetrics and Gynaecology in 1990 to estimate the probability that an adnexal (pelvic/ovarian) mass is malignant. The index combines three clinical variables into a single numeric score: (1) an ultrasound score (U) based on the number of suspicious features present on pelvic ultrasound; (2) menopausal status (M); and (3) serum CA-125 level (in U/mL). The product RMI = U × M × CA-125 generates a continuous score where higher values indicate greater malignancy risk. A threshold of RMI ≥200 is the most widely validated cutoff, yielding sensitivity approximately 75–85% and specificity approximately 90–97% for discriminating benign from malignant adnexal masses in prospective series. The RMI was developed from a cohort of 143 patients and validated in numerous international studies encompassing thousands of patients. Three subsequent iterations exist — RMI II (Tingulstad 1996), RMI III (Tingulstad 1999), and RMI IV (Yamamoto 2009) — but RMI I remains the most widely used, endorsed by RCOG, BGCS (British Gynaecological Cancer Society), and incorporated into NICE guidelines for ovarian cancer assessment. The primary clinical use is triaging patients with adnexal masses for referral to gynaecologic oncology centers (RMI ≥200) vs. management at general gynecology units (RMI <200).

When to use

• Preoperative evaluation of any woman with a newly identified adnexal mass being considered for surgical management
• Triage decision: referral to gynecologic oncology (RMI ≥200) vs. management at general gynecology center (RMI <200)
• Complementing clinical assessment in women with incidental adnexal mass on imaging
• Prior to laparoscopic cystectomy or oophorectomy — to ensure appropriate center and surgeon expertise
• NOT validated for women currently on chemotherapy, women with known endometriosis-associated CA-125 elevation, or acute abdominal presentation requiring emergency surgery
• NOT a diagnostic test for ovarian cancer — histopathology remains the only definitive diagnosis
• NOT validated in premenarchal girls (different CA-125 ranges and tumor biology)

Formula

Pearls & pitfalls

• The RMI should be used together with clinical assessment, not in isolation — a woman with RMI 180 but strong family history (BRCA1/2 mutation), bilateral masses, or ascites should still be referred to gynecologic oncology despite borderline score.
• CA-125 elevation has many non-malignant causes: endometriosis, fibroids, pelvic inflammatory disease, benign ovarian cysts, peritonitis, liver disease, and normal menstruation — context is critical, especially in premenopausal women.
• The menopausal status multiplier (M=1 premenopausal, M=3 postmenopausal) appropriately weights the higher baseline malignancy risk in postmenopausal women with adnexal masses — any new adnexal mass in a postmenopausal woman warrants more aggressive investigation.
• RMI II uses M=4 for postmenopausal and U=4 for multiple ultrasound features, while RMI III uses U=3 for any features present. Meta-analyses suggest RMI I and RMI II perform comparably; RMI I is most widely studied and recommended by BGCS/RCOG.
• The five ultrasound features scored in RMI are: (1) multilocularity; (2) solid areas/components; (3) bilateral lesions; (4) ascites; (5) intra-abdominal metastases. Each feature present adds to the U score.
• CA-125 has lower sensitivity in mucinous ovarian carcinoma (CA-125 elevated in only ~40% vs. ~80% for serous carcinoma) — HE4 (human epididymis protein 4) or the ROMA score (Risk of Ovarian Malignancy Algorithm combining CA-125 + HE4) may be more sensitive in CA-125-negative cases.
• The IOTA (International Ovarian Tumour Analysis) group has developed ultrasound-based classification systems (IOTA simple rules, ADNEX model) that may outperform RMI for ultrasound characterization — these are increasingly adopted in specialist centres.
• In women with suspected recurrent ovarian cancer (previously treated), CA-125 rising above nadir is a strong indicator of biochemical recurrence — use serial CA-125, not RMI, in the follow-up setting.
• Dermoid cysts (mature cystic teratomas) typically appear characteristic on ultrasound (hyperechoic, shadowing) with normal CA-125 — low RMI expected but ultrasound pattern allows confident benign diagnosis without surgery in many cases.
• BRCA1/2 mutation carriers and BRCA-positive family history should be flagged regardless of RMI — these women have lifetime ovarian cancer risk of 15–45% (BRCA1) and 10–20% (BRCA2), and risk-reducing salpingo-oophorectomy is recommended at age 35–45.

Next steps

RMI <25 (Low Risk): Very low probability of malignancy. Conservative management or elective surgery at general gynecology center. Reassess with repeat ultrasound in 3–6 months if managing conservatively. RMI 25–199 (Moderate Risk): Moderate risk — cannot exclude malignancy. Discuss management at general gynecology unit with appropriate surgical staging preparedness, OR refer to gynecologic oncology depending on additional clinical features (patient age, family history, ultrasound complexity, CA-125 trend). RMI ≥200 (High Risk): High risk of malignancy. Refer to specialist gynecologic oncology center urgently. Surgery should be performed by or with a trained gynecologic oncologist to allow comprehensive staging and optimal cytoreduction. Avoid laparoscopic rupture of likely malignant cysts — associated with upstaging.

Evidence

Jacobs I, Oram D, Fairbanks J, Turner J, Frost C, Grudzinskas JG. A risk of malignancy index incorporating CA 125, ultrasound and menopausal status for the accurate preoperative diagnosis of ovarian cancer. Br J Obstet Gynaecol. 1990;97(10):922-929. PMID:2223684. Original derivation: n=143, sensitivity 85%, specificity 97%, PPV 78% at RMI cutoff ≥200. Tingulstad S et al. Evaluation of a risk of malignancy index based on serum CA125, ultrasound findings and menopausal status in the pre-operative diagnosis of pelvic masses (RMI II). Br J Obstet Gynaecol. 1996;103(8):826-831. PMID:8760719. Systematic review: Geomini P et al. The accuracy of risk scores in predicting ovarian malignancy: a systematic review. Obstet Gynecol. 2009;113(2 Pt 1):384-394 — RMI sensitivity 78%, specificity 87% in meta-analysis of 47 studies. NICE NG12 Guideline: Suspected cancer: recognition and referral (2015, updated 2023) — endorses RMI for ovarian cancer triage. RCOG Green-top Guideline 62: Management of Suspected Ovarian Masses in Premenopausal Women (2011).

References

1. Jacobs I et al. A risk of malignancy index incorporating CA 125, ultrasound and menopausal status for the accurate preoperative diagnosis of ovarian cancer. Br J Obstet Gynaecol. 1990;97(10):922-929. — link
2. Tingulstad S et al. Evaluation of a risk of malignancy index based on serum CA125, ultrasound findings and menopausal status in the pre-operative diagnosis of pelvic masses (RMI II). Br J Obstet Gynaecol. 1996;103(8):826-831. — link
3. Geomini P et al. The accuracy of risk scores in predicting ovarian malignancy: a systematic review. Obstet Gynecol. 2009;113(2 Pt 1):384-394. — link
4. NICE Guideline NG12. Suspected cancer: recognition and referral. National Institute for Health and Care Excellence. 2015, updated 2023. — link
5. RCOG Green-top Guideline No. 62. Management of Suspected Ovarian Masses in Premenopausal Women. Royal College of Obstetricians and Gynaecologists. 2011. — link
6. Van Calster B et al. (IOTA group). Evaluating the risk of ovarian cancer before surgery using the ADNEX model. BMJ. 2014;349:g5920. — link