As these tumours are by definition benign, their prognosis is very good.

Traditional practice recommends removal on the grounds that this prevents torsion and malignant transformation. However, many small MCTs found incidentally on transvaginal ultrasound can be managed conservatively and if slow growing are unlikely to cause clinical symptoms [6, 7, 12]. Factors which increase the likelihood of women managed expectantly then choosing to have surgery include multiparity, young age, larger cysts, bilateral cysts and past history of ovarian cyst [12]. Larger symptomatic cysts may need excision by the open or laparoscopic route. If predominantly cystic, these cysts can usually be managed laparoscopically, but care needs to be taken to avoid spillage of the contents into the abdominal cavity as this can stimulate a peritonitic reaction [16]. The use of endoscopic retrieval bags can help to minimise this. As small asymptomatic cysts are being found with increasing frequency, these are increasingly being managed conservatively with success [6, 7] (Fig. 3).

MCT is bilateral in between 10 and 15 % of cases [1]. Multiple teratomas are seen in the ipsilateral ovary in less than 1 % of cases [10]. The tumours are round or ovoid and range in size from 1 to 30 cm, but most are within the 5–10 cm diameter range [25]. They have thick white capsules, which are usually free of adhesions. They are usually unilocular cysts filled with thick greasy sebaceous material and matted hair [25]. These sometimes round up to form sebum and hair balls (Figs. 4 and 5). The sebaceous contents have a fluid consistency above 35 °C which becomes doughy or solid at room temperature [11, 32]. Some cysts however are filled with predominantly mucoid or clear fluid. Most are unilocular, and between 5 and 10 % are multilocular. In 87 % of cases [11], a protuberance can be seen projecting into the cyst cavity called a dermoid protuberance (or Rokitansky tubercle/dermoid mamilla) [32]. The cyst lining is usually smooth like skin and the hair generally grows from this tubercle. Teeth are present in up to one third of cases. Other tissues commonly identified are cartilage, bone, brain and adipose tissue. Older lesions may exhibit marked dystrophic calcification [25]. Hemangiomas are unusual and give rise to highly vascular structures [11]. More organised structures such as intestine; skeletal parts, especially jaw bone; and eyes may be present [25] (Figs. 6 and 7).

Typically, the inner surface of the cyst is lined by epidermis which desquamates into the lumen [11]. Skin adnexal structures such as hair follicles and sebaceous and sweat glands are often present on the dermal papilla [25] and a large amount of mature tissues are usually found at and around this area [11]. Other ectodermal derivatives such as cerebrum, cerebellum, choroid plexus and retina may also be found [24]. The most common mesodermal derivatives are smooth or striated muscle, adipose tissue, bone, teeth and cartilage. Less common tissues are thyroid, peripheral nerve, respiratory and gastrointestinal epithelium, teeth and salivary gland [25].

Transvaginal ultrasonography has an established role in the evaluation of adnexal masses and provides an accurate assessment of ovarian morphology. It is also well known that dermoid cysts have distinct sonographic grayscale morphological features, which can be used to accurately diagnose dermoid cysts (‘pattern recognition method’) [14, 33–35] (Figs. 8 and 9).

Previous studies examining the ultrasound morphology of MCT have tried to analyse the appearance of the whole tumour. Laing et al. [18] in 1981 analysed 51 MCTs and found that ‘33 % of the cases demonstrated “typical” findings, i.e. “an echogenic focus with acoustic shadowing in a predominantly cystic mass”, 23.5 % were predominantly solid, 20 % were almost entirely cystic, and 23.5 % were not visible’ on ultrasound scan (Fig. 10). Bronshtein used an in vitro simulation with hair and oil in a glove and showed that this replicated the ‘white ball’ of dermoid very well. They also showed that hair in water can produce the same ‘dot and dash’ echo pattern seen in some dermoid cysts. Cohen [8] in 1993 and Caspi [5] in 1996 attempted to categorise the morphology of MCT into different groups based on the presence of acoustic shadowing and the amount of echogenic material within the cyst cavity. It is our opinion that this approach is rather complicated and that once individual features of hair or sebum are recognised, these alone are sufficient to make a diagnosis in the vast majority of MCT.

Immature teratomas, in common with the mature teratomas, contain tissues from the three germinal layers, but in contrast to mature teratomas, they contain immature or embryonal structures [31]. These tumours arise from postmitotic germ cells and contain immature (embryonal) structures as well as mature elements in most cases [13]. The additional presence of other malignant germ cell tumour elements places the tumour in the teratomas group, and these are excluded from this study. Pure immature teratomas are rare tumours which comprise <1 % of ovarian teratomas overall [15, 22, 31, 32, 37]. They represent 20 % of malignant germ cell tumours and approximately 1 % of malignant ovarian tumours [3] and account for up to 25 % of malignant ovarian tumours presenting in girls <17 years of age [4].