Bone Cyst of the Pelvis and Hips
Cystic or hollow-looking areas in the bones of the pelvis and hips are common imaging findings, and most are either a consequence of joint wear or benign lesions of no consequence. This region differs from the limbs in two ways that matter. The bones are thick and overlapped by bowel and soft tissue, so lesions are harder to see on X-ray and are frequently detected first on cross-sectional imaging performed for another reason. And the pelvis is a common site for secondary cancer deposits in adults, so a destructive lesion here carries different weight than the same appearance in a child’s knee. Age, location, and the sharpness of the lesion’s border do most of the work in distinguishing these.
What is it?
The pelvis is a ring of bone formed by two large paired bones joined to the sacrum at the back and to each other at the front. The hip joint sits where the socket of the pelvis receives the head of the thigh bone, and the upper thigh bone—its head, neck, and the bony prominences below—is considered alongside the pelvis because the two function together.
A bone cyst is a fluid-filled cavity within bone, and as elsewhere in the skeleton, several distinct processes produce cystic or lucent appearances here.
Two features of this region shape how such findings are approached.
The bones are thick and are overlapped by bowel, gas, and soft tissue. This makes X-ray assessment considerably less reliable than in the limbs, and lesions that would be obvious in a forearm can be invisible in the pelvis. A consequence is that many pelvic bone lesions are first identified on CT or MRI performed for an abdominal, urological, or gynaecological reason, where the bones are visible incidentally.
The pelvis is one of the most common sites in the skeleton for secondary cancer deposits. The marrow here is red marrow—active, blood-forming marrow with a rich blood supply—throughout adult life, and this is where circulating cancer cells tend to lodge. In an adult over 40, a destructive lesion in the pelvis or upper thigh bone raises the possibility of spread from a cancer elsewhere considerably more than the same appearance would in a young person’s limb.
The lesions commonly encountered include the following.
Subchondral cysts form in the bone beneath a worn hip joint surface, on either the socket or the head of the thigh bone. They are a feature of osteoarthritis rather than an independent condition, and they are particularly associated with the structural abnormalities that drive hip arthritis—a cyst at the junction of the head and neck of the thigh bone is a recognised feature of femoroacetabular impingement, where repeated contact between the two damages the cartilage at that point.
Fibrous dysplasia has a particular association with this region. In this condition, areas of normal bone are replaced by fibrous tissue, producing a hazy, ground-glass appearance rather than a clear fluid cavity. The femoral neck is a characteristic site, and extensive involvement there can cause the bone to bend progressively under load, producing a curved deformity sometimes described as a shepherd’s crook. It may affect one bone or several, and in the latter case can be associated with a broader syndrome involving skin pigmentation and hormonal abnormalities.
Simple bone cysts occur in the upper thigh bone in children and adolescents, this being the second most common site after the upper arm bone. Their significance is structural, since a cyst in the femoral neck weakens a region under high load.
Aneurysmal bone cysts occur in the pelvis and upper thigh bone in people under 20, expanding the bone and requiring active treatment.
Intraosseous ganglia occur adjacent to the hip joint in adults.
Enchondromas and chondromas occur in the pelvis, and distinguishing a benign cartilage lesion from a low-grade cartilage cancer is more difficult here than in the limbs. Cartilage tumours of the pelvis have a higher likelihood of being malignant than those in the hands, and a cartilage lesion in the pelvis is therefore assessed more carefully than the same appearance elsewhere.
Giant cell tumours occur in the pelvis and in the upper thigh bone, requiring treatment given their local aggressiveness.
Several normal variants and benign findings are worth knowing because they appear on reports.
The upper part of the thigh bone contains a region where the internal struts are naturally sparse, since forces pass around rather than through it. This appears as a lucent area on X-ray and is normal anatomy, analogous to the equivalent appearance in the heel bone.
Intraosseous lipomas occur in the upper thigh bone and have a characteristic appearance.
Bone islands—small areas of dense bone—are common in the pelvis and are entirely benign.
Degenerative cysts around the joints at the back of the pelvis are common with age and are usually unimportant.
Malignant lesions are the principal reason for assessment.
Secondary deposits from cancers elsewhere are the leading cause of a destructive bone lesion in an adult over 40 in this region. The commonest primary sources are breast, prostate, lung, kidney, and thyroid. Deposits may be destructive, bone-forming, or mixed, with prostate cancer characteristically producing dense rather than lucent lesions.
Myeloma produces multiple well-defined lucent lesions, commonly involving the pelvis, and is an important consideration in older adults, particularly with anaemia, kidney impairment, or raised calcium.
Ewing sarcoma occurs in children and young adults and has a particular predilection for the pelvis, where it can grow substantially before causing symptoms because the region accommodates expansion without obvious deformity. It typically produces pain including at night, sometimes with fever and feeling generally unwell, and can be mistaken for infection.
Chondrosarcoma, a cancer arising from cartilage, occurs in the pelvis more commonly than at most other sites, generally in adults over 40.
The features distinguishing benign from concerning lesions are the same as throughout the skeleton, though harder to assess here.
Reassuring features are a sharply defined border, a surrounding rim of denser bone, an intact cortex, no reaction of the bone surface, and no soft tissue mass.
Concerning features are an ill-defined border, a patchy or moth-eaten pattern of destruction, loss of the cortex, aggressive bone surface reaction, a soft tissue mass, and change over a short interval.
Age remains the most powerful discriminator. In a child or adolescent, lesions here are most often simple or aneurysmal bone cysts, fibrous dysplasia, or—in the case of aggressive appearances—Ewing sarcoma. In an adult over 40, a destructive lesion raises the possibility of secondary cancer or myeloma before anything else.
Symptoms are frequently absent, and when present are often vague.
Deep groin, buttock, or hip pain is typical, and is poorly localised because the pelvic bones are deep and not readily palpable. This vagueness contributes to delayed diagnosis, since the pain is easily attributed to muscular strain, the hip joint, or the lower back.
Pain may be referred to the thigh and knee, following the pattern of hip joint disease.
Pain on weight bearing and a limp occur with lesions affecting the hip or upper thigh bone.
Sudden pain with inability to weight bear suggests fracture through weakened bone, which in the femoral neck is a significant injury.
The symptom pattern warranting attention is pain that is constant rather than activity-related, night pain, and progressive worsening. In the pelvis these carry particular weight, because the region’s capacity to accommodate a growing lesion without obvious signs means that symptoms may be the only indication for a considerable time.
Assessment differs from the limbs in sequence.
X-ray remains the starting point for characterisation, but its limitations here are substantial, and a normal pelvic X-ray provides considerably less reassurance than a normal X-ray of a limb.
CT demonstrates pelvic bone detail far better and is frequently the study on which a lesion is first seen. It shows cortical integrity, mineralisation within a lesion, and the pattern of destruction clearly.
MRI is the most sensitive technique for marrow lesions, assesses the extent within the bone, identifies soft tissue components, and detects lesions invisible on other modalities. It is essential where a lesion is not confidently benign.
Where secondary cancer is a consideration, bone scan or PET-CT assesses the rest of the skeleton, and investigation for a primary source follows.
Blood tests including a full blood count, calcium, kidney function, and protein studies are appropriate where myeloma is considered.
Comparison with previous imaging is frequently decisive, and given how often pelvic bones appear incidentally on abdominal scans, earlier studies are often available.
The procedural point about biopsy applies with particular force here. The pelvis is a technically demanding region for surgery, and an unplanned biopsy through an inappropriate approach can compromise a later operation substantially. Referral to a specialist bone tumour service before any biopsy is recommended where a lesion is not confidently benign.
Important to Know
Most bone lesions found in the pelvis and hips are benign and require nothing, but this region warrants a slightly lower threshold for investigation than the limbs because lesions are harder to see, symptoms are vaguer, and secondary cancer is more common here. Care is managed by orthopaedic services, with specialist bone tumour centres where required.
Several points help interpret a report.
A well-defined lesion with a sclerotic rim, in a characteristic location, is reassuring here as elsewhere, and frequently requires no action.
A lucent area in the upper thigh bone where the bone’s internal struts are naturally sparse is normal anatomy rather than a lesion.
Subchondral cysts around the hip indicate that the cartilage above them has worn. The relevant finding is the state of the joint, and a cyst at the head-neck junction specifically suggests impingement, which may itself be worth assessing in a younger person since treating it can alter the course.
Lesions in the femoral neck deserve particular attention regardless of their nature, because this region carries high load and a weakened neck risks fracture. A fracture here is a significant injury with the possibility of damage to the blood supply of the femoral head. Where a lesion occupies a substantial proportion of the neck, prophylactic treatment—curettage and grafting, or fixation—is considered even where the lesion itself is benign.
Fibrous dysplasia of the femoral neck is managed according to its extent and the degree of deformity, with observation appropriate for limited involvement and surgical correction considered where bending is progressive or fracture has occurred. Medication affecting bone turnover reduces pain in some people.
Treatment of symptomatic or structurally significant lesions follows the usual approach of curettage with filling, though access in the pelvis is more demanding than in the limbs.
Aneurysmal bone cysts and giant cell tumours require active treatment and follow-up given their tendency to recur.
Cartilage lesions in the pelvis warrant specialist assessment rather than routine observation, given the higher likelihood of malignancy at this site compared with the hands and limbs.
Where a lesion proves to be a secondary deposit, management is directed by the oncology team. Local treatment including radiotherapy and, where a bone is at risk of fracture, surgical stabilisation is considered. A lesion in the femoral neck or the area above the hip socket that threatens structural integrity may be fixed prophylactically, since fixing a bone before it breaks produces considerably better outcomes than fixing it afterwards.
Several situations warrant prompt investigation.
A new bone lesion in anyone with a known cancer requires assessment, since the pelvis is a common site for spread.
Multiple lucent lesions in an older adult, particularly with anaemia, kidney impairment, raised calcium, or back pain, warrant investigation for myeloma.
Persistent deep pelvic, groin, or buttock pain in a child or young adult, particularly with night pain, fever, or feeling unwell, should be investigated rather than attributed to muscular strain. Ewing sarcoma occurs here, grows substantially before producing obvious signs, and can be mistaken for infection or for a sporting injury.
A pelvic lesion with any aggressive feature warrants referral regardless of how well the person feels.
A note about incidental findings is particularly relevant here, because pelvic bones appear on every abdominal and pelvic CT and MRI, and lesions are frequently identified in people being scanned for entirely unrelated reasons. Most are longstanding and benign. The appropriate response is characterisation, usually achieved by reviewing the appearance against the features described above and comparing with any previous imaging, rather than either dismissal or alarm.
Care is managed by orthopaedics with specialist bone tumour services as required, and with oncology where secondary disease is identified. Imaging features together with age and location carry more weight than size, and the limitations of X-ray in this region mean that cross-sectional imaging frequently contributes more than it would in a limb.
Patient education plays an important role. Understanding that most pelvic bone lesions are incidental and benign, that X-rays are less reliable here than in the limbs, that lesions in the femoral neck warrant attention because of fracture risk, that secondary cancer is a more common consideration in adults over 40, and that vague deep pelvic pain with night pain in a young person warrants investigation all contribute to appropriate care.
Red flag symptoms include pain that is constant rather than related to activity; pain that wakes you at night; pain that is progressively worsening over weeks; deep groin, buttock, or pelvic pain in a child or young adult, particularly with fever or feeling generally unwell; sudden pain with inability to bear weight, suggesting fracture; a new bone lesion in anyone with a history of cancer; unexplained weight loss, night sweats, anaemia, or raised calcium; back or pelvic pain with multiple lucent lesions, suggesting myeloma; and any lesion reported as having ill-defined margins, cortical destruction, aggressive periosteal reaction, or a soft tissue mass. These warrant prompt medical evaluation.