- Email: [email protected]
Involuted intraosseous lipoma of the sacrum showing high signal intensity on T1-weighted magnetic resonance imaging (MRI)
J Orthop Sci (2001) 6:183–186
Involuted intraosseous lipoma of the sacrum showing high signal intensity on T1-weighted magnetic resonance imaging (MRI) Satoru Kamekura1, Kozo Nakamura1, Hiromi Oda1, Koichi Inokuchi1, Takuo Iijima1, and Tsuyoshi Ishida2 1 2
Department of Orthopaedic Surgery, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan Department of Pathology, Faculty of Medicine, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
Abstract Involuted intraosseous lipoma with extensive fat necrosis resulting in cyst formation (Milgram stage III) is distinguishable from lesions without necrosis (stage I) or lesions with focal fat necrosis (stage II), based on differences in signal intensity on magnetic resonance imaging (MRI). Fat tissue has a high signal intensity on both T1- and T2-weighted MR images, whereas the extensive fat necrosis that results in cyst formation shows high signal intensity on T2-weighted images and low intensity on T1-weighted images. We report a patient in whom an intraosseous lipoma with high signal intensity on both T1- and T2-weighted MRI was found to be extensively involuted on histopathologic examination. Intraosseous lipoma appears to undergo spontaneous involution. In some patients, therefore, surgical excision may not be necessary. A correct preoperative diagnosis should reduce the necessity for a biopsy or surgery. Although lesions classified as stage I or II are easily identified by MRI, those of stage III are difficult to diagnose preoperatively by this method. Key words Intraosseous lipoma · Sacrum · Magnetic resonance imaging · Fat necrosis · Involution
Introduction Intraosseous lipomas are benign tumors that occur in approximately 1 of 1000 bone tumor patients.5 The lesion primarily consists of fat tissue. Intraosseous lipomas frequently involute spontaneously through the processes of infarction, calcification, cyst formation, and reactive bone formation. In some instances, the lipo-
cytes of the original lesion may not be identifiable histopathologically, but lipomas with involutional changes have characteristic radiographic and microscopic morphologic features.7 Based on his clinicopathological study of 66 cases, Milgram7 advocated classifying intraosseous lipoma into three stages by the degree of involution: stage I, solid tumors composed of viable fat cells; stage II, transitional phase tumors with partial fat necrosis and focal calcification, but regions of viable fat cells as well; and stage III, late phase tumors in which the fat cells have died and there is a variable degree of cyst formation, calcification, and reactive woven bone formation. He reported that local treatment was usually sufficient for stage III intraosseous lipomas and that these lesions were treatable by observation alone. Because intraosseous lipoma appears to undergo spontaneous involution, surgical excision may not be necessary when an accurate assessment of the degree of involution and accurate preoperative stage diagnosis are possible.7 Blacksin et al.1 used MRI for a more definitive diagnosis in their radiologic-pathologic correlation study: lesions consisting of variable fat tissue had a high signal intensity on both the T1- and T2-weighted MR images, whereas those with extensive fat necrosis had a low signal intensity on the T1-weighted scans and high intensity on the T2-weighted scans. This finding has yet to be confirmed, because the MRI manifestation of stage III has been described in only two patients. We report a patient with a stage III intraosseous lipoma, whose MRI findings were representative of stage II.
Case report
Offprint requests to: S. Kamekura Received: May 12, 1998 / Accepted: November 7, 2000
A 49-year-old Japanese man was referred to us because of a sacral tumor seen on a plain radiograph taken for the presenting complaint of back pain. There was no
184
S. Kamekura et al.: Involuted intraosseous lipoma
Fig. 2. Computed tomography (CT). Note both the lowdensity area in the lesion (arrow), consistent with fat tissue, and the central calcification
high intensity within the lesion (Fig. 3a,b). The lesion was cold on a Tc-99m-metylene diaphosphonate (MDP) bone scan. Surgical procedure Fig. 1. Lateral plain radiograph. Note the well defined, radiolucent, round lesion accompanied by central calcification (arrow) in the anterior of the sacrum
significant medical or family history. Rectal examination detected a palpable hen-egg-sized, bony hard, smooth-surfaced tumor in the anterior aspect of the sacrum. The intraosseous radiolucent lesion present on the lateral plain radiograph was round and well demarcated, and protruded into the pelvic cavity. The anterior cortex in the fourth and fifth sacral vertebrae was eroded, and calcification was visible in the center of the lesion (Fig. 1). Computed tomography (CT) detected a well marginated, low-density lesion and areas of central calcification (Fig. 2). MR examinations were done with a permanent magnet system at a field strength of 0.3 T (MRP-7000-0; Hitachi Medical, Tokyo, Japan). Section thickness was 9 mm, with a 1-mm gap between sections. Standard spin echo sequences were obtained for the coronal and sagittal planes. Coronal T1-weighted imaging was done at TR 700/TE 25, and sagittal T2weighted imaging at TR 2000/TE 90. The T1-weighted coronal plane image showed a high signal intensity, and the T2-weighted sagittal plane image showed slightly
Surgery was performed 2 weeks after the preoperative radiographic examinations. The tumor was resected using a posterior approach, fenestrating the lateral side of the hiatus sacralis. The entire lesion appeared to be a cystic cavity with sclerotic rims of reactive new bone containing serous fluid, rather than a solid mass of tissue. After the fluid was suctioned off, no solid mass of tissue which might give a high signal intensity on a T1weighted MRI was detected. As much as possible of the thin soft brownish tissue lining the cystic cavity wall was scraped away, and the small tissue fragments obtained were submitted to histopathological examination. Pathological observations The lining tissue was composed of mature fat cells and extensive degenerative, necrotic fat cells, interspersed with irregular woven bone trabeculae (Fig. 4). Histologically, these findings were consistent with stage III intraosseous lipoma. Discussion MRI is a low-specificity imaging method, but its differentiation of tissue components, such as hematoma, fat,
S. Kamekura et al.: Involuted intraosseous lipoma
185
a
Fig. 4. Photomicrograph of the cyst wall, showing that it consists of degenerative fat cells with collagenous stroma and woven bone (arrowheads). The arrows point to the inner surface of the cyst wall. H&E, 310 b Fig. 3. a Coronal T1-weighted magnetic resonance imaging (MRI) (TR 700/TE 25). Note the high signal intensity in the lesion (arrow) consistent with fat tissue. b Sagittal T2weighted MRI (TR 2000/TE 90). Note the high signal intensity in the lesion (arrow)
necrosis, and cystic areas, may lead to the specific diagnosis of some bone tumors.2 Fat generally produces a high signal intensity on MRI; therefore, lesions composed primarily of fat should be readily identifiable by this method.6 Stage I and II intraosseous lipomas, which consist mainly of viable fat cells, show low density on CT scans and high signal intensities on T1- and T2weighted MRI.1 These radiographic features indicative of fat tissue are useful for making a preoperative diagnosis. Blacksin et al.1 reported that, in their two patients, the cystic regions of stage III lesions showed a low signal intensity on T1-weighted MRI and a high signal intensity on T2-weighted MRI because they had undergone extensive cystic/myxomatous changes.
In our patient, the plain radiograph that showed an expansile radiolucent lesion of the sacrum with central calcification was typical of a lipoma. The lesion had low density with focal calcification on CT, and high signal intensity on both T1- and T2-weighted MRI, indicative of fat tissue. These radiographic features are consistent with a stage II intraosseous lipoma. Although the images indicated the presence of proliferating fat tissue, no solid tissue mass was found, only a cystic cavity containing serous fluid. The histopathologic examination showed degenerative fat cells with collagenous stroma and woven bone. Based on the extensive involution of this lesion, we classified it as a stage III intraosseous lipoma. The lipocytes remaining in this stage III lesion, however, also provide evidence that totally involuted lesions may be derived from a stage I lesion or from proliferating lipocytes.7 Although MRI is considered a low-specificity imaging method, lesions which show high signal intensity on T1-weighted images are relatively limited and charac-
186
teristic. This high signal intensity usually points to the presence of fat tissue, hematoma, or a cyst with a high concentration of protein.2–4,8 The lesion in our patient did not have a solid mass of tissue that would produce a high signal intensity on T1-weighted MRI, rather, it was a cystic cavity containing only serous fluid. Because the cyst’s contents were not examined, the reason that this lesion showed high signal intensity on T1-weighted MRI is not known. Fat suppression pulse sequence imaging, a noninvasive modality, may be useful in differentiating fluid from fat tissue. There has been no previous report of a patient in whom an intraosseous lipoma with high signal intensity on both T1- and T2-weighted MRI was subsequently found to be extensively involuted on histopathologic examination. Involuted intraosseous lipoma should be considered in the differential diagnosis of a lesion with such MRI findings. Curettage or local resection is usually sufficient treatment for stage I and II intraosseous lipomas, and stage III lesions are treatable by observation alone.7 A correct preoperative diagnosis should reduce the necessity for a biopsy or surgery. Lesions classified as stage I or II are readily identified by MRI, whereas those of stage III are difficult to diagnose preoperatively because accurate
S. Kamekura et al.: Involuted intraosseous lipoma
assessment of the degree of involutional change is not possible, even with MRI.
References 1. Blacksin MF, Ende N, Benevenia J. Magnetic resonance imaging of intraosseous lipomas: a radiologic-pathologic correlation. Skeletal Radiol 1995;24:37–41. 2. Bohndorf K, Reiser M, Lochner B, et al. Magnetic resonance imaging of primary tumours and tumour-like lesions of bone. Skeletal Radiol 1986;15:511–17. 3. Brown JJ, vanSonnenberg E, Gerber KH, et al. Magnetic resonance relaxation times of percutaneously obtained normal and abnormal body fluids. Radiology 1985;154:727–31. 4. Cohen JM, Weinreb JC, Maravilla KR. Fluid collections in the intraperitoneal and extraperitoneal spaces: comparison of MR and CT. Radiology 1985;155:705–8. 5. Dahlin DC. Lipoma and liposarcoma. In: Dahlin DC, Unni KK, editors. Bone tumors. 4th ed. Springfield, Illinois: Charles C Thomas; 1986. pp. 181–5. 6. Dooms GC, Hricak H, Sollitto RA, et al. Lipomatous tumors and tumors with fatty component: MR imaging potential and comparison of MR and CT results. Radiology 1985;157:479– 83. 7. Milgram JW. Intraosseous lipomas. A clinicopathologic study of 66 cases. Clin Orthop 1988;231:277–302. 8. Sundaram M, McGuire MH, Herbold DR, et al. High signal intensity soft tissue masses on T1 weighted pulsing sequences. Skeletal Radiol 1987;16:30–6.
Involuted intraosseous lipoma of the sacrum showing high signal intensity on T1-weighted magnetic resonance imaging (MRI) Satoru Kamekura1, Kozo Nakamura1, Hiromi Oda1, Koichi Inokuchi1, Takuo Iijima1, and Tsuyoshi Ishida2 1 2
Department of Orthopaedic Surgery, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan Department of Pathology, Faculty of Medicine, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
Abstract Involuted intraosseous lipoma with extensive fat necrosis resulting in cyst formation (Milgram stage III) is distinguishable from lesions without necrosis (stage I) or lesions with focal fat necrosis (stage II), based on differences in signal intensity on magnetic resonance imaging (MRI). Fat tissue has a high signal intensity on both T1- and T2-weighted MR images, whereas the extensive fat necrosis that results in cyst formation shows high signal intensity on T2-weighted images and low intensity on T1-weighted images. We report a patient in whom an intraosseous lipoma with high signal intensity on both T1- and T2-weighted MRI was found to be extensively involuted on histopathologic examination. Intraosseous lipoma appears to undergo spontaneous involution. In some patients, therefore, surgical excision may not be necessary. A correct preoperative diagnosis should reduce the necessity for a biopsy or surgery. Although lesions classified as stage I or II are easily identified by MRI, those of stage III are difficult to diagnose preoperatively by this method. Key words Intraosseous lipoma · Sacrum · Magnetic resonance imaging · Fat necrosis · Involution
Introduction Intraosseous lipomas are benign tumors that occur in approximately 1 of 1000 bone tumor patients.5 The lesion primarily consists of fat tissue. Intraosseous lipomas frequently involute spontaneously through the processes of infarction, calcification, cyst formation, and reactive bone formation. In some instances, the lipo-
cytes of the original lesion may not be identifiable histopathologically, but lipomas with involutional changes have characteristic radiographic and microscopic morphologic features.7 Based on his clinicopathological study of 66 cases, Milgram7 advocated classifying intraosseous lipoma into three stages by the degree of involution: stage I, solid tumors composed of viable fat cells; stage II, transitional phase tumors with partial fat necrosis and focal calcification, but regions of viable fat cells as well; and stage III, late phase tumors in which the fat cells have died and there is a variable degree of cyst formation, calcification, and reactive woven bone formation. He reported that local treatment was usually sufficient for stage III intraosseous lipomas and that these lesions were treatable by observation alone. Because intraosseous lipoma appears to undergo spontaneous involution, surgical excision may not be necessary when an accurate assessment of the degree of involution and accurate preoperative stage diagnosis are possible.7 Blacksin et al.1 used MRI for a more definitive diagnosis in their radiologic-pathologic correlation study: lesions consisting of variable fat tissue had a high signal intensity on both the T1- and T2-weighted MR images, whereas those with extensive fat necrosis had a low signal intensity on the T1-weighted scans and high intensity on the T2-weighted scans. This finding has yet to be confirmed, because the MRI manifestation of stage III has been described in only two patients. We report a patient with a stage III intraosseous lipoma, whose MRI findings were representative of stage II.
Case report
Offprint requests to: S. Kamekura Received: May 12, 1998 / Accepted: November 7, 2000
A 49-year-old Japanese man was referred to us because of a sacral tumor seen on a plain radiograph taken for the presenting complaint of back pain. There was no
184
S. Kamekura et al.: Involuted intraosseous lipoma
Fig. 2. Computed tomography (CT). Note both the lowdensity area in the lesion (arrow), consistent with fat tissue, and the central calcification
high intensity within the lesion (Fig. 3a,b). The lesion was cold on a Tc-99m-metylene diaphosphonate (MDP) bone scan. Surgical procedure Fig. 1. Lateral plain radiograph. Note the well defined, radiolucent, round lesion accompanied by central calcification (arrow) in the anterior of the sacrum
significant medical or family history. Rectal examination detected a palpable hen-egg-sized, bony hard, smooth-surfaced tumor in the anterior aspect of the sacrum. The intraosseous radiolucent lesion present on the lateral plain radiograph was round and well demarcated, and protruded into the pelvic cavity. The anterior cortex in the fourth and fifth sacral vertebrae was eroded, and calcification was visible in the center of the lesion (Fig. 1). Computed tomography (CT) detected a well marginated, low-density lesion and areas of central calcification (Fig. 2). MR examinations were done with a permanent magnet system at a field strength of 0.3 T (MRP-7000-0; Hitachi Medical, Tokyo, Japan). Section thickness was 9 mm, with a 1-mm gap between sections. Standard spin echo sequences were obtained for the coronal and sagittal planes. Coronal T1-weighted imaging was done at TR 700/TE 25, and sagittal T2weighted imaging at TR 2000/TE 90. The T1-weighted coronal plane image showed a high signal intensity, and the T2-weighted sagittal plane image showed slightly
Surgery was performed 2 weeks after the preoperative radiographic examinations. The tumor was resected using a posterior approach, fenestrating the lateral side of the hiatus sacralis. The entire lesion appeared to be a cystic cavity with sclerotic rims of reactive new bone containing serous fluid, rather than a solid mass of tissue. After the fluid was suctioned off, no solid mass of tissue which might give a high signal intensity on a T1weighted MRI was detected. As much as possible of the thin soft brownish tissue lining the cystic cavity wall was scraped away, and the small tissue fragments obtained were submitted to histopathological examination. Pathological observations The lining tissue was composed of mature fat cells and extensive degenerative, necrotic fat cells, interspersed with irregular woven bone trabeculae (Fig. 4). Histologically, these findings were consistent with stage III intraosseous lipoma. Discussion MRI is a low-specificity imaging method, but its differentiation of tissue components, such as hematoma, fat,
S. Kamekura et al.: Involuted intraosseous lipoma
185
a
Fig. 4. Photomicrograph of the cyst wall, showing that it consists of degenerative fat cells with collagenous stroma and woven bone (arrowheads). The arrows point to the inner surface of the cyst wall. H&E, 310 b Fig. 3. a Coronal T1-weighted magnetic resonance imaging (MRI) (TR 700/TE 25). Note the high signal intensity in the lesion (arrow) consistent with fat tissue. b Sagittal T2weighted MRI (TR 2000/TE 90). Note the high signal intensity in the lesion (arrow)
necrosis, and cystic areas, may lead to the specific diagnosis of some bone tumors.2 Fat generally produces a high signal intensity on MRI; therefore, lesions composed primarily of fat should be readily identifiable by this method.6 Stage I and II intraosseous lipomas, which consist mainly of viable fat cells, show low density on CT scans and high signal intensities on T1- and T2weighted MRI.1 These radiographic features indicative of fat tissue are useful for making a preoperative diagnosis. Blacksin et al.1 reported that, in their two patients, the cystic regions of stage III lesions showed a low signal intensity on T1-weighted MRI and a high signal intensity on T2-weighted MRI because they had undergone extensive cystic/myxomatous changes.
In our patient, the plain radiograph that showed an expansile radiolucent lesion of the sacrum with central calcification was typical of a lipoma. The lesion had low density with focal calcification on CT, and high signal intensity on both T1- and T2-weighted MRI, indicative of fat tissue. These radiographic features are consistent with a stage II intraosseous lipoma. Although the images indicated the presence of proliferating fat tissue, no solid tissue mass was found, only a cystic cavity containing serous fluid. The histopathologic examination showed degenerative fat cells with collagenous stroma and woven bone. Based on the extensive involution of this lesion, we classified it as a stage III intraosseous lipoma. The lipocytes remaining in this stage III lesion, however, also provide evidence that totally involuted lesions may be derived from a stage I lesion or from proliferating lipocytes.7 Although MRI is considered a low-specificity imaging method, lesions which show high signal intensity on T1-weighted images are relatively limited and charac-
186
teristic. This high signal intensity usually points to the presence of fat tissue, hematoma, or a cyst with a high concentration of protein.2–4,8 The lesion in our patient did not have a solid mass of tissue that would produce a high signal intensity on T1-weighted MRI, rather, it was a cystic cavity containing only serous fluid. Because the cyst’s contents were not examined, the reason that this lesion showed high signal intensity on T1-weighted MRI is not known. Fat suppression pulse sequence imaging, a noninvasive modality, may be useful in differentiating fluid from fat tissue. There has been no previous report of a patient in whom an intraosseous lipoma with high signal intensity on both T1- and T2-weighted MRI was subsequently found to be extensively involuted on histopathologic examination. Involuted intraosseous lipoma should be considered in the differential diagnosis of a lesion with such MRI findings. Curettage or local resection is usually sufficient treatment for stage I and II intraosseous lipomas, and stage III lesions are treatable by observation alone.7 A correct preoperative diagnosis should reduce the necessity for a biopsy or surgery. Lesions classified as stage I or II are readily identified by MRI, whereas those of stage III are difficult to diagnose preoperatively because accurate
S. Kamekura et al.: Involuted intraosseous lipoma
assessment of the degree of involutional change is not possible, even with MRI.
References 1. Blacksin MF, Ende N, Benevenia J. Magnetic resonance imaging of intraosseous lipomas: a radiologic-pathologic correlation. Skeletal Radiol 1995;24:37–41. 2. Bohndorf K, Reiser M, Lochner B, et al. Magnetic resonance imaging of primary tumours and tumour-like lesions of bone. Skeletal Radiol 1986;15:511–17. 3. Brown JJ, vanSonnenberg E, Gerber KH, et al. Magnetic resonance relaxation times of percutaneously obtained normal and abnormal body fluids. Radiology 1985;154:727–31. 4. Cohen JM, Weinreb JC, Maravilla KR. Fluid collections in the intraperitoneal and extraperitoneal spaces: comparison of MR and CT. Radiology 1985;155:705–8. 5. Dahlin DC. Lipoma and liposarcoma. In: Dahlin DC, Unni KK, editors. Bone tumors. 4th ed. Springfield, Illinois: Charles C Thomas; 1986. pp. 181–5. 6. Dooms GC, Hricak H, Sollitto RA, et al. Lipomatous tumors and tumors with fatty component: MR imaging potential and comparison of MR and CT results. Radiology 1985;157:479– 83. 7. Milgram JW. Intraosseous lipomas. A clinicopathologic study of 66 cases. Clin Orthop 1988;231:277–302. 8. Sundaram M, McGuire MH, Herbold DR, et al. High signal intensity soft tissue masses on T1 weighted pulsing sequences. Skeletal Radiol 1987;16:30–6.