PET in the Diagnostic Management of Soft Tissue Sarcomas of Musculoskeletal Origin

INTRODUCTION

Soft tissue sarcomas (STSs) are a relatively rare group of heterogeneous tumors derived from mesenchymal tissue elements. An STS can occur at any age, accounting for less than 1% of all adult solid tumors and about 7% of pediatric malignancies. As a result, STSs are the cause of 2% of all cancer-related deaths.^1,2

Typically, the clinical manifestation of an STS is of a heterogeneous soft tissue mass that grows over time. Symptoms usually develop due to the mass effect on nerves, vessels, and other adjacent structures. The anatomic locations at which STSs of musculoskeletal origin most often occur are the extremities (70%), followed by the thoracic wall.³ Within these locations, the muscular compartments are the most common spaces. Distinguishing between the more than 50 discrete histologic subtypes of STSs is possible through tissue biopsy. In adults, the most common histologic subtypes are liposarcoma, malignant fibrous histiocytoma (MFH), and leiomyosarcoma. In children, almost all STSs are rhabdomyosarcomas at 40%.^4,5 Prognosis of disease is subsequently determined through the combination of the histologic subtype, the tumor’s grade, the size and depth of the primary tumor, the stage of the disease at its initial presentation, and the patient’s age. Following treatment, additional indicators of prognosis are incorporated, including the presence of disease at the margins of the resected specimen and the recurrence of disease on successive follow-up imaging studies. Treatment protocols often focus on surgical resection with the addition of adjuvant chemotherapy and radiotherapy. With current management strategies, the resulting 5-year survival for patients with an STS is 50% for adults and 71% for pediatric patients.³

With the advent of PET–computed tomography (CT), many clinicians and researchers have explored the use of (18) fluorine-2-fluoro-2-deoxy-d-glucose (FDG) PET imaging to improve the management of STSs. This article reviews the current evidence for use of FDG PET-CT in the general diagnosis, staging or prognosis, and treatment monitoring of STSs. Additionally, a brief overview of several of the most common histologic subtypes of STS are discussed with more specific information regarding the use of FDG PET-CT in the management of each subtype.

VALUE OF PET IN THE DIFFERENTIAL DIAGNOSIS OF PRIMARY SOFT TISSUE MASSES

FDG PET-CT is rarely the modality of discovery for a mass concerning for a STS. However, FDG PET-CT may be used for specific patient populations as a method for detecting malignant transformation of benign lesions into biologically aggressive lesions. One example of this is the case of plexiform neurofibroma transformation into a malignant peripheral nerve sheath tumor (MPNST).^6,7 On finding a suspected malignancy, evaluation proceeds with tissue sampling and histologic grading. Though FDG PET-CT cannot replace a direct tissue sampling, it can significantly increase the diagnostic yield of the biopsy by targeting the hypermetabolic part of a heterogeneous lesion.³

Grading a tumor is the most reliable predictor of a tumor’s biological behavior and the patient’s ultimate clinical outcome. The most commonly used grading system for STSs is the French Federation of Cancer Centers Sarcoma Group (Fédération Nationale des Centers de Lutte Contre le Cancer [FFNLCC]) grading system. The FFNLCC system categorizes tumors based on the mitotic rate, cellularity, and degree of differentiation. Recently, many studies have explored the complementary role of FDG PET-CT in the grading of STSs.^8–11 Benz and colleagues¹² analyzed 120 subjects with 12 different STS subtypes. Their study revealed a significant relationship between the standard uptake value (SUV) at maximum SUV (SUV_max) of a lesion and the histologic grade given by the 3-tiered FFNLCC system when using a cutoff of 6.6 g/mL. On a meta-analysis examining a total 441 tumoral lesions that attempted to distinguish malignant STSs from benign lesions with FDG PET, Ioannidis and Lau¹³ reported a sensitivity and specificity of 87% and 79% using an SUV_max threshold of 2.0, and 70% and 87% using an SUV_max threshold of 3.0, respectively. In their study, 100% of the intermediate and high-grade sarcomas were detected, whereas 74% of lower grade sarcomas and 39% of benign lesions were correctly characterized. Furthermore, several additional studies have shown similarly high sensitivities in distinguishing high-grade sarcomas from lower grade tumors.^14,15 Another meta-analysis, which included 341 subjects with STSs, revealed a sensitivity and specificity of 88% and 86%, respectively, when using the mean SUV to discriminate between low-grade sarcomas and high-grade sarcomas.¹⁶

Several recent studies have attempted to achieve better performance in STS and benign tumor differentiation by examining the lesion FDG kinetics. Lodge and colleagues¹⁷ reported that malignant STSs achieve the maximal FDG uptake 4 hours following the radiotracer injection, whereas benign lesions reached peak uptake after only 30 minutes. They found that these indices had a sensitivity and specificity of 100% and 76%, respectively. In another approach, Dancheva and colleagues¹⁸ studied the method of dual time point imaging for the detection of recurrent tumor in restaging FDG PET-CT studies. They reported that an increase in SUV greater than 10% on delayed imaging could detect high-grade sarcomas with a sensitivity and specificity of 100% and 80%, respectively.

With the many potential benefits of evaluating a primary tumor with FDG PET-CT, it is important to know its limitations. Though FDG PET-CT has shown the ability to differentiate between high-grade and benign tumors on multiple studies, there is lack of evidence of its ability to differentiate between low-grade and benign soft tissue lesions.³ One study found that false-negative interpretations of low-grade sarcomas was found to be primarily related to their low metabolic rate, whereas false-positive results of benign lesions were often the result of associated inflammation.¹⁶

PET IN INITIAL STAGING OF SOFT TISSUE SARCOMAS

Staging a patient’s STS is among the most important prognostic indicators for a patient’s clinical outcome. The most common site for STS metastasis is the lung (75%), whereas metastasis to lymph nodes and bones occur less frequently or more often with specific subtypes. The American Joint Committee on Cancer (AJCC) system of staging, based on the evaluation of the primary tumor, lymph node involvement, and distal metastasis, is the most commonly used staging system for STSs. Typically, the AJCC system uses CT scan and MR imaging to evaluate the primary tumor and assess for spread. More recently, the use of whole-body FDG PET-CT has been shown to play a complementary role in the staging and restaging of many cancers, including STSs. Lucas and colleagues¹⁹ reported a sensitivity and specificity of 86.7% and 100%, respectively, for detecting pulmonary metastases with FDG PET compared with 100% and 96.4%, respectively, for CT scan alone. Furthermore, FDG PET-CT found 13 additional, unexpected sites of metastases. Researchers, Volker and colleagues,²⁰ and Ricard and colleagues,²¹ showed that PET-CT can detect a greater number of lymph nodes and bone lesions in the initial staging of an STS than conventional imaging alone; however, it had a lower sensitivity and specificity for detecting pulmonary metastases. A similar limitation was found by Fortes and colleagues²² in a study of 154 subjects with pulmonary nodules, 18 of which were STSs. They concluded that lack of high FDG uptake in a suspicious pulmonary nodule on a CT scan cannot exclude malignancy. Therefore, use of dedicated chest CT scan on full inspiration has been advocated.^20,21

As mentioned previously, FDG PET-CT has been shown to be effective at detecting malignancy deposited in lymph nodes.²³ This has proven useful for excluding distant metastases in patients who may otherwise be surgical candidates.³ In the evaluation of osseous metastases, a meta-analysis reported a greater sensitivity and specificity for detecting bone metastases with FDG PET-CT than with CT scan alone but with equal sensitivity and specificity to that of MR imaging.^24–26 In either case, it is important to analyze both the PET and CT components to avoid missing lesions that demonstrate minimal metabolic activity, such as in densely sclerotic lesions.

RESTAGING AND TREATMENT RESPONSE ASSESSMENT

FDG PET imaging has also been found to be useful in both restaging the disease and assessing response to treatment. Kole and colleagues²⁷ performed an analysis of 14 subjects involving the detection of recurrence of an STS using FDG PET-CT and reported 93% sensitivity. Al-Ibraheem and colleagues²⁸ found that FDG PET-CT offered a higher accuracy of detection for recurrent bone or soft tissue tumors compared with conventional CT scan. This superiority is mainly due to its ability to discriminate local tumor recurrence from scar tissue in a treated area.^28,29 However, an important finding to note when comparing FDG PET-CT in restaging versus the initial staging of a tumor is that calculated SUV cannot reliably predict a tumor’s grade at recurrence.³⁰

Furthermore, it is becoming common knowledge that observing for a decrease in tumor size is a poor anatomic metric for the evaluation of treatment response. In particular, heterogeneous tumors can contain various tissue types with differential sensitivity to chemoradiation and may demonstrate no significant change in size even with effective treatment. Often, a tumor may even appear to grow in size due to changes such as increased edema or internal hemorrhage. Additionally, with the advent of new targeted therapies and cytostatic versus cytotoxic agents, classic treatment assessment based only on size is even less accurate. In a study by Evilevitch and colleagues,³¹ the reduction of metabolic activity, as measured by FDG PET-CT, was shown to be a more accurate predictor of tumor response to chemotherapy than the change in size as evaluated by the Response Evaluation Criteria In Solid Tumors (RECIST) criteria. Schuetze and colleagues³² found that a 40% decline in the SUV_max of a tumor in which the baseline SUV_max was greater than or equal to 6 g/mL could discriminate responders from nonresponders and help predict patient outcome. Similarly, another study stated that a 35% reduction in SUV_max is capable of predicting a histopathologic response in high-grade sarcomas even after only 1 cycle of chemotherapy.³³ Moreover, Eary and colleagues³⁴ showed that the percentage reduction of FDG uptake after 2 cycles of chemotherapy was also a good prognostic indicator. Though current evidence for FDG PET-CT is promising for use in evaluation of treatment response, additional studies with larger subject groups are needed.

HISTOLOGIC SUBTYPES

The following sections briefly review some of the most common STSs and the subtype-specific use of FDG PET-CT in their management.

Liposarcoma

Liposarcoma is among the most common STSs. They arise most often from the deep soft tissues of the extremities and retroperitoneum, and can demonstrate multiple histologic subtypes, including myxoid, pleomorphic, and dedifferentiated tumor cells.^35–39 Considered a low-grade to intermediate-grade sarcoma, pleomorphic type accounts for 20% to 50% of all liposarcomas (Fig. 1) with a 5-year survival of about 90%.^38,40,41 The standard treatment is complete tumor resection with negative surgical margins. The myxoid subtype demonstrates chemoradiotherapy sensitivity and often receives neoadjuvant therapy. Additionally, novel methods, such as photodynamic therapy with acridin, have been proposed as alternative therapy.⁴²

Fig. 1.

A 63-year-old man presented with a growing right inguinal mass. CTscan demonstrated a fat-containing heterogeneous soft tissue mass (A, B). The lesion is heterogeneous with both fat and nonfat components on T1-weighted image (C), fat-saturated T1 (D) weighted image, heterogeneous short tau inversion recovery (STIR) hypersignal intensity (E), and postcontrast enhancement (F). The lesions is mildly hypermetabolic on PET-CTwith an SUV_max of 3.6 (G–I). STIR, short t1 inversion recovery. The pathologic assessment of the lesion confirmed pleomorphic liposarcoma.

CT and MR imaging are often sufficiently able to visualize the fat and nonfat tissue components (see Fig. 1A–F) of a liposarcoma and predict a sub-type; however, anatomic imaging is unable to distinguish the well-differentiated liposarcomas from benign lesions.⁴³ Suzuki and colleagues⁴⁴ found that both visual and quantitative analysis of FDG PET images could allow for differentiation of liposarcomas from lipomas. They stated that the mean SUV of the myxoid-type lipomas, as well as other types of liposarcoma, were significantly higher than that of well-differentiated liposarcoma by 2-fold and 3-fold, respectively.⁴⁵ Another study by Schwarzbach and colleagues⁴⁶ showed that more well-differentiated myxoid liposarcoma present with a lower FDG uptake than a dedifferentiated or pleomorphic tumor. They also demonstrated that an SUV_max greater than or equal to 3.6 was associated with a significantly reduced progression-free survival. Suzuki and colleagues⁴⁶ offered a cutoff SUV of 0.81 to discern benign lipomatous lesions from sarcomas with a high level of accuracy. Lucas and colleagues,¹⁹ and Tateishi and colleagues,⁴⁷ further reported that FDG uptake in liposarcoma depends on specific tumoral histologic features. Conill and colleagues⁴⁸ proposed that the pleomorphic, mixed, and/or higher grade liposarcomas should be selected preferentially for FDG PET evaluation, whereas the The National Comprehensive Cancer Network (NCCN) guidelines have recommended that FDG PET-CT be used only for high-grade tumors larger than 3 cm.⁴⁹ FDG PET-CT has a lower sensitivity of pulmonary metastasis detection and often underestimates the extent of osseous metastatic disease. The combination of FDG-PET-CT and MR imaging for the staging of myxoid-type liposarcomas may be helpful in this clinical setting.⁴⁸

Malignant Fibrous Histiocytoma

MFH, also known as pleomorphic undifferentiated sarcoma or fibrosarcoma, is the most common STS in adults. This STS arises from histiocytic and fibroblastic cells or directly from more primitive mesenchymal cells.^50,51 This aggressive tumor typically occurs in the deep fascia and skeletal muscles of the extremities, most commonly occurring in the thigh or retroperitoneum. Of the 5 histologic types of MFH, storiform-pleomorphic type is the most common subtype (50%–60%). It is composed of spindle cells (fibroblastic-like) and round cells (histiocytic-like) arranged in a storiform pattern with intervening inflammatory cells.⁵² Similar to liposarcomas, MFH is treated with surgical resection with or without adjuvant treatment.^53–59 However, MFH is considered a higher grade sarcoma and frequently metastasizes, leading to a poorer prognosis with an overall 5-year survival of about 14%.⁵²

Diagnosing MFH can be difficult because neither the clinical features nor its gross appearance distinguishes this tumor from the other subtypes of STS. With 25% of MFH tumors demonstrating a highly myxoid composition, the tumor may mimic myxoid liposarcoma.⁵² Though few studies describe MFH explicitly, this subtype has been included in other studies of malignant STS, demonstrating that it is also hypermetabolic.^60,61 Unlike MFH of the torso, only few reports exist describing MFH in extremities; the limited available publications describe MFH and other STSs as hypermetabolic masses.^60,61 The study of Kern and colleagues,⁸ among the first studies showing the value of FDG-PET in STSs, established that FDG-PET is among the most useful tools for STS metabolic evaluation. Since then, studies have been done that applied FDG-PET for grading,^{13,17,19,62–67} staging, assessment of response to treatment,^{31–33,60,68–72} and surgical planning of STSs. Hoshi and colleagues,⁶⁰ in an analysis of 113 subjects with STSs, including MFH, demonstrated that an SUV_max greater than or equal to 2 (Fig. 2) and a tumor size greater than or equal to 5 cm (Fig. 3) would infer a worse prognosis and would likely benefit from more aggressive therapy. Again, this generalized knowledge can be helpful in the management of MFH; however, more research specifically directed toward MFH and FDG PET-CT use is necessary.

Fig. 2.

A 72-year-old woman presented with a growing left leg fungating mass. MR imaging demonstrated a 2.3 cm heterogeneous T1 hyposignal (A), Proton Density (PD) intermediate (B), and STIR hypersignal (C) intensity lesion with heterogeneous postcontrast enhancement (D) compared with precontrast imaging (E). The lesion was highly hypermetabolic on PET (F) with an SUV_max of 12.7. The CT component of PET demonstrates a nonspecific fungating soft tissue density mass (G). The lesion was pathologically diagnosed as MFH or pleomorphic undifferentiated sarcoma (PUS).

Fig. 3.

MFH or PUS: multilobulated T1 intermediate signal intensity mass with areas of hyperintensity suggestive of internal hemorrhage (A, B), centered in the subcutaneous adipose tissues of the left anterolateral thigh, measuring approximately 18.1 by 9.9 by 11.4 cm, with areas of central T1 hyperintensity, which do not suppress on the fat-saturated sequences, likely representing areas of necrosis and hemorrhage. The mass abuts a short segment of the rectus femoris muscle and anterior aspect of the vastus lateralis muscle with no evidence of neurovascular involvement. The lesion demonstrates heterogeneous postcontrast enhancement (C, D). There is no evidence of bone invasion to the underlying osseous structures on radiograph (E). On ultrasound, the lesion is heterogeneous in echogenicity with mild internal vascularity (F, G). On PET-CT, the lesion is heterogeneously hypermetabolic with an SUV_max of 14.3 (H, I).

Rhabdomyosarcoma

Rhabdomyosarcoma is the most common STS in children and adolescents. It arises from mesenchymal cells during skeletal muscle differentiation.^73–75 The most common anatomic locations for rhabdomyosarcoma are the head and neck, genitourinary tract, and limbs. Histologically, the embryonal form is the most common subtype. The embryonal form accounts for about 57% of all childhood rhabdomyosarcoma versus about 23% and 20% for the alveolar form and all other types of this sarcoma, respectively. The treatment most often includes surgical removal of the tumor with accompanying systemic chemotherapy followed by local radiotherapy.⁷³ The therapeutic management almost completely depends on initial staging, highlighting the need for a reliable tool for accurate evaluation of the tumor.

Ricard and colleagues²¹ concluded that FDG PET-CT is a useful method for staging and restaging of pediatric rhabdomyosarcoma with particular emphasis on the detection of lymph node and bone involvement.²¹ Multiple studies have also demonstrated the utility of FDG PET-CT in the detection and evaluation of primary lesions with a higher sensitivity (95%–100%) and specificity (80%–100%) than that achieved by conventional imaging (17%–83% and 43%–100%, respectively).^{20,21,76–80} For nonpulmonary and distal lymph node involvement, FDG PET-CT has shown a higher sensitivity with the same specificity compared with conventional imaging tools. In many of these studies, a greater total number of lymph nodes were detected using FDG PET-CT. In their 41 subject study of rhabdomyosarcoma, Baum and colleagues^21,77–83 noted that nodal involvement, primary tumoral metabolic activity, and other metastatic site involvements are the major determinants of patient survival in rhabdomyosarcoma. Concerning treatment response, Eugene and colleagues^{26,77–80,82} demonstrated FDG PET-CT has a greater ability to detect complete response compared with conventional imaging. However, similar to other STSs, the major limitation of FDG PET-CT in rhabdomyosarcoma is related to the evaluation of pulmonary metastases.^77,79

Angiosarcoma

Angiosarcoma is an uncommon malignant tumor of vascular and lymphatic endothelial origin, accounting for less than 5% of all STSs.⁸⁴ Most often appearing in thighs and retroperitoneum, angiosarcomas can occur anywhere in the extremities, trunk, and head and neck regions.⁸⁵ Clinically, an angiosarcoma often presents as an enlarging, painful mass frequently associated with a vascular disorder such as anemia or a coagulopathy. They are aggressive tumors that tend to reoccur and metastasize widely. Prognosis is poor, as demonstrated in a case series study of 49 subjects that showed a 53% median survival at 11 months.⁸⁴ Epithelioid angiosarcoma is the most common subtype of angiosarcoma. Similar to other STSs, wide excisional resection remains the treatment of choice, with the addition of chemotherapy and radiotherapy if complete resection is not possible. Early diagnosis is of great importance in providing the greatest chance of total tumor resection.

There are only a few case reports in literature regarding the usefulness of FDG PET-CT in the management of angiosarcoma, primarily due to its low incidence; however, FDG PET-CT has provided some reliability in the early detection of distant metastases, staging, and its prognostication of disease.^86–92 Benign vascular lesions, such as hemangiomas and hemangioendotheliomas, demonstrate significantly lower FDG avidity compared with angiosarcoma, allowing distinguishability.^93–95 In a study by Lee and colleagues,⁹³ FDG uptake levels, as expressed by SUV_max, were able to effectively predict the prognosis of subjects with vascular tumors. Their study showed a significant reduction in survival with an SUV_max greater than or equal to 3 g/mL.⁹³ Clearly, much more research is needed to examine the value of FDG PET-CT in this setting.

Synovial Cell Sarcoma

Synovial cell sarcoma is the third most common STS, accounting for 6% to 10% of STSs. Synovial cell sarcoma occurs primarily in the extremities of young adults with a predilection for the periarticular regions, with the popliteal area being most common. However, rarely, synovial cell sarcoma may involve the trunk, near the joints (Fig. 4). Clinically, they present as slow growing and often painless masses. Like most STSs, synovial cell sarcoma metastasizes to the lungs.^96,97 Although, local surgical excision and radiotherapy give excellent local control of the tumor, it is a metastatic disease that is more difficult to treat.

Fig. 4.

A 61-year-old woman presented with a 10.4 cm soft tissue density mass within the right upper lobe, which abuts the pleural, pericardial, and mediastinal surfaces (A, B). The lesion is hypermetabolic on PET-CT with an SUV_max of 17.7 (C–E), the biopsy of which confirmed synovial cell carcinoma of the chest wall.

MR imaging is the primary imaging modality for diagnosis and initial tumor staging, with CT scan useful for detection of distant disease.⁹⁸ There are very few studies that have evaluated FDG PET-CT in synovial cell sarcomas. Rayamajhi and colleagues⁹⁹ demonstrated that FDG PET-CT is a useful method for staging and restaging of patients with synovial cell sarcoma. Other studies suggested that a pretherapy SUV_max could be used for prediction of survival and pathologic response to neoadjuvant therapy.¹⁰⁰ It was shown that an SUV greater than 4.35 g/mL is associated with shorter progression-free survival and increased risk for developing recurrence or metastases of synovial sarcoma.¹⁰⁰ General STS studies that included synovial cell sarcoma have demonstrated an ability to distinguish low-grade from high-grade sarcomas with 80% sensitivity.^101,102

Malignant Peripheral Nerve Sheath Tumors

MPNSTs are a rare group of STSs that account for 5% to 10% of all STSs with an expected incidence of 0.1 out of 100,000 per year in the general population.¹⁰³ MPNSTs are of ectomesenchymal origin, deriving from Schwann cells or pluripotent cells of the neural crest and they arise along peripheral nerve branches or their sheaths.

About 25% to 50% of observed MPNSTs present in patients with neurofibromatosis (NF)-1, and 2% to 29% of patients with plexiform NF will develop MPNST,^6,7 whereas the total lifetime prevalence of MPNST in NF1 is only 10%.^104,105 These high numbers, along with the poor prognosis associated with MPNST, stress the need for early detection. Detection of the malignant transformation of plexiform NF to MPNST is difficult using clinical history and conventional CT and MR imaging alone. Clinical symptoms, such as an enlarging, painful mass, are nonspecific and unreliable. Similar to other STSs, the blind tissue sampling of a small portion of the heterogeneous tumor cannot reliably determine an entire tumor’s biological behavior because a higher grade component may have been missed. Additionally, excisional biopsies may not be feasible due to the high resultant morbidity from the involvement of the adjacent structures. Luckily, FDG PET-CT has shown the ability to detect malignant transformation in the patients with NF1, offering a promising tool for surveillance.^3,105–108 Ferner and colleagues¹⁰⁸ reported a significant difference between the SUV_max of malignant and benign lesions using an SUV_max cutoff of 2.5 g/mL for NF1 subjects 200 minutes following FDG injection (Fig. 5). Additional potential metrics for the assessment of malignant transformation to MPNST and tumor burden are total lesion glycolysis and total metabolic tumor volume.¹⁰¹ One study even used a combination of FDG PET-CT and 11C-Methionine to demonstrate an even greater sensitivity and specificity.¹⁰⁹

Fig. 5.

Malignant nerve sheath tumor: A 70-year-old woman presented with low back pain. MR imaging of lumbar spine incidentally found a 4.7 by 4.4 by 4.8 cm lobular, well-defined heterogeneous T1 hyposignal (A, B), T2 intermediate to hypersignal (C), and STIR hypersignal intensity (D) mass centered at the left sacrum vertebra (S1) foramen, containing foci of central necrosis. On further postcontrast imaging, the lesion shows heterogeneous enhancement (E, F). The lesion underwent CT-guided biopsy with which osseous erosion with extension of the mass into the left S1 vertebral body and the left aspect of the sacral spinal canal was identified (G). PET-CT showed an intensely FDG avid lesion centered at left S1 foramen with an SUV_max of 8.9 (H–K).

Despite its ability to detect malignant transformation from NF to MPNST, FDG PET-CT is unable to differentiate schwannomas from malignant sarcomas. Schwannomas are benign and often solitary peripheral nerve sheath tumors unassociated with NF1 and can present with a wide range of FDG uptake values. It is for these reasons that FDG PET-CT must be used with caution in distinguishing MPNSTs and other tumors from schwannomas.^110,111

SUMMARY

FDG-PET-CT offers important complementary information that can be used in the diagnosis, staging, restaging, treatment response monitoring, and prognostication of STSs. Additional research is needed to strengthen the current evidence and to further elaborate on the application of FDG PET-CT, particularly for rare subtypes of STS.

KEY POINTS.

• A soft tissue sarcoma (STS) is relatively rare. (18) Fluorine-2-fluoro-2-deoxy-d-glucose (FDG) PET–computed tomography (CT) offers complementary information in the management of an STS.

• Additional research is needed to strengthen the current evidence and to elaborate on the application of FDG PET-CT, particularly for rare subtypes of STS.

• Though FDG PET-CT cannot replace direct tissue sampling, it can significantly enhance the biopsy diagnostic yield by targeting the hypermetabolic part of lesion.

• FDG PET-CT can be used to detect malignant transformation of a benign lesion into an aggressive lesion.

• Because classic size-based assessment of treatment response is inadequate, metabolic FDG PET data is valuable in posttreatment evaluation of cancer, including STS.