Retroperitoneal sarcomas are characterized by poor prognosis. More than half are high-grade and adequate surgical margins are rarely obtained. The standard imaging procedure is a chest-abdominal CT scan (V, A). An extraperitoneal image-guided percutaneous core needle biopsy is used for histologic diagnosis (IV, A). Nevertheless, it is reasonable to avoid biopsy if the imaging is pathognomonic (heterogeneous dedifferentiated/well-differentiated LPS) and no preoperative treatment is planned (V, A) En bloc resection of the tumor, including adherent structures even if not overtly  infiltrated at the time of primary presentation, is the only curative treatment for RPS (III, A). Post-operative radiation therapy is of limited value and not a standard treatment, could be associated with great toxicity, and may be an option in highly selected patients with well-defined risk areas of recurrence (IV, C). Adjuvant and neoadjuvant chemotherapy should not be routinely employed in RPS due to lack of evidence of benefit (IV, C).
Uterine sarcomas are composed of different tumor entities: leiomyosarcomas, high-grade uterine sarcoma, and endometrial stromal sarcoma (ESS). Carcinosarcomas behave like epithelial carcinomas and are not covered by the following guidelines. Standard surgery of localized US consists of total abdominal hysterectomy (plus double oophorectomy only in ESS) with full abdominal cavity exploration. Lymphadenectomy is not indicated. Adjuvant radiotherapy is controversial. Most available data are retrospective and suggest an improvement in local relapse control but not consistent improvement in overall survival . Thus, adjuvant radiotherapy it is not routinely considered, but it could be recommended in selected cases with a high relapse risk (II, C). There is not enough evidence to support the use of adjuvant chemotherapy, but it could be individually planned in some patients with high risk of systemic relapse (III, B). Hormonal therapy with megestrol acetate, gonadotropin-releasing hormone (GnRH) analogues, and aromatase inhibitors can delay progression for long periods of time in low-grade oestrogen receptor-positive ESS, and it is preferred over chemotherapy as front-line palliative treatment (IV, C). Doxorubicin is an active single agent for US and is less toxic than combination regimens; for that reason, it constitutes the standard first-line treatment for advanced US (I, A). Positive results have been published for LMS patients treated with gemcitabine plus docetaxel as first- or second-line treatment. It is acceptable to select this regimen as first-line palliative chemotherapy (III, B). Systemic treatment in second and further line is similar to other STS.
Desmoid tumors represent a mesenchymal neoplasm of intermediate behavior. They do not metastasize, but show a marked tendency to local relapse. Surgery has classically been the mainstay of DT curative treatment. It is usually straight forward in the case of limb and chest-wall tumors, but can be much more challenging in abdominal disease. The aim of surgery is the macroscopic removal of the whole tumor while minimizing morbidity . Wide margins, even microscopically negative ones, do not justify on their own mutilating surgeries or functional sequels, as the prognosis of macroscopically resected (R1) patients do not depend on the microscopical status of the margins (III, A). Given the unpredictable natural history of the disease and functional problems implied by some tumor locations, a watch-and-wait approach is also acceptable (III, B). For progressing cases, the optimal treatment needs to be individualized. RT is able to control even bulky disease for long periods of time. (III, B). Systemic treatment is appropriate for unresectable tumors, Gardner-related cases with multiple recurring DT, progressions in areas previously irradiated, and functionally or aesthetically unacceptable surgery. Evidence-based options include non-steroidal anti-inflammatory drugs, such as sulindac (IV, D), anti-oestrogens (tamoxifen and toremifene) (IV, D), chemotherapy (low-dose methotrexate plus vinblastine or vinorelbine, liposomal doxorubicin and vinorelbine monotherapy) (III, B), imatinib (III, B), sorafenib (III, B), and full-dose chemotherapy (III, C). We recommend to employ the less toxic monotherapy options in the first place.
Dermatofibrosarcoma protuberans is a cutaneous mesenchymal tumor of intermediate behavior that rarely metastasizes but is locally aggressive. The treatment of localized DFSP is wide surgical excision with wide margins (2–4 cm). Mohs surgery can be planned to avoid major cosmetic defects (III, B). Adjuvant radiation therapy should be considered when margins are positive and re-resection is not feasible (IV, B). In unresectable, recurrent or metastatic DFSP, imatinib is recommended  (III, B). Imatinib activity is related to the presence of t(17;22) translocation. However, objective responses have been documented in translocation-negative tumors. If transformation to high-grade sarcoma occurs (less than 15% of DFSP), management is similar than a conventional high-grade STS.
Other rare specific subtypes
In metastatic or locally advanced malignant, solitary fibrous tumor antiangiogenic agents, such as sunitinib (III, B) or the combination of temozolomide plus bevacizumab, constitute active options  (IV, B). Chemotherapy following the common guidelines for STS could be administered but its efficacy is low (III, B). Alveolar soft part sarcoma is not particularly sensitive to classic chemotherapeutic agents. However, ASPS has an upregulation of angiogenesis elements, and cediranib has proven to be active in advanced disease (III, B). Several partial responses to sunitinib and bevacizumab have also been reported (IV, B). Clear cell sarcoma tends to metastasize to lymph nodes, unlike other STS. In advanced cases, the efficacy of chemotherapy is low. However, some isolated responses have been described with antiangiogenic agents, such as sorafenib or sunitinib (V, D). The PEComa family of tumors consists of related mesenchymal neoplasms that share a distinctive cell type, the perivascular epithelioid cell. It includes angiomyolipoma, clear cell “sugar” tumor of the lung, lymphangioleiomyomatosis, clear cell myomelanocytic tumor of the falciform ligament/ligamentum teres, abdominopelvic sarcoma of perivascular epithelioide cells, and extrapulmonary sugar tumor. Although most PEComas are benign, a subset exhibits malignant behavior. Frequently, tumors of the PEComa family share dysregulated activation of the mechanistic target of rapamycin (mTOR) signaling through mutations in the TSC1 or TSC2 genes. This is the basis for the use of mTOR inhibitors, sirolimus, or everolimus in the treatment of locally invasive or metastatic PEComas (IV B). The Inflammatory myofibroblastic tumor (IMT) is associated with rearrangements of the ALK (anaplastic lymphoma kinase) locus on chromosome 2p23.13. In advanced IMT ALK-translocated, ALK-inhibitors, as crizotinib, produce sustained responses and constitute the best option (IV, B). Finally, Angiosarcoma (AS) is a heterogeneous type of sarcoma due to its age of presentation and location. In advanced cases, systemic chemotherapy with either anthracyclines or taxanes is acceptable treatment options (II B). However, in the AS of the scalp, frequently seen in elderly patients, weekly paclitaxel is the drug of choice, because it seems to have a better response rate than anthracyclines. Antiangiogenic drugs, such as bevacizumab and sorafenib, have also been tested in metastatic AS with moderate activity  (III, B).