colorectal cancer is one of the leading causes of cancer - related death worldwide ( 1 ) . skeletal metastases in patients with colon carcinoma are rare , and the occurrence of isolated an solitary skeletal metastasis in the absence of visceral metastases is even rarer ( 2 ) we herein describe a case in which the post - chemotherapy imaging features of a metastatic lesion from a patient with colorectal carcinoma mimicked the radiologic appearance of osteosarcoma . a 26-year - old woman presented to the outpatient department with a chief complaint of pain in the left lower limb of 1 month duration . the patient had a known case of rectal carcinoma for which she had undergone a diagnostic laparoscopy and transverse colostomy and was on neoadjuvant chemotherapy thereafter . a radiograph of the femur was obtained which showed ill - defined lytic area in the proximal meta - diaphyseal region of the left femur with spiculated periosteal reaction ( fig . the patient underwent a computed tomography ( ct ) study of the pelvis and the proximal lower limbs , which showed a lytic lesion with a circumferential spiculated sunburst type of periosteal reaction and an ill - defined associated vascular soft tissue component in the proximal third of the meta - diaphysis of the left femur ( fig . a provisional diagnosis of osteogenic sarcoma was made based on the radiograph and ct scan findings . a magnetic resonance imaging ( mri ) examination of the thigh was performed to evaluate the local disease . mri revealed altered marrow signal intensity in the proximal meta - diaphysis of the left femur with a circumferential soft tissue mass . hypointensity was seen in the radial distribution on gradient echo sequences , corroborating the " sunburst " calcification noted on the radiograph and ct scan ( fig . these findings supported a diagnosis of a second primary lesion in the form of osteogenic sarcoma . a ct - guided biopsy was performed to confirm the diagnosis and revealed an extremely tiny focus malignant tumor composed of scattered large polygonal cells with a moderate amount of vacuolated cytoplasm , hyperchromatic nuclei , and occasional prominent nucleoli . the differential diagnoses considered were high grade carcinoma and lymphoma ( anaplastic large cell lymphoma or plasmablastic lymphoma ) but this was unlikely to be osteogenic sarcoma . the tumor cells were immunopositive for epithelial markers , including cytokeratin ( ck ) and epithelial membrane antigen but negative for lymphoma and plasma cell markers , i.e. , leukocyte common antigen , cd30 , and cd138 . the tumor cells were also immunopositive for ck20 and cdx2 , but negative for ck7 , confirming metastatic poorly differentiated adenocarcinoma primarily of colonic origin ( fig . 1d ) . based on the histology , we reviewed the positron emission tomography ( pet)-ct staging scan and prior radiographs taken at the time of presentation 6 months previously . the radiographs showed an ill - defined lytic lesion in the proximal femur that appeared to be an ill - defined lytic area on the pet - ct images and was mildly f - fluorodeoxyglucose ( fdg)-avid , with a standardized uptake value ( suv ) of 3.2 ( fig . the patient underwent a re - staging pet - ct scan , which showed no other skeletal or visceral metastasis , and the lesion in the proximal meta - diaphyseal region of the femur had a decreased suv value compared to that of the previous staging pet - ct , and it now measured 1.2 . thus , the radiological appearance represented healed metastases from the primary rectal carcinoma with calcification , which mimicked the classic pattern of a spiculated sunburst periosteal reaction with a lytic sclerotic appearance of an osteogenic sarcoma . colorectal cancer is the third most commonly diagnosed cancer in males and the second most common in females ( 3 ) . it is the most common malignancy in the gastrointestinal tract and is the fourth leading cause of cancer - associated death in the world ( 1 ) . colorectal cancer is a disease of middle age and elderly ; younger patients often have an advanced stage at presentation and present with more poorly differentiated lesions leading to a worse prognosis ( 1 ) . skeletal metastases in patients with colorectal carcinoma incidence is particularly higher in rectal carcinomas compared to that of other parts of the colon and in carcinomas with a non - mucinous histopathology ( 4 ) . the most common sites for metastases include the vertebral column , pelvis , and long bones ( 4 ) . metastases to the bone are primarily via the paravertebral venous plexus of batson ( 5 ) after direct invasion of the veins by the lesion , which further explains the higher incidence of skeletal metastases from recto - sigmoid carcinoma ( 6 ) . the prognosis of patients with skeletal metastases from colorectal carcinoma is poor ( 6 , 7 ) . isolated solitary skeletal metastases from primary colonic carcinoma are even rarer . a study conducted by roth et al . ( 7 ) using whole - body f - fdg pet showed that no patient had osseous metastasis at the time of diagnosis , and no patient had isolated skeletal metastases without other organ development . a few earlier studies reported a 1 - 2% incidence of solitary osseous metastases , although none of them utilized pet - ct scans for the evaluation ( 8 , 9 ) . osseous metastases from colon carcinoma are predominantly osteolytic in nature , and osteoblastic or mixed osteoblastic - osteolytic lesions are rare ( 4 ) . the lytic lesions are associated with reactive changes within the bone , periosteum , and adjacent soft tissue and often simulate a primary bone lesion ( 10 ) . notably , patients with isolated skeletal metastases have a better prognosis than those with multiple skeletal or visceral metastases ( 3 ) . the lesion in our case was predominantly osteolytic on ct images and radiographs obtained prior to chemotherapy . the same lesion now appeared osteoblastic with significant soft tissue and a sunburst type of periosteal reaction mimicking a primary bone lesion , such as osteosarcoma , rather than metastases . however in our setting where the patient had a known primary ; even though the imaging appearance as suggestive of osteogenic sarcoma , the first differential considered was metastasis , as primary bone tumors are relatively rare and should be considered only after metastasis has been ruled out . the characteristic pseudo - sarcomatous appearance of the lesion was attributed to post - treatment changes . the purpose of our case report is to highlight the unusual radiological appearance of these osseous metastases after chemotherapy . we emphasize that the occurrence of a second osseous primary malignant lesion in an already diagnosed case of colorectal carcinoma is far less likely than a solitary metastasis , although the incidence of the latter is very low . in conclusion , it is imperative for radiologists to analyze imaging findings in light of available clinical data to arrive at the correct diagnosis . skeletal metastases can have varied imaging appearances , particularly after chemotherapy ; thus , it is essential to diagnose such lesions at the earliest for accurate staging and prompt management to ensure a better prognosis .
solitary metastases from colorectal carcinoma in the absence of hepatic or pulmonary metastases are rare . 
 these can have a diverse imaging appearance , particularly after chemotherapy . 
 it is important identify patients with solitary skeletal metastases , as they have a better prognosis than those with multiple skeletal or visceral metastases . 
 we describe an unusual case of a solitary metastasis to the femur in a case of colon carcinoma that went undiagnosed and later presented with imaging features of osteogenic sarcoma .