Germ cell tumors (GCTs) account for nearly all (95%) primary testicular tumors.  The GCT line is fairly evenly split between seminomas (45%) and nonseminomas (50%) (see the images below). Of the nonseminomas, mixed GCTs are most common (40%). Teratomas and teratocarcinomas are considered malignant in adults and constitute 30% of nonseminomatous lesions. The embryonal cell tumor is the classic pure-cell, nonseminomatous tumor and is relatively uncommon (20%). Choriocarcinoma represents the most lethal but least common (1%) nonseminomatous type.
Yolk sac tumors, which are also known as endodermal sinus tumors, are the most common prepubertal GCTs. They may be benign, but most often, they are malignant. Most affected patients require surgery and chemotherapy because of the aggressive nature of the tumors, but the overall prognosis is excellent.
Among nongerminal testicular tumors, stromal Leydig cell tumors and Sertoli cell tumors constitute the remaining 5% of primary testicular tumors; these are rare tumors that are malignant in only about 10% of the cases.
Lymphoma, leukemia, and melanoma are the most common malignancies that metastasize to the testicle. 
The exact etiology of testicular cancer is not clearly defined; however, recent data show a dramatically increasing incidence rate in developed countries, which has lead to increasing interest in factors affecting hormonal balance, with speculated association to a variety of endocrine disruptors. 
In the United States, the age-adjusted incidence rate for testicular cancer has been estimated at 5.5 cases per 100,000 men per year.  Testicular cancer is common in the United States, with almost 8,000 new cases presenting each year, although the death rate is relatively low at 370 deaths per year. 
With regard to racial distribution in the United States, testicular cancer has a clearly higher predisposition in whites. The incidence is 6.6 cases per 100,000 men per year, compared with 4.7 cases per 100,000 men per year for Latinos, 4.5 cases per 100,000 men per year for Native Americans, 1.9 case per 100,000 men per year for Asians, and 1.4 cases per 100,000 men per year for blacks. 
With regard to age-related statistics, the median age for diagnosis for cancer of the testes in the United States has reported at 33 years and for death from testicular cancer at 40 years.  Twenty percent of patients present with metastatic disease.
Testicular cancer rates in developed European countries are slightly higher than in the United States, at 8-9 cases per 100,000 men per year. Rates are lowest in Asian and African populations, at less than 1 case per 100,000 men per year. 
The key anatomical factor in testicular cancer is that the lymphatic drainage of the scrotum is into the lower legs, while the lymphatic drainage of the testicles is into the abdomen and retroperitoneal lymph nodes. For this reason, transscrotal testicular biopsy is not recommended. In addition, scrotal masses that do not involve the testicle are rarely malignant. 
Screening for testicular cancer
Screening for testicular cancer is not a current recommendation, because patients with stage 1 or 2 (2A or 2B) testicular cancer have a long-term, disease-free survival rate greater than 95%; in patients with bulky retroperitoneal nodes and/or pulmonary or visceral metastasis, this rate is 60-80%.
Ultrasonography is the standard imaging technique used to identify testicular carcinoma. It has a high sensitivity, but it must be combined with physical examination to achieve the best specificity. More than 95% of testicular parenchymal abnormalities are identifiable on routine sonograms, but several other lesions commonly mimic testicular cancer. Ultrasonography is more specific in the presence of a palpable mass. Nonpalpable intratesticular lesions larger than 1 cm are unlikely to be malignant. [8, 9]
The false-negative rate for ultrasonographic identification of testicular lesions is extremely low.
The American Institute of Ultrasound in Medicine (in conjunction with the American College of Radiology, Society for Pediatric Radiology, and Society of Radiologists in Ultrasound) have published guidelines on testicular and extratesticular examination. Indications include the following  :
Evaluation of scrotal pain, including but not limited to testicular trauma, ischemia/torsion, and infectious or inflammatory scrotal disease.
Evaluation of palpable inguinal, intrascrotal, or testicular masses.
Evaluation of scrotal asymmetry, swelling, or enlargement.
Evaluation of potential intrascrotal hernias.
Detection/evaluation of varicoceles.
Evaluation of male infertility.
Follow-up of prior indeterminate scrotal ultrasound findings.
Localization of nonpalpable testes.
Detection of occult primary tumors in patients with metastatic germ cell tumors or unexplained retroperitoneal adenopathy.
Follow-up of patients with prior primary testicular neoplasms, leukemia, or lymphoma.
Evaluation of abnormalities noted on other imaging studies (including but not limited to CT, MRI, and positron emission tomography [PET]).
Plain radiographs have no role in the initial diagnosis of testicular cancer. Metastasis to the chest is fairly common and often seen on screening chest radiographs. However, computed tomography (CT) scanning is more sensitive and specific for staging the disease.
CT scan imaging is routinely performed as part of the initial staging process, but it is not sensitive or specific enough to be useful in evaluating undiagnosed testicular masses. Chest, abdominal, and pelvic CT scan studies are indicated for the evaluation of retroperitoneal and mediastinal metastases. (See the image below.)
CT scanning of the chest is especially useful when mediastinal or parenchymal lung disease caused by testicular cancer is suspected. This modality or magnetic resonance imaging (MRI) is also indicated in patients with neurologic signs or symptoms.
The most common site of disease recurrence is the retroperitoneum; thus, CT scanning is the best tool to detect recurrence.
Lymphoma can be difficult to distinguish from metastatic testicular cancer. Use tissue sampling from the abnormal testicle to make this distinction.
Magnetic Resonance Imaging
MRI is not used as the initial modality to evaluate testicular masses.
Degree of confidence
Increased signal intensity in the testicle is seen on T2-weighted images of testicular malignancies, but it is not specific for the disease.
Testicular ultrasonography is used to determine the location of a palpable mass when testicular cancer is suspected. Generally, palpable extra-testicular lesions are benign. On the other hand, intratesticular masses, especially if they are palpable, are likely to be malignant and must be surgically explored. Therefore, ultrasonography is useful for localizing palpable abnormalities and to triage them for surgical repair when indicated.  (See the images below.)
The examination is usually performed with a high-frequency linear transducer to compare the echotexture of the 2 testicles for areas of heterogeneity. Testicular cancers are hypoechoic relative to the surrounding parenchyma in about 95% of cases. Published findings suggest that seminomas are often more homogeneously hypoechoic and that nonseminomatous lesions are often more cystic, with interspersed areas of calcification. [12, 13] For scans of malignant tumors of the testicle, see the images below.
The tumor tissue type cannot be reliably differentiated solely by its ultrasonographic appearance. Commonly, seminomas are well defined within the tunica albuginea and homogeneously hypoechoic. Embryonal cell cancers typically are hypoechoic, with interspersed cystic components. Teratomas and choriocarcinomas are often heterogeneous with multiple internal calcifications present. Stromal cell tumors (eg, Leydig and Sertoli cell tumors) are generally well defined and hypoechoic, but calcifications are frequently found. Lymphoma and leukemia of the testicle generally present as an ill-defined, diffuse process of decreased echogenicity.
When multiple lesions are present, the differential diagnosis should be expanded to include metastatic processes, such as leukemia and lymphoma, and inflammatory processes, such as sarcoid. Testicular lymphoma can be difficult to diagnose when both testes are homogeneously hypoechoic.
Testicular microlithiasis (≥ 5 or more microcalcifications within a testicle) results from concentric cores of calcification of intrasubstance collagen fibers. Case studies of patients with testicular tumors suggest a high rate of microlithiasis, but prospective evaluations of patients with microlithiasis have failed to demonstrate more than a minimal increase in the frequency of such tumors. Annual ultrasonographic screening of patients with microlithiasis has been suggested by some authors, but prospective studies have failed to demonstrate a positive cost-benefit ratio at this time. [14, 15, 16]
Azzopardi tumor is the name used for a presumed "spontaneously burned out" tumor, wherein malignant cells spontaneously necrose and calcify, perhaps related to outgrowing the blood supply.
Degree of confidence
Although the specificity and sensitivity have not been reported, general consensus exists that an ultrasonographic finding of a solid or mixed cystic and solid intratesticular mass is an indication for surgical exploration.
False-negative results are most common in the infiltrative malignant processes. When a condition such as leukemia or lymphoma causes bilateral diffusely decreased echogenicity, the infiltrative malignant process can be difficult to recognize.
False-positive results are seen in a variety of conditions. Dilated rete testes can be masslike, and they can simulate a predominantly cystic mass. The imaging characteristics of epidermoid tumors can be indistinguishable from those of germ cell lesions. Cho et al report that the classic appearance for an epidermoid is a heterogeneous mass, possibly with concentric hyperechoic and hypoechoic layers forming a ring.  The epidermoid is often avascular. An abscess or phlegmon of the testicle is hypoechoic and often associated with increased vascularity. Testicular infarction can present as ill-defined decreased echogenicity in a testicle, suggesting a diffusely infiltrative malignant process. (See the images below.)
A troublesome scenario can occur when a patient presents with a history of trauma and the sonogram shows a hypoechoic focus presumed to be a hematoma. Distinguishing a hematoma from a testicular tumor can be impossible on the initial images. Because such tumors can be detected after incidental trauma, ultrasonographic follow-up of suspected hematomas is recommended to ensure their complete resolution.
PET and PET-CT
According to the American Society of Clinical Oncology, there is currently no role for PET or PET-CT in the initial staging of testicular cancer. 
PET is very useful in evaluating the response to chemotherapy, especially in pure seminoma. It can be used in nonseminomatous malignancy, but the specificity is decreased because of false negative results in mature teratomas. 
Retoperitoneal lymph node dissection is to be considered in patients with PET-positive residual masses, which are 3 cm or greater . In patients with nonseminomatous germ cell tumors, the cut-off size is much lower, at 1 cm.