Testicular tumors are subdivided into four major categories: germ cell tumors, sex cord/stromal tumors, mixed tumors, and nonprimary tumors [1] (Table 4.1). Germ cell tumors account for most of testicular tumors (90–95 %) and further subdivided into seminoma and nonseminomatous germ cell tumors (NSGCT). Differentiation of seminoma from nonseminomatous germ cell tumor plays a critical role in the selection of appropriate treatment plan. Seminoma is extremely sensitive to radiation therapy, and its treatment is composed of radical orchiectomy followed by radiation therapy. On the contrary, nonseminomatous germ cell tumor is usually treated by radical orchiectomy with retroperitoneal lymph node dissection followed by chemotherapy. Most common histologic types of germ cell tumors include seminoma (35 %), embryonal carcinoma (20 %), and mixed germ cell tumor (40 %). The average age of onset of testicular cancer is slightly different according to the testicular tumor subtypes: yolk sac tumors (0–10 years), choriocarcinomas (20–30 years), embryonal carcinomas and teratomas (25–30 years), and seminomas (30–40 years) [2]. Tumor markers including AFP or hCG are helpful in the diagnosis, staging, and treatment response evaluation of testis tumors. The elevation of AFP suggests yolk sac tumor and the elevation of hCG means seminoma or choriocarcinoma.

Sex cord–stromal tumors account for 4 % of all testicular tumors [1], and the majority of sex cord–stromal tumor is Leydig cell tumor or Sertoli cell tumor. Other sex cord–stromal tumors including granulosa cell tumors and fibroma–thecomas are very rare. Although sex cord–stromal tumors are usually benign, it is often difficult to determine the biologic behavior of sex cord–stromal tumors with histologic analysis. Sex cord–stromal tumors can produce estrogen or androgen so patients, especially with Leydig cell tumors, manifest as precocious virilization, gynecomastia, or decreased libido.

Nonprimary testicular tumor includes lymphoma, leukemia, and metastasis. Testicular lymphomas accounting for 5 % of all testicular tumors occur in a much older population and are the most common testicular neoplasm in men over 60 years of age. Primary testicular leukemia is very rare, but the testis is a common site of leukemia recurrence in children because the blood–testis barrier protects leukemic cells from chemotherapeutic agents.

Testis tumor most commonly spreads via the lymphatic pathway, but choriocarcinoma can also spread hematogenously. The retroperitoneal lymph nodes are the most common site for metastatic disease, and evaluation of sentinel nodes is important in the assessment of testis tumor. Sentinel nodes of the right testicular cancers are located in the aortocaval area inferior to the renal hilar vessels, and those of the left testicular cancers are located in the para-aortic area bordering with left renal vein, ureter, aorta, and inferior mesenteric artery. As metastasis advances, metastasis in sentinel nodes spreads to adjacent retroperitoneum. Crossover drainage to the contralateral lymph nodes from the sentinel nodes is encountered in 15–20 %. Crossover drainage from aortocaval lymph nodes to the para-aortic lymph nodes is more common than vice versa [3, 4].

US is the first imaging modality in the evaluation of intrascrotal mass. US is a very sensitive imaging tool for identification of testis mass (almost 100 % in the sensitivity) and is helpful in the differentiation of testicular mass from paratesticular mass. The first step in the evaluation of the scrotal mass is to differentiate testicular mass from paratesticular mass. Testicular mass is usually malignant in its nature, but paratesticular mass is usually benign in its nature (Fig. 4.2). If the scrotal mass is regarded as testicular germ cell tumor, the next step is to differentiate seminomas from nonseminomatous germ cell tumors. US imaging findings of the tumor reflect its histologic characteristics. Therefore, seminoma appears as homogenous low echoic mass due to its uniform internal architecture, whereas nonseminomatous germ cell tumor appears as a mixed echoic mass due to its necrosis or internal hemorrhage. If older men present with testis mass, the first impression of the testis mass is lymphoma or metastasis. On US, testicular lymphoma appears as homogenously hypoechoic lesions, and color Doppler evaluation shows markedly increased intralesional blood flow irrespective of lesion size (Fig. 4.3) [5]. Testicular metastasis is generally seen in the setting of widespread metastasis of primary cancers so the US diagnosis of testicular metastasis is not difficult although the US appearance of testicular metastasis is quite varied. Because the blood–testis barrier protects leukemic cells from chemotherapeutic agents, testicular leukemia should be considered in the differential diagnosis of testis mass in children with leukemia history. The US appearance of testicular leukemia is similar to the US appearance of testicular lymphoma that presents as homogenously hypoechoic lesions with markedly increased intralesional blood flow.

Because most of intratesticular mass are malignant tumors and rare benign tumorous condition of the testis shows similar clinical and radiological manifestations to the malignant testicular tumors, intratesticular mass is usually treated by radical orchiectomy with tentative diagnosis of malignant testicular tumor. However, some of benign tumorous conditions like epidermoid cyst or testicular cyst show specific imaging findings and familiarity with those imaging features that can be helpful in the selection of testis-sparing enucleation in the treatment of intratesticular mass. The internal content of the epidermoid cyst is composed of cheesy laminated material. On US, the epidermoid cyst appears as onion-skin appearance due to its internal architecture. Preoperative diagnosis of epidermoid cyst makes urologists to select less invasive enucleation instead of more invasive radical orchiectomy. Testicular cyst is usually detected incidentally in men at least 40 years of age. On US, testicular cyst appears as a pure cyst without a perceptible wall or internal solid portion. These cysts are usually solitary but can also be multiple. Testicular cysts adjacent to or within the mediastinum testis are associated with extratesticular spermatoceles. Testicular cysts do not require treatment.

The staging system of the American Joint Committee on Cancer (AJCC) is commonly used. AJCC staging is a TNMS-based classification (T = tumor, N = node, M = metastases, S = serum markers) (Table 4.2) [6]. Because T staging is determined through surgical pathology, the role of imaging is focused on the assessment of N and M staging. Abdominopelvic CT is the reference standard for the assessment of N and M staging of testis tumors (Fig. 4.4). Testicular cancer has a high propensity for nodal micrometastasis [7]. Because the identification of metastatic lymph nodes is based on size criteria, abdominopelvic CT provides sensitivity of about 70–80 %. Chest CT is performed if metastatic adenopathy is noted in the abdominopelvic CT and additional imaging like brain imaging is warranted if clinical suspicions are present. Testicular cancers can be grouped into three major categories according to the TNMS-based classification: tumors limited to the testis are stage I, those with retroperitoneal nodal involvement are stage II, and those with distant disease are stage III [6].

Testicular microlithiasis is defined as five or more microliths on US image (Fig. 4.5) [1]. Microcalcifications within the lumen of the seminiferous tubules are responsible for testicular microlithiasis, and defect of the phagocytic activity of Sertoli cells is regarded as being responsible for testicular microlithiasis [8]. The prevalence of testicular microlithiasis is reported as being between 0.6 and 9 % of symptomatic men [9]. Although the risk of testicular cancer is increased up to 21.6 times [10], it is doubtful whether US follow-up would substantially change the outcome [11].

Burned-out germ cell tumor is defined as a germ cell tumor with spontaneous regression. The pathogenesis of burned-out germ cell tumor is uncertain but is presumed to be due to rapid outgrowth beyond the blood supply [1]. Because burned-out germ cell tumor presents with metastasis in the form of extragonadal germ cell tumor, we should consider two possibilities when faced with extragonadal germ cell tumors. One is a primary germ cell tumor originated from the extragonadal germ cells that is aberrantly migrated from the yolk sac, and the other is an extragonadal metastasis of burned-out germ cell tumor. Primary extragonadal germ cell tumors occur commonly in the pineal gland, mediastinum, retroperitoneum, or sacrum [12]. Because retroperitoneal germ cell tumors are more likely to be a metastasis from a primary testicular tumor, the testis should be thoroughly interrogated to rule out the possibility of an occult testicular primary lesion.

There is a strong association between undescended testis and testicular carcinoma, especially seminoma. Therefore, the possibility of testicular cancer should be considered in the evaluation of undescended testis. The pathophysiology of malignant transformation in undescended testes is uncertain, and orchiopexy, performed even at an early age, does not decrease the risk of testicular carcinoma. Furthermore, the risk of testicular cancer is not limited to the undescended testis, and the risk extends to the contralateral normal testis [1]. MR is the most helpful imaging modality in the detection of associated testicular carcinoma in men with undescended testis. US or CT is not good due to its poor tissue contrast. Undescended testis of over 2 cm in US or CT should consider the possibility of associated testicular carcinoma. The majority of the cases present as homogenous mass developed in the abdominally located testis. The demonstration of ipsilateral spermatic cord absence makes specific diagnosis of testicular tumor associated with undescended testis.

Tumorous conditions of the epididymis and spermatic cord are rarely encountered in the clinical practice, and most of them are benign in their nature. The adenomatoid tumor is the most common benign tumor of the epididymis. Leiomyoma or papillary cystadenoma is the next common benign tumor of the epididymis. Adenomatoid tumor is a benign neoplasm of uncertain origin. Adenomatoid tumor occurs frequently in the epididymal tail and appears as a well-defined round mass on US evaluation. The internal echogenicity of the adenomatoid tumor seems to be homogenous and slightly hyperechoic compared with the epididymis. Papillary cystadenoma is frequently associated with von Hippel–Lindau disease. On US evaluation, papillary cystadenoma shows nonspecific imaging features indistinguishable from adenomatoid tumor or leiomyoma, but papillary cystadenoma has the tendency to be more cystic compared with adenomatoid tumor or leiomyoma. Sperm granuloma is a nontumorous condition mimicking epididymal tumor. Most of epididymal tumor needs enucleation, but sperm granuloma is treated conservatively or by epididymectomy for relief of postvasectomy pain syndrome. Therefore, differentiation of sperm granuloma from epididymal tumor is clinically important. Sperm granuloma itself shows imaging feature indistinguishable from epididymal tumor, but previous vasectomy-related findings including dilated epididymal system can be helpful in the differential diagnosis.

Lipoma is the most common extratesticular neoplasm and can occur anywhere within the scrotal sac. Lipoma accounts for more than half of the neoplasms that developed in the spermatic cord. Lipoma appears as echogenic mass on US evaluation (Fig. 4.6). However, this US imaging feature is a nonspecific finding that can be also seen in other disease entities such as inguinal hernia or sarcoma. Cross-sectional imaging like CT or MR might be helpful for specific diagnosis of lipoma by demonstrating internal fat content. When the mass of the spermatic cord is not lipoma, the possibility of malignant spermatic cord mass is increased. A report says that spermatic cord mass other than lipoma is a malignant mass in about 56 % [13]. Rhabdomyosarcoma is the most common malignant tumor of the spermatic cord in children, and liposarcoma is the most common malignant tumor of the spermatic cord in adults. Because the malignant mass of the spermatic cord is usually large and the relationship with adjacent structure can be helpful in the assessment of its origin, MR with large FOV and good tissue contrast is helpful in the evaluation of malignant spermatic cord tumor.

In the treatment of testis tumor, surgery remains as a core part of the management. However, the advance in chemotherapy and improvement in clinical staging due to the exquisite imaging modality and serum tumor markers reduced their role in the past few decades [22].

Because testis tumors are rapidly growing, surgery should be performed immediately without delays longer than 1–2 weeks. However, unfortunately, delays in the diagnosis and treatment of testis tumors are very frequent [23]. Bosl et al. reported that there are significant differences in delayed intervals for diagnosis between the early (CS I) and advanced (CS II and III) stage testis tumor. Painless scrotal mass is often ignored up to 18 % [24]. Careful history taking and physical examination, as well as imaging modality and serum tumor markers, are important for correct diagnosis.