Epidemiology

Penile carcinoma is a rare disease, accounting for only 1640 new cases diagnosed in the United States in 2014. The incidence is higher among men in developing countries of Asia, South America, and Africa. Risk factors for penile carcinoma include chronic inflammation, lichen sclerosis, phimosis, and tobacco use ; one-third of cases can be attributed to human papillomavirus (HPV) infection. Because of its localization, this malignancy can be a source of potentially devastating psychosexual distress, in addition to the other factors associated with malignancies.

Pathology

Penile squamous cell carcinoma (pSCC) is the most common malignant disease of the penis, accounting for more than 95% of cases of penile carcinomas. The American Joint Committee of Cancer recognizes 4 subtypes of SCC:

• •
  

  Verrucous

  
  
• •
  

  Papillary squamous

  
  
• •
  

  Warty

  
  
• •
  

  Basaloid

The verrucous subtype is known to be of low malignant potential, whereas the other types have a worse prognosis.

Management

The current guidelines of the American Urologic Association recommend a surgical amputation of the primary tumor. Prognostic factors after treating the primary lesions mainly depend on the presence of regional lymph nodes metastases (LNM), which are found in 25% of men at presentation. Hence, establishing the lymph node status for each patient is relevant at baseline to determine the prognosis and, thus, the need for further treatment and following.

The biology of pSCC is such that it exhibits a prolonged locoregional phase before distant dissemination, also providing a rationale for the therapeutic value of lymphadenectomy, what has demonstrated survival benefits. Yet, it has been suggested that 20% of men with pSCC have nodal metastases that are not clinically evident at the time of initial presentation.

Clinical examination to determine the lymph node status is unreliable in inguinal lymph nodes; the incidence of occulted metastases varies between 2% and 66%. The sensitivity and specificity of computed tomography or MRI are not sufficient as 50% of enlarged nodes show no malignancy. Other methods like the PET with fludeoxyglucose 18 scanner have been suggested for nodal staging, but the results have been inconclusive. Minimally invasive techniques, such as fine-needle biopsy or sentinel node biopsy, have been used for nodal staging. Based on recent results, dynamic sentinel lymph node biopsy (DSNB), which relies on the injection of the primary lesion with blue dye and a radioactive tracer, was shown to have a false-negative rate of 7.0% and a complication rate of 4.7%. Association with fine-needle aspiration of suspicious lymph nodes can help to improve DSNB sensitivity. But these techniques require a multidisciplinary team consisting of urologists, nuclear medicine physicians, and pathologists in addition to specialized equipment and training. So far, the gold standard to determine the inguinal lymph node status is inguinal lymph node resection, which is an invasive procedure associated with significant morbidity. Even if the therapeutic benefits outweigh these complications for patients who have lymph node involvement, only 20% of those with clinically nonpalpable lymph nodes will harbor occult metastasis.

To select candidates for inguinal lymphadenectomy for patients with clinically negative inguinal nodes, guidelines are mainly based on primary penile histology, including stage, lymphovascular invasion, and grade of primary tumor ; but a recent prospective study stipulates that these criteria alone are still insufficient, with many patients (82%) being subjected to unnecessary lymphadenectomy with this method of selection.

These limitations to penile cancer stage provide the underpinning for the identification of more reliable penile cancer biomarkers.

Currently, there are no existing biomarkers for pSCC routinely available; but ongoing research has identified biomarkers, which can be classified into several types:

• •
  

  Plasmatic biomarkers

  
  
• •
  

  Proliferation associated markers

  
  
• •
  

  HPV-related markers

  
  
• •
  

  P53

  
  
• •
  

  Cytogenetic markers

Epidemiology

Penile carcinoma is a rare disease, accounting for only 1640 new cases diagnosed in the United States in 2014. The incidence is higher among men in developing countries of Asia, South America, and Africa. Risk factors for penile carcinoma include chronic inflammation, lichen sclerosis, phimosis, and tobacco use ; one-third of cases can be attributed to human papillomavirus (HPV) infection. Because of its localization, this malignancy can be a source of potentially devastating psychosexual distress, in addition to the other factors associated with malignancies.

Pathology

Penile squamous cell carcinoma (pSCC) is the most common malignant disease of the penis, accounting for more than 95% of cases of penile carcinomas. The American Joint Committee of Cancer recognizes 4 subtypes of SCC:

• •
  

  Verrucous

  
  
• •
  

  Papillary squamous

  
  
• •
  

  Warty

  
  
• •
  

  Basaloid

The verrucous subtype is known to be of low malignant potential, whereas the other types have a worse prognosis.

Management

The current guidelines of the American Urologic Association recommend a surgical amputation of the primary tumor. Prognostic factors after treating the primary lesions mainly depend on the presence of regional lymph nodes metastases (LNM), which are found in 25% of men at presentation. Hence, establishing the lymph node status for each patient is relevant at baseline to determine the prognosis and, thus, the need for further treatment and following.

The biology of pSCC is such that it exhibits a prolonged locoregional phase before distant dissemination, also providing a rationale for the therapeutic value of lymphadenectomy, what has demonstrated survival benefits. Yet, it has been suggested that 20% of men with pSCC have nodal metastases that are not clinically evident at the time of initial presentation.

Clinical examination to determine the lymph node status is unreliable in inguinal lymph nodes; the incidence of occulted metastases varies between 2% and 66%. The sensitivity and specificity of computed tomography or MRI are not sufficient as 50% of enlarged nodes show no malignancy. Other methods like the PET with fludeoxyglucose 18 scanner have been suggested for nodal staging, but the results have been inconclusive. Minimally invasive techniques, such as fine-needle biopsy or sentinel node biopsy, have been used for nodal staging. Based on recent results, dynamic sentinel lymph node biopsy (DSNB), which relies on the injection of the primary lesion with blue dye and a radioactive tracer, was shown to have a false-negative rate of 7.0% and a complication rate of 4.7%. Association with fine-needle aspiration of suspicious lymph nodes can help to improve DSNB sensitivity. But these techniques require a multidisciplinary team consisting of urologists, nuclear medicine physicians, and pathologists in addition to specialized equipment and training. So far, the gold standard to determine the inguinal lymph node status is inguinal lymph node resection, which is an invasive procedure associated with significant morbidity. Even if the therapeutic benefits outweigh these complications for patients who have lymph node involvement, only 20% of those with clinically nonpalpable lymph nodes will harbor occult metastasis.

To select candidates for inguinal lymphadenectomy for patients with clinically negative inguinal nodes, guidelines are mainly based on primary penile histology, including stage, lymphovascular invasion, and grade of primary tumor ; but a recent prospective study stipulates that these criteria alone are still insufficient, with many patients (82%) being subjected to unnecessary lymphadenectomy with this method of selection.

These limitations to penile cancer stage provide the underpinning for the identification of more reliable penile cancer biomarkers.

Currently, there are no existing biomarkers for pSCC routinely available; but ongoing research has identified biomarkers, which can be classified into several types:

• •
  

  Plasmatic biomarkers

  
  
• •
  

  Proliferation associated markers

  
  
• •
  

  HPV-related markers

  
  
• •
  

  P53

  
  
• •
  

  Cytogenetic markers

Squamous Cell Carcinoma Antigen

Squamous cell carcinoma antigen (SCCAg) is a tumor-associated glycoprotein and a member of the serine protease inhibitor family. It can be measured using a microparticle enzyme immunoassay. In cervical and anal carcinoma, SCCAg has been successfully used in both staging and following the course of the disease.

There are few clinical studies examining the reliability of SCCAg in penile cancer.

Laniado and colleagues found that SCCAg was elevated in patients with LNM and demonstrated that an elevated SCCAg level had a sensitivity of 57% and a specificity of 100% for predicting nodal metastases at the 1.50 μg/L cutoff. This same study demonstrated that SCCAg was a useful tool in following patients after excision of the primary lesion. In this study, levels of SCCAg increased exponentially in patients who subsequently developed nodal metastases and before any clinical or radiological evidence of nodal involvement.

More recently, a prospective study evaluated the clinical use of SCCAg in 16 patients with penile cancer and suggested that the sequential measurement of plasma SCCAg might indicate the presence of LNMs and/or distant metastases before they are detected clinically or by imaging, allowing early and potentially curative inguinal nodes dissections. It also demonstrated that SCCAg was useful for monitoring patients’ response to treatment.

In a larger series including 54 patients, Hungerhuber and colleagues reported a correlation between tumor burden and SCCAg, which increased significantly with extensive lymph node involvement and metastatic disease. They also reported that SCCAg was powerful for monitoring treatment control and early detection of recurrence after treatment.

The prospective series of Zhu and colleagues, based on the data of 63 patients with penile cancer, showed that despite the confirmation that SCCAg is significantly associated with nodal status, it could not accurately predict the presence of occult inguinal metastasis. These investigators further demonstrated that SCCAg was an independent prognostic factor in node positive cases and was an important predictor of progression-free survival in patients with penile cancer.

However, despite these promising results, these data are not yet sufficient to include the use of SCCAg in the guidelines for the management of patients with penile cancer, primarily because of the low number of patients involved in theses studies. It seems that SCCAg correlates with tumor burden and is useful for following the disease course and detecting early recurrence. Regarding the ability of this biomarker to predict the presence of occult LNM, data are controversial. There is a need for a larger prospective study to ascertain the clinical utility of this biomarker.

C-Reactive Protein

C-reactive protein (CRP) is a protein produced by the liver in response to microbial infection, trauma, infarction, autoimmune diseases, or malignancies. Its production occurs in response to an inflammatory stimulus involving increased cytokine expression ; it is already a cheap, simple, and routinely performed marker for infection. Various mechanisms can explain the elevation of CRP during malignancies: inflammation caused by tumor growth, immune response to tumor antigen, or the chronic inflammation itself, which can be an etiologic factor for carcinogenesis.

High levels of plasma CRP have been linked to a poor prognosis in various cancers, including urologic cancers, such as renal cell carcinoma and urothelial carcinoma.

Recently, Saito and Kihara suggested that CRP could be a useful biomarker for many urologic cancers and that the analysis of dynamic changes in CRP concentrations over time could predict tumor aggressiveness and potential treatment efficacy.

Two studies evaluated CRP blood levels as a prognostic marker in penile cancer. Steffens and colleagues, in a retrospective study counting 79 patients with penile cancer, found that an elevated preoperative serum CRP level is significantly associated with reduced cancer-specific survival but failed to define CRP as an independent prognostic factor.

Another retrospective study based on data from 51 patients found that CRP serum level correlated significantly with LNM.

These preliminary data based on retrospective studies suggest that CRP could be an interesting noninvasive prognostic marker and that it could help identify patients with pSCC who may benefit from an inguinal lymph nodes dissection. This finding needs to be confirmed by further studies.

Ki-67

Ki-67 is a nonhistone nuclear matrix protein expressed in all cell-cycle phases except G0. An assessment of Ki-67 protein expression is a reliable mean of the evaluation of tumor cell proliferation. It has already been found as a prognostic factor in various cancers, but the data remain controversial in the case of penile carcinoma.

In a retrospective study of 44 samples of pSCC, there was a significant association only between Ki-67 and tumor grade and a tendency toward higher Ki labeling index with tumor stage, nodal metastasis, and clinical disease progression. In a larger retrospective study on 148 patients, Stankiewicz and colleagues found a similar significant association between Ki-67 and tumor grade but failed to find an association with LNM or survival. Similarly, in the larger multicentric study with 153 patients with pSCC, it has been found that Ki-67, even if it reflects a more aggressive behavior with a positive correlation with histologic tumor grade, is not an independent prognostic factor for the cancer-specific survival.

For instance, only one small study found a positive association between Ki-67 and LNM. Conversely, Guimarães and colleagues found an inverse association between LNM and the Ki-67 expression level in both univariate and multivariate analysis.

According to these results, Ki-67 seems to have a positive correlation with tumor grade; but its prognostic value for LNM or survival remains elusive.

Proliferating Cell Nuclear Antigen

Proliferating cell nuclear antigen (PCNA) is a protein found in the nucleus and is essential for replication. Thus, it is as Ki-67, a marker of cell proliferation, which has also been studied in penile cancer. In a quantitative analysis, Martins and colleagues found that PCNA expression was significantly associated with LNM but did not find a correlation between PCNA immune expression and prognosis.

More recently, in a study of 125 patients with penile carcinoma, Guimarães and colleagues found that PCNA was an independent prognostic factor for LNM but not survival.

There are few data on the prognostic significance of this biomarker; it is important to mention that there are no standardizations in the execution and interpretation of PCNA, making comparison of results challenging.

Human Papillomavirus: a Risk Factor for Penile Cancer

HPV is a family of double-stranded DNA viruses, which is generally sexually transmitted. It is associated with several malignancies, including cancer of the penis, cervix, anus, and oropharynx; it seems to be an important factor in the development of in situ and invasive epithelial tumors.

It is estimated that one-third of pSCCs can be attributed to HPV-related carcinogenesis. The reported prevalence of HPV DNA in penile cancer ranges between 20% and 80%, this variation is likely due to the method used for the detection and the geographic variance.

It has been established that prevalence varies with histologic subtype: high HPV prevalence is found in basaloid subtypes and warty pSCCs, whereas verrucous and papillary pSCCs are associated with HPV in one-third of cases.

Thus, it has been suggested that penile carcinogenesis can follow 2 distinct pathways: one associated by HPV infection and the other related to nonviral factors, such as phimosis, chronic inflammation, or lichen sclerosus.

Studies have demonstrated that HPV 16 and 18 are the most common types of HPV found in penile cancer.

The studies on cervical SCC have provided the most convincing model for understanding the molecular mechanism by which HPV promotes carcinogenesis. It seems to rely on the integration of the viral genome into the host cell’s chromosomal DNA. It results in an overexpression of 2 viral oncogenes, E6 and E7, which disturb cellular differentiation, proliferation, and apoptosis through their involvement with p53 tumor suppressor and the retinoblastoma RbE2F, which in turn leads to accumulation of P16 ^INK4a . This is thought to be the most likely mechanism of HPV-related penile cancer.

Human Papillomavirus: Prognostic Marker for Penile Cancer

The Prognostic Significance of a Human Papillomavirus Marker: P16 ^INK4a

P16 ^INK4a is a tumor suppressor gene that induces cell cycle arrest and prevents cell division by G1 cyclin-dependent kinase 4 and 6 inhibition. It is known to accumulate in HPV-related tumors, in response to inactivation due to viral E7 protein, and seems to be a reliable marker for high-risk HPV infection in pSCC cases. However, recent studies have stipulated that this diagnosis must be confirmed by P16 ^INK4a immunostaining that is diffuse, continuous, and strongly nuclear and cytoplasmic.

As a marker of the presence of HPV, and because of the promising prognostic value of HPV in penile cancer, P16 ^INK4A has been thought to have a prognostic significance in this disease. In a small series, Ferrándiz-Pulido and colleagues found a tendency toward a positive association between P16 ^INK4a overexpression and a better prognosis. This finding is supported by a multi-institutional study including 92 cases of pSCC that found that P16 ^INK4a independently predicted improved cancer-specific survival. Similarly, a Canadian study of 43 cases found that the lack of p16 immunostaining was an independent factor for decreased survival. Recently, Steinestel and colleagues confirmed that P16 ^INK4a and high-risk HPV status indicated a less aggressive behavior but failed to provide prognostic contribution of P16 ^INK4a status for node involvement or survival.

This prognostic role might be explained by its implication in cell cycle control, notably by mediating G1 arrest; this could explain the positive significance in terms of prognosis of HPV positivity found in some of the studies mentioned earlier.

Regarding the association between P16 ^INK4a and lymph node status, Poetsch and colleagues found an inverse relation, with a lack of P16 ^INK4a predicting an increased risk of LNM.

In a retrospective cohort including 119 patients, Tang and colleagues found that a lack of P16 ^INK4a expression was significantly associated with recurrences only when there were positive lymph nodes found at diagnosis. Despite the large number of cases, they failed to find an association with survival. This finding suggests that P16 ^INK4a could have an important role in the subgroup of patients with positive lymph nodes at baseline to predict the prognosis and help determine the choice of further treatment.

These results support the fact that this protein marker could be an important prognostic marker regarding the natural history of the disease, with the limitation that all these studies used a variable definition of P16 ^INK4a positivity.