Similar to the Basal Cell Carcinoma, cutaneous Squamous Cell Carcinoma typically occurs in areas of skin receiving the greatest sun exposure. The etiology appears to be the result of UVB radiation (wavelength range of 290-320 nm), which produces thymidine dimers in the DNA of the p53 tumor-suppressor gene.
Fair-skinned individuals, albinos and those with xeroderma pigmentosum seem to be at particularly increased risk. Other risk factors include infection with human papillomavirus, chronic immunosuppression such as that seen in the organ-transplant population, exposure to chemical carcinogens such as arsenic, and exposure to ionizing radiation.
Chronically inflamed or damaged skin may predispose to carcinoma as well, termed Marjolin's ulcer. These are most commonly squamous cell carcinomas, and they can arise in long-standing, chronic wounds such as pressure sores, fistulae, venous ulcers, lymphedema and burn scars.
The metastatic potential of SCC is greater than that of BCC, and the relative risk of recurrence and metastasis can be assessed according to characteristics of the lesion. The most important predictor is size of the tumor, with lesions greater than 2 cm in diameter recurring at a rate of 15% and resulting in metastasis at a rate of 30%. Anatomic location predicts greater malignant potential, especially the lip and ear, but also the scalp, forehead, eyelid, nose, dorsum of the hands, penis and perineum.
Other features associated with higher risk of recurrence and metastases are rapid tumor growth, host immunosuppression, prior local recurrence, depth of invasion greater than 4 mm or into the subcutaneous tissues and location in a Marjolin's ulcer. Perineural invasion denotes a particularly poor prognosis and is lethal in a majority of patients by five years.
With the exception of cryotherapy, treatment options for SCC are similar to those for BCC. However, there have been no randomized controlled trials comparing the efficacy of the various techniques. Direct surgical excision has demonstrated a recurrence rate of approximately 8% and metastatic rate of 5% at five years. Some surgeons have advocated surgical margins of 4 mm for low risk lesions and 6 mm for high-risk lesions whenever feasible.
Mohs surgery has demonstrated the highest cure rates, about 97% for primary SCC, and is especially recommended for high-risk lesions. Due to lack of histological control and unacceptable cure rates, curettage with cautery, cryosurgery and radiotherapy are not recommended.
Mohs micrographic surgery (MMS) was developed by Dr. Frederic Mohs in the 1930s. Dermatologists with specialized fellowship training in Mohs surgery typically perform the technique today. Fresh-tissue horizontal frozen sections are the hallmark of the procedure. Another salient feature is that the surgeon who performs the excision also interprets the histological results. The process can be summarized briefly in the following steps:
This approach combines the highest cure rates for nonmelanoma skin cancer with maximum preservation of normal tissue and function. It provides the theoretical benefit of being able to examine 100% of the surgical margin in a single sitting, compared to the standard "bread-loafing" histological examination of surgical specimens. The disadvantages are that is can be time-consuming, labor intensive and expensive
Indications for Mohs micrographic surgery over other treatments include location of the lesion in the "H-zone," tumors greater than 2 cm in diameter, aggressive tumors, recurrent tumors, tumors in previously irradiated areas, incompletely excised tumors, clinically ill-defined tumors, presence of perineural invasion and aggressive rare tumors such as dermatofibrosarcoma protuberans, microcystic adnexal carcinoma or sebaceous gland carcinoma.
Follow-up after treatment of skin cancers consists of monthly whole-body skin self-examinations and twice yearly skin examinations by a dermatologist or other qualified medical provider for at least five years to check for local recurrence as well as the occurrence of a second primary. Patients with larger SCC should be examined every three months, including palpation of regional lymph nodes, for several years and then followed at six-month intervals for the rest of their lives.
The two main pitfalls in treating skin cancers are missed or delayed diagnosis and inadequate biopsy. Although some skin cancers can be diagnosed by visual inspection alone, histologic confirmation is essential. The adage "when in doubt, cut it out" should be applied when dealing with skin lesions. A corollary to that rule is "if the patient wants it out, cut it out." If one fails to biopsy a lesion, which later turns out to be a malignancy, the delay in treatment can lead to a higher risk of recurrence, metastases and malpractice suits.
An excisional biopsy is preferable because it gives the pathologist the entire lesion to examine. If the lesion is large, a representative incisional or punch biopsy, taking the full thickness of the skin, is preferable to a shave biopsy, particularly if the lesion turns out to be a melanoma.
As in other areas of tumor surgery, reconstructive concerns should not compromise surgical margins. One way to avoid this "conflict of interest" is to have one surgeon excise the lesion and another reconstruct the defect. Mohs surgery is particularly well suited to this two-surgeon approach.
For extensive tumors treated by standard excision, delayed reconstruction should be considered in order to wait for the final histological margins, unless the defect can be closed primarily. One should not proceed with a complex flap reconstruction solely on the basis of an intraoperative frozen section examination of the margin because of the potential for a false negative.
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