External irradiation of epithelial skin cancer

Abstract

In 1989 more than 500,000 new cases of nonmelanomatous skin cancer were dialgnosed in the United States. The majority of these skin cancers are basal cell carcinomas and the majority of the remainder are squamous cell carcinomas. From an etiological point of view the most common cause is exposure to ultraviolet light and many treatment modalities are used in the management of these lesions. They are readily accessible to examination, thereby offering a great potential for early diagnosis and cure. Radiation therapy plays a significant role as the major treatment modality in lesions involving the eyelid, the nose, the lips, and the ea.rlobes. The major reason for the utilization of radiotherapy in these areas is the preservation of normal soft tissues, thereby giving rise to excellent cosmetic results and excellent control rates. Radiotherapy is equal to surgical management in other sites and is therefore an equally competing treatment regimen. The basic advantages for well-designed programs of radiation therapy are directed toward excellent cosmetic results and a major potential for cure. In this issue Lovett ef al. report their experience from the Radiation Oncology Center of the Mallinckrodt Institute of Radiology at the Washington University School of Medicine (1). This experience extended from January 1966 through December 1986 and comprised 339 consecutively treated biopsied proved squamous cell and basal cell carcinomas of the skin. There were 242 basal cell carcinomas, 92 squamous cell carcinomas, and 5 variances of squamous cell carcinomas in various locations. Radiotherapy was the initial treatment modality in 2 12 patients, and 127 were .treated after having failed initial surgical excision. Lymph node metastases in patients with basal cell carcinoma occurs in less than 0.5% of all patients whereas metastatic disease to lymph nodes from primary treated squamous cell carcinomas occurs in 15% and 39% of patients with recurrent sqeamous cell carcinoma of the skin. Distant metastatic disease was found in only one patient in the entire group. The data presented by Lovett et al. indicate local tumor control was achieved in 86% of the patients treated, 9 1% of those having basal cell carcinomas and 75% of those having squamous cell carcinoma. The data presented clearly stress the impact of tumor size on the potential for local control: 97% for basal cell carcinomas less than 1 cm in diameter and 9 1% for squamous cell carcinomas less than 1 cm in diameter; the local control rate diminished as the tumor size increased. For tumors greater than 5 cm, the tumor control for basal cell carcinomas was 87% and 56% for squamous cell carcinomas. The modality used in treating the patients was clearly a significant and important predictor in the expected outcome. When superficial X rays were used, the tumor control was excellent for all tumor sizes. The utilization of superficial X rays allows for a wide variety in choice of field size and half-value layers, and provides excellent dose distribution within the volume being treated. Superficial X ray techniques allow for sophisticated shielding procedures to be used thus shielding normal structures with a more precise fitting of the central portion of the beam to the area being treated and for choice of energy and filtration to allow for homogeneous adequate dose distribution within the volume of the tumor. In the utilization of electron beam, however, major factors emerge in the treatment planning that can have a significant and important impact upon the outcome. The data from Lovett et al. indicate that tumor control was 88% for tumors less than 1 cm, 72% for those that were l-5 cm in size, and 78% for those that were greater than 5 cm in size, results that were clearly less good than those achieved by superficial X rays. Treatment planning in the utilization of electron beam must consider the utilization of highly sophisticated techniques for shielding normal structures but also must take into consideration the specific characteristics of the electron beam and the need for a bolus in the field being treated to allow for homogeneous