The Effects of External Beam Irradiation on Ear Cartilage: An Animal Model

摘要

Adjuvant therapy of the head and neck has become the mainstay for treatment of head and neck malignancies. Although important for local control, complications do arise with the use of this adjuvant therapy. We have recently noted changes in the ear architecture in patients that have undergone postoperative irradiation therapy. In an attempt to identify changes in the cartilage thickness, we conducted an animal model to determine the etiology of our clinical observation.Eighteen New Zealand white rabbits were utilized. Three experimental irradiated groups were designed for single dose external cobalt60 beam therapy at 20, 25 and 30 Gy. The right ear served as matched nonirradiated control. After two months the cartilage underwent quantitative histomorphometric analysis using an inverted microscope, digitized camera and computer software. Irradiation did not significantly affect the thickness of cartilage upon comparing control and irradiated groups. There was no change seen in vascularity or tissue composition that could be identified.In conclusion, it appears that alterations in thickness of the cartilage did not result in the clinical examples demonstrated. The etiology to the changes noted clinically are unknown, but may be related to fibrosis and scarring of the dermis/epidermis. Further evaluation is warranted. Introduction With the discovery of X-rays by Wilhelm Konrad Roentgen (1895), ionizing radiation became both a diagnostic tool and therapeutic modality. The goals of ionizing radiation are local regional tumor control combined with functional preservation alone or in conjunction with surgical extirpation. High doses of irradiation can be delivered currently with specialized techniques in an attempt to minimize the late complications of normal tissue. Radiation portals precisely outline tissues at risk and exclude uninvolved regions by applying individualized treatment blocks.1,2 With the advent of this adjuvant therapy, the approach to the patients with surgical extirpation has changed. Local control has been improved, however, complications related to wound healing have increased. The daily dose of radiation is considered to be one of the most important factors in the development of sequelae. We have recently noted that patients undergoing pre and/or postoperative radiation therapy of the head and neck have presented alterations in their ear architecture (Figure 1). This presentation is irrespective of surgical alterations within the area. Figure 1 demonstrates the clinical appearance seen in many of these patients. Unfortunately we can not correlate this effect with dose or other factors clinically. Protrusion of the ear anteriorly with a constriction similar to congenital ear anomalies has lead us to explore this clinical observation in an animal study. Consequently, it was the purpose of this study to determine alterations in cartilage thickness in a rabbit model with animals undergoing single dose irradiation therapy. It was hypothesized that cartilage thickness would be effected in a radiation dose dependent manner. Material And Methods Eighteen New Zealand white rabbits (3.0-3.5 kg) were used in this study. Three experimental irradiated groups were designed. Under general anesthesia, radiation was applied to the left ear using a cobalt60 external beam source. The dose was not fractionated and three distinct dosing levels were employed. It is a common practice to utilize single dose irradiation as a determinate of an effect. Further studies with fractionated dosing can then be employed.2-6 These included six animals each that received 20, 25 and 30 Gy. The right ear served as matched, nonirradiated control. The right ear was protected during the irradiation dosing to abrogate scatter irradiation. Preoperative dosimetry was determined allowing the maximum dosing to the area in question, and to prevent associated morbidity and damage to the surrounding tissues. After two months, rabbits were euthanized. Commonly