New techniques in irradiation: clinical implications of perioperative high-dose rate brachytherapy

Abstract

Brachytherapy (derived from the Greek term brachios, meaning short) is defined as the treatment of tumors through the placement of radioactive sources into or close to the malignant lesion. The main advantage of brachytherapy is based on the inverse squared law inherent to radioactive sources, which allows the delivery of a very high dose of radiation to the tumor with a substantial sparing of the surrounding normal tissues. Historically, brachytherapy has evolved from hot loading (i.e. manual placement of radioactive devices into the patient) to remote afterloading. In remote afterloading, non-radioactive applicators are inserted into the tumor; these applicators are then loaded using mechanically driven, computer-controlled equipment after dosimetry is approved. From the radiation safety standpoint, remote afterloading eliminates the radiation hazards for both the hospital staff and visitors, and allows more convenient access to the patient. Brachytherapy techniques, largely obscured with the advancement of modern teletherapy equipment after 1960, have received increasing attention over the last 15 years. The reasons that have contributed to this brachytherapy renaissance are multiple. (i) Advances in brachytherapy software, particularly the integration of computed tomography (CT) image acquisition and computer-driven dosimetry, now allow a fast and meticulous analysis of the area to be treated with brachytherapy. The spatial relationship between the radiation isodoses and the volumes of interest is now visualized in a three-dimensional environment and displayed through dose volume histograms. Similarly, small imperfections in the implant quality can be corrected by changing the dwell times of the multistepping sources used in modern brachytherapy. As a result, tumor coverage is improved, and radiation dose to critical organs minimized. (ii) Brachytherapy equipment has been improved with the design of small, stepping flexible sources with high specific activity (Ir) that can navigate through thin plastic or metal applicators of up to 5 French in diameter (1.5mm). These sources can be designed to deliver treatments at a high dose rate (>12Gy/h) that substantially shortens the period for which the patient remains radioactive. (iii) Low-energy, low-activity gamma radionuclides (I, Pd) are now available and can be used in permanent implants. These sources present a negligible radiological hazard when used for deep-seated tumors. Permanent implants have the advantage of requiring a much shorter hospital stay than other types of radiation therapy, and they are perceived by the patient as a less aggressive and more convenient type of therapy. (iv) Finally, developments in medicine and health education have increased the number of patients with malignant tumors who are diagnosed at an earlier stage. As a result, many patients now present with small tumors that have a high probability of being organ-confined and that may be treated with organ therapies, such as brachytherapy.