PART II
Modern Technologies in Radiotherapy (Cont'd)
1. Improvements on the technical aspects of radiotherapy
Generally, radiation oncologists attempt to reduce the injury to normal tissues surrounding the target tumor by limiting (or blocking) the radiation field. In recent years, a number of significant technical developments and modifications in radiotherapy have been developed in order to increase the efficacy of radiotherapy and/or reduce radiotherapy-induced injury. These technology improvements include using multiple ports, higher radiation doses, interstitial brachytherapy, stereotactic radiosurgery, and TomoTherapy. In general, therapists use these approaches in attempts to increase the radiation dose to the tumor tissue while decreasing the dose to the surrounding normal tissue(s). The main goals are to enhance/maintain the therapeutic effect while minimizing the damages to normal tissues.
A representative example of these newer technologies is stereotactic radiotherapy. Stereotactic radiotherapy utilizes small fractions of high-dose radiation (as compared to conventional dose) from different angles to destroy tumor tissue. By giving multiple radiation doses from different angles, it can improve the radiotherapy outcome while minimizing potential side effects on normal tissues, especially the skin. Stereotactic radiotherapy is now frequently used to treat brain tumors and other types of solid tumors.
A specific form of stereotactic radiotherapy used for brain tumors is called stereotactic surgery, which utilizes high dose radiation (x-ray or gamma ray) to precisely focus on the specific area in the brain to destroy tumor tissues in the brain, very similar to surgery, thus often referred to as radiation surgery. In this treatment, the dose and the treatment area have to be precisely coordinated to ensure that normal tissues in the proximity suffer the least damage by the high dose radiation. It is also called fractionated stereotactic radiosurgery wherein the total dose of radiation is divided into several doses and delivered in several days.
Another popular new technology is called TomoTherapy. It is a computed tomograph (CT)-guided intensity modulated radiation therapy (IMRT) that delivers radiation to the tumor slice-by-slice, which is different from other forms of external beam radiation therapy in which the entire tumor volume is irradiated. TomoTherapy is essentially an image-guided radiation therapy (IGRT). The “beam on” time for TomoTherapy (3 to 5 minutes) is similar to normal radiotherapy. However, it does require additional daily CT to precisely locate the tumor because tumor size and position may shift due to the shrinkage over time. The daily CT will help the operator in directing the radiation beam to modify the treatment (adaptive radiotherapy). TomoTherapy has been used in many solid tumors including prostate cancer, lung cancer, as well as head and neck cancer.
In recent years, stereotactic
radiotherapy is being combined with TomoTherapy. Right before the radiotherapy
procedure, a patient is positioned on an examination table and a
high-resolution CT scan is used to determine the precise location, size, and
shape of the tumor. The data is then transferred to the computing system
wherein an exact treatment plan is defined in real time. Once the patient is
positioned, the radiation source is rotated and/or orientated by a computer to
different positions/angles, and a predetermined dose of radiation is targeted
at the tumor while sparing the healthy tissues. Commercially, several advanced
radiotherapy systems utilizing similar principle have been developed and used.
Examples of modern radiotherapy systems include Discovery CT590 RT and Optima
CT580 made by GE Healthcare, and CyberKnife® made by Accuray. The most recent
comprehensive radiosurgery system, Edge™, is made by Varian Medical
Systems. Edge™ is just entering the
market and there are only few units installed in the United States. It is a
dedicated system for performing advanced radiosurgery using new real-time tumor
tracking technology.
