External beam radiation therapy uses a linear accelerator to deliver fast-moving sub-atomic particles to tumors. The intent is to direct as much energy as possible to tumor cells while minimizing the damage to normal cells (and the associated side effects). The most common type of particle used is the photon, but neutrons, protons and carbon ions are also used. In addition, the particles may be delivered from different platforms that shape the energy to more precisely match any particular tumor.
ACC patients have external beam therapy in various scenarios:
- After surgery has removed the primary tumor. The intent is to kill any remaining tumor cells and thereby reduce the chances of recurrence.
- Instead of surgery when the primary tumor is not accessible due to nearby vital structures. The intent is to kill the cells in the primary tumor.
- For patients with metastases to the brain, lung, liver and elsewhere. Surgery may sacrifice too much normal tissue so radiation therapy may be less painful and just as effective. Also, radiation may shrink a tumor that is causing pain or interfering with breathing or eating.
There are limits to how much radiation may be used in any site, so radiation oncologists are careful to measure and limit the dosages. Typically, post-operative photon radiation consists of daily treatments, five times per week for 6 weeks or so. Each of 30-35 treatments may involve 2 Grey (Gy) of radiation, resulting in total radiation exposure to the targeted tissues of 60-70 Gy. Relatively low radiation doses over an extended time period increase the chances that (1) tumors cells will be killed when they happen to be dividing and are most sensitive, and (2) normal cells will repair themselves from the radiation damage.
For inoperable tumors and metastases, the more precise platforms offered by Stereotactic Body Radiation Therapy (SBRT) are appropriate. Given the more targeted nature of the treatment, fewer and more powerful doses may be used than with standard photon therapy. SBRT is described at RadiologyInfo.org and a list of facilities in the United States is provided here.
Also called “brachytherapy”, internal radiation therapy delivers radiation through seeds that are placed in the body with needles or catheters. The seeds typically are left inside or near tumors with their radioactivity wearing off within weeks or months. As with external beam radiation therapy, the intent is to target only the tumors and minimize radiation exposure to normal tissue.
Brachytherapy has been used in ACC patients with lung metastases with reportedly little discomfort. The main risk is a collapsed lung (pneumothorax).
Although not technically radiation, thermal ablation is a frequently used alternative for patients dealing with metastases. Interventional radiologists use radiofrequency ablation (RFA), microwaves or ultrasound to heat up tumors to about 100 degrees Fahrenheit. At that temperature, tumor cells tend to die while normal cells are able to recover eventually. CT and MRI scans are often used to guide the placement of the electrodes for RFA.
Patients who are unable or unwilling to undergo surgery or radiation therapy are candidates for RFA, as are those whose tumors have been resistant to radiation or systemic therapy. Typically, tumors must be 3cm or smaller to be treated effectively, though some practitioners will treat larger tumors. More information is available at www.radiologyinfo.org, www.rfablation.co.uk and www.cancernews.com. As with brachytherapy, the main risk for those with lung metastases is a collapsed lung (pneumothorax).
The National Cancer Institute has published a fact sheet, Radiation Therapy for Cancer: Questions and Answers, that provides greater detail on the rationales, types and side effects of radiation therapy.