Ultima Vez Modificado: 1 de enero del 2003
Dear OncoLink "Ask The Experts,"
My son had a squamous cell carcinoma removed from his tonsil. Radiation has been recommended. New IMRT equipment has come into the equation but it is not yet available within the facility where my son is being treated. Given the advice that salivary glands may well be impacted by "regular" equipment, is IMRT equipment at a stage where it should be vigorously perused as a treatment of choice? Many thanks for any thoughts you can share with us.
Mitchell Machtay, MD, Assistant Professor of Radiation Oncology at the Hospital of the University of Pennsylvania and Deputy Chairman of Radiation Therapy Oncology Group, responds:
IMRT has been the catchphrase of the 21st century in the field of radiation oncology. To some radiation oncologists, IMRT (Intensity Modulated Radiation Therapy) represents technology at its best and a revolution in cancer therapy; to others, IMRT is just a four-letter word.
IMRT is difficult to define, but its differences from conventional radiotherapy are glaringly obvious. In conventional radiation therapy, patients receive a combination of 2 to 6 radiation beams (also called "portals" or "fields") per day. These beams are aimed - with a variable degree of precision - at the patient's tumor, and efforts are made at shielding surrounding normal healthy tissues. However, because of the laws of physics, the amount of radiation that hits most nearby tissues is fairly similar to that received by the tumor.
IMRT is a very high-technology way of delivering radiation such that the amount of radiation given to the tumor can be maintained or even increased while actually decreasing (sometimes astonishingly so) the radiation dose received by normal tissues. This is done by combining a huge number (up to hundreds) of mini-beams (also called "segments") of varying radiation intensity, but all focused on part or all of the tumor (or an area suspected of containing tumor). IMRT has only been possible in recent years, the result of enormous advances in computer technology and in the mechanical hardware that administers radiation.
The case that is asked about is not uncommon. People with cancer of the tonsil and/or other sites within the head and neck often need extensive radiation therapy, which has many side effects, especially permanent and uncomfortable dry mouth. Dry mouth (xerostomia) is not just an annoying by-product of radiotherapy; it can lead to serious nutritional problems, dental, infectious, and psychosocial problems. In the worst case, dry mouth can lead to osteoradionecrosis of the mandible, a rare but life-threatening condition where the jawbone becomes destroyed. Can IMRT prevent - or at least minimize - this and other risks of radiation to the head and neck?
Some leaders in the field of head and neck radiation oncology assert that IMRT can indeed dramatically decrease the problem of dry mouth. Arguably, the most important organs for saliva production are the two parotid glands, which are embedded in the cheeks. These glands are devastated by conventional radiation therapy. With IMRT, the radiation dose received by the parotid glands can be reduced by 50 to 75%. At the same time, the radiation dose to the tumor and lymph nodes is the same or higher than with conventional radiation. On paper, at least, this should result in preservation of some or most of the function of at least one parotid gland, enough to keep the mouth moist. Small studies from centers of excellence have been encouraging. In one study from the University of California San Francisco, IMRT for cancer of the nasopharynx appeared to significantly improve the tumor control rate while significantly decreasing long-term side effects of radiation. However, there have not been large-scale studies to prove this. The Radiation Therapy Oncology Group (RTOG), a consortium of elite radiation oncology centers, is running a prospective clinical trial of IMRT for oropharynx (including tonsil) cancer. A similar RTOG study for nasopharynx cancer is expected to open this year as well.
Since there are such exciting preliminary data with IMRT, and since it is becoming available at many community radiation oncology centers, is there anything to lose? Perhaps not, but there are some practical and theoretical downsides to IMRT and other forms of super-high-technology radiotherapy.
The first is that IMRT requires a large increase in time and resources, on both the health care provider and the patient. A single daily IMRT treatment may take as much time to deliver as 3 or 4 regular radiation treatments. It may take 10 to 20 times the amount of physician and radiation physicist effort to plan a case of IMRT compared with conventional radiation. From the patient's point of view, this may result in lengthy, uncomfortable daily sessions in the radiation departments and/or delays before or during treatment. IMRT is covered by most health insurance plans, but as always, there is still the potential for added out-of-pocket costs.
The second concern about IMRT is the hypothetical risk that its increased complexity could paradoxically increase the risk of cancer recurrence at the edges of a tumor. IMRT requires extraordinary precision in planning and delivery, with very little margin for error. If there is an unanticipated problem - such as a patient or tumor that moves a lot, or difficulty in interpreting the patient's CT scan - there could be under dosing of tissues where tumor cells lurk. Reports from very experienced IMRT centers suggest that this is unlikely to be a problem, but these studies are based on simulated IMRT on "dummy" patients plus short-term follow-up of actual treated patients. Long-term clinical data proving that IMRT is better than (or at least equal to) conventional radiation at curing tumors is not available yet.
A third concern is the possibility that IMRT may cause significant "hot spots" of high-dose radiation. This results from overlap of multiple small beams of radiation in an effort to spare a normal structure like the parotid glands. Usually these "hot spots" are not relevant, but if these are in a sensitive area, such as the skin, a very severe reaction can result.
A final concern about IMRT is that it exposes large portions of the entire body to low-moderate doses of radiation. This is partly the result of the need for very prolonged radiation "beam-on" treatment time to deliver IMRT. These radiation doses won't cause early and noticeable radiation damage, but may increase the risk of a new cancer forming 10 to 20 years later. Radiation-induced cancers are quite uncommon after conventional radiotherapy, but will IMRT change this?
In my view, these worries about IMRT are more theoretical than real, particularly when IMRT is administered by a treatment team with expertise and a rigorous quality assurance program. While it would be preferable for patients to receive IMRT as part of a clinical research study, I think it is perfectly acceptable to use it for selected cases of head and neck cancer (and many other types of cancer). However, there are few situations where IMRT should be considered mandatory. Oropharynx or tonsil cancer is not one of these situations. For a typical cancer of the tonsil, I would not recommend seeking out a radiation oncology center specifically for IMRT. There are too many other ingredients that are important to the overall outcome, most notably the overall experience of the entire treatment team - physicians, physicists, nurses, therapy technologists, nutritionists and other cancer care providers.Imprima English