Pain is one of the most awful symptoms of cancer, and the one that most people talk about when discussing the value  of physician assisted suicide.  While I (thankfully) have never had a patient directly ask me for such aid, I have had many conversations with patients and their families about pain management and how we can do a better job.

   This post cannot possibly tackle the entire field of pain syndromes and pain management.  Instead, here I want to emphasize a specific area, that of tumor impinging on the spinal cord.  Lung cancer frequently metastasizes to bone, and the bones of the spine are a prime target.  Those metastases can be directly painful, but even more worrisome from an oncologist’s perspective, the tumor can impinge on the nerves leaving the spine (by growing into the foramina, or holes by which nerves branch off from the spine and go out through the body to all muscles, organs etc) or compressing the spinal cord itself.

Spinal Anatomy (spinal cord in yellow):

 (click to enlarge)

   Symptoms of spinal cord compression depend on the level of tumor pressure.  Usually, the first thing that patients notice is change in sensation—things feel funny, like a foot that has fallen asleep.  Later, muscle weakness comes and often change in the ability to control urine or bowel function.  Once it reaches that level, making the diagnosis is straightforward and a CT or MRI of the spine will be ordered by your treating doctor.  Treatment is typically radiation, as soon as feasible, but sometimes surgery first is needed for rapid relief of the pressure on the spinal cord.

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   Targeting cancer cells and missing all of the normal tissue is the Holy Grail of cancer therapy.  It is the cancer equivalent to the perfect diet:  eat everything you want, never exercise and stay perfectly skinny and fit.  Doesn’t happen in metabolics and doesn’t happen in cancer therapy.  Yet, to hear radiation oncologists or medical oncologists talk, you would think that all of our therapies are super precise.   Let’s consider each in turn, but being a radiation oncologist I will spend more time on that.

   Therapeutic radiation beams given from outside the body (as opposed to brachytherapy –otherwise known as implanted radiation or “seed therapy” in prostate cancer, which will not be considered here at all) are like visible light waves, but much higher energy.  Like visible light, the physics of therapeutic radiation beams has the same wave/particle duality that make theoretical physicists lay awake at night.  From a therapeutic perspective, though, the fact that the beam “travels in a straight line” is useful for it allows us to point the radiation beam at the tumor and miss the adjacent normal tissue (like using a knife to cut a bruised part out of a piece of fruit).  However, like that bruised piece of fruit, the knife has to start on the outside of the fruit and cut deep enough to get out the bruise—if the bruise is deep, all of the fruit from the skin down to the bruise will also get cut away.  In cancer therapy, this means that all the tissue from the skin down to the tumor “sees” the radiation beam. 

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   Someone recently asked this question, which sounds like the start of a bad joke, but it’s actually an excellent question for it gets to the heart of the challenge of fractionated radiation therapy (given over multiple treatments, or fractions): “Why do we what we do, and can’t patient comfort or convenience play a larger role in radiation therapy?” 

   To begin to answer this, we need to step back and consider what radiation therapy is, and what it is used for:  radiation therapy uses high energy photon beams (like supercharged light) to target cancer cells and kill them.  It is the true prototype of “targeted therapy”, which is now a phrase used a lot to discuss specific chemotherapy agents that “target” a specific molecule.  The absolute fundamental difference, however, is that a chemotherapy is inside the body when it targets a molecule, and thus the delivery (via a tablet or via the vein) doesn’t require people to take it exactly the same way every day.  You can stand and swallow the pill or you can sit.  You can sit in any of the infusion chairs at the chemotherapy suite, sitting still or wiggling around.  In contrast, targeting radiation therapy is like having to trace exactly the same line everyday, without changing it at all, for weeks.  The pencil cannot slip, cannot change direction or thickness, cannot merely be “close enough”.

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   Esophagitis is a symptom that occurs in patients undergoing radiation for lung cancer.  It is not uncommon for patients to blame the radiation for this side effect.  Radiation esophagitis if often described as a “sunburn on the inside of the esophagus.”  The esophagus it the long swallowing tube that sits in the middle of the chest usually right next to the trachea (the windpipe).  The tube connects the mouth to the stomach.  Unfortunately, it is very difficult to avoid this structure when delivering radiation because it is intimately associated with the lung, central lymph nodes, and the trachea.  Avoiding the esophagus would mean undertreating the tumor in many instances.

   Patients who develop esophagitis will often complain of some heartburn like discomfort or pain with swallowing.  These symptoms come on gradually and get worse as they complete treatment.  They typically peak sometime after the radiation ends.  In the most severe form, an ulceration can form in the esophageal wall (this happens very rarely).

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   Despite the acute side effects, it is important to try and deliver the radiation treatment without any interruptions or delays in treatment.  Experiments in the laboratory with cancer cell lines demonstrate quite convincingly that interrupting the radiation treatment even for a few days allows the cancer cells to grow back.  A large retrospective study has demonstrated that the survival is significantly worse if patients had an interruption in their treatment of longer than 5 days.  These results have also demonstrated that patients that go through the treatment without an interruption have a statistically higher chance of beating the disease than patients that experience an interruption in their treatment.  Sometimes the interruption in treatment is simply unavoidable (i.e., because a patient is simply too sick), but treatment interruptions scheduled out of convenience should be avoided if at all possible.

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   Image Guided Radiotherapy, which his also known as IGRT, is a new and emerging technology in radiotherapy.

   In its broadest sense, IGRT applies to any of a number of technologies that improve the ability of the radiation oncologist to validate the patient’s exact position prior to initiating radiotherapy.  For many of years, the standard approach was to apply tattoos (little dots, not the interesting kind) to the patient’s body and then line the patient up on the treatment table by using wall-mounted lasers to verify the patient’s position.  For some treatments, a customized mold or cradle is created for the patient to lie in for each and every treatment.  This customized mold conforms to the patient’s shape and position and then solidifies.  In this way, the patient is thought to be in the same position for each and every treatment.  Periodically during the treatment, the patient undergoes a “port film”.  This is a simple x-ray that is taken during the course of radiotherapy.  The radiation oncologist evaluates this x-ray to ensure that the patient is accurately positioned on the treatment table.

   More recently, IGRT has come to mean the use of a CT scan performed periodically prior to the initiation of radiotherapy.  There are several different equipment platforms that perform this function.  The Elekta Synergy and the Varian Trilogy are linear accelerators that have a built in imaging device that resembles a CT scan.  The Tomotherapy unit is a CT scanner that has a built in linear accelerator.  Although there are subtle differences between the different platforms, the purpose of each is to image the patient’s soft tissues and more accurately evaluate the patient’s position prior to treatment.  By obtaining images that are nearly CT quality, there is a wealth more information than what can be seen on a simple x-ray image.  In addition, the CT that is obtained prior to treatment can be analyzed by a computer to compare how closely the patient is positioned on the table at the exact time of current treatment to the position of the patient on the table at the time of previous treatment planning and simulation.  The computer will also specify how to adjust the table in terms of height (up or down), right or left, and front to back in order to more accurately align the patient in three dimensions.  The accuracy of this CT matching is thought to be less than 3 mm.  The additional CT scans that are obtained during the treatment course add to the radiation exposure, but this is thought to be only a small amount of additional exposure.

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   The treatment of lung cancer with radiotherapy is rapidly changing as new technologies make the treatment safer and more effective.  One of the more recent developments has been the development of tools that allow for designing radiation fields that account for a tumor’s specific motion, or it’s change in position over time, the fourth dimension.

   In conventional radiotherapy treatment planning, patients are positioned in the CT simulator room in the position that they will be treated in.  The patients then actually undergo a CT scan.  The images are transferred to a treatment planning computer.  Specialized treatment planning software is used that processes the CT images for analysis.  A 3D image of the patients target region is created on the computer.  In patients with lung cancer, this region typically includes the lungs, spinal cord, heart, esophagus, ribs and other tissues.  The primary tumor and regional lymph nodes can often be identified on the CT scan and the 3D rendering.  The radiation oncologist and team will identify the targets that need to be treated and proceed to design fields that encompass the targets while minimizing exposure to the normal tissues.  This approach, however, doesn’t account for the motion of the tumor do to respiratory motion (from breathing). 

   The treatment planning CT scan represents a “snapshot” in time.  In essence, it lets the radiation oncologist and team identify the tumor’s location at that time and in that particular portion of the respiratory cycle.  In order to account for the tumor motion, most practicing radiation oncologists will add a “safety margin” around the target to account for this motion that is occurring while the patient is breathing.  In effect, a larger region is exposed to radiation to make sure that wherever the tumor might actually be it will still receive radiotherapy.  The size of the “safety margin” is typically about 1-2cm in every direction.  Read the rest of this entry »