Boy....how to even TRY to answer this satisfactorily???? (especially after having had a couple of glasses of wine)!!
The rationale for delivering a fractionated course (i.e. over numerous, equal dose treatments) of radiation therapy, is to eradicate the harmful cells while "sparing" the healthy surrounding tissue. Because the radiation is delivered externally, of course the healthy surrounding tissue DOES receive a portion of the dose. When optimizing a patient's individualized "treatment plan", every possible precaution is taken to ensure that the maximum dose is delivered to the target volume without compromising the critical surrounding structures. Different tissues and organs in the body have different "tolerance doses".
Cancer cells are "rogue" cells that have, for some reason, changed in ways that they no longer follow the "normal" pattern of what their function is supposed to be....they ignore the rules. A cancer grows by cell division (i.e. you start out with one bad cell, it divides, then you have two--then four---then eight....etc.) What the radiation does (to both bad cells and to healthy cells) is to cause damage to the cellular DNA that makes the cell capable of dividing (reproducing). Fortunately, healthy cells are better able to repair the radiation damage than the bad cells, so the healthy cells are able to repair themselves and do their job, while the bad cells are not as capable of repairing the damage. By delivering the course of radiation in equal daily doses over a number of treatments, it allows time for the healthy cells to repair some of the damage. When the cells reach the end of their normal cell life cycle, the healthy cells divide as they are supposed to, but the bad cells just die off (ideally).
This is also the reason why side effects from treatment do not generally appear immediately.....it takes time for the radiation effects on both the healthy and the bad tissue to manifest. Side effects to different tissue occur at different times during the course of treatment, depending on the radiosensitivity of the tissue.
As with any curative treatment, no doctor can EVER give you a guarantee that you will not experience acute and/or long-term side effects. All they can do is give you the statistics. It is impossible to predict who will experience side effects, or to what degree of severity they will experience them.
There is also a risk of secondary malignancies arising from EBRT. Again, statistics are all one has to go by when determining their best course of action regarding treatment. It is definitely a case of risk vs. benefit.
The best advice I could give anyone is to BE YOUR OWN ADVOCATE!! There is absolutely NOTHING wrong with bringing in a list of questions when you consult with your doctor. At least once you have had the opportunity to ask your questions (even if you don't get the precise answers you'd like to hear), you will be making a more informed decision regarding your treatment.
As far as daily positioning for treatment, there are numerous ways positioning can be verified for day-to-day reproducability. External marks such as tattoos are often used to give the therapists a "starting point" for daily positioning. Distances and measurements are often confirmed with the use of an ODI (optical distance indicator). When images are taken, they can be overlaid and compared with the images provided from the original treatment plan, by use of fiducial markers implanted within the prostate (e.g. gold seeds) or by matching bony anatomical structures such as the symphisis pubis and the obturator foramen. Adjustment to positioning can then be made, if necessary, depending on the numerical results of the image matching.
One thing to point out is that the TIME it takes to verify positioning can vary GREATLY from day to day. Some days a person can lie down almost perfectly and positioning verification can be done very quickly. Other days, it takes much fiddling around and adjusting to get everything lining up just right. This really has NOTHING to do with how experienced the therapists are.....it is basically the fact that we are living, breathing human beings, with mobile skin and bones.....we are not a bunch of bricks that can just be plopped onto the treatment couch in the exact same position from day to day, without changing shape, gaining/losing weight, retaining fluid, etc., etc., etc.
Again, it certainly doesn't hurt to ask what system your treatment centre uses.
I'm tired, wined-out, and ready for bed!! I'll probably read this post in the morning and realize that it doesn't make any sense at all!!! Oh well.....I tried!!
Post Edited (kuls) : 2/4/2011 11:59:39 PM (GMT-7)