Radiotherapy Side Effects
If you read the article about the way radiotherapy works you are on your way to better understand the treatment side effects. In that article, we had already gone through how radiation interacts with our cell’s DNA, damaging it and compromising the cell’s life.
It is the treatment’s objective to kill cancer cells, but since we cannot pinpoint which cell is cancerous and which one is healthy, we always have safety margins to account for that uncertainty as well as for others arising from organs internal motion and the ability to exactly reproduce the patient’s treatment position.
These safety margins will expand the treatment volume and often encompass neighbouring healthy tissues that may lead to side effects. Also, since the vast majority of centres in the world use photon radiation beams, side effects can also arise from the nature of these “particles” since they leave the majority of their energy on the path to reach the tumour.
Therefore, unlike chemotherapy, radiotherapy side effects tend to be related to the area being treated and are correlated with the irradiated volume of healthy tissue within or near the treatment site.
Does that mean that healthy cells are also killed?
Yes and no. Of course, there will always be healthy cells damaged by the radiation and some may die. But that’s where we take advantage of the uncontrolled cell reproduction in cancerous cells. Since these reproduce much faster than the healthy ones, they tend to be more exposed to radiation damage because it is during reproduction cell cycle that they are the most sensitive. While treatment is being delivered and the dose accumulated through time, cancer cells go by the cell cycle more frequently than the healthy ones, accumulating more damage, dying faster thus creating a time/dose interval called therapeutic window. In simpler terms, since cancer cells reproduce faster, they also take more damage from radiation and die faster than healthy cells. This means that there is a dose threshold in which a cancer cell dies but a healthy one doesn’t.
Is it possible to minimize side effects?
As mentioned in beginning of this article it is the amount of healthy tissue in the surrounding healthy organs inside or near the treatment site that will determine whether and which side effects may appear. They may appear in different forms depending on the organ being irradiated.
It is possible to minimize side effects using different strategies. One of them is the treatment’s technique choice as well as the level of the planner experience. When using treatment techniques as Intensity Modulated Radiation Therapy (IMRT) or Volume Modulated Arc Therapy (VMAT) it is possible to reduce the accumulated dose to health tissues thus minimizing potential side effects. When compared to the traditional 3D Conformal Technique (3DCRT), both IMRT and VMAT potentially provide less dose to healthy surrounding tissues, since they can produce sharper dose gradients and greater levels of dose conformity providing better protection.
How can we predict side effects?
The goal of treatment planning is always to find the better way to treat what we want to treat and protect what needs to be protected. During this stage all the relevant organs surrounding the tumour are contoured on the treatment planning image alongside with the volume to be treated. Using treatment planning systems, it is possible to analyse, for each of the surrounding organs, how much radiation reaches any given volume. These dose-volume relationships are studied through radiobiological models along with evidence-based scientific studies giving us dose-volume thresholds and probabilities for different side effects to occur.
It is during treatment planning that it is possible to harness the full potential of any treatment machine. As most of the more recent equipment allow for different treatment techniques, it is of the outmost importance to have experienced planners to take advantage of such different possibilities. Having new equipment in a department can be a bit overwhelming for the staff with so many new things to know, understand and learn. If you feel like you need assistance in such a transition, check out our ePlanning service where you can find different possibilities to fit in to your needs.
André Pereira
