Introduction

Radiotherapy is a technique used as a treatment for cancer by using high doses of radiation such as photons and electrons to shrink or kill cancer cells and prevent further growth. The main goal is to break the cancer cell DNA as cancer cells have lost their ability to repair DNA damage or has a deficiency in repairing its DNA adequately and successfully. The radiation also causes indirect damage by creating toxic radicals from oxygen known as reactive oxygen species (ROS). The cancer cell thus slows down in replication or simply dies due to too much damage. The body then removes the dead cancer cells by phagocytosis from phagocytes and utilises powerful enzymes and acid/base agents to destroy and dissolve the cancer cells.

Radiotherapy can be given as a neoadjuvant therapy, which is the treatment of cancer cells before surgery (such as shrinking the cancer mass) or before the main treatment. Radiotherapy can also be given as an adjuvant therapy, which is after the main treatment to maximise its effectiveness and kill any leftover cancer cells.

The type of radiation therapy given such as internal/external beam or photons/electrons is dependent on a few factors. These are:

• Size of the tumour.

• Location of the tumour.

• Curability and type of cancer (some are more resistant than other).

• Proximity of tumour to sensitive tissue.

• Palliative or curative care.

• Past medical history, general health of patient and patient’s wishes.

• Objective data of the patient such as age, weight, gender, et cetera.

What is cancer?

Cancer cells are mutated cells that have lost the tight control of replication, repair and the normal cell death function. The DNA is found to be faulty, which is a crucial step in oncogenesis (onco meaning tumour or mass, and genesis meaning origin or source). Having acquired too many mutations results in the cancer cells to use specific techniques to stay alive successfully. These mutations contribute to the ‘hallmarks of cancer’ which describes the behaviour and techniques that cancer cells utilise. These are:

• Hide and avoid the immune system.

• Enabling replicative immortality, meaning cancer cells do not become old or do not have a set amount of replication like normal cells.

• Sustaining proliferating signalling indicating the cancer cells can proliferate without any external stimuli.

• Evading growth suppressors means that cancer cells manage to not respond to molecules that would stop or inhibit proliferation.

• Resisting cell death.

• Inducing angiogenesis, meaning that cancer cells promote the formation of a vascular system to nourish itself.

• Activating invasion and metastasis, meaning cancer cells are able to go from one organ to another.

• Deregulation of cells energetics, meaning that cancer cells change the cell internal reaction to favour itself.

• Promoting inflammation and favouring genome instability and mutation.

The outcome of cancer cells on the body is: damage to nearby organs due to its growth (such as pinching neurons or pushing against the brain), loss of organ function (for example if cancer grows on the heart muscles, the muscle will be weaker and loses its original function), using crucial resources away from normal cells, releasing of certain hormones at an inappropriate amount which acts on the body, promotes inflammation which causes damage and unusual immune response which may attack healthy cells.

The 5Rs of radiotherapy describes how the tumour and normal cells will respond to fractionated therapy. Fractionated therapy describes different doses of radiation in a certain amount of time. Depending on if the tumour is curative or palliative, the amount of dose and time given is different. For example, 2 Gy is given per day for 5 days in a week, and there could be between 4-8 weeks prescribed. Gy is the unit for Gray, which is the radiation dose. There are different types of fractionated therapy such as Continuous Hyperfractionated Accelerated Radiotherapy (CHART) where instead of doing 2 Gy once a day over 6 weeks, it is 1.5 Gy three times a day over 2 weeks. Extreme hypofractionation may also be used where large fraction size of 12-18 Gy per sessions is given in less than a week. In extreme hypofractionation there could be between 3-4 sessions. Each method has its advantages and disadvantages over each other, and some favour curative or palliative more than the other.

Radiosensitivity

Certain tumour cells are more sensitive to radiation than other tumours. The amount of radiation given (intensity and time) may be changed to try and achieve tumour cell damage. However, the toxicities and side effects will, therefore, increase if a tumour cell is particularly resistant.

Radio-sensitive cells:

• Normal cells: haematological cells, epithelial stem cells, hair follicles, gametes.

• Tumour cells from haematological, sex organ origin, lymphoma, small cell cancer.

Radio-resistant cells:

• Normal cells: myocytes, neurons, connective tissues, conjunctiva.

• Tumour cells such as melanoma, renal/pancreatic tumours.

We can deduce from the normal radio-sensitive cells the side effects patients may have from radiotherapy. Early radiation toxicity side effects are most likely going to be: alopecia (loss of hair), denudation of mucosa/skin, inflammation, erythema, desquamation and cytopenia (if radiation was aimed near the bone marrow). Early and minor side effects heals quite successfully after 4 weeks.

Repair

As mentioned earlier, cancer cells struggle to repair their DNA. This is more evident when fractionated therapy is utilised as cancer cells are less likely to repair DNA when radiation has direct and indirect (through ROS) impact. Ionising radiation has three types of damage to cells; these are:

• Lethal damage: the cell is damaged beyond repair and is fatal, resulting in the death of the cell.

• Sublethal damage: damage that can be repaired if given some time to do so. This is crucial in fractionated therapy as by giving radiation in smaller radiation for a certain amount of time, it allows the healthy and normal cell to recover and repair or initiate the safe and regulated cell death. The cancer cell cannot efficiently repair, thus dies. Each tissue has a repair half-life which describes how long a tissue can recover. This is a factor oncologist and staff need to think before initiating further therapy.

• Potentially lethal damage: under certain circumstances, the cell can recover.

Interestingly, the healthy cells surrounding the cancer cells and lies in the radiation pathway equally get damaged. However, the healthy cells do recover by either successfully repairing their DNA or inducing cell death (apoptosis). Apoptosis is a tightly regulated and controlled system where the cell can safely die and be eaten away by phagocytes. Hence there is no inflammation or necrosis present and there is no nearby damage to other cells. In rare cases, there is a chance where cancer may be produced from a damaged healthy cell due to radiation therapy. This occurs in 1:1000 patients and patients should know about this risk; however, they should also understand that the beneficial outcome far outweighs the risk of developing further cancers.

Reoxygenation

To cause and insure permanent damage, oxygen needs to be required and present with the radiation. This allows the radicals (ROS) to be formed and cause lethal damage as oxic cells (cells with oxygen being present) can be killed. It is important to note that hypoxic cells are therefore resistant to radiotherapy. However, here lies the problem. Cancer cells grow very fast and can outgrow the production of blood vessels known as angiogenesis. This leads to cancer clusters to have a necrotic and hypoxic core, where the core is away from any blood vessels, which leads to no oxygen delivery. The opposite can also be mentioned as cancer surrounding the blood vessels are oxic with oxygen that is readily available and the outer core may be in a hypoxic state. On a histological slide, it is seen as small round blue tumour cells (primarily derived from endocrine or neuroendocrine origin) that are surrounding the blood vessels. They are very basophilic with small round nuclei and quickly outgrow their blood supply, see Figure 1. Hence, notice the pink necrotic cells surrounding the small round blue tumour cells.

Fractionated therapy comes into play by delivering a small amount of lethal radiation at a time over a few weeks (different fractionated therapy may be used such as CHART, normal fractioning or extreme hypo-fractionation/CHISEL). This method peels the tumour cluster like an onion by removing the tumour layers that are oxic (containing oxygen). The recovery time allows the tumour to be reoxygenated and allows the blood vessels to deliver oxygen. Then radiation is yet again used, peeling another oxic layer. This method is useful for shrinking the tumour before a surgery where cancer seemed to be too dangerous to be removed prior to its shrinkage. Figure 2 below demonstrates a cluster of cancer cells surrounding a blood vessel and a cluster of cancer cells that are surrounded by blood vessels.

There has been some method that should be mentioned where oxygen delivery to cells is increased. These methods utilise carbogen gas (95% oxygen and 5% carbon dioxide) with nicotinamide which is a vasodilator. The idea is to release as much oxygen to cells and make the tumour cells oxic and sensitive to radiotherapy. Currently, studies are suggesting that it may improve tumour cells being killed and that in some cancers (such as bladder and rectal cancer) it is significantly successful but further clinical and randomise trial should be done.

Repopulation

Describes the increase in cell division of normal and cancer cells after radiation in response to a decreasing number of cells. It was found and noted that after 4 weeks of radiation, the amount of radiation is ‘wasted’ on tumour cells as it is being utilised to kill the overnight tumour repopulation. Depending on the tissue, repopulation occurs at a different speed. The early responding tissue comes into play after 4 weeks (such as tumour stem cells or normal skin, GI tract and oral mucosa). In contrast, late responding tissues occurs after the radiation course has been completed. Repopulation has little consequences in late responding and slowly proliferating tissues.

Some tumours may undergo the accelerated repopulation phenomenon where, after 4-5 weeks, the growth fraction and doubling time are significantly increased. This is because at each radiotherapy treatment, there are less clonogenic tumour cells (cells that can repopulate). However, the surviving clonogenic tumour cells increase their rate of proliferation and decrease cell loss, which reduces the radiotherapy treatment effectiveness and introduces resistance. We can deduce that increase overall treatment time will lead to a higher dose being given to achieving local control of tumour cells and inevitably introduces side effects. Of course, this all depends on the cancer type.

Redistribution

The cells in your body are dividing at their own rhythm; thus we can safely assume that a cluster or population of cells are not at the same part of the cell cycle. Some of the stages in the cell cycle is radiosensitive and likewise can be said to be radio-resistant. G2/M (G2 stands for growth and preparations for mitosis and M stands for mitosis) are found to be the most sensitive stage to radiotherapy. S/G0 (S stands for DNA synthesis, and G0 stands for resting phase or is viewed as either an extended of G1 phase, where the cell is neither dividing nor preparing to divide or a distinct quiescent stage that occurs outside of the cell cycle) to be the most resistant cell stage. G2/M is the stage where the DNA comes together for the cell to split; hence it is the stage where DNA is at its most exposed to radiotherapy as it is now condensed and is subjected to mechanical stress.

Fractionated therapy can be advantageous by delivering radiation to sensitive cells, then await for other tumour cells to go into the G2/M phase and be irradiated once more. This method is particularly effective at targeting tumour cells at their most sensitive cell cycle stage while waiting for the resistant stage cells to continue their cell cycle into the sensitive stage. Another interesting approach is to use medications that stop cells at their most sensitive cell cycle stage. These can be the Vinca Alkaloids medication or the Taxans medications, which stops the cells at the mitosis cell cycle, making them at their most sensitive stage.

Published 30th May 2020. Last reviewed 1st December 2021.

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