Radiobiology and Rationale of Fractionation in Liver and Kidney Cancers
Cancer is disease that can affect any part or system of the body. The cancer cell s produce a self masking effect in the body such that the body’s immunity cannot identify these cells as harmful or not its own part
Cancer cells are not sitting in isolation in the human body.They are surrounded by normal cells,blood vessels and tissues from where they derive their nutrition.The challenge during radiation therapy treatment is to knock out the cancer cells without harming the normal tissues in which the cancer is housed.
The “four R’s” of radiobiology include repair, redistribution, re-oxygenation, and repopulation. They form the core principles of radiotherapy
Radiation treatments works by damaging the backbone of cancer cells the DNA-helix.A higher number of exposures (fractions) in small doses of radiation cause direct DNA damage aggregation resulting in increased cancer cell destruction.
Conversely, small doses of radiation allow normal cells to “Repair”, thereby reducing toxicity. Radiation oncologists try to find a level of radiotherapy that balances tumor cell kill and prevents normal tissues from being damaged.
The Royal College of Radiologists radiotherapy dose-fractionation guidelines provide fractionation schemes for different organs since various organs have different sensitivity to radiation, so the same fractionation schedule cannot be used in all cancers because different tissues repair at different rates.
For “Redistribution”, radiotherapy that takes place over multiple sessions is more effective because it allows the cells in the G2 phase (sleeping phase) of the cell cycle to move around to mitotic phase(division phase ,where radiation is more effective) of the cycle.
Fractionation with radiation doses that are timed apart can target previously resistant cells. As the cancers grow and enlarge they have blood supply at the periphery of their circumference, however, central core starts decaying due to less nutrition and low blood supply.
This condition is known as “hypoxia”(low oxygen levels in cancer cells).Hypoxic cancer cells can survive for longer time and are one of the main causes for cancer recurrence or regrowth.Fractionated radiation delivered over time allows for “Re-oxygenation”. Finally, as the total treatment time to deliver a dose of radiation lengthens, cells within a tissue have the ability to “Repopulate”and become more sensitive to radiation..