What is TMR independent of?

Tissue Maximum Ratio (TMR) is a key concept in radiation therapy that measures the dose delivered at a certain depth in tissue relative to the dose delivered at a reference depth, typically at the maximum dose. TMR is independent of Source-Surface Distance (SSD) because it is a ratio that specifically compares doses at two points in tissue rather than being influenced by the distance from the radiation source to the surface of the tissue.

As SSD changes, it can affect the absolute dose delivered to a point in tissue but does not affect the ratio of doses at different depths within the same tissue type. This is crucial in treatment planning as it allows dosimetrists to compare dose distributions based on their intended treatment depths without needing to worry about variations in distance from the source of irradiation to the patient’s skin surface.

In contrast, the other factors such as field size, energy level, and patient thickness do influence the TMR. Field size impacts the amount of radiation and the distribution of that radiation in the treatment area, energy level affects the depth at which radiation is absorbed, and patient thickness can alter the amount of tissue for which dose calculations are made. Thus, these components are critical considerations for calculating accurate dose distributions and dose planning in radiation therapy.