Partial tolerance is the situation where an organ is treated to a level below its tolerance doce. This leaves open the potential for further treatment in the future without a large risk of late effects. Care must be taken when considering partial tolerance, as the remaining tolerance of the organ may change over time:
- Some tissues show no significant change in tolerance over time (bladder mucosa)
- Some tissues show worsening of tolerance dose over time (kidney)
- Some tissues show improvement of tolerance dose over time (spinal cord)
Tissues showing neither recovery or loss of tolerance
The bladder epithelium is a good example of this type of tissue. Following an initial dose of radiation, the remaining tolerance of the tissue appears to be closely related to the original tolerance minus the delivered dose. In animal studies, this holds true for several years. This means that bladder retreatment is possible only if the sum of the initial and retreatment dose are less than the tolerance of the bladder.
Tissues showing loss of tolerance over time
The kidney is the commonly quoted organ in this category. If retreatment is delivered relatively soon (1 - 2 months) after the initial treatment there is minimal loss of tolerance. As time passes, the dose required to cause renal failure falls, indicating that the kidney tolerance continues to fall after late radiation effects have begun.
Tissues showing recovery of tolerance over time
Monkey and mouse studies have shown that the total dose delivered to the spinal cord can exceed 150-160% if the dose is separated by at least a year. This is consistent with loss of remembered dose in the order of 50-60%. If (big if) this data can be carried over to humans, it means that retreatment of tumours near the spinal cord is possible, particularly since the spinal cord is frequently in a lower dose region of treatment.