Dog Parvo Symptoms

Fractionation affects fast and slowly proliferating cells in different ways. This is summarized in the table below:

Table II – Effects of radiation dose fractionation on different cell types:
Tumor cells (fast proliferating)	Normal fast proliferating (Early responding tissues)	Normal slowly proliferating (Late responding tissues)
REPAIR ¯tumor control.	¯side effects and complications. ­tissue tolerance.	¯¯¯side effects.­­­tissuetolerance.
RE-ASSORTMENT ­tumor control.	­cell death and acute side effects.	Very little effect since so slowly proliferating. Requires a huge time interval before effect is seen.
RE-POPULATION ¯tumor control.	¯acute side effects.	Very little effect since so slowly proliferating. . Requires a huge time interval before effect is seen.
RE-OXYGENATION ­tumor control.	Has no effect.	Has no effect.

Fractionation – which requires a higher total dose than single port protocols – allows for an increased repair capability of late responding tissues. This, in turn, results in greater tumor control, and different levels and types of radiation side effects.

Radiation side effects are generally categorized as acute and late. Acute effects are defined as those occurring during the course of radiation treatment or soon after the completion of treatment. Late effects only become clinically evident at least six months or more following the completion of treatment.

The greater tumor control created by fractionation creates in a beneficial decrease in late complications. However, fractionation can also slightly increase the severity of acute side effects. Despite this, the protocol is justified because early side effects tend to be reversible (if the damage to the cells is repairable). The damage caused by late side effects, however, tends to be permanent as it results from slowly to non-proliferating cell loss. Once these cells have died, the chance of replacement is virtually non-existent (this is especially important when irradiating critical tissues like the brain and spinal cord). For this reason, late effects are of more concern medically than early ones.

As a general rule, a prescribed treatment protocol should have no more than a 10% probability of inducing serious late complications for most tissues, and no more than a 5% probability of late complications for nervous tissue. Due to improved time-dose fractionation, computer treatment planning, and the use of megavoltage therapy, a serious complication rate of less than 5% exists in most veterinary radiation therapy today.