Definition of Shrinking Field Technique
Treatment using at least two phases, where latter phases use smaller fields than the former phases.
Use of Shrinking Field Technique
Shrinking fields are used when different volumes within the patient are thought to contain different quantities of tumour stem cells, in an effort to reduce the volume of normal tissue treated to high dose.
- The initial fields distribute dose to all areas of concern, up to the dose required in the areas thought to be at 'least risk'.
- Smaller fields are then used to increase the dose to the smaller volume believed to be at higher risk.
It is possible to have several phases with shrinking fields between each.
Anal Cancer Example
Anal cancer may be treated with a three phase technique:
- Phase I uses large fields to treat all the nodal regions at risk (internal iliac, presacral, and inguinal nodes) as well as any involved nodes and the primary tumour
- Phase II constricts the fields to treat the involved nodes and anal canal
- Phase III delivers the final few treatments to the anal canal only
Rationale of Shrinking Field Technique
The chance of eliminating all tumour cells is partly dependent on the number of cells to start with (based on Poisson statistics). Therefore, areas of subclinical risk require less total dose to eliminate the tumour clonagens present.
Shrinking field is less required in adjuvant settings as the main tumour bulk (which would require the highest dose) has been removed. Adjuvant treatments do not require the shrinking field technique as frequently.