Spinal cord varies in depth below the surface. Patient positioning helps - but cord always deeper at neck vs sacrum.
Compensation in planning needed if dose variation is > 5%. A compensation technique is used to improve the non-uniform dose distribution at the spinal cord caused by varying depth, and tissue inhomogeneity.
Custom dose compensator defined along surface of anterior spine
- Each individual patient has his/her CT of spine which defines the depths at multiple points along the spine of that patient
- Based on those depths - a computer program calculates the compensation necessary to get the most homogeneous dose distribution across these points
- A compensator is milled in styrofoam based on these calculations
- The styrofoam mould is filled with 2.5mm diameter stainless steel granules
- This is inserted onto LINAC machine for each individual patient
- Each patient compensator is verified with film and ionization chamber measurements.