GAP CALCULATIONS

Gap Calculations are used to:

• Accommodate for a field size that is to long to fit in 1 treatment field
• If a first field treatment is completed at an earlier time and a new field is added at a later date
• Two different fields have a common border

Gap Calculation is based upon the Law of Similar Triangles:

The Gap Calculation will tell us the seperation needed between the two butting fields on a patient because we do not want the divergence of the 2 fields to create a hot spot to the patient.

• This is especially important because we could use this a lot of the time on medulloblastoma cases for the spine field and we donot want to overdose the spinal cord
• Fields even though are gaped should be moved occasionally to prevent accidental overdose

To find the gap calculation we need to know:

•  the   length of the 2 fields

•  the depth of the 2 fields

•  the SAD of the 2 fields

To Solve for a Gap, we need to solve both equations (Gap 1 and Gap 2) and then add the 2 values together to get the gap needed between the 2 fields.

The Equations for the Gap Calc are :

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Example: