Question

Definitions

Present the Tissue Phantom Ratio and Tissue Maximum Ratio equations. Describe

how the two quantities are related.

Define, percentage depth dose (include a sentence description, equation and

diagram).

Describe how %D at depth (dref) is affected (i.e. increased or decreased) by

increasing field size, increasing SSD, increasing energy. Give a reason for the effect in each case.

Answer 1

Definitions

1.TISSUE PHANTOM RATIO:

The TPR is defined as the ratio of the dose at a given point in phantom to the dose at the same point at a fixed reference depth usually 5 cm

Formulas

Relationship between the TPR and TMR are

A simple model is developed which suggest that the degree to which the TPR and TMR are independent of distance from the source depends largely on the collionator flattaning/filter scatter.

2.TISSUE MAXIMUM RATIO:

It may defined as the ratio of the dose at a given point in phantom to the dose at the same point at a fixed depth OF MAXIMUM DOSE .

Formula

Defination

1.PERCENTAGE DEPTH DOSE:

The quanity percentage depth dose may be defined as the quotient expressed as a percentage of the absorbed dose at any depth 'd' to the absorbed dose at a fixed reference depth 'do' along the central axis of yhe beam

Diagram

Where

Now effects

1.effects of field size and shape

PDD data for radiotherapy beams are usually tabulated for square field

In clinical practice require rectangular and irregularly shaped fields.

A system of equating square fields to different field shapes is required equivalent square.

(1). WHEN INCREASING IN FIELD SIZE:

As the field size INCREASE ,the contribution of the scattered radition to be absorbed dose INCREASE.

(2). INCREASING SSD :

When SSD will be increase the PPD will be increase because it os directly proportional to it

(3). Increasing energy

PDD will be increase with beam energy ,higher energy beam have greater penetrating power and thus deliver a higher percentage depth dose.