Question

If a patient is to be treated at 140cm SSD on a linear accelerator and the treatment field size is 22cm× 40cm at this extended distance, what would be the required collimator setting?

A. 22×40
B. 16×29
C. 11×20
D. 8×16

Answer 1

To find the required collimator setting for a treatment at an extended SSD of 140cm, the treatment field size is divided by the ratio of the extended SSD to the standard SSD, resulting in a setting of approximately 16cm by 29cm.

Step-by-step explanation:

Calculating the Collimator Setting for a Linear Accelerator

In radiation therapy using a linear accelerator, the collimator setting is crucial to correctly shape the treatment beam to match the size of the treatment field at the patient's skin surface. To determine the correct setting when treating at an extended Source to Skin Distance (SSD) of 140cm, the inverse square law must be applied to compensate for beam divergence.

At the standard SSD (100cm), the field size would equal the collimator setting. Moving to a 140cm SSD, the field size increases proportionally with distance. Thus, to find the collimator setting, we divide the treatment field size at 140cm by the ratio of the extended SSD (140cm) to the standard SSD (100cm), which gives:

Collimator setting (Width) = 22cm / (140cm/100cm) = 15.71cm, and
Collimator setting (Length) = 40cm / (140cm/100cm) = 28.57cm.

The nearest option to these calculated sizes is 16×29, which would be the appropriate collimator setting for this scenario.