Isocenter, ODI, SSD, and Distance

Question 1

What does SSD stand for in radiation therapy?

Answer 1

SSD stands for Source to Surface Distance, which is the distance from the radiation source to the patient’s skin.

Question 2

What does SAD stand for?

Answer 2

SAD stands for Source to Axis Distance, typically set at 100 cm, which is the distance from the radiation source to the isocenter.

Question 3

What is the isocenter in radiation therapy?

Answer 3

The isocenter is the point where the gantry rotates around, the collimators focus, and the lasers intersect. It is usually positioned at the tumor’s location within the patient.

Question 4

Are all treatments isocentric

Answer 4

Not all treatments are isocentric

Question 5

What is the role of the ODI in radiation therapy?

Answer 5

The ODI (Optical Distance Indicator) shows the SSD on the patient’s skin, helping align the radiation beam accurately.

Question 6

What is the difference between SAD and SSD?

Answer 6

SAD remains constant at 100 cm, while SSD changes depending on the depth of the tumor and the angle of the gantry.

Question 7

Why can’t you see the isocenter when treating a patient?

Answer 7

The isocenter is inside the patient’s body, usually at the location of the tumor, so it is not visible during treatment.

Question 8

What is the purpose of calipers in radiation therapy?

Answer 8

Calipers are used to measure the patient’s thickness at the treatment site.

Used for finding patient separation of two surfaces.

Can be used with the SSD to help locate the tumor at Iso-center.

SAD and separation can confirm the SSD.

Question 9

How is patient separation (thickness) calculated using SSD?

Answer 9

Subtract the patient’s SSD reading from 100 cm. The difference is the patient’s separation at that location.

Question 10

Example: Calculate the patient’s separation if the SSD is 94 cm.

Answer 10

Step 1: 100 cm SAD – 94 cm SSD = 6 cm from the anterior surface.

Step 2: The patient’s separation at this point is 6 cm.

Question 11

What does it mean when the gantry is at 180 degrees?

Answer 11

When the gantry is at 180 degrees, it is positioned directly below the patient, pointing upward.

Question 12

Example: Calculate the tumor depth if the patient’s separation is 21.5 cm and the SSD is 94 cm.

Answer 12

Step a: 100 cm SAD – 94 cm SSD = 6 cm from the anterior surface.

Step b: 21.5 cm separation – 6 cm = 15.5 cm from the posterior surface.

Step c: The tumor is 15.5 cm deep from the posterior surface.

Question 13

What is triangulation in patient setup?

Answer 13

Triangulation is the process of aligning the patient using three lasers, which intersect at the isocenter for precise targeting.

Question 14

How is SSD affected by different gantry angles?

Answer 14

SSD changes with every gantry angle because the distance from the treatment head to the patient’s skin varies with the angle.

Question 15

What is the role of the field light during setup?

Answer 15

The field light shows the size and shape of the radiation treatment field on the patient’s skin, helping ensure accurate alignment.

Question 16

How does modern CT technology replace traditional calipers?

Answer 16

Modern CT simulation provides detailed imaging that can determine the patient’s separation (thickness) more accurately than manual calipers.

Question 17

What is the relationship between SSD and tumor depth?

Answer 17

Tumor depth is determined by subtracting the SSD from the SAD and considering the patient’s total separation (thickness).

Question 18

Example: Calculate the tumor depth if the SSD is 96 cm and the patient’s separation is 20 cm.

Answer 18

Step a: 100 cm SAD – 96 cm SSD = 4 cm from the anterior surface.

Step b: 20 cm separation – 4 cm = 16 cm from the posterior surface.

Step c: The tumor is 16 cm deep from the posterior surface.

Question 19

What does SAD of 100 cm mean in an isocentric treatment?

Answer 19

A 100 cm SAD means that the distance from the radiation source to the isocenter remains constant at 100 cm, regardless of gantry angle.

Question 20

How is SSD displayed during treatment?

Answer 20

The ODI projects the SSD value onto the patient’s skin, showing the distance from the treatment head to the surface.

Question 21

What is the significance of an isocentric setup in radiation therapy?

Answer 21

An isocentric setup places the tumor at the isocenter, ensuring that the distance from the treatment head to the tumor remains constant as the gantry rotates.

Question 22

What is the typical SAD in modern linear accelerators?

Answer 22

The typical SAD in modern linear accelerators is 100 cm.

Question 23

Example: Calculate the patient’s separation if the SSD is 92 cm.

Answer 23

Step a: 100 cm SAD – 92 cm SSD = 8 cm from the anterior surface.

Step b: The patient’s separation at this point is 8 cm.

Question 24

How does patient positioning affect SSD readings?

Answer 24

The SSD changes depending on the patient’s position and the depth of the tumor, which affects the distance between the radiation source and the skin.

Question 25

How does SSD influence the delivery of radiation treatment?

Answer 25

SSD influences the dose distribution and precision of the treatment, as the depth of the tumor affects how the radiation beams are aimed and the dose delivered.

Question 26

What does SID stand for in imaging?

Answer 26

SID stands for Source to Image Distance, and it refers to the distance between the radiation source and the image receptor (not used in treatment).

Question 27

What does SOD stand for in imaging?

Answer 27

SOD stands for Source to Object Distance, referring to the distance from the radiation source to the object being imaged (not used in treatment).

Question 28

What is a graticule, and how is it used in treatment?

Answer 28

A graticule is a grid projection sometimes used in radiation therapy to help project the radiation field onto the patient’s surface.

Question 29

What is the maximum field size on modern Linacs?

Answer 29

The maximum square field size is 40x40 cm, but larger fields can be created using corner-to-corner alignment or extended distance.

Question 30

You are treating on a Varian linac with a standard maximum field size of 40 cm by 40 cm, and a 100-cm isocenter. Your patient is lying prone on the treatment table and you will only be irradiating them with the gantry at the 0-degree position. You need to treat their entire spine, which is 42 cm in length. What are your options for getting a 42 cm field length from a collimator with a maximum setting of 40 cm?

Answer 30

A: Treat at an extended distance beyond the normal 100 cm isocenter and allow beam divergence to do the work.

B: Treat the spine with two abutting fields while accounting for any field divergence to avoid hot spots where the 2 fields meet.

C: Rotate the collimator from a square to a diamond position because the distance from diagonal corners is greater than any of the side lengths

All of these are an option

Question 31

You are treating on a Varian linac with a standard 100 cm isocenter configuration. Your patient is lying supine on the table. The carbon fiber table is 3 cm thick. His head is resting on a “B” HR (headrest). His arms are above his head, supported by a wingboard, with his hand holding the poles. His anterior/posterior chest separation is 15 cm. The ODI shows the anterior SSD to be 93 cm. If you rotate the gantry to 180 degrees, under the table, what will the SSD read on the bottom of the table?

Answer 31

89cm

Because:
Seperation=15cm

SSD=93 anterior

SAD=7cm

Posterior=8cm

Under table=8cm plus 3cm =11cm

100-11=89cm