radiotherapy imaging devices

Question 1

what is verification and what are the two types? Provide examples.

Answer 1

Verification in RT is undertaken to ensure that the RT plan produced is what is being delivered.

1. non-imaging verification: screens, ID (IM(ME)R), patients, and the right kid on the couch
2. imaging verification: radiographic film, electronic portal imaging devices, cone beam CT (CBCT) and ultrasound

Question 2

what are the two types of errors in radiotherapy and give examples.

Answer 2

1.Random–patient positioning errors, machine variability and anatomical variation (patients lose/gain weight during treatment)

2.Systematic – usually an error at some stage during the workflow i.e. pre-treatment document error, technical failure, prescribing errors, planning errors.

Question 3

what is the difference between offline and online imaging

Answer 3

offline imaging the patient is not in the room whilst the image is being developed whereas online the patient is present.

Question 4

describe portal imaging

Answer 4

1.KV X-ray was taken during the simulation.
2.The patient was set up to this, a 2d MV plain film x-ray portal image was taken during the treatment.
3.The MV portal image was developed after the patient went home and would be compared to the simulation to readjust the patient the next day.
Form of offline imagining – the patient was not there whilst it was being developed.

Question 5

explain why when using MV images are very poor quality and contrast compared to using kV.

Answer 5

photoelectric effect and Compton scatter dominate at kV energies, these attenuation processes depend on the atomic number of the material, meaning that there is clear differentiation between bone (lots of attenuation) and soft tissue (comparatively less attenuation). At MV energies, the only attenuation process present is pair production which isn’t affected (much) by the material type. This means there is barely any difference in how much the photons are attenuated meaning the image is a fairly uniform grey blob.

Question 6

describe linac imaging - MV

Answer 6

1. Incident photon strikes the phosphor layer which is detected by the amorphous silicon layer that contains an array of photodiodes.
2. The fluorescence is converted into light in the photodiodes which “captures the charge” which is then converted to the electrical current.
3. Each diode creates a level of current that is unique and representative of the amount of light that hits it, this is amplified and converted to digital format where the computer software enhances and interprets the image.

Question 7

when is MV imaging used today?

Answer 7

for breast cancer to identify air, dose constraint caused of the lung volume, no more than 2cm of lung volume posterior

Question 8

what is unique about MV linac imaging?

Answer 8

the dose that comes out is the dose you want to deliver i.e. if we have a port image dose of 180mu and we use 3mu for imagining we use 177mu for the treatment.

Question 9

describe the components of CBCT

Answer 9

• Generated by a rotating anode X-ray tube mounted on a retractable arm.
• Has bowtie filters which allow the beam to be homogenous and eliminate low energy photons.
• Has a scintillation layer – emits fluorescence when strikes with an ionising material.
• Had an anti-scatter grid as KV can degrade image quality

Question 10

what imaging device has the most accurate verification

Answer 10

kv imaging - CBCT

Question 11

what imaging device allows you to make a decision on the spot to treat palliative patient

Answer 11

linac imaging -MV