Semi-Conductors and In-Vivo Dosimetry

Question 1

What are the 2 types of semi-conductor?

Answer 1

P and n types
p-type doped with Al or boron to have excess of holes, only 3 electrons in the valence band. When an electron fills the hole it becomes a negative ion.
n-type doped with phosphorous so 5 electrons in the valence band, with one free to wander around lattice. It is a positive ion.

Question 2

What happens at a P-N junction?

Answer 2

N and P type semiconductors are put together.

Electrons flow across the gap until an equilibrium is reached due to the build up in charge.

Question 3

What is the region with no electrons in called?

Answer 3

The depletion region

Question 4

What happens when you apply a forward bias?

Answer 4

The potential hill becomes lower, making it more likely for electrons to cross the junction

Question 5

How are most P-N junctions made?

Answer 5

A P-type silicon semiconductor with SiO2 regions has phosphorous added. The SiO2 protects most of the base semiconductor from the phosphorous but it does dope a small area. This makes a P-type semiconductor with a small region of n-type semiconductor.

Question 6

How do semiconductors detect radiation?

Answer 6

Radiation creates electron/hole pairs
Electrons, in a P-type, and holes, in a N-type, that make it to the depletion region without recombining add to charge.
Current is proportional to dose.
The diffusion length is proportional to carrier concentration so longer for P-type (50um compared to 1um)

Question 7

What is the process of radiation damage?

Answer 7

As radiation interacts is dislocates atoms forming traps. They shorten the diffusion length and active detection volume, reducing the sensitivity of the diode.
Damage is proportional to radiation energy so 20MeV electrons are 20x more damaging than 8MV photons.

Question 8

Are P-type or N-type more affected by damage? Why?

Answer 8

N-type, as boron is smaller than phosphor so it is less likely to be displaced.

Question 9

Are P-type or N-type more linear over dose rate?

Answer 9

P-type. N-types are more susceptible to traps so will over respond at high dose rates.

Question 10

What is the saturation effect?

Answer 10

Where all the traps are full so you see a non-linear response.

Question 11

Why is the PDD shallower at depth for N-types?

Answer 11

Because they over respond at Dmax

Question 12

What causes the temperature variation in semiconductors?

Answer 12

There is more energy so carriers are more likely to escape traps. This plateaus at ~5kGy so use pre-irradiated diodes.

Question 13

What are the advantages of using diodes?

Answer 13

High signal per unit volume
High resolution
Immediate dose readout

Question 14

What is the stopping power ratio in the clinical range for photons and electrons?

Answer 14

0. 8% for photons

0. 5% for electrons

Question 15

What is added to photon diodes to reduce the scatter and low energies incident on it which would cause an over response?

Answer 15

Shielding behind the measuring volume is added

Question 16

What are diodes used for?

Answer 16

Small fields. Large field only if validated with an ion chamber.
Measuring profiles in relative dosimetry

Question 17

What is the main cause of errors for diodes?

Answer 17

Not being perpendicular to the beam as they have a strong angular dependence

Question 18

How do EPIDS work?

Answer 18

Amorphous silicon photodiodes, with 1024x1024 pixels in a 40x40cm area. Each pixel is a photodiode and TFT. The TFT releases the charge which has been held in the EPID so that blocks at a time can be read out.

Question 19

What is in-vivo dosimetry?

Answer 19

Direct measurement of the dose to the patient to identify gross errors. Uncertainty ~5%.

Question 20

Who recommends IVD?

Answer 20

WHO
Towards Safer Radiotherapy
Manual for Cancer Standards, RT Manual

Question 21

Why do IVD?

Answer 21

Detects major errors in patient set-up
Identifies problems with understanding
Identifies planning system problems
Independent check of MU
Defence in depth
Intracavity doses
OAR doses
Test non-standard SSD doses

Question 22

What are typical IVD equipment?

Answer 22

MOSFETS
TLDs
Diodes
Film
EPID

Question 23

How is the dose for IVD with diodes calculated?

Answer 23

D = R.diode calibration.angle correction.collimator setting and SSD correction.temperature correction

Question 24

What are the advantages of Diode IVD?

Answer 24

Simple
Instant results
SSD errors change the dose

Question 25

What are the disadvantages of Diode IVD?

Answer 25

No IMRT
Single dose point
Only entry point
Takes time to attach
Hard to do posterior

Question 26

How does EPID IVD work?

Answer 26

Measures the fluence transmitted through the patinet.
The images are deconvolved with the EPID kernel, which recovers the scattered fluence in the EPID and gives in air fluence map.
This is converted to RMUs using calibration image.
This is then back projected onto the planning CT using a PB alogrithm so the dose can be compared.

Question 27

What are the advantages of EPID IVD?

Answer 27

IMRT (although not mandated)
Dose anywhere in field found
Not intrusive on workflow
Simple to commission
3D dose distribution
Uses existing equipment

Question 28

What are the disadvantages of EPID IVD?

Answer 28

Complex
Uses a PB algorithm
Large field sizes so it irradiates EPID electronics
Risk of collision
Not for electrons
Kernels need updating as EPID degrades, expensive
Insensitive to SSD errors
Not instant

Question 29

How is QA performed on EPID?

Answer 29

Expose with a 100MU, 10x10 beam, send to vendor