In Vivo Diode Dosimetry

What TG-report covers in vivo diode dosimetry

TG-62

What clincial scenarios would you commonly use in vivo dosimetry

1. Measuring total skin electron patients,

2. Monitoring dose approximately at a pacemaker for patients with implanted cardiac pacemakers,

3. Monitoring dose to approximate fetus positions for pregnant patients (which should be monitored at three in vivo positions: the fundus, the umbilicus, and the pubis).

4. Just routine checking of patient field dosimetry.

How much more sensitive is a diode from an air ion chamber

18,000x - very small volumes give measurable signal. Silicon is 18000x denser than air and charge carrier creation takes about 10x less energy (W/e is only about 3.5 eV)

Should you apply bias to diodes?

NO can seriously damage them

Is there an energy or dose limit on diodes?

Yes - exposure above 1 MeV or 1 kGy damage it and reduce sensitivity

Is there an SSD dependence with diodes?

yes due to the fact that the dose rate per pulse changes with SSD.  This affects the rate of recombination in the diode.

Is there a dose rate dependance with diodes

on a LINAC not as much The diode collection time is much shorter than the time between pulses.  Therefore, since the pulse dose rate remains constant over various LINAC dose rates, diodes are relatively insensitive to changes in dose rate at the console.

1. Changes that are seen are due more to the monitor unit chamber’s recombination changing with dose rate and affecting the output of the linac.

2. The sensitivity varies approximately 1% over 80-120cm SSD distances

summarize the dependancies of diodes

1. Dose rate per pulse (SSD) (1%).

2. Dose damage (sensitivity decrease with increasing dose requires kGy).

3. Temperature (0.5% per ^oC).

4. Angular (very diode dependant 3-4%).

5. Field Size (5%).

What type of radiation damages diodes the most?

Electrons damage diodes faster than photons, but high energy photons (15-18 MV) produce neutrons which also induce a lot of damage to the diode.

are there different diodes for different purposes?

1. Adequate build up is necessary to shield contaminate electrons.  This helps reduce the SSD dependence of the diode.

2. Having too much build up leads to an unacceptable level of a dose shadow below the diode (diode acts as a shield).  This is especially pronounced in electron therapy (with photon diodes) where differences can reach 20% (typically about 5-6% for photons).

3. If only one photon diode is to be used, select the high energy diode to remove contaminant electrons in all energies.

1. This ensures better SSD independence and better field size independence.

2. There will be a larger dose shadow at the low energy than would otherwise be necessary.

In general there are 3: electrons, 6-10MV and 15-18MV diodes

Is there a diode angular dependance?

Yes: from the following

1. Different transmission thickness to the sensitive zone of the diode.

2. Change in backscatter from the patient.

2 types of diodes

1. Cylindrical - better for oblique fields since they exhibit radial symmetry but they have a strong tilt dependence.

2. Hemispherical - easier to place on flat anatomy, doesn’t have strong tilt dependence but greater angular dependence than cylindrical diodes

diode field size dependance

1. Sensitivity increases with increasing field size.

2. Up to 5% greater than the field size dependence predicted by an ion chamber.

Special diode concerns with TBI

1. Leakage is a large concern due to the very long irradiation times involved.

2. Need a specific calibration due to the very large SSDs involved (low dose rate).

3. Temperature dependence correction of the diode or automatic correction by the software is needed since the diode will be in contact with the patient’s skin for a long duration.

QA for diodes recommendations

1. The diode calibration should be checked monthly and should be within 2%.

2. Readings should be reproducible within 1%.

3. Leakage should be less than 0.5% of the reading.

4. The dose rate dependence should be less than 2% (note: this is the same as TG-142’s output versus dose rate tolerance.  This dependence is due to the dose rate dependence of the MU chamber).

5. Diode readings should be linear within 2%.