Dosimetry Flashcards
Define absorbed dose
Absorbed dose is defined by ICRU Report 60 as the mean energy imparted to matter of mass dm, i.e. the energy deposited per unit mass (with units of Gy).
Explain the dosimetry calibration chain
[ENTER MODEL ANSWER FROM NEWCASTLE NOTES]
What would you do if you saw the output from a linac trending upwards
Report this to the section or department head and ensure that the results of the QA are noted in the QMS along with a note stating upward trending. i.e. follow local reporting policies. A discussion would be needed with more experience and competent physicists to decide if the machine’s upward trend would breach a tolerance before the next QA session (i.e. does the machine require taking out of use immediately).
Likely given the trend the decision would be to call in the engineers or possibly the manufacturer to take a look. The timescale would be dictated by how fast the upward trend was i.e. can it wait until the next scheduled maintenance involving the engineers and/or manufacturer.
What linac checks would you perform if the linac output is high?
First remeasure the output (in case I’ve made a mistake); if still showing a higher than expected output (i.e. out of tolerance), try different equipment in case the test equipment is faulty.
If a high output is still high then I would need to seek advice from a more experienced medical physicist. I suspect that the tests that would be recommended are: [insert list of tests]
What is the field size tolerance for a 10x10cm field?
[ENTER ANSWER]
How do you convert from PDI to PDD?
[ENTER ANSWER]
Why don’t you need to convert the PDD for photons?
[ENTER ANSWER]
Why do centres follow a code of practice when doing intercomparisons? Why don’t they just do it their own way?
Current code of practice is the IPEM COP for high-energy photon therapy dosimetry based on the NPL absorbed dose calibration service (2020), an update from the Lillicrap 1990. This is to ensure that all centres are calibrating their dosimetry equipment in the same way to allow for traceability back to the NPL; this allows comparisons of dose measurements between centres nationally.
How do we know the dose we measure at Sheffield is the same as the dose measured at another centre?
As all dosimeters are calibrated to the NPL primary standard through the calibration chain (Field instrument to departmental standard, departmental standard to hospital secondary standard and hospital secondary standard back to NPL primary standard).
How would you check the dose you are delivering is correct?
[enter answer> measure through verification, deliver to a phantom]
