2011-02-09

CT Dose Profiler for Point Dosing

RTI was present at the NACP meeting in Århus in the fall. The theme of the meeting was "CT Dosimetry and image optimization for medical physicists". Our CT Dose Profiler attracted a lot of attention, in particular for its capabilities in measuring point doses. There was some discussion on whether the CTDI concept was outdated and whether measuring point doses was the new way of being able to estimate patient doses correctly. The measurement problems in Cone Beam CT were also discussed, and that point doses might be the right way to go.

If you're thinking about starting to measure point doses, make sure you have the latest version of the CT probe, that is, the shorter CT Dose Profiler, instead of the longer CT-SD16. The latter is not suitable for this type of measurement.

FAQs about the data in the CT Dose Profile Analyzer database:

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1. What do I do if I can't find the CT scanner that I'm going to perform the measurements with in the database?

You should choose a database with measurements made using a scanner from the same manufacturer and with the same type of phantom. As you can see from the list, the K-factor does not vary greatly between different manufacturers, but there are big differences between the body and head phantoms.

2. How precise is the K-factor? Can you really substitute measuring in all five holes?

RTI strongly recommends that you use the single point method with K-factor for measuring CTDIw, because it's much faster to collect the values you need, and there are limits on measuring in the peripheral holes with CT Dose Profiler.

The variation between different scanners and bowtie filters is around to ‏±5%. This difference introduces a smaller error than if you were to measure all five holes separately. Factors contributing to the uncertainty in the different holes are energy-dependent, a low signal, and a large dynamic variation as a result of the rotation. When you use CT Dose Profiler in the peripheral holes, the pitch must be less than 0.5 to avoid dose information being lost.

The K-factor has been extracted from statistics of measurements made by CTDIw and CTDI100 with an ion chamber over 2-3 years.

3. How do I check that the K-factor is appropriate?

To double-check if the K-factor seems reasonable, you can do your own calculation based on old data measured with an ion chamber. All you need to do is divide CTDIw by CTDI100, c. If you want to measure it with CT Dose Profiler instead to get the equivalent values, that's okay, but in that case use a pitch of <0.5 and make sure that the dose profile in peripheral holes doesn't bottom out. CT-SD16 doesn't work for this.

4. How can I add my own K-factor?

If the K-factor you are after can't be found in the program's database, or if you want to add a new one, you can do so. Contact RTI for more instructions