If you are a clinical medical physicist, what are the major challenges in your department in order to perform invivo dosimetry for each individual patient when he/she is receiving radiotherapy or undergoing for a diagnostic radiological procedure?
My MD thesis was on invivo dosimetry of bladder and rectum during brachytherapy of Cancer cervix, but done very long ago, in 1980. I am writing from my experience then; things may have changed now
The first challenge was selection of method. We selected thermolumiscence dosimetry (TLD). Next was how to ensure that the substance got measurable amount of radiation which can be later measured as light. We packed the TLD units inside IV tubes and kept it inside rectum and bladder, about 10 units, at 1/2 to 1 cm in each tube, and it stayed there for 4-5 hours. But patient should not change position then and you have to ensure that urine is not collected much, ie empty the bladder very well before that. We used Calcium sulphate dysprosium and Lithium fluoride, but LiF is ideal and the other is not good. Later we took each and measured light emission and plotted the exposure against the distance from the reference point, which was taken as urethral opening/anal verge.
This TLD method can be used for other sites such as oesophagus etc. For other deeper sites which do not have a tract, it will be difficult (any method will be difficult) since the device needs to be implanted with surgical help.
My suggestion is you start with TLD and device your further strategies as you gain experience
It is a good area.
Best Wishes
Dear Shakardokht,
I append an official report about practical dosimetry in brachytherapy. It´s written in German but you will find through. Please look into chapter 1.4. The main problem is positioning of the probes and to avoid displacement during irradiation.
Hi Shakardokht,
I don't have any experience with dosimetry in diagnostic radiology but I have been doing radiotherapy in vivo dosimetry (TLDs and GaF film) for a while now. Narayan's work is really intra cavitory dosimetry, which, these days is not performed regularly as modern planning systems can predict/estimate the doses to OARs quite accurately. I have found eye lens dosimetry to be the most challenging one. In vivo dosimetry in this case uses surrogates on the skin to estimate dose to lens at depth. The other issue the uncertainty in dose estimation using TLDs due to the angular dependence of TLDs. Not sure if a phantom exists where one could place TLDs in place of lens and then performs dosimetry by putting TLDs on the skin and comparing the doses from on the skin TLDs to the TLDs placed inside the phantom where the lens is located. The other anatomical areas where in vivo can be challenging are the nose and ear, especially when MV electron are used to treat skin tumours in these areas. The issues to consider here are the electron backscatter, uneven surface and the lack of tissue. There probably are other areas/issues such as dosimeter preparation readout etc. which can also be challenging.
Cheers
Vinod
Hello, Shakardokht!
We are doing INVIVO dosimetery for rectum during HDR brachytherapy for few years. Detector is 5 diodes in catheter (PRW-Freiburg). The main task to correctly calibrated this detector: to find angle to wchich detector don't have pik of sensitivity, to do calibration. Then to rotate detector to angle 180 deg, calibrate again. And at the and to take avarege if these two calibration.
Article 528 oral ANGULAR CORRECTION OF THE IN-VIVO RECTUM PROBE READ...
Article EP-1136: Angular dependence of IN-VIVO bladder detector read...
- Badges
- Science topic