In-Vivo Dosimetry Concepts

[Chase]

In the context of radiation oncology, there are a number of treatment techniques that are frequently accompanied with some form of in-vivo dosimetry method to verify dose delivered after the first fraction in case modifications to the treatment need to be made. Unfortunately, one of the biggest go to methods for this, Landauer's optically stimulated luminescent dosimeters (OSLDs) were just discontinued abruptly, leaving most clinics with nothing. Our clinic was fortunate enough to be using thermoluminescent dosimeters (TLDs) instead, though these are somewhat archaic and very time consuming to deal with. At least using more popular and traditional methods, this is just about the only alternative to OSLDs but requires a fair bit of knowhow, lots of expensive equipment, and tons of time spent commissioning and calibrating it.

With this background in mind, I thought I'd just see if any of you on here happen to have any ideas for a possible dosimeter concept that could do the trick and to see what you all thought of mine. I'm especially curious to hear what people come up with who are not in the medical physics field as I think we can have a tendency to get stuck in the ruts of the traditional methods and end up oblivious to other potentially good ideas. Here are the specifications it needs to be able to achieve:

Dose range: ~1cGy to ~10 Gy
Dose rate: extremely high (microsecond pulses combining to deliver 1Gy in 15s or so, basically needs to be dose rate independent)
Accuracy: ~5%
Energy range: nearly full Brem spectrum with peak of 20 MeV or so (preferably roughly tissue equivalent density material)
Preferably needs to be quick to use and safe for patients to be in contact with.
Needs to be equally sensitive to electrons and photons
Needs to not significantly alter the fluence of photons or electrons as to compromise the treatment.
Needs to be small, about 1cm^2 in detection area

What I was considering was small squares of radiochromic film, then build a transmission sensor that can read it off using a photodiode and maybe a red laser with a diverging lens to give a nice monochromatic light source in the red spectrum that is ideal for reading dose to radiochromic film.

Anyways, if you all have any ideas or thoughts about it, I'd love to see what sorts of detectors or methods are out there that I haven't even considered.

[Sesselmann]

You pose a challenging question, such high dose rates is way beyond what most of us will ever encounter. The dosimeters you talk about are all passive dosimeters, has anyone made an active devise which provides feedback in real time?

I wouldn't know what kind of real time detector one could use, but there would have to be a whole bunch of materials that change property in a measurable way to such radiation, how about just measuring temperature change in a tissue equivalent ccl of water or a small scintillation crystal with fiber optic link to a detector. ?

Just brainstorming of course 😉

[Chase]

how about just measuring temperature change in a tissue equivalent ccl of water or a small scintillation crystal with fiber optic link to a detector. ?

I'm not sure about the water. In theory it would be good, but in practice I think the patient's natural body temperature would heat it up orders of magnitude more than the dose would, so that would pose a challenge.

Interestingly enough you nailed it with the scintillation crystal. That actually exists: https://medscint.com/scintillation-dosi ... tillation2 Though I suspect it could be made better. Apparently they use a plastic based scintillation coupled to a fiber optic, but the fiber or the plastic, not sure which though I suspect it might be the plastic, starts to go opaque with enough dose so they need constant recalibration and replacement (and they aren't cheap). I believe they are typically used more in the context of beam profile scanning measurements and small field dosimetry, but I don't see why it couldn't be repurposed for an in-vivo measurement.

I've wondered if something other than a polymer could work, but I don't have enough experience with the fundamentals of how crystals respond to feel confident in choosing a good material. Also I doubt I'd get approval to start buying a bunch of expensive equipment that I would need to make one sadly so this might have to live in my head for the foreseeable future.

Somewhat along the same line of thinking, there are diodes that are sometimes used. I think they get accuracies within the 10% range, but it does give real-time feedback and they often come in an array of a bunch of them. I've heard of them mostly in the context of full body radiation treatments to make sure that there aren't any missed spots.

Good ideas though!

[Rob Tayloe]

I do not have expertise with dosimetry for radiation oncology. It appears an interesting field and one in which there is considerable demand (which means this should be a profitable enterprise for someone). One wonders what caused Landauer to withdraw from this market.

I found some open source and available information on OSLs. One is a Japanese paper and another is a presentation on an US NRC website. This information provided some interesting background on OSLs.

[broken link removed - steven]

https://iopscience.iop.org/article/10.3 ... 065/ac9106 or https://iopscience.iop.org/article/10.3 ... ac9106/pdf

[Sesselmann]

Chase,

This recent presentation by Dr Anatoly Rozenfeld from Woolongong University might be of interest to you.

https://www.youtube.com/watch?v=MEnT_o2QKIg

Way outside my field of expertise.

Steven