[loarake]

Just defended my PhD thesis in medical physics. Worked on radiation therapy treatment planning, which combines optimisation theory with the physics of Monte Carlo particle transport engines (and more macro energy deposition modeling as well) to simulate millions of different radiation dose distributions in patients and figure out which combination will lead to the right outcome based on what the radiation oncologist prescribes.

People in my field are fairly fortunate as there is a career track as a clinical medical physicist that is highly paid and pretty low stress, so most people end up going there. The work consists of maintaining and calibrating the radiation therapy machines, along with implementing new technologies in the clinic, and fixing problems that don't fall within the job description of the radiation therapists. Like what to do when a radioactive seed falls on the floor instead of going inside the patient where it's supposed to go. There's also a separate track as an imaging physicist where you maintain and QA the diagnostic imaging machines.

I'm personally doing a postdoc at the junction between optimisation, machine learning and radiation therapy. Just starting out though. Basically just extending my PhD work to automate the treatment planning process and remove the variability in treatment plan quality due to the level of experience of the people making the plans.

[blevin]

Sounds fascinating. What coordinate systems are used for treatment planning? Given how much bodies can change over time, and the difficulty of re-achieving a specific pose, I'm curious if there are interesting ways to correlate measurements over time. Certainly, medical training involves learning lots of prepositional anatomy words like "antecubital" but is there anything more precise, a GPS system for bodies? This seems very challenging for e.g. the gastrointestinal tract -- but I could imagine something using lots of relative reference points, the way I assume surgeons orient themselves.

[loarake]

It's much more primitive than you think. Dose distributions are simulated based on a CT/MRI that was acquired before treatment (treatment often lasts weeks). Only minor corrections are made when anatomy changes during the course of treatment, even though the patient is often losing tons of weight due to chemo, etc. There are quite a few tools that help with patient positioning, like vac-lok bags or literally molding a mask and drilling it down on the treatment couch (an example is shown here: https://newsnetwork.mayoclinic.org/discussion/new-radiothera...).

Motion during treatment can be tracked with cameras or IR sensors or subcutaneous probes but that doesn't tell you about internal organs moving. The topic of deformable registration, where you find a non-rigid mapping between initial imaging conditions and the current ones, is still a topic of active research. Adaptive planning, where you actively change the treatment plan every N sessions based on the most up to date information, is also actively researched / implemented in some good research centers.

For treatment planning you just use a standard Cartesian grid, or a "beam's eye view" coordinate system that's aligned with the radiation beam axis as it rotates around the patient.

[blevin]

Makes sense; thanks. I'm out of my depth but it seems neurosurgery may just have it easier here, being able to fix a rigid stereotaxy head frame and fiducial markers across both imaging and therapy. Not to mention less tissue deformation enabling a gamma knife intersection-of-beams approach (i.e. ~200 collimated, mm-wide gamma sources).

Not to be glib but on behalf of the thousands of people going into a radiotherapy clinic today for treatment, thanks for working to improve these techniques.

[bananaowl]

So what happens to the seed??

[loarake]

Don't quote me on this because I only covered the topic briefly in some applied classes before doing 6 years of research, but if I remember correctly, you grab it with long tweezers and dump it in a shielded "garbage can" type container. And fill tons of paperwork that involves estimating the radiation dose delivered to everyone that could've been exposed. And probably present a post mortem at conferences about how you dealt with it.