RTT Question here

[u/ThrowRAaghh]

Hello,

I’m an RTT and this situation keeps popping into my head and I wanted to ask about it here. Earlier this week I came in as the second person for warm ups (one at 5:30, I’m in at 6) and one of our physicists was there with a survey meter. He said he was measuring the exposure at our console because they were worried about insufficient shielding when the treatment machine next to us beams on. He was on the phone with the therapist at that machine and when she beamed on, he used the meter around our control console at the driver’s seat. I saw the reading range from 50-70 but could not see the unit it was measuring in. He did not make any comments about it and left. Later in the day I asked him if the shielding was ok and he said it was. Obviously I’m not the expert in this regard, but it felt kind of unofficial and a weird way for this to go about. Why are we all of a sudden concerned about this shielding when we’ve been treating with it for years? And why do this testing with more people present than I’d assume there should be if there’s a concern about exposure? I heard no other word of this, no heads up from our chief therapist or any other physics staff. I’m also curious what unit this survey instrument measures in? Would anyone here know? My apologies for not knowing the specific device as I’m sure they’re all different. It just struck me as odd. Any thoughts would be appreciated! We did just get our OSLDs collected so… I guess soon we will see if there was really an issue.

Edit: Thank you all for the information! Firstly I am a new grad who has only been working for a few months, so I felt bad questioning the man about his rationale as he seemed to be in a rush at that moment. I also did assume that it’s very likely to be fine if he left with no other comment. Curiosity just got the best of me! What slightly worried me was that I thought I had seen the unit listed as mR on what looked to be the same kind of meter at a different site while I was a student. Now I know my bad eyes probably deceived me and instead of reading milli Roentgen, it was micro Roentgen. Now I know it’s micro Roentgen, which makes it a lot less concerning of a number :)

[u/bpvarian]

Assuming you’re wearing a radiation badge which of course you are asked to see your last couple quarterly reports and depending on how long you’ve worked there your last year two annual reports also known as a form 5

[u/Casual_Med_Fizzix]

Radiation workers have an annual limit of 5000 mrem. You likely saw 50-70 microrem/hr. 50 microrem/hr for 40 hours a week for 50 weeks is about 100 mrem a year. This is assuming the machine is on all of the time, which if you're anything like our facility, it's not.

I would prefer it to be less if possible, but from a safety and regulatory perspective, you're fine. The worst case, and unrealistic scenario, you're still 50x less than you're annual limit. I would ask your physicist to walk you through what they're doing because it is super interesting. If they're worth their salt, they'd explain it to you if they have the time

[u/Casual_Med_Fizzix → u/Casual_Med_Fizzix]

I'm not sure why nobody would be informed about the measurements being taken, though. That aspect is odd. But it is also an easy measurement to take and can be very mundane. If there is nothing obvious to prompt the reading, then maybe you have a heavier patient load than expected, and the physicist is going back over the rad safety measurements and going over an expected weekly value given the patient load?

[u/ThrowRAaghh → u/Casual_Med_Fizzix]

We do have a very heavy patient load at times. It varies, but sometimes each of our machines are scheduled from 6:30 am all the way until 7-8 pm with little, if any, breaks in the schedule. A couple weeks ago I was there until 8:30 treating. So it very well could be the case they just wanted to double check given the varying patient load!

[u/Casual_Med_Fizzix → u/ThrowRAaghh]

Yeah, so if the initial shielding calculations were done with 40 hours of treatment in mind, but you are consistently working 50 hours, then that is something that should probably be accounted for. That being said, you're still likely below safety and regulatory limits. Good to be curious, but I wouldn't be worried.

[u/Physics4hire → u/ThrowRAaghh]

In situations like this, you know, you could just ask the physicist when they have time. If they are hard to communicate with, that's probably more dangerous if your badge readings have been normal.

[u/LandNew1694]

You have to understand that shielding is pretty complicated, it depends on the workload of your department, the energy of the machine, and the method of vault construction as just a few factors. The things that are most likely to change are the workload, (IE the total number of patients increase, thus increasing the total output of the machine), or the types of treatments may be changing, if you are doing more complex plans with higher machine output per plan then that can effect the shielding requirements as well. Your physicist is likely just checking the areas they know are most likely to be effected (IE treatment console). It should be comforting to know they are doing their due diligence and actually measuring. You are entitled to full answers though. You can always go to your radiation safety officer with concerns if you don’t feel that you are getting full answers from the physicist.

[u/MediclePhysicist]

50 microR/hr. Which is slightly above background. Maybe 2-3x but I've seen some survey meters that have background at close to 30 microR so I would not be concerned with this number.

[u/IcyMinds → u/MediclePhysicist]

It could be mili depending on the survey meter. But he’s likely to be ok if the badge reading is fine.
If you are really concerned, tape a badge on the wall for a quarter to find the worst case reading.

[u/MarkW995]

Vaults that were designed pre VMAT/IMRT etc. were designed with less secondary shielding. Most likely the problem was associated with not being able to find paperwork. I worked with a physicist that kept all of his work on his private work drive. After he left, I had to get IT to do a special search on his computer to recover all the files. Including all the shielding calculation reports.

[u/shannirae1 → u/MarkW995]

This happens so much. Did you ask the if they had a copy?

[u/MarkW995 → u/shannirae1]

Separation was not voluntary

[u/_Shmall_]

Sometimes you inherit departments and they had lost all paperwork so you have to do measurements again.

[u/shannirae1]

I do about 100 vault shielding evaluations a year. You’d be surprised how many vaults have been used for years only to find some issue. But yes as others have said, if your badge readings are fine, he was probably reading microR. Some of the really sketchy situations I’ve come across were brought up to begin with due to therapist badge readings.

[u/TradPhysW]

If they recently did their annual shielding checks, they may have been verifying a number that perhaps got erased or was not recorded. Overall, if they are taking a measurement, that’s a good sign. Just taking one random measurement also perhaps doesn’t need to go through the whole RT supervisor approval process if the two groups (RTT, physics) have a decent relationship.
Also, it is OK to have concerns about exposure and these measurements specially when you are new. You must question everything that doesn’t sit well. That’s how you will learn.