Dosimetry during residency

[brendav]

Hi guys -

How closely do you work with your dosimetrists during residency? And what particularly should you be trying to learn from them?

At our program, we just place volumes and hand them a constraint sheet. They do the rest. I just feel I should be doing more.

B.

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[Gfunk6]

I think a dedicated dosimetry rotation (for at least a month) should be mandatory in residency.

There are so many important things you can learn from dosimetrists:

1. 2D plans - you should be able to plan these yourself (whole brain, palliative spine); learn how to place optimal beam angles, experiment with different beam energies to see what the dose distribution looks like, learn how to use MLCs to shape your field on DRRs

2. 3D plans - for breast you should understand calc points (what they are used for, why they are placed in certain locations), learn how to do a breast mono-isocentric setup (or whatever equivalent your institution uses); for other sites you should learn how to use basic beam modifiers (like wedges), generate appropriate beam angles to meet your dose constraints

3. IMRT plans - be familiar with general principles (optimal beam angles for common sites like prostate, how the inverse planning algorithm works, how you can place weight on different OAR variables)

Also, if your department uses different planning software you should be very familiar with each one.

By the end of my residency, I insisted on planning all 2D plans and simple 3D plans by myself. Of course, they were still subject to routine dosimetry/physics QA.

 

[lazers]

Can you tell me what a calc point and control point is? Sorry for these stupid questions.

 

[Gfunk6]

For breast plans, a calc point is used to equalize dose distribution in the treated breast. The idea is to use a proxy point so that you make make the dose as homogeneous as possible. If you place the calc point in a location too deep (for instance at the level of the chest wall) then you will end up overdosing the breast in your attempts to deliver ~ 50 Gy to the calc point. If you place the calc point in a too superficial location (near the nipple) then the entire breast will be underdosed.

The calc point is typically placed 1 cm anterior to the chest wall midway between your superior and inferior breast borders. This will generally give you the most homogeneous dose distribution.

Similar types of points can be used in a variety of 3D conformal plans. If you are not prescribing to a PTV, then you can select a calc/control point to deliver a nice, uniform dose to your target of interest.

 

[lazers]

For breast plans, a calc point is used to equalize dose distribution in the treated breast. The idea is to use a proxy point so that you make make the dose as homogeneous as possible. If you place the calc point in a location too deep (for instance at the level of the chest wall) then you will end up overdosing the breast in your attempts to deliver ~ 50 Gy to the calc point. If you place the calc point in a too superficial location (near the nipple) then the entire breast will be underdosed.

The calc point is typically placed 1 cm anterior to the chest wall midway between your superior and inferior breast borders. This will generally give you the most homogeneous dose distribution.

Similar types of points can be used in a variety of 3D conformal plans. If you are not prescribing to a PTV, then you can select a calc/control point to deliver a nice, uniform dose to your target of interest.

Thank you for your reply. So is a calc point used only for plans you're not prescribing to a volume? Also in breast cancer are you prescribing to a volume or to a point?

 

[RadSki]

At the University of Chicago we do not have a formal dosimetry rotation because we spend a full year doing research (ACGME requires 36 months of clinical work). We have developed "self-directed" dosimetry modules to fill this educational need. Each module takes ~1 hour for a resident to complete. The objective is to teach the resident how to use pinnacle while also teaching important dosimetric concepts (calc point, how to prescribe dose, IMRT optimization parameters, etc). I am happy to share these with anyone interested. In return I would only ask that you complete evaluations about the modules and return them to me. I would also include the CT and pinnacle contour data... it just requires loading these into Pinnacle. Any physicist or dosimetrist should be able to do this. PM me for more info.

 

[Gfunk6]

Thank you for your reply. So is a calc point used only for plans you're not prescribing to a volume?

Correct

Also in breast cancer are you prescribing to a volume or to a point?

In most breast plans you prescribe to a calc point (at least I do). If you REALLY like to contour then you can certainly draw out all of the breast tissue, designate it as a PTV and then prescribe to it. Personally, I think that's a lot of work for virtually no gain.

If you are doing breast IMRT however, you will need some type of volume to prescribe to.

 

[medgator]

In most breast plans you prescribe to a calc point (at least I do). If you REALLY like to contour then you can certainly draw out all of the breast tissue, designate it as a PTV and then prescribe to it. Personally, I think that's a lot of work for virtually no gain.

It can help with field design some of the 3- and 4- field setups (to make sure you're getting all the breast tissue with a mono-isocentric technique when you customize your blocking for the tangents. Also, as you said, it's necessary for IMRT planning.

Also, for 3D billing with whole brain, I typically contour the whole brain as my target volume. Wouldn't the same apply for a 3D breast plan? I've never gotten a straight answer on that one.

 

[lazers]

Thank you all, this was helpful.

 

[Gfunk6]

It can help with field design some of the 3- and 4- field setups (to make sure you're getting all the breast tissue with a mono-isocentric technique when you customize your blocking for the tangents. .

Good point. When I do 3 or 4 field breast I draw the suprclav nodes and axillary nodes respectively. Generally, I still don't prescribe to a volume but when you draw in the contours you can tell exactly what dose your target structures receive.

 

[lazers]

I assume the calc point and control point are the same thing. Is my assumption correct?

 

[Gfunk6]

Basically, yes.

 

[OTN]

As a guy who married a dosimetrist I met while in residency...spending plenty of time in dosimetry is not necessarily a bad idea.