Procedures in radiation oncology

[Chemdude]

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Can someone elaborate on the scope of procedures radiation oncologists can preform? I read in one of the FAQ threads that rad oncs perform brachytherapy procedures; I always thought that the IR folks were the ones doing the brachytherapy procedures.

 

[Chemdude]

Did you answer your own question? you thought wrong.

Alright...thanks for your input

 

[Ray D. Ayshun]

From what I've seen IR does brachy, if you'd call it that, in the liver. It's actually radioembolization. Radoncs do HDR and LDR GU brachy, sometimes in conjunction with another specialist, and may be present for placement of other things (plaques for ocular melanoma, intracavitary breast radiation, etc.). I think it's technically correct to say that HDR and LDR brachy for genitourinary cancers comprises the vast majority of procedures a radiation oncologist would ever perform.

 

[OTN]

From what I've seen IR does brachy, if you'd call it that, in the liver. It's actually radioembolization. Radoncs do HDR and LDR GU brachy, sometimes in conjunction with another specialist, and may be present for placement of other things (plaques for ocular melanoma, intracavitary breast radiation, etc.). I think it's technically correct to say that HDR and LDR brachy for genitourinary cancers comprises the vast majority of procedures a radiation oncologist would ever perform.

Not exactly. Brachytherapy is a mainstay of gynecologic cancer treatment, and at least in our center we do far more gyn brachy than prostate brachy (thanks, urorads!). You can also do brachy for H+N malignancy and sarcoma, which would involve a procedure for placement. Additionally, some radoncs place interstitial catheters for accelerated partial breast radiation (more common in Europe, but there are some institutions in the US doing it), and can place SAVI/Mammosite catheters as well.

While Y90 technically is brachytherapy, it's not commonly referred to as such. We used to "push" the beads at the time of delivery, but the IR docs now are allowed to handle and push themselves, so we don't have be there during the procedure.

 

[napoleondynamite]

Other procedures we commonly perform:

NPL for head and neck
GYN Pelvic exams & DRE's
A few centers do IORT

 

[medgator]

Other procedures we commonly perform:

NPL for head and neck
GYN Pelvic exams & DRE's
A few centers do IORT

DREs are procedural? 😀

 

[napoleondynamite]

a bit tongue in cheek - but I don't put my white coat on for nothin! lol

 

[seper]

my favorite one is occult blood test

 

[NS_hopeful]

I have a question that sort of goes along with this topic.

I'm doing rad-onc research at the moment, and there are a few concepts I'm having trouble wrapping my mind around. Can someone explain to me (in basic terminology) the importance of needing Hounsfield units and electron densities from CT imaging for IMRTs?

Also, can someone explain what anisotropic analytical and Monte Carlo algorithms are used for?

 

[OTN]

I have a question that sort of goes along with this topic.

I'm doing rad-onc research at the moment, and there are a few concepts I'm having trouble wrapping my mind around. Can someone explain to me (in basic terminology) the importance of needing Hounsfield units and electron densities from CT imaging for IMRTs?

Also, can someone explain what anisotropic analytical and Monte Carlo algorithms are used for?

I'll take the easier first question and let some one else closer to physics boards tackle the second. Hounsfield units and electron densities from CT imaging help treatment planning systems know how much attenuation the photons will face traveling through the body. You can imagine that in the lung, for example, you would see far less attenuation than you would in the pelvis, due to the increased density of the pelvic tissues compared with the lung. Computerized treatment planning systems need to take this into account when determining dose distribution for a particular photon energy in a particular part of the body.

 

[NS_hopeful]

I'll take the easier first question and let some one else closer to physics boards tackle the second. Hounsfield units and electron densities from CT imaging help treatment planning systems know how much attenuation the photons will face traveling through the body. You can imagine that in the lung, for example, you would see far less attenuation than you would in the pelvis, due to the increased density of the pelvic tissues compared with the lung. Computerized treatment planning systems need to take this into account when determining dose distribution for a particular photon energy in a particular part of the body.

Thanks for the feedback OTN! A few minor Q's for u now.

So are CT electron densities converted to HUs?
Are the HUs basically a value you you get from CT imaging then do a Plug&Chug into some type of pre-made dose calculation formula?

 

[Phantom1]

Thanks for the feedback OTN! A few minor Q's for u now.

So are CT electron densities converted to HUs?
Are the HUs basically a value you you get from CT imaging then do a Plug&Chug into some type of pre-made dose calculation formula?

Not really. Modern dose calculation algorithms use very complex mathematical models to predict the dose to a given area, including the use of heterogeneity corrections. Heterogeneity corrections can be used because different tissues can absorb or scatter electrons differently. Photons deposit dose in tissues via the generation of electrons. In order to calculate an accurate dose most algorithms assume "electronic equilibrium" in which the same number of electrons are entering an area as are leaving it. To achieve electronic equilibrium there is typically a buildup region because high energy photons generate electrons which are forwardly scattered. So you can imagine that when treating a lung tumor the photons have to traverse soft tissue, bone, lung, and then deposit dose in tumor. Each of these tissues may have different HU and the algorithm must account for that to accurately model the dose. It is probably one of the more complex mathematical applications in medicine.

Monte Carlo modeling is a very different approach. Monte Carlo modeling attempts to model individual particles as they traverse through tissues used established parameters and probabilities for how they would interact or scatter in tissues. In order to be accurate you must model millions or tens of millions of single particle paths in order to be confident that the dose distribution reflects reality. Using modern computers this can now be done fast enough to be clinically useful and is being applied.

So the short answer to your question is no. We don't generally use pre-made plug and chug formula for dose calculation, that was done decades ago. Now that being said there are "hand calculations" that can be done to verify or check that the number of monitor units are correct. This could be used for emergency situations such as whole brain radiation. Maybe that is what you are referring to?

 

[Neuronix]

Also, can someone explain what anisotropic analytical and Monte Carlo algorithms are used for?

Dose modeling is a complicated, specialized topic. In addition to the excellent answers already given, check out this presentation which should answer your questions: http://www.medicaldosimetry.org/pub/397f61d4-2354-d714-5172-744e9b92e649

Can someone explain to me (in basic terminology) the importance of needing Hounsfield units and electron densities from CT imaging for IMRTs?

Hounsfield units represent the linear attenuation coefficient (common exam question) of a medium. Once the scanner is calibrated (i.e. that the HU for air and water are what they should be), the linear attenuation coefficient is converted to electron densities by a table built into the planning system. This then gets fed into the dose calculation algorithms.

So are CT electron densities converted to HUs?

Are the HUs basically a value you you get from CT imaging then do a Plug&Chug into some type of pre-made dose calculation formula?

A hounsfield unit is a measure of how much an x-ray is "attenuated" at some point in space in an image. No more, no less. That is then converted into other information like electron density based on correlations between the two known for tissues of the human body.

Without CT based planning, for institutions that still use non-CT based plans (we do for some palliative cases), radiation can be delivered based on dose calculation formulas. I would venture to say that this is not routinely done for definitive cases in the modern era in the US. For 3D based planning, dose is modeled based on the presentation I linked.