Medical Physics is an exciting field

[Clinac]

Can anyone please explain the basic concept of dose gradients as applied to the gamma index where used with Dose difference and DTA etc.

 

[Choppy]

Can anyone please explain the basic concept of dose gradients as applied to the gamma index where used with Dose difference and DTA etc.

Conceptually, the gamma concept is used for comparing two data sets. Examples might be a 2D portal image prediction vs. a measured image, or a water tank scan taken presently vs. one taken at the time of commissioning. Either way you have a reference data set
D_r(r_r)
and a comparison data set
D_c(r_c).

The concept suggests defining an ellipsoid of acceptance in the dose phase space around the reference data set. (You can imagine it for a 1D scan by plotting dose on the y-axis against position on the x-axis. Then draw ellipsoids around the reference data set with your dose acceptance criterion and distance-to-agreement criterion on the semi-major and semi minor axes.) For each point in the reference set you then search the comparison data set for the point the minimum "phase-distance" away (ie. try to see if the closest point in the comparison data set falls inside the acceptance ellipsiod). Mathematically you ascribe a value to this - your gamma - which is less than or equal to 1 for acceptance and > 1 otherwise.

\gamma(r_r) = min|_{r_c}{\GAMMA(D_c,r_c)}

where
\GAMMA(D_c,r_c) = SQRT{ (\delta r)^2/(dta)^2 + (\delta D)^2/(AD)^2}

dta = acceptable distance to agreement
AD = acceptable dose difference

(Apologies for the crappy formatting. I don't type out expressions in HTML too often.)

It's worth noting that there are more rigorous algorithms out there. As I've described it, the gamma is suceptable, for example, to sampling artefacts.

 

[Clinac]

Thanks choppy that helps a lot but how and what are high and low dose gradients as used with DTA and percentage dose difference respectively, thanks, just hard to get my head around this topic.

 

[Choppy]

In a high dose gradient region (say on a field edge) it's likely that if you define acceptance by a percentage of the central axis dose only, that all points in that high gradient region will fail. Because of the high gradient, moving only a few mm results in changes in dose that are larger than your acceptance criterion. But we need to recognize issues such as error in detector positioning, and tolerances in field position.

So, the gamma concept basically says that if there is a point within your dose tolerance that is within a specified DTA (rather than right at the reference point) the metric will return an acceptable value.

In a low dose gradient region, differences between reference and comparison data sets will fail if the differences are greater than the allowable dose difference because the nature of the low dose gradient means you generally won't find an agreeable point within the DTA.

 

[Clinac]

Thanks for that choppy, that's makes more sense than explained by low et al.
I wonder form a practical perspective to which i applying the gamma index why the following occurs: I am evaluating two dose matrices calculated and measured obviously using a 2 d array. When using a global reference the gamma index for the matrices is in agreement for 95% -100% for 3mm/3% criteria- happy with that.
However when applying local dose reference the comparison fails due to DTA. I am also suppressing dose below 5% due to use of the local reference. I am unsure why I am not getting similar results to that in the global reference. I would have thought it would be due to dose difference because it is a a stricter criteria in local dose. Much of the failure is cold spots on the periphery or outside the PTV. This surely is not a bad thing.

Could be be to the degeneration of the gamma index to DTA due to the low dose gradient? or the resolution of the device (10 mm). but why is this not happening in gobal gamma reference.

Any thoughts would be much appreciated.

Thanks for all your insights.

 

[Choppy]

We're getting a little off the topic of the thread here. I'll PM you.

 

[qball2]

This post is to everyone who claimed there is a shortage of jobs in medical physics. It's to everyone who claimed that it should be very easy to get a job in medical physics after graduation.

There was a memo from the American Board of Radiology dated July 29, 2010. It reads:

"ABR RP Trustee Statement
a.) Waiver for Part 1 Employment Requirement
b.) Reminder of Application Submission Deadline
Date: July 29, 2010

The ABR has become aware that because of current economic circumstances many recent graduates are experiencing difficulty finding employment in medical physics. Because of this situation, the ABR is waiving the requirement for employment in medical physics as a condition for taking Part 1 effective immediately. All other Part 1 requirements remain in effect.

The requirement for clinical experience prior to taking Part 2 remains unchanged.

Please note also that the deadline for submitting complete applications for the 2011 written examinations in Radiologic Physics is September 30, 2010. With the exception of the Special Circumstances category [http://theabr.org/ic/ic_rp/ic_rp_process.html] , applications received after that date will be returned to the candidate unopened with the next opportunity for submission being in July 2011 under the requirements in place for the 2012 examinations. " [emphasis mine]

You are all wrong.

You can ban me all you want, but you cannot silence the truth.

Game. Set. Match. And QED.

 

[rakhaa]

Worrying, especially for someone about to start grad school in this field!

How do things look for the coming five year period, or ten year period? What are the reasons for this lack of demand? Is it temporary or a systematic oversupply of medical physicists?

And finally, how does all of this fit in with the new CAMPEP rules applicable from 2012 and 2014?

 

[Choppy]

I'm not sure who was arguing that it's easy for recent gradutes to get a job in medical physics right now.

To answer Rakhaa's questions, I don't think it will come as a surprise to anyone that the economy is rather slow right now and healthcare is not immune from that. But the population is aging, which means over the next decade, there is going to be a significant increase (to the tune of 40%) in the number of people needing radiation therapy. Estimates of the number of qualified medical physicists needed to meet the anticipated demand for 2020 range from as high as 400/yr¹ to about 150/yr.²

Of course the concern is that the requirement that these new physicists come through CAMPEP programs will create a bottle neck in which this demand will not be met. Further, for graduating students, the existing residencies will be extremely competative.

1. TG-133 Report at: http://www.aapm.org/pubs/reports/RPT_133.pdf
2. Mills et al., "Future trends in the supply and demand for radiation oncology physicists," JACMP 11 (2) 209-219 (2010).

 

[MSstrawberry]

Hi, I'm a newbie here, but am I glad I found you :-)
I wonder what would you advise to a Canadian medical physicist entering the job market next summer? CAMPEP accredited MS, ditto accredited residency... And we are thinking about moving south of the border.
Thanks for every bit of information.

 

[Choppy]

Hi, I'm a newbie here, but am I glad I found you :-)
I wonder what would you advise to a Canadian medical physicist entering the job market next summer? CAMPEP accredited MS, ditto accredited residency... And we are thinking about moving south of the border.
Thanks for every bit of information.

Do you have any specific questions?

The single biggest factor to have in your favour is membership with the CCPM, or if you specifically are interested in the US, certification with the ABR.

My understanding is that the US job market for medical physicists is tough right now. Canada is somewhat more stable - following general economy trends. Also, from what I've observed it's more difficult for MSc-level graduates to land positions compared to PhD conterparts in Canada, and that's generally speaking because there is a stronger emphasis on research in Canadian institutions. That's not to say that there aren't opportunities though. There are several new institutions being built in the western provinces over the next few years.

 

[MSstrawberry]

Thank you, Choppy. I'm familiar with the Canadian job market, so I'm trying to learn more about the situation in the US. There appear to be more job ads on the major sites (like AAPM) lately, but it's rare that one ad lasts longer than two months (we might say one month is rather typical)
There were financial and personal reasons why I couldn't afford a PhD at the time, however I still managed to land a residency. True, I'm not a research guy either. I enjoy clinical work and problem solving on a daily basis.
I hope to pass the Canadian Board exam next year, but as you mentioned lack of a PhD makes it harder to find a career job here.
Anyways, if anyone here is familiar with the situation south of the border, I would love to read your opinions. Does the bottleneck still exist? What lies ahead for a Canadian with board exam and a very strong resume, lack of PhD, and so on...

 

[Choppy]

I've noticed more job postings on the medical physics list-server lately too. New ones seem to come out every week or so. I don't know that this necessarily signals the end of the drought though. Numerous posters on this site and others have expressed frustration with the job situation in the US.

Another avenue might be to contact some of the medical physics head hunters.

The best places for networking, I've found, have been conferences. The ASTRO meeting is coming up at the end of October.

 

[rakhaa]

I'm looking for a used version of "The Essential Physics of Medical Imaging" from Bushburg (2nd edition-2002).

Anyone willing to sell this book? Or if you can recommend a place I could find it, would be great!

Thanks,
R.

 

[GuillaumeA]

Hi everyone !

I need some help about high energy electron beams...

The SURFACE percent depth dose increases while energy beam increases. I did not find any courses or papers establishing the reasons of this well-know phenomenon (I mean known by people working in radiation therapy). I suppose it is due to contamination electrons from the linac head but I am not sure at all. If anyone knows, please let me know and if you can give me your reference/paper it would be awsome.

I am desperate :-) because I have been searching for two weeks...

Thanks in advance

GuillaumeA

 

[qball2]

Hi everyone !

I need some help about high energy electron beams...

The SURFACE percent depth dose increases while energy beam increases. I did not find any courses or papers establishing the reasons of this well-know phenomenon (I mean known by people working in radiation therapy). I suppose it is due to contamination electrons from the linac head but I am not sure at all. If anyone knows, please let me know and if you can give me your reference/paper it would be awsome.

I am desperate :-) because I have been searching for two weeks...

Thanks in advance

GuillaumeA

It does increase with increasing energy and the reason is due to the increased energy of the scattered contamination electrons.

I cannot direct you to the source of this information because I do not know it. Most medical physicists I know of memorize it and then take it as gospel. This is one of the major problems with medical physics education.

 

[Doubell]

I enjoy mechanics and things of this nature. It is really easy to forget the other branches of physics like the medical field. ATB for starting a thread on it. Medical physics has lead to great devices like the MRI. IMO it is among the most interesting devices in physics only topped by tesla coils and particle accelerators. Anyway, besides the hair raising name like Magnetic Resonance Imaging, it has really aided in the diagnosis of ailments which is never a bad thing.

so how long would one have to study at the university level for a profession in medical physics

 

[Choppy]

Hi everyone !

I need some help about high energy electron beams...

The SURFACE percent depth dose increases while energy beam increases. I did not find any courses or papers establishing the reasons of this well-know phenomenon (I mean known by people working in radiation therapy). I suppose it is due to contamination electrons from the linac head but I am not sure at all. If anyone knows, please let me know and if you can give me your reference/paper it would be awsome.

I am desperate :-) because I have been searching for two weeks...

Thanks in advance

GuillaumeA

It sounds like you're asking about clinical electron beams, in which case I'm not sure you would really define something as a "contaminant" electron.

The shape of the electron PDD is determined by the scattering properties of the electrons themselves - the differential cross-sections of which are dependent on energy. You should be able to look these up in any standard medical physics textbook like Johns and Cunningham and that will have references to their original derivations. Solving the system for a PDD is a rather complex task to perform analytically. Your best bets for solving them for a specific geometry are the Monte Carlo method or grid-based numerical techniques.

 

[Choppy]

so how long would one have to study at the university level for a profession in medical physics

There are a number of threads about this.

Minimum: 4 year BSc + 2 year MSc = 6 years
This is not typical however. Most positions will expect board certification which will require a 2 year residency and it's difficult to get into a residency without a PhD. To go from high school to a practicing, certified medical physicist you're typically looking at 8-12 years.

 

[qball2]

It sounds like you're asking about clinical electron beams, in which case I'm not sure you would really define something as a "contaminant" electron.

The shape of the electron PDD is determined by the scattering properties of the electrons themselves - the differential cross-sections of which are dependent on energy. You should be able to look these up in any standard medical physics textbook like Johns and Cunningham and that will have references to their original derivations. Solving the system for a PDD is a rather complex task to perform analytically. Your best bets for solving them for a specific geometry are the Monte Carlo method or grid-based numerical techniques.

The contaminants are the scattered electrons since they enter at oblique incidences and contribute to the surface dose.

 

[GuillaumeA]

Thank you for all your answers. I found something very interesting. Look at the fourth page of this pdf. The increase of Ds/Dm with the energy in electron beams is well explained.

GuillaumeA

 

[GuillaumeA]

jpg not pdf :-)

 

[AtomicPile]

This post is to everyone who claimed there is a shortage of jobs in medical physics. It's to everyone who claimed that it should be very easy to get a job in medical physics after graduation.

There was a memo from the American Board of Radiology dated July 29, 2010. It reads:

"ABR RP Trustee Statement
a.) Waiver for Part 1 Employment Requirement
b.) Reminder of Application Submission Deadline
Date: July 29, 2010

The ABR has become aware that because of current economic circumstances many recent graduates are experiencing difficulty finding employment in medical physics. Because of this situation, the ABR is waiving the requirement for employment in medical physics as a condition for taking Part 1 effective immediately. All other Part 1 requirements remain in effect.

The requirement for clinical experience prior to taking Part 2 remains unchanged.

Please note also that the deadline for submitting complete applications for the 2011 written examinations in Radiologic Physics is September 30, 2010. With the exception of the Special Circumstances category [http://theabr.org/ic/ic_rp/ic_rp_process.html] , applications received after that date will be returned to the candidate unopened with the next opportunity for submission being in July 2011 under the requirements in place for the 2012 examinations. " [emphasis mine]

You are all wrong.

You can ban me all you want, but you cannot silence the truth.

Game. Set. Match. And QED.

There is no shortage in the medical physics field nor will there be in the foreseeable future. I realize that I might seem like the "glass half empty" type, but think about this:

1. The "slow" economy is not the source of the current problem as some would portray in other threads. There was never a shortage. Some guys, say Peter Balter (MD Anderson Cancer Center), or his brother James Balter (Univ Michigan) , or their father Stephen Balter (Columbia University Medical Center) made this statement up to draw in new recruits to their training programs around the US. Their programs, and others, are certification mills that feed each other. Once you are out of the queue, you are unemployed.

2. The aging population argument that some wish to place their "faith" in is a red herring. It is true that the population IS aging. But it would negate the "slow" economy supposition of #1. Consider that the population is aging and consequently, because they are baby boomers, expanding the need for medical physicists. Then the slow economy could not hold back the floodwater of demand for these same physicists.

The one place that has even tried to justify a shortage of medical physicists by publishing a study is looking for, u-hum, now pay attention, a health physicist. This is the University of Louisville advertising on the American Association of Physicists in Medicine website.

QED

 

[Choppy]

Sorry - you've resurrected this topic to argue that the economy isn't slow right now?

I think I'm missing your point. If you've got some hard data please post.

 

[AtomicPile]

There is an oversupply of medical physicists! The ABR has publicly acknowledged that there are more medical physicists than jobs.

The economy IS slow. However pundits who wish to grow more training programs falsely argue a need for growth based on the fact that the baby boomer population is aging.

Suppose for the moment that the aging baby boomer population argument is sound. Disease progression isn't subject to the vagaries of a poor economy. So the need to hire more people to treat this growing pool of disease would be immune to economic circumstances. But that has not been the reality of the job market for quite a number of years.

I argue that there has always been an oversupply of medical physicists. And anyone who claimed there is was a shortage of medical physicists consciously engaged in an act fraud.

 

[Choppy]

I argue that there has always been an oversupply of medical physicists. And anyone who claimed there is was a shortage of medical physicists consciously engaged in an act fraud.

Why are you so full of animosity that you would accuse people of a criminal act just to further your argument?

At my institution I live daily with the fact that we are grossly understaffed.

Further, your argument doesn't hold much water. If a hospital administrator has no money to hire a physicist, an increase in clinical demand won't change that - at least not immediately. Funding timescales work on the order of years.

You also have to separate qualified medical physicists from people who want to become medical physicists. I don't believe there is a shortage of the latter. But you might want to talk to someone who's been involved in the recruitment of the former.

 

[qball2]

Why are you so full of animosity that you would accuse people of a criminal act just to further your argument?

At my institution I live daily with the fact that we are grossly understaffed.

Further, your argument doesn't hold much water. If a hospital administrator has no money to hire a physicist, an increase in clinical demand won't change that - at least not immediately. Funding timescales work on the order of years.

You also have to separate qualified medical physicists from people who want to become medical physicists. I don't believe there is a shortage of the latter. But you might want to talk to someone who's been involved in the recruitment of the former.

Choppy, can you explain this? If you are so understaffed why don't you hire someone? This shouldn't be a problem if the market is over-saturated (and it is an indisputable fact that it is). And if you're not looking to hire someone then that job doesn't really exist as far as the market is concerned.

 

[NeoDevin]

Choppy, can you explain this? If you are so understaffed why don't you hire someone? This shouldn't be a problem if the market is over-saturated (and it is an indisputable fact that it is). And if you're not looking to hire someone then that job doesn't really exist as far as the market is concerned.

They can't hire anyone without the money to do so. The money is not there because of the economy. Once the economy recovers, and the budget is increased, one would expect that they will fill the position.

 

[AtomicPile]

They can't hire anyone without the money to do so. The money is not there because of the economy. Once the economy recovers, and the budget is increased, one would expect that they will fill the position.

I would like to know who "they" are. I always worry when someone puts in an ambiguous descriptor like "they".

Do you work with Choppy? Your post implies that you have some kind of intimate knowledge of Choppy's workplace environment and the future hiring practice of Choppy's workplace.

I would add that I have the CV's of at least 20 medical physicists seeking work. Some of them have been board certified for over 5 and 10 years.

 

[NeoDevin]

I would like to know who "they" are. I always worry when someone puts in an ambiguous descriptor like "they".

I would think it's abundantly clear that "they" in my post refers to the hospital administration (whomever is in charge of hiring for the particular positions that are short-staffed at the moment). I don't think my use of "they" was at all ambiguous.

Do you work with Choppy? Your post implies that you have some kind of intimate knowledge of Choppy's workplace environment and the future hiring practice of Choppy's workplace.

I know what the economic situation is like where Choppy works.

I would add that I have the CV's of at least 20 medical physicists seeking work. Some of them have been board certified for over 5 and 10 years.

That's a wonderful anecdote, I'm glad you shared. Was it posted for a purpose?

 

[sanadan]

So, I have an undergrad in EE from U of Alberta, and I have thought a lot about going back to school for a more physics centric education. Obviously medical physics is an interesting option, but I am unsure exactly how much upgrading I may require to get accepted into an MSC program. FYI, if I do go back to school, I will likely go somewhere other than UofA or UofC.

For the past 9 years I have been doing industrial automation, which is pretty far removed from any of the signal processing or electromagnetics classes that I took within my degree.

 

[Choppy]

So, I have an undergrad in EE from U of Alberta, and I have thought a lot about going back to school for a more physics centric education. Obviously medical physics is an interesting option, but I am unsure exactly how much upgrading I may require to get accepted into an MSC program. FYI, if I do go back to school, I will likely go somewhere other than UofA or UofC.

For the past 9 years I have been doing industrial automation, which is pretty far removed from any of the signal processing or electromagnetics classes that I took within my degree.

It would depend on the specific program whether or not they would accept you or if you would need some upgrading. I know people who've entered with an engineering physics background and done fine. Further, most of the people who entered medical physics through a BME graduate route started out with an EE undergrad.

I'm curious why wouldn't want to go through either of the schools in Alberta. They have extremely good programs.

 

[NeoDevin]

I would second the recommendation. If you want to do MSc in Medical Physics, University of Alberta is an excellent place to be. There is lots of interesting research going on regarding the Linac-MR (most of which is covered by an NDA, so don't bother asking me for details), and an EE background would likely be an asset at the moment. You may need to take a few undergrad physics courses to get any background you're missing, but it shouldn't be too much. Feel free to PM me for more information, I'm a grad student here right now.

 

[sanadan]

Well, it's not that I am anti-UofA or UofC, but, I didn't realize they were both so highly regarded in this regard and primarily my reason for discounting both is that I have lived in Alberta my entire life and experiencing something new and gaining additional perspective on the world is appealing to me.

Sounds like I shouldn't discount them too early though. If I needed to do some physics upgrading, which is what I suspected, what classes are most likely needed? I am doing some self directed learning atm to refresh myself in certain areas and although I am currently mostly refreshing I am also interested in learning something new. For new things I have considered learning Lagrangian mechanics and then parlaying that into Quantum Mechanics, but I am honestly floundering a bit at the moment.

 

[qball2]

Well, it's not that I am anti-UofA or UofC, but, I didn't realize they were both so highly regarded in this regard and primarily my reason for discounting both is that I have lived in Alberta my entire life and experiencing something new and gaining additional perspective on the world is appealing to me.

Sounds like I shouldn't discount them too early though. If I needed to do some physics upgrading, which is what I suspected, what classes are most likely needed? I am doing some self directed learning atm to refresh myself in certain areas and although I am currently mostly refreshing I am also interested in learning something new. For new things I have considered learning Lagrangian mechanics and then parlaying that into Quantum Mechanics, but I am honestly floundering a bit at the moment.

You don't actually need to know any physics to be successful in medical physics so don't waste your time.