Physics Questions

[RadtionVacation]

What would people say about having a thread where new residents can post questions they have while taking their first year physics courses? Junior residents can post their questions, and senior residents preparing for boards in the next year or so, can help? I think we all know that Khan is not a friendly, first read for those of us who have not taken physics since undergrad!

Let me start with one:

What are the main differences and advantages between a Magnetron and Klystron?

---

Members don't see this ad.

 

[deleted4401]

Both are ways to generate power for the linear accelerator to accelerate electrons.

A magnetron actually generates the waves utilized by the waveguides to accelerate the electrons. A klystron is an amplifier of said waves.

For the questions I saw in Raphex and other places, that was the relevant difference - magnetron generates the waves, klystrons amplify them.

I know - the obvious questions is - then what generates the waves in machines that utilize the klystron if they are just amplifiers? I don't know - I'm a lowly physician (who has passed the boards, though).

-S

 

[medgator]

Magnetrons are good for 6 MV machines, where the higher energy machines require klystrons IIRC?

 

[TarHeelRadOnc]

Actually, the Elekta machines use a Magnetron and offer 6mV, 10mV, amd 15mV photon energies. Newer Magnetrons can accelerate RF in the same range as Klystrons. The old Raphex exams do say, however, that Magnetrons are only good up to 4-6mV...

 

[BraggPeak]

So in a Linac, you have a power supply which proveds AC voltage that isconverted to DC with a rectifier; a filament, which is heated up and releases electrons; and a magnetron (or in some cases a klystron) that produces waves to accelerate the electrons to the speed of light; and a target of tungsten which halts the motion of the electrons, producing either characteristic or brehmstraulang (excuse the spelling) x-rays? And along the way, there may be bending magnets to change the direction of the electrons in a bigger linac?

 

[RadtionVacation]

Thanks for your help!
Now, another probably basic question:

Why are cobalt-60 units considered inferior to the Linacs that we now use. Is it because of the contaminaation of the primary gamma radiation caused by interaction of the gamma ray with itself, source, capsule, etc?

 

[Brim]

Thanks for your help!
Now, another probably basic question:

Why are cobalt-60 units considered inferior to the Linacs that we now use. Is it because of the contaminaation of the primary gamma radiation caused by interaction of the gamma ray with itself, source, capsule, etc?

Lower energy beam (although in some cases, the lower energy of 60-Co is desirable).
Larger beam penumbra.

 

[medgator]

Lower energy beam (although in some cases, the lower energy of 60-Co is desirable).
Larger beam penumbra.

plus you have to change the source out every 5 years or so.

 

[RadtionVacation]

Why should we care about kerma?

 

[RadtionVacation]

This is a really stupid question, but can anyone explain what an MU is and why it is important?

 

[TarHeelRadOnc]

An MU (monitor unit) is a measurement of machine workload, ie the energy expended by a linear accelerator. MUs are measured by the ion chamber in the head of the linear accelerator. Linacs are calibrated to deliver of 1cGy/MU for a 10cm x 10cm field at 10cm depth and 100cm SAD. The MU required to give a fixed dose is dependent upon the dose rate, distance from the source (ie inverse square law; TBI requires a LOT of MUs), TMR/TPR, the field size factor, and tray factors (wedges, compensators, etc).

If more MUs are required to deliver an equivalent dose, then the treatment is inherently less efficient. One example of this decrease in efficiency is IMRT, which requires greater monitor units that 3D-conformal treatment to deliver an equivalent dose. MUs are also associated with integral dose, due to increased scatter and leakage between leaves.

MUs are important because linear accelerators don't understand absorbed dose (ie Gy), they only understand monitor units. Therefore, for each treatment plan, the desired dose (as prescribed by the radiation oncologist) has to be converted to monitor units (by the physicist) for the treatment to be delivered. The MUs are quantifies for each field, arc or segment (in the case of IMRT)

 

[clintpark]

Translating it to laymen's language...

MU is the amount of radiation the machine is supposed to emit. Because of different blocking, MLCs moving in and out of field, distance from source, etc. it's not directly proportional to the absorbed dose. As TarHeel pointed out, for the same given dose, IMRT plan (because at each moment, the aperture is smaller than an open field) would have higher MU than a 3D plan.

Kerma is the amount of energy released in the region (per mass) by incoming radiation. Some are absorbed (= dose), some are wasted (Bremsstrahlung, etc.).

These are all simplified, physician's version of the terminologies. Any physicist should be able to point out multiple errors on these statements.

 

[DeadCactus]

As an MS I with an undergraduate degree in Electrical Engineering, this thread really makes me want to go into Rad Onc.

 

[RadtionVacation]

Could a klystron be considered a microwave power source? Or would that only be a magnetron?

 

[Brim]

Could a klystron be considered a microwave power source? Or would that only be a magnetron?

A klystron is a microwave amplifier but does not generate RF waves. It is dependent on another source for generating the RF waves. Basically it takes an input of low power microwaves and produces high power microwaves for use in the accelerator.

A magnetron, on the other hand, is a microwave generator.

 

[RadOncMegatron]

I'm trapped in my prelim internal medicine year! Someone help me get out here and into Rad Onc and learn this stuff!!!

Learning about klystrons >> learning Vent settings >>>>>> Good Bowel Regiments

 

[Zap]

I'm trapped in my prelim internal medicine year! Someone help me get out here and into Rad Onc and learn this stuff!!!

Learning about klystrons >> learning Vent settings >>>>>> Good Bowel Regiments

Vent settings you can forget. The bowel regimens you will want to remember when your palliative patients on mounds of opiates tell you on the weekly on-treatment visits that they haven't pooped since the last saw you....

 

[iradi8u]

I love this topic and will have plenty of questions very soon!

 

[RadtionVacation]

These are probably really stupid questions, but can someone explain the importance of:

1) hinge angle
2) gantry angle
3) wedge angle
4) couch kick

5) when do you want to use (and what are benefits of) half-beam block

6) when setting up patients clinically for electron set up, do you use the Dmax of electrons or the R90?

 

[G'ville Nole]

These are probably really stupid questions, but can someone explain the importance of:

1) hinge angle
2) gantry angle
3) wedge angle
4) couch kick

5) when do you want to use (and what are benefits of) half-beam block

6) when setting up patients clinically for electron set up, do you use the Dmax of electrons or the R90?

1) Hinge angle refers to the angle difference used in a wedge-pair set up. It becomes important when selecting an appropriate wedge angle to optimize dosimetry (optimal wedge angle=90 deg-[hinge angle/2]).

2) Gantry angle is simply the angle of the gantry relative to the patient. E.g. a standard AP/PA plan will have gantry angles of 0 and 180 deg.

3) Wedge angle refers to the angle observed b/w the isodose line and central axis at d=10cm with a wedge in place. The wedge itself is given this angle designation (i.e. 15 deg, 30 deg etc.).

4) Couch kick refers to any yaw rotation of the couch. We most commonly use it to flatten the superior or inferior border of a field that will be junctioned with another field, thus reducing any hot spots that would otherwise arise from beam divergence.

5) Half beam block is another way of avoiding beam divergence. It can be used in any situation where this is desirable, with the only limitation being field size (field will be a maximum of 20cm for standard 40x40cm collimators, unless you use extended SSD).

6)It depends. For things like mastectomy scar boost, I use 80% as it corresponds most closely with the block edge, and a 20% hot spot will only run at 12 Gy instead of 10. For small skin cancers (especially things like eyelid), I think 90% is OK as long as you give the tumor an appropriate margin. My older partners will use Dmax in these situations to reduce skin reaction, but if you do this, you have to recognize that everything outside of Dmax at central axis is going to be running cooler.