prescribing to an isodose line

[twesting]

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hi there,
just starting out in rad onc. have a bunch of basic/silly questions. i look things up a lot and read on things before i post but sometime don't get the complete answer...probably b/c the questions are so simple and basic. .just being able to ask the dumbest questions, in anonymity, would be so helpful. please forgive me if i am posting in the wrong forum or irritating anyone. i will stop asking if not appropriate.

first: what does it mean when someone says "i want to prescribe to the 95% isodose line." does it mean that the total dose you want to give needs ot be delivered to 95% of the contoured area? how do you decide what isodose line to pick?

second: as far as energies...if you have photons being delivered at 15mev vs 9mev..does that mean the the dose is getting in deeper with the 15? how do you know what energy to choose? how can the same machine be deliver different energy beams...are they all coming form electrons hitting the target and making energy?

thanks so much. again, I'm awfully sorry about the dumb questions. i hope i can continue to ask them as i do basic reading and get a better grip on things.

thanks!

 

[Gfunk6]

first: what does it mean when someone says "i want to prescribe to the 95% isodose line." does it mean that the total dose you want to give needs ot be delivered to 95% of the contoured area? how do you decide what isodose line to pick?

Let's take a straightforward palliative case to help illustrate the point. You have a 80M w/ a lung cancer obstructing his bronchus resulting in pneumonia and you are palliating it with 30 Gy in 10 fractions.

After simulation, your dosimetrist makes a simple plan with two beams, on AP and one PA each delivering 1.5 Gy each per fraction based on the tumor that you contour. Once the plan is ready to review, the prescription or 100% isodose line (30 Gy in this case) is shown as well as hot spots (105% isodose line) and cold spots (90% isodose line).

Let's say that the prescription isodose line (again 100%) is not covering the tumor well enough. However you think that the 95% isodose line does. Thus you tell the dosimetrist, "let's prescribe to the 95% isodose line". In other words, the total dose goes up by 5% and the hot spots and cold spots increase accordingly.

Hopefully that was clear, it's difficult to illustrate without a visual aid.

second: as far as energies...if you have photons being delivered at 15mev vs 9mev..does that mean the the dose is getting in deeper with the 15? how do you know what energy to choose? how can the same machine be deliver different energy beams...are they all coming form electrons hitting the target and making energy?

Photons are skin-sparing. They rely on secondary electrons to deliver their dose of radiation and the higher the energy, the deeper the maximum dose (dmax is) and greater the skin sparing. For 6 mV the dmax is about 1.5 cm at which point the dose slowly tapers off and for 18 mV it is about 3.0 cm.

The energy you choose is based on the depth of the tumor. For deep seated pelvic and abdominal tumors, some of the beams have to be higher energy whereas for H&N you can use lower energies.

The energy of the beam can be modulated by varying the power of the electric field in the linear accelerator. After electrons are produced by thermionic emission, they need to be accelerated before they hit their target and produce photons. Modulating the acceleration by changing the electric field is how one machine can produce multiple photon energies.

 

[Radonc11]

Let's say that the prescription isodose line (again 100%) is not covering the tumor well enough. However you think that the 95% isodose line does. Thus you tell the dosimetrist, "let's prescribe to the 95% isodose line". In other words, the total dose goes up by 5% and the hot spots and cold spots increase accordingly.
.

As an Intern, it was a pleasure writing, "we are treating the patient to 50 Gy in 5 fractions prescribed to the 90% iso-dose line using 10 non-coplanar beams" during my radonc elective last month. My pal, the intern covering the patient, sent me a text page "Go to Hell"

Bliss....

 

[twesting]

cant thank you enough gfunk6!

 

[Pewl]

I like to think of isodose curves as "everything within this curve is receiving at least this percentage of the total dose being delivered." So, everything within this curve is getting 95% of what we're shooting out. Then, you're basically normalizing your dose prescription to one of these curves.

 

[Palex80]

I like to think of isodose curves as "everything within this curve is receiving at least this percentage of the total dose being delivered." So, everything within this curve is getting 95% of what we're shooting out. Then, you're basically normalizing your dose prescription to one of these curves.

Good explanation.

I would just add, that prescribing to a lower isodose you may be aiming to have a inhomogenous dose distribution within your target volume. The best example for this would be prescriptions in SART (i love the new term).

 

[RadOncDoc21]

You guys do a better job then my physics book ever did... anybody care to breakdown calculating Monitor Units in a simple way?

This is what I have thus far:

MU=dose/ Everything under the sun


-R