Radiation Therapy: Dosage, Duration & Effects

[nesp]

Not sure if this belongs here or in the nuclear engineering forum.

Many cancer patients in radiation therapy for cancer receive as much as 170-190 rads of daily radiation for 7-9 weeks. When I looked at a radiation chart I found that equates roughly to the dose at 1 mile from a Hiroshima type explosion on a daily basis. The former is, hopefully, curartive. The latter is, presumably, lethal.

What's the difference? Is it the duration? Milliseconds for a nuclear blast vs minutes for radiation therapy? Or is it area covered? Whole body for nuclear blast vs focused for radiation therapy?

 

[syhprum]

I think this fear of radiation was in a large part cold war propaganda to reduce the fear of a nuclear attack you weren't going to blown to bits or burnt to death as most of the victims at Hiroshima were but the main danger was RADIATION that could be stopped by some newspapers covering the windows.

 

[daveb]

To answer your question, yes, it is whole body vs focused radiation. Treatment plans involve using some kind of imaging (MRI,CT, etc.) to locate the tumor, then the physician prescribes a treatment area which includes the tumor site. Unfortunately, there is healthy tissue around the tumor, and no device can currently target only cancerous tissue. So the physician sets the treatment area to slightly larger than the tumor area so as to make sure the tumor is irradiated while as much as possible limiting the irradiation of healthy tissue. Depending on the device, treatments areas can focus radiation to 1mm or even less.

The main problem with this is that the tumor can move between or during treatment (depending upon body location), so the doctor usually sets the treatment area to several centimeters larger than the tumor area, and even more healthy tissue gets irradiated. [Blatant plug here] ViewRay has received FDA clearance for a device that uses MRI for real time imaging of the tumor, which could potentially reduce the area that needs irradiation.

 

[nesp]

davep, thanks for the reply.

So with something like that view ray system, the targeted area is visualized real time as it's being radiated? And it's done with MRI?

I assume the MRI is used to detect movements of the tumor being targeted. How do you know that the linear accelerator is depositing its energy at precisely the targeted area? Is there a separate feedback system for the radiation beam?

It's amazing that the beam can be focused to mm precision.

 

[daveb]

The MLC (Multi Leaf Collimator) defines the target area shape, and the MLC attenuates the beam outside the target area to a fraction of the dose delivered to the target area.