# Volumetric modulated arc therapy

1. Sep 14, 2009

### Rafi7

1. The problem statement, all variables and given/known data

volumetric modulated arc therapy (vmat) is a new technology in radiation therapy. It gives radiation treatment in a single 360 degree or less arc. During VMAT delivery on medical linear accelerator dose rate, gantry speed and MLC shapes can be simultaneously varied when radiation beam is on. during vmat treatment gantry is continuously moving in an arc with dynamic multileaf cllimator motion.

In an article on vmat the authors mention that 'In practice monitor unit weight that causes the maximum dose rate setting to be exceeded can be delivered by reducing gantry rotation speed. Deceleration of the gantry is undesirable because it will result in increased treatment time as well as potentially less accurate delivery due to the substantial angular momentum of the linac apparatus'

2. Relevant equations
my question is how decelaration of the gantry speed will result in less accurate delivery due to angular momentum of the linac apparatus?

3. The attempt at a solution

i understand angualr momentus is the ability of a body to keep moving and is equal to mass multiplied by velocity i.e. angular momentum = mass * veocity

the linac apparatus has high mass (as its quite heavy and massive) so does it mean if we reduce gantry rotation speed (variable gantry speed) the angular momentum will reduce and the ability to keep moving will be low?

or does it mean as speed (distance/second) is reduced, mass will increase to keep the momentum constant?
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Sep 16, 2009

### gabbagabbahey

Hi Rafi7, welcome to PF!

I'm not familiar with that apparatus, but if you provide a link to the article you referred to, I may be able to help.

No, linear momentum is mass times velocity. Angular momentum is defined as $\textbf{L}=\textbf{r}\times\textbf{p}$ for a point particle at position $\textbf{r}$, with linear momentum $\textbf{p}$.

Without some information on how the apparatus actually works, it is impossible to answer these questions.

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