Wedges in Treatment planning

[brendav]

Can someone explain why we use lateral wedges (with heels post) when treating rectal cancer using 3 fields? It is to improve dose distribution and move the hot spot anteriorly away from the surface and towards the tumor?

Thanks!

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[probiotic]

3 field setup (parallel opposed beams and the third beam perpendicular to them for example gantry angles 0, 90 & 270) results in an inhomogeneous dose distribution in any site because of asymmetry. And we need wedges usually 30 degrees with heels toward the 0 angle beam to create a homogeneous plan.

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[RadSki]

1) The PA beam is hotter posteriorly compared with anteriorly due to dose falloff.

2) The patients usually curve posteriorly (the buttocks) and therefore there is less tissue for the lateral beams to pass through making the posterior part of the plan hotter.

For these two reasons, you need lateral wedges with the heels posterior to cool down the posterior part of the field and make the plan more homogenous. Take a few minutes and ask your friendly neighborhood dosimetrist to show you a plan without and with the lateral wedges and it will make sense. You should also compare this to a 4 field box plan where wedges frequently are not required.


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[Scatter]

Remember, the patient is prone, on a belly-board.

 

[Palex80]

"When a rectum comes along... You must wedge it!"

 

[Neuronix]

On a circular phantom, three 10x10cm open fields are applied with equally weighted MUs prescribed to give 100cGy to the isocenter (center red circle). The Blue half box in the center corresponds to the 100% isodose line. There is a dose gradient within the areas of field overlap from anterior to posterior. Of course this is not the shape of a human body, but the principle is the same.

To make the dose more homogenous to the area of three field overlap, 60 degree lateral wedges are applied (again equal MUs for each beam here to deliver 100 cGy to isocenter). A near rectangle is now formed by the 100% prescription isodose region.