IRREGULAR FIELD SIZES

Taking blocking into account:

• When we use a block we must calculate a tray factor
• When we use a block and include the tray factor into the BOT calculation the BOT must increase
• Once the tray is in, no matter what size the block is the same tray factor is always used (remember the tray factor takes into account the attenuation of the beam from the tray the block is mounted on)
• Also note that the field size must be changed therefore a different output factor will be needed and the PDD, TAR, or TMR must be adjusted for the new field size that we have because of the block itself
• If field size gets smaller the BOT must always increase !!! (if field size and the tray factor were not taken into account we would underdose the patient by a significant amount

In order to take blocking into account we need to know the components to a dose to a point and what effects them.

Components of dose to a point are (this is true for every point in a beam of radiation):

• Primary
• Scatter

(so a dose to a point is always made up of the primary radiation plus the scatter radiation)

• As the field size is reduced (by blocking in this case) the scatter radiation is also reduced BUT the primary radiation to that point stays the same
• At a 0 X 0 field size there is no scatter radiation and only the primary radiation contributes to the dose at that field size

For Irregular Field Sizes we use Clarkson Integration:

• PDD uses Scatter Function tables (SF)
• TAR uses Scatter Air Ratio tables (SAR)
• TMR uses Scatter Maximum Ratio tables (SMR)

These tables show scatter only

(These tables are for circular field size to accommodate for the errors that would be seen if they were calculated for regtangular fields.   We want to cut the field into slices and if that is done on a rectangular field size then the slices towards the edges would be larger than those slices towards the middle)

Example of SAR table:

• Radius of circular field size is at the top
• Depth of the field is on the side

This is an example of how we would use these tables:

• As seen above we find the dose to the shaded area by subtracting the 4 cm circle SAR from the 5cm circle SAR and then dividing that number by 36 (10 degree angle on the slice, 360 degrees in a circle)
• we would do this for every slice for a blocked field to find the amount of scatter that will be factored in

Example with a mantle field:

• this is how we would find the scatter reaching the center of the field for a blocked field.  Using 10 degree slices for the whole field we would find the amount of scatter each slice contributes to the center of the field.
• For this slice we can see that we must break the slice up into blocked area and unblocked area.  First we would measure the length of the slice (20cm).  We would find the Value for a 20cm radius circle on the SAR table ( .318 ) then subtract the value for a 13cm radius circle (.271 ,because this is where the blocking starts) finally we would ass back in 4 cm radius circle from the SAR table (.136 , because this is where the block ends on this slice and the tissue starts again)
• remember order is important because scatter towards the center contributes more than the scatter further away on the slice
• this would be repeated for all 36 slices (36 because of 10 degree slices and 360 degrees around a circle)
• all the values would be added up at the end and then divided by 36 to find the total amount of scatter contributed by the blocked field

Note: for this field the areas that are just free air would be considered blocked since they do not contribute to the scatter!!!!