treatment planning calculations handout

if adjacent fields abut on the skin surface, the fields will overlap to an increasing degree with depth because of divergence

abutment

fields can abut at depth as opposed to the skin surface leaving a skin gap or separation at the surface

separated fields

Involves using the treatment planning computer to place fields in a manner in which isodose distribution is uniform at the desired depth and the cold and hot spots are within acceptable ranges.

dosimetric isodose matching

Involves blocks or use of asymmetric jaws or MLC to block one side of the field up to the central axis

half beam blocking

Based on the fact that the geometric field size is defined by the 50% isodose line

geometric matching

A skin gap can be calculated to allow the _____% lines or field edges to match at depth

50

Used to change the point of junction on a scheduled basis during the course of treatment in order to spread out or “feather” over a distance.

junction shifts

junction shifts require a change in

field size

“tissue absorption” factor used in SSD MU calculations

PDD

a ratio of dose at a specific depth to the dose at Dmax depth along the central axis of the beam

PDD

As beam energy increase, PDD _____.

increases

As field size increases, PDD _____

increases

as SSD increases, PDD _____

increases

as tumor depth increases, PDD ______

decreases

ratio between the dose at a given depth in phantom (w/ full scatter) and the dose without phantom (w/out full scatter) at the same depth

TAR (tissue air ratio)

increase energy = _____ TAR

increase

increase field size = _____ TAR

increase

increased depth = ______ TAR

decreased

TAR is dependent on SSD

false

Ratio of dose rate with scattering medium to dose rate in air at Dmax

BSF (backscatter factor)

As beam energy increases, BSF _____ up to a point (about 2MV)

increases

when a beam is >2MV the scatter becomes forward and BSF begins to decrease with increased energy. it is negligible above

8MV

as field size increases, BSF ____

increases

BSF is not dependent on SSD

true

Ratio of dose at a given depth in phantom to the dose at the reference depth of Dmax

TMR (Tissue maximum ratio)

as energy increases, TMR _____

increases

as field size increases, TMR

increases

as depth increases, TMR

decreases

TMR is dependent on SSD

false