module 0: Geometry of Photon Beams

fields size is _____ proportional to the distance from the source to the point of measurement

directly

field size relationship is based on what principle

similar triangle principles

the projection of the collimator's distal end on a plane perpendicular to the beam axis.

geometric field size

this is determined by the light field

geometric field size

the distance between the 50% intensity points of the beam

Dosimetric Field Size (Physical Field Size)

how is the dosimetric field size reported

width x length (x * y)

this determines the treatment field size at the isocenter

dosimetric field size (physical field size)

Field size at the collimator equals field size at the isocenter for any modern-day unit at a distance of _____cm

100

MLC increase ______ compared to cerrobend blocks

penumbra

MLC consists of 60-120 motorized ____ leaves

tungsten

how are MLC leafs controlled

computer system

when using MLC what ensures minimal beam transmission (<2%)

tungsten leaves

MLC allows for

IMRT

Occurs when images appear larger than their actual size due to geometry and imaging setup.

magnification

factors that influence magnification

depth of object, SID, SOD

greater depth ___ magnification

increases

larger SOD ____ magnification

reduces

Larger SID _____ magnification

reduces

formulas for magnification

1. F_mag = (field size on image) / (actual field size)

2. F_mag = (size on image) / (actual size)

3. F_mag = (SID) / (SOD)

When adjacent fields are aligned at the skin surface

abutment

Due to beam divergence when using abutting fields, fields will overlap more as depth ____

increases

Adjacent fields are abutted at a specific depth rather than the surface

separated fields

separated fields creates a skin gap or separation at the surface to avoid overlap at ____ levels

deeper

when would we use field separation

treating near previously irradiated areas, treating regions with different beam energies, & treating a long field

abutment or separation must be effectively

documented

Achieves uniform isodose distribution at the desired depth while keeping hot and cold spots within acceptable ranges

dosimetric isodose matching

dosimetric isodose matching uses what to place fields

treatment planning computer

dosimetric isodose matching achieves a uniform isodose distribution at the _____ _____ while keeping hot and cold spots within acceptable ranges

desired depth

Prevents overlap of photon fields at depth while ensuring a uniform dose at the surface

half-beam blocking (beam splitting)

this uses blocks, asymmetric jaws, or MLCs to block one side of the field up to the central axis.

half-beam blocking

half-beam blocking removes ____ along one side of the field and allows two-photon fields to match at the surface

divergence

Calculates the gap between fields to ensure proper alignment of the 50% isodose lines at depth

geometric matching

this uses geometric data to calculate the gap

geometric matching

geometric matching defines the field size dosimetrically by the

50% isodose line

geometric matching calculates a skin gap to allow the 50% lines or field edges to match at ___

depth

(d/2) x [(L₁/SSD₁) + (L₂/SSD₂)]

geometric gap calculation

A systematic or scheduled change in the location of the abutment or separation (skin gap)

junction shift

this is used to change the point of junction during treatment

junction shift

junction shift helps to spread out or “___” the dose distribution over a distance.

feather

Avoid placing the junction

over the area of the tumor or critical organs/structures

The match line must be drawn in ________ to ensure proper field alignment

each treatment session

If the tumor is superficial and fields match at depth, do not use

a skin gap

if you use a skin gap for superficial tumors it will create a

cold spot above the junction

Use a beam splitter to match the fields on the _____, stopping divergence at the junction point to prevent overlap at depth.

skin surface