RIT 470: Radiation Therapy Treatment Planning: Beam Modifiers.

Approximate Rectangular Fields

- "guestimate" method - useful for double checking MU's, but not for treatment

Disadvantages of MLCs

- MLCs can't replace an "island block" (example: blocking a kidney, and used in mantle fields to block lung) -MLC leaves can only enter in one direction (X-jaw) - jagged field edges

Field Shaping - Jaws

- Methods: jaws, hand blocks, cast blocks, multileaf collimators (MLCs) - a linac has 2 sets of collimator jaws for a total of 4 jaws - asymmetric jaws -- X and Y jaws move independently of each other -- allows for beam-split techniques (half-beam blocks), or asymmetric rectangular fields without blocks - matching fields, such as AP SCV and breast tangents, or lateral H&N and AP SCV beams, are aligned with one isocenter using asymmetric jaws - the movement of the Y jaws creates the dynamic wedges - the transmission through jaws is typically 0.5% or less; solid jaws should be close to the edge of the MLC to avoid interleaf transmission

Dose at Off-Axis Points

- OAR = Off-Axis Ratio - flattening filters flatten an accelerator beam at one depth. At depths deeper and shallower, the profiles are under or over-flattened - OAR's are measured from isodose charts or given in chart form

Field Shaping - Cast Blocks

- are custom made for each individual patient and are bolted onto the block tray - lipowitz metal (cerrobend is the brand name), alloy-158 used because of low melting point - are focused, or divergent edges, to avoid penumbra

Electron Blocks

- blocks are typically 1-2 cm thick, therefore no focused edges - electrons scatter easily (even in air) so it's necessary to collimate the beam close to the patient - applicator (cones): attachable to the linac that extends down close to the isocenter - electron cutouts = small block that shapes the field that are placed in the distal end of the electron applicator - shape of e- field is traced onto transparency or printed out at e- tray distance (if e- block distance is 95cm, field must be de-magnified to 95% of size at isocenter) - when electron blocks are made, the shape of the field is cut from styrofoam and poured around

Advantages of Multileaf Collimators (MLCs)

- castblocks are time-consuming, laborious and contain toxic metals - MLC shapes can be changed quickly in the treatment planning system and re-exported to the R&V system - no need to lift or store heavy blocks - beam transmission (intra-leaf) is reduced through MLCs (< 2.0%), although inter-leaf transmission is an issue - MLC's allow for the use of IMRT and VMAT

Simple Rotation Therapy

- gantry rotates during beam-on. Calculation depth constantly changes with contour - produces dose distributions that are very conformal in the axial plane - "past pointing" = used in partial arcs, because the maximum dose is displaced toward the irradiated sector - no longer routinely used since they create a cylindrical shape (do not taper at ends) -- replaced by static custom-shaped fields (exception is Brain SRS) -- IMRT and Tomotherapy are sophisticated extensions of simple rotational therapy - calculate average TAR by dividing contour into 24 equal segments - measure each radii, calculate TAR and average

Cerrobend Safety

- lead and cadmium vapors from melted alloy is a concern in block rooms. They are both toxic metals (blocks should be poured under exhaust vents) - ingestion of cerrobend is another concern - cerrobend blocking adds a tertiary level of beam collimation (sharper beam edges because diaphragm is closer to patient) *- block and tray must be at least 15 cm from skin surface to reduce contamination from 2 degree electrons generated from tray* (also, this provides clearance to avoid collisions

Field Shaping - Cerrobend Blocks

- melting point = 70 degrees C (158 degrees F); composed of 50% Bismuth, 26.7% Lead, 13.3% of Tin, and 10% of Cadium (BLT+C)

Superficial & Orthovoltage Blocks

- placed on the patient's skin surface - usually 1-2mm lead sufficient to block energy x-rays. A thin layer of wax should be placed behind the block to absorb 2 degree electrons scattered from the block - lead shields should be used in other places to protect normal tissue from 1 degree transmission or 2 degree scatter (example: behind the ear, in nasal cavity, over eyes, behind lip, or adjacent to teeth to block scatter from fillings) - 2 degree electrons can also produce backscatter, so shields should be covered on both sides with bolus material (wax)

Field Shaping - Hand Blocks

- prior to 1980, most irregular fields were shaped by placing lead bricks on a tray inserted in the path of a beam; rectangular blocks were often stacked to provide curved shapes - for lateral beams, loose blocks were attached to a tray with nuts and bolts; hand blocks were non-divergent and therefore caused penumbra - reproduciblity was very poor with hand blocks; the light field is used to position the block over the desired region of the patient's anatomy by looking at the shadow of the block on the patient - blocks are typically 5 HVL, or HVT, or about 3% (0.5^5 = 0.0313)

Clarkson's Method

- used to estimate the dose on CAX within an irregularly shaped field, or at points away from CAX of any field - primary and scatter components of dose are computed separately and then added together, using TAR0 and SAR tables - TAR for 0x0 field represents primary beam only (no scatter). TAR or TMR for any other field size represents primary and scatter (difference is scatter component or SAR) - scatter of any field is determined by measuring distances every 15 degrees from point to edge of field and looking up SAR values from chart (average SAR total SAR/24)

Cerrobend attenuates 85% of lead's attenuation, so blocks must be _______ thicker than lead for same attenuation. Blocks are usually 7.5 cm thick (3").

15%

The MLC leaf width, projected to the isocenter, ranges from:

3mm to 1 cm

The tray attenuates typically ______ or less (use the tray transmission factor AKA tray factor)

5%

The number of MLC leaves ranges from:

52-160

MLCs replace the x-jaws in Siemens and are a tertiary field-shaper (below the jaws) in:

Varian Linacs (tertiary limits clearance)

Film must be at correct TFD and block tray must be at correct tray distance or ____________ ___________ will be wrong

beam divergence

Multileaf Collimators (MLCs)

motorized tungsten leaves which replace castblocks they are rounded or focused

Electron Cutouts

small block that shapes the field that are placed in the distal end of the electron applicator

Blocks are traced on a block-cutting machine and cut out of:

styrofoam

MLCs: Interleaf Transmission

transmission between the leaves = ~3% (reduced by tongue-and-groove arrangement)

MLCs: Intraleaf Transmission

transmission through a leaf = 2%

MLCs limit the:

usable field size

MLCs: Overtravel

when MLC leaves travel over midplane (2-10 cm past iso) (allow for MLC use in mono-isocentric setups)