2. NEED OF MLC
• To achieve the aim of radiotherapy
• Maximum dose to tumor and minimum dose to OARs
• Shaping of the beam is an important way of minimizing the
absorbed dose in healthy tissue and critical structures.
3. How to shape beam
• Collimator jaws are used for shaping a regular treatment
field; but the treatment volume is not regular
• Lead blocks or individually made Cerrobend blocks are
attached onto the treatment head under standard
collimating system.
4. MLC
A multileaf collimator (MLC) is a device made up of individual
"leaves" of a high atomic numbered material, usually
tungsten, that can move independently in and out of the path
of a beam in order to block it.
6. • Tungsten alloy
• Pure tungsten has a density of 19.3 g/cc and brittle
• alloys have densities that range from 17.0 to 18.5 g/cc3, with
varying mixtures of nickel, iron, and copper to improve
machinability.
9. UPPER JAW REPLACEMENT
• This configuration entails splitting the upper jaw into a set
of leaves (Elekta)
10. ADVANTAGE
Range of motion of the leaves required to traverse the
collimated field width is smaller, allowing for a shorter leaf
length and therefore a more compact treatment head
diameter.
11. Disadvantage
•Leaf width must be somewhat smaller and the tolerances on
the dimensions of the leaves as well as the leaf travel must be
tighter than for other configurations.
•Average radiation leakage increase
32. • Limit switches:
• are used in bi-state MLCs such as that developed by NOMOS,Inc.
• The open or closed state can be detected depending on which switch is turned
on by the leaf.
• Linear Encoders:
• high precision potentiometers.
• Two potentiometers are used with correlated readings.
• The advantages of using linear encoders include simple read-out, less
susceptibility to radiation damage, and good linearity and accuracy.
• The drawbacks are more wiring in the head structure, and more occupied space
in the head. Since defective potentiometers are sometimes hard to detect,
redundant ones are often required to ensure correct measurements.
34. • The advantages of the video-optical system include real-
time display of the leaf positions, less wiring, and high
spatial resolution.
• When using solid-state devices such as charge-coupled
device (CCD) cameras, the system also provides high
position linearity.
• However, most CCDs are not radiation resistant. Frequent
camera replacements are therefore required.
37. DYNAMIC MLC
Leaf A Leaf B
Position
Intensity
Continuous modulation
Delivered dose with
dMLC is directly dependent
on the precision of the gap
between opposed leaves.
