Download IOM and IMRT - Heather L. Schultz

IMRT
 Goals:
o IMRT or Intensity Modulated Radiation Therapy is used to improve the
therapeutic ratio by reducing the toxicity of radiation while ensuring the
effective doses are delivered to the target volume.
o Another goal of IMRT is to increase tumor dose without increasing
morbidity or sacrificing tumor control
 What is it?
o A form of 3D conformal therapy where the shape of the field and angle
of the beam changes as the gantry rotates around the patient
o Radiation therapy where the intensity is modulated on a small scale
making it necessary to use inverse planning
o Requires computer controlled multi-leaf collimators with leaf sequences
that can be automated
o Allows for precise and accurate radiation treatments
 Types:
o Segmented or static field MLC Delivery (SMLC)
 Leaves are positioned and then the beam is initiated
 After the first beamlet is delivered, the beam turns off
 The accelerator then moves the leaves to the next position, turns
the beam on and off and so forth as it goes through each leaf
position until the treatment is complete
 The linac controls the beam on/off throughout the treatment
o Dynamic MLC Delivery (DMLC)
 The MLC leaves move through one beam on/off sequence
 The leaves of the MLC move during the delivery of the dose
(coining the term ‘sliding window’)
 After the dose of the first beam is delivered, the gantry moves to
the next position and treatment is delivered
o IMAT/VMAT Delivery
 MLC shapes field with the gantry is rotating
 Similar to step and shoot, however, the MLC shapes each subfield
while the gantry is rotation and the beam is on all the time
 Multiple overlapping arcs are delivered with leaves moving to new
positions and regular intervals. This creates the modulation of
fields at each beam angle
o Tomotherapy Delivery
 Consists of a linear accelerator mounted on a helical/spiral CT
scanner. It combines treatment planning, CT image-guided patient
positioning, and treatment delivery into one integrated system
 The DMLC generates IMB’s (intensity modulated beams) for 3D
treatment of the tumor slice by slice as the gantry rotates
longitudinal axis of the patient in a fan beam
 A 1mm error in couch positioning can cause a difference of up to
25% in junction dose
 Summary
o IMRT used to permit delivery of high doses of radiation while minimizing
dose to sensitive organs
o Multiple beams are focused on the target volume
 Each bema has segments and the intensity of each segment is
varied according to the treatment plan
VIDEO: medical animation of IMRT
http://www.youtube.com/watch?v=US7CtE2fGmA&feature=related
References
1. Khan FM. The Physics of Radiation Therapy. 4th ed. Philadelphia Lippencott Williams
& Wilkins; 2010: 430-451.
2. Washington CM., Leaver D. Principles and Practice of Radiation Therapy. 3rd ed.
Mosby, Inc; 2010: 329-345, 538.
IOM
Intrafractional organ movement is movement during treatment.
Research shows that internal organs, especially those located in the thorax and
abdomen tend to have significant inter and intrafractional organ motion.1 This
motion can greatly impact the localization of clinical target volumes.1
Ways to account for these motions is to utilize image guidance, such as Image
Guided Radiation Therapy IGRT, respiratory gating to obtain 4DCT data, or
internal implanted beacons to localize and trace interfraction and intrafraction
motion.2 These types of imaging and monitoring systems can allow for beam
interruption should the tumor move outside the path of the treatment beam.2
Monitoring internal target motion throughout the entire treatment process will
ensure adequate dose coverage of the target while sparing the maximum healthy
tissue, and further enabling dose escalation.1
Attached are two videos that illustrate internal organ movement, especially
during breathing. The second video does a good job of showing other organ
movement, and the direct effect on target volumes. These videos were obtained
from the physicist at my site.
References
1. Rietzel E, Rosenthal SJ, Gierga DP, Willet CG, Chen GT. Moving targets:detection and
tracking of internal organ motion for treatment planning and patient set-up. Radiother
Oncol. 2004 Dec;73 Suppl 2:S68-72. PubMed PMID:15971313.
2. Coleman AM. Principles and Practice of Radiation Therapy. 3rd ed. Mosby, Inc; 2010:
329-345, 169.