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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.