Coordinates, Movement of Radiotherapy Machines: Varian IEC 601-2-1

July 24, 2010 · Posted in Equipment, Treatment Planning · 5 Comments 

You can move the table, collimator, XY jaws, and gantry in the right direction without entering the treatment room if you familiarize yourself with the IEC scale convention of your machine. I have mostly used Varian machines during my career in medical physics and recently commissioned a Varian Trilogy linear accelerator at our cancer center. During commissioning, it became necessary (for productivity’s sake) to move the jaws, table, gantry and collimator from outside treatment room. Normally, Varian machines conform to either the Varian standard, Varian IEC (601-2-1) or IEC1217. Our Trilogy coordinates, movements and scales follow the Varian IEC (601-2-1) scale. Therefore, here are a few tips on this Varian IEC 601-2-1 scale that I generated during the commissioning of my machine. For a better understanding, these “tips” are explained in question and answer format. However, since some of us may not know what IEC stands for, here is a short paragraph taken from the IEC website before proceeding to the main topic:

IEC (International Electronic Commissioning) is the world’s leading organization that prepares and publishes International Standards for all electrical, electronic and related technologies.  Wherever you find electricity and electronics, you find the IEC supporting safety and performance, the environment, electrical energy efficiency and renewable energies.  The IEC also manages conformity assessment systems that certify that equipment, systems or components conform to its International Standards.

Out of the four components (the couch, jaws, collimator and gantry), we cover the couch and the gantry in this post and will discuss the jaws and the collimator in next post.

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The answer is 4.5 cm.
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The answer is 998.5 cm.
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The answer is 8.5 cm.
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The answer is 994.5 cm.
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The answer is 79 cm.
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The answer is 55 cm.
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The answer is 25 degrees.
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The answer is 105 degrees.

Couch: Vertical Movement
The couch vertical is at 8.5 cm. You need to move the table 4 cm upward toward the ceiling. At what do you set the new vertical position of the table from outside the treatment room: 12.5 cm or 4.5 cm?
Click on this link for the answer. Depending on your screen resolution, you may have to scroll up to see the answer window.
The couch vertical is at 2.5 cm. You need to move the table 4 cm upward toward the ceiling. At what do you set the vertical position of the table from outside the treatment room: 6.5, -1.5 or 998.5 cm?
Click on this link for the answer.
Tip: Table vertical position is at 0 degrees at iso-center. Moving the couch toward the floor down from iso-center, the number increases on this coordinate axes. If you move the couch toward the ceiling up from iso-center, the number decreases from 1000.

Couch: Lateral Movement
The couch lateral position is at 4.5 cm. You need to move the table 4 cm toward the right of the gantry if you face the gantry. What do you set the new lateral position of the table from outside the treatment room: 8.5 cm or -8.5 cm?
Click on this link for the answer.
The couch lateral position is at 2.5 cm. You need to move the table 8 cm toward left of the gantry if you face the gantry. What do you set the new lateral position of the table from outside the treatment room: -5.5 cm or 994.5 cm?
Click on this link for the answer.
Tip: Table lateral position is at 0 degrees at iso-center. When moving the couch toward the right (facing the gantry) from iso-center, the number increases on this coordinate axes. If you move the couch toward left (facing the gantry) from iso-center, the number decreases from 1000.

Couch: Longitudinal Movement
The couch longitudinal is at 55 cm. You need to move the couch 24 cm toward the gantry. What do you set the new longitudinal position of the table from outside the treatment room: 79 cm or 31 cm?
Click on this link for the answer.
The couch longitudinal is at 105 cm. You need to move the couch 50 cm away from the gantry. What do you set the new longitudinal position of the table from outside the treatment room: -55 cm or 55 cm?
Click on this link for the answer.
Tip: Table longitudinal Position increases if the couch moves towards the gantry and decreases if the couch moves away from the gantry.

Gantry
The gantry is at 115 degrees. You need to move gantry 90 CCW. What do you set the position of the gantry from outside the treatment room: -25, 25, 205, or 335 degrees.
Click on this link for the answer.
The gantry is at 15 degrees. You need to move gantry 90 CW. What do you set the position of the gantry from outside the treatment room: -75, 105, or 255 degrees.
Click on this link for the answer.
Tip: The gantry on the Varian IEC 601-2-1 scale is at 0 degrees when the gantry is up at vertical position. The degree increases as gantry rotates CW. It goes to 90 degrees when the gantry is to the right (facing the gantry), to 180 degree when gantry is down at vertical position, to 270 degree when gantry is to the left (facing the gantry) and increases as it rotates CW to 359.9 and then 0 degree up at vertical position. Reversely, the angle decreases as gantry rotates CCW.

References:

  1. Varian resources
  2. S C Lillicrap et. al. “Radiotherapy equipment standards from the International Electrotechnical Commission” BJR 71 (1998), 1225-1228.
  3. Correspondense – BJR 72 (1999), 623
  4. Radiotherapy equipment-Coordinates, movements and scales. AS?NZS 4495:1997, IEC 1217:1996
  5. http://www.iec.ch/

Streak Artifact Reduction and Enhancement of Pelvic CT Image in a Patient with Hip Replacement

February 28, 2010 · Posted in Treatment Planning · 1 Comment 

Our Radiation Oncologist recently showed me a pelvis CT image of a male patient who had undergone bilateral hip replacement with forged titanium alloy implants. The patient had early stage prostate cancer, and the doctor wanted to treat this patient with IMRT. The difficulty was to delineate the prostate, seminal vesicles and other organs at risk because of the streak artifacts in the CT images due to high-Z material in the patient. The question came up about how this kind of artifact can be removed or minimized so that the target organ, as well as the organs at risk and other organs can be delineated for treatment planning. I did some research and was able to come up with some good articles on this topic. I am summarizing my findings below for those who are interested to learn or as a reference for those who may have such cases in the clinic.

Streak artifacts in CT images are generated in conventional CT when implanted objects of high atomic number exist in the patient. The artifact and image degradation associated with the kilovoltage (kV) CT imaging in the presence of high atomic number material greatly hinders the ability to delineate tumors and certain organs, particularly in the treatment planning of a prostate patient with hip prostheses. Such a situation, therefore, precludes precise dose calculation. There are several techniques reported that, if used, can minimize such artifacts, thereby enhancing image visualization for the delineation of tumor and other organs.

1- Charmley et al. (1) suggested that the use of CT-MR image registration to define target volumes in pelvic radiotherapy in the presence of bilateral hip replacements could facilitate target definition of prostate patient with hip replacements. However, a number of factors were found to affect image quality and/or the accuracy of target definition. The standard MR couch, different from a CT or linac treatment couch, might result in different patient positions, and the presence of the metallic implants may create significant distortion.

2- Yazdia M. (2) suggested an adaptive approach to metal artifact reduction in helical computed tomography for radiation therapy planning. At that time, they may require manual image post-processing and most CT scanners available in radiation oncology department are not equipped with these features.

3- The artifact image and degradation associated with the kilovoltage (kV) CT imaging in the presence of high atomic number material is greatly reduced with Megavoltage Cone Beam Computed tomography (MV-CBCT). MV-CBCT has been used in image-guided radiotherapy (IGRT) to correct patient setup immediately before treatment. Hansen et al (3) used this technique to treat paraspinous tumors in the presence of orthopedic hardware. It allows rapid acquisition of 3D images that can be registered with the planning CT with millimeter precision and enhance image visualization by exploiting the predominantly Compton scattering of high-energy photons delivered in the MV-CBCT system. Aubin et al. (4) of the Department of Radiation Oncology at UCSF did a study with the support of Siemens Oncology Care systems on the use of Megavoltage Cone Beam CT to complement CT for target definition in pelvic radiotherapy in the presence of hip replacement. They found the MV-CBCT image could be used to clearly visualize the hip prostheses and provide sufficient soft-tissue contrast to help delineate the prostate, bladder and rectum. The artifacts on the kV CT obscure the border between the prostate and anterior wall of the rectum and the interface between the prostate base and the bladder neck. However, the MV-CBCT images were particularly useful to help delineate these structures as well as the lateral extension of the prostate in the axial plane, the seminal vesicles and the lymph nodes. Also, normal anatomy such as pelvic bones, penile bulb, bladder, femoral heads, rectum and small bowel can be delineated with higher accuracy as well. They evaluate this technique for seven patients. For each patient, the MV-CBCT images were imported into the treatment planning system and registered with the original CT using body anatomy contoured on each image set. The target volumes and organs at risk for prostate treatment were contoured using both the CT and the MV-CBCT for single hip replacement, and using only the MV-CBCT for bi-lateral hip prostheses. For the full article, click on: http://bjr.birjournals.org/cgi/reprint/79/947/918

The following two figures taken from Aubin M. at el (4) show the difference between conventional CT and MV-CBCT images:

Picture 7

Picture 8

  1. Chamley N. et al. The use of CT-MR image registration to define target volumes in pelvic radiotherapy in the presence of bilateral hip replacements. BJR 2005; 78:634-636.
  2. Yazdia M. et al. An adaptive approach to metal artifact reduction in helical computed tomography for radiation therapy planning: experimental and clinical studies. Int. J. Radiation Oncol Biol Physics 2005; 62(4): 1224-1231.
  3. Hansen, E.K. et al. Image guided radiotherapy using Megavoltage Cone-Beam Computer Tomography for treatment of paraspinous tumors in the presence of orthopedic hardware. Int. J. Radiation Oncol Biol Physics 2006; 66(2): 323-326.
  4. Aubin M. at el. Use of Megavoltage Cone-Beam CT to complement CT for target definition in pelvic radiotherapy in the presence of hip replacement. Short Communication: British Journal of Radiology 2006.

Prostate Seed Implant, IMRT or SBRT for Prostate Cancer Treatment?

July 26, 2009 · Posted in Treatment Planning · 1 Comment 

Recently, a dear friend of mine, a scientist, called and told me he had been diagnosed with early stage prostate cancer and needs to undergo radiation therapy for his treatment. His physician had gave him three options: prostate seed implant (PSI), Intensity Modulated Radiation Therapy (IMRT), and Stereotactic Body Radiation Therapy (SBRT) via Cyberknife. He asked me for advice. I gave him my thoughts, but didn’t recommend one particular option over another; I told him he should make his final decision with his physician. Regarding PSI versus IMRT, I referred him to my post on mdphysics.com without getting into the specifics over the phone. I also brought up the same point left by a recent commenter (medphysphd) that uncertainty in treatment planning exists with all the treatment modalities. However, the human error will be higher in PSI than other modalities. Regarding choosing IMRT over SBRT/Cyberknife (or vice-versa), below is a summary of what I told my friend. I am also posting this here as a response to the recent comment left by a reader inquiring about this very issue: Read more

List of DVH Limits

March 23, 2009 · Posted in Treatment Planning · 1 Comment 

Nathan Childress has put together a list of DVH limits (with preference given to RTOG limits, when available). The file is called “Dose-volume constraints for OARs” and is in Excel format on the medphysfiles.com website:

http://www.medphysfiles.com/index.php?name=Downloads&file=details&id=4

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