Image guidance technology and radiation therapy for localized prostate cancer

Prostate cancer patients who are to be treated with any type of intensity-modulated radiation therapy (IMRT) for localized prostate cancer today can be pardoned if they are confused by the many ways in which that radiation therapy can be aimed and targeted through the use of image guidance (IGRT) systems. The variety of methods is significant and growing.

Over the past decade, all of the following methods have been approved as methods to direct radiation therapy accurately to predefined conformations of a patient’s prostate:

  • Daily cone beam computed tomography
  • Ultrasound
  • Orthogonal x-ray systems that take advantage of implanted fiducial markers
  • Implanted radiofrequency (RF) markers that can localize localize and track motion of the prostate during radiotherapy (the Calypso 4D system)
  • Cinematic magnetic resonance imaging

Of course we have no good, long-term data to tell us whether any one of these methods produces better outcomes over time than any other. Individual radiotherapy practices make decisions on the type of image guidance to acquire based on the equipment they already have in place, the cost of adding on the image-guidance system (which is measured in hundreds of thousands of dollars), and the numbers of patients that they see (and therefore how fast they will be able to recover the initial acquisition costs of the guidance system they select).

A new article by Das et al. has provided the first critical review of the advantages, disadvantages, and limitations of each of the currently available image-guidance technologies, and of the existing literature relevant to comparison of existing types of image guidance. The full text of an article addressing some of the relevant issues is available on line and was first published in 2012 by Forster et al.

From the patient’s perspective, at this time, all we can really tell you is that if you decide to have IMRT as first-line treatment for localized prostate cancer, you do want to be sure that some form of image guidance is being used in conjunction with the radiation delivery system, and that the Calypso system is the only one that is actually able to track the positioning of the patient’s prostate during radiation delivery (which may be important because the prostate can and does move slightly all the time within the patient’s lower abdomen).

Since insurance companies and Medicare generally cover all the relevant costs of image guidance in association with IMRT, this should not be a major issue for most patients.

What you probably don’t want today as first-line radiation therapy for localized prostate cancer is the older three-dimensional, conformal radiation therapy (3D-CRT) that was “new” in the early to mid 1990s. 3D-CRT can still be appropriate for the treatment of larger areas of the pelvis in men who have locally advanced prostate cancer, but for localized cancer it simply isn’t as accurate as you should want if you are the patient, and it comes with too much risk for side effects that are now (largely) avoidable.

3 Responses

  1. It is not correct that “the Calypso system is the only one that is actually able to track the positioning of the patient’s prostate during radiation delivery.” Several systems are capable of tracking intra-fractional motion including Tomotherapy, CyberKnife, and Novalis Tx. The Novalis Tx system I was treated with was fitted with an ExacTrac X-Ray 6D System that operated intra-fractionally rather than just inter-fractionally. Inter-fractionally, I was aligned with 3D cone beam CT. One CyberKnife radiation oncologist I spoke to explained to me that, unlike CyberKnife, Calypso doesn’t track pitch or rotational motion, only translational motion. All of these SBRT systems claim sub-millimeter accuracy.

  2. I will take your word for it. This was not covered in the article I referred to, which is just one more indicator of the speed with which these systems are getting upgraded.

  3. Thanks for reporting this paper and providing the reference link. I’ve read it with layman’s eyes, though with skin in the game (literally) as today I completed my 20th of 39 sessions being delivered by a TomoTherapy system that includes gold fiducial seeds for the CT imaging part.

    While a fair amount of the technical detail was over my head, the main points were clear enough. As always, I appreciate your summary.

    The full report makes the point that the Calypso system uniquely offers target location information throughout each session (if desired by the therapy team), referred to as intrafaction (within fraction) motion, enabling adjustment of radiation delivery if needed, and the report documents there is slightly more target intrafraction uncertainty using the cone beam CT approach (used by TomoTherapy). Importantly, the report found the slightly increased uncertainty was not clinically significant, confirming the impression I had formed from my own research before making my final decision to have TomoTherapy.

    As the actual delivery of the set of radiation shots in each session is rapid, with just a fraction of a second between one shot and the next, and more than 500 shots delivered in the space of a few minutes, I’m curious about the practicality of adjustment of targeting by a therapist based on updated Calypso information about extremely small movements during each session. I’m trying to visualize how the therapist (or software) would notice the differences, appreciate the corrections needed, and implement the corrections. I trust that happens, but I’m having trouble seeing how it would work. My impression for my TomoTherapy sessions, in contrast, which do not use dynamic intrafaction imaging, is that the final adjustments are made after the cone beam CT scan and that the therapist just makes sure the whole system is functioning properly until the session is complete.

    The full report also mentions an advantage of cone beam imaging over Calypso: cone beam CT gives good detail regarding “organs of interest”, such as the rectum and bladder, enabling better minimization of unplanned doses to those organs; in contrast, Calypso does not provide information about the locations of the organs of interest. In fact there is a line that puzzles me at the end of the “conclusion” section. It says the Calypso system does not provide information about the target; obviously it provides location information, but what is it not providing?

    Thanks again for posting this.

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