DOS 523 - Week 5 Discussion
- Choose an article from a previous issue of the Medical Dosimetry Journal that interests you and connects to the week five lectures (IMRT, IGRT, Tomo).
- Choose two web sites (give the URLs) that provide useful information about the concepts of the journal article.
- Write 2-3 paragraphs that describe why you chose that topic, article, and web sites.
- Write one question that came to your mind when completing the assignment.
Initial Post: Evaluating IMRT vs VMAT vs Tomotherapy
The Medical Dosimetry journal contains a wealth of useful papers on a variety of topics. One that caught my eye is a three way comparison of Intensity Modulated Radiation Therapy (IMRT), Volumetric Modulated Arc Therapy (VMAT), and helical tomotherapy by Lee et al.1 These techniques can be thought of as cousins, since they share a common concept of modulating dose across a cross sectional profile by actively controlling the positions of multilead collimators (MLCs). The primary difference between the techniques is that IMRT uses a set of fixed angle beams, while both VMAT and tomotherapy deliver dose while the treatment head is in motion around the patient. VMAT treats an entire volume in one or more coplanar arcs, while tomotherapy uses a fan beam that delivers dose to thin sections of the target while the table moves through the machine, resulting in the dose being delivered along a helical path that spirals around the patient many times.
The techniques were compared by Lee et al based on dose conformality to the target, dose to organs at risk (OR), total monitor units (MUs) used for delivery, and length of treatment delivery time. The study used 10 patients who had stage III or IV nasopharyngeal carcinomas. Tomotherapy was found to have better dose conformality that linac-based techniques. Among the OR structures evaluated, both VMAT and tomotherapy each scored clear wins on some but not others. VMAT was the clear winner for the least number of MUs used for delivery, in most cases beating tomotherapy by an order of magnitude. VMAT was also the clear winner on treatment time.
To expand on the analysis, I went in search of additional articles to expand on the comparison. I found two articles: one in a scholarly publication and one in the popular press, that were useful and relevant. The scholarly article by Oliver et al is also a comparison of IMRT, VMAT, and tomotherapy.2 It can be found at http://www.jacmp.org/index.php/jacmp/article/view/3068/1740. This article is interesting because it uses phantom datasets rather than real patient data to evaluate planning time, delivery time, and plan quality. Adding the dimension of planning time is interesting because it has a direct impact on staffing levels needed to support the machines. The time required for IMRT planning on the phantom studies averaged 7.5 minutes, VMAT needed 48 minutes, and tomotherapy needed 59 minutes. This paper also addressed plan quality, based on ability to meet dose volume histogram (DVH) constraints, radiation conformity index, dose homogeneity within the target volume, integral dose, and low dose exposure as measured by the volume receiving 2 Gy and 5 Gy. This is quite a different set of evaluations than the first paper. The conclusion was that IMRT can be planned much more quickly and can deliver the lowest integral dose; VMAT can be delivered the fastest and has the best dose conformation to the target volume, and tomotherapy has the best ability to avoid ORs and also has the best uniformity of dose across the target volume. I find it interesting that there is no clear winner, since each patient's needs are unique and what is best for one patient may not be the best for the next patient. I also find it interesting that the planning time findings by Oliver et al contradict those from Lee et al, who anecdotally thought that TomoTherapy planning was the fastest of the three techniques.1 Planning times were not formally measured by Lee et al due to lack of experimental controls for that factor, whereas Oliver et al incorporated time as part of the experiment.2
The popular press article by Bolan can be found at http://www.itnonline.com/article/vmat-vs-tomotherapy. It focuses on a comparison of VMAT to tomotherapy with a nod to IMRT.3 It mentions the technical differences between the techniques and addresses dose conformality, but it adds a dimension that the other scholarly articles missed. Instead of simply reporting differences in treatment times, it tries to explain why shorter treatment times are useful. Workflow efficiency is certainly one aspect, but motion management is another key area neglected by the other articles. The longer a patient stays on a treatment table, the higher the chance that they may become uncomfortable and begin moving. Furthermore, for organs that may move due to body processes such as digestion, shorter times between setup and the end of delivery will reduce the effects of intrafraction motion of organs.
The logical question that comes up upon review of these articles is "which one is best"? As already mentioned, there doesn't seem to be a clear winner between VMAT and tomotherapy because each has relative merits that the other doesn't have. IMRT is also not a clear loser, since it has advantages in planning simplicity and integral dose. Standard linear accelerators may have had an advantage over a TomoTherapy system in the past because moderns linacs can perform VMAT, IMRT and 3D conformal treatments while TomoTherapy systems used to only be able to perform tomotherapy treatments. That argument is disappearing now that Accuray is offering TomoDirect,4 which is a delivery mode that allows TomoTherapy systems to treat with static fields as well as rotational fields. It is nice to see that competition is a rising tide that lifts all boats.
- Lee FK, Yip CW, Cheung FC, Leung AK, Chau RM, Ngan RK. Dosimetric difference amongst 3 techniques: TomoTherapy, sliding-window intensity-modulated radiotherapy (IMRT), and RapidArc radiotherapy in the treatment of late-stage nasopharyngeal carcinoma (NPC). Med Dosim. 2014;39(1):44-9. http://dx.doi.org/10.1016/j.meddos.2013.09.004
- Oliver M, Ansbacher W, Beckham WA. Comparing planning time, delivery time and plan quality for IMRT, RapidArc and Tomotherapy. J Appl Clin Med Phys. 2009;10(4):3068. http://dx.doi.org/10.1120/jacmp.v10i4.3068
- Bolan CC. VMAT vs tomotherapy. Imaging Technology News Website. http://www.itnonline.com/article/vmat-vs-tomotherapy. Published June 2, 2010. Accessed March 25, 2015.
- TomoDirect. Accuray Website. http://www.accuray.com/solutions/treatment-delivery/tomotherapy-treatment-delivery/tomodirect. Accessed March 25, 2015.