Journal of the National Cancer Institute Advance Access originally published online on August 26, 2008
JNCI Journal of the National Cancer Institute 2008 100(17):1265-1266; doi:10.1093/jnci/djn235
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© The Author 2008. Published by Oxford University Press.
CORRESPONDENCE |
Response: Re: Intensity-Modulated Radiation Therapy Dose Prescription, Recording, and Delivery: Patterns of Variability Among Institutions and Treatment Planning Systems
Affiliations of authors: Department of Radiation Oncology, University of Pennsylvania, Philadelphia, PA (IJD, EG); Morristown Memorial Hospital, Morristown, NJ (CWC); Kennedy Health System, Sewell, NJ (KLC); Ochsner Clinic Foundation, New Orleans, LA (RKM); Reid Hospital & Health Care Service, Richmond, IN (SPS)
Correspondence to: Indra J. Das, Radiation Oncology, University of Pennsylvania, 2 Donner Bldg, 3400 Spruce St, Philadelphia, PA 19104 (e-mail: das{at}xrt.upenn.edu).
We thank Olch and Lavey, Deye et al., Galvin et al., and Chien et al. for their interest in our paper (1) and their willingness to be provoked into a response. The discussion of some of the issues raised is healthy, and it is worthwhile to challenge the assumptions inherent in this topic. We agree that both clinical significance and achievability should be carefully considered in developing the dose–volume specifications used in clinical trials, and that is exactly what we mentioned in the Discussion section of our article [(1); paragraph 2, lines 6–10].
Contrary to what Olch and Lavey have implied, we did not endorse the use of any single point dose but rather the use of the entire dose–volume histogram. On the other hand, it is an undeniable fact that the ICRU-50 (2) has put a lot of emphasis on the reporting of isocenter dose for three-dimensional conformal radiotherapy (3D-CRT). Unfortunately, as we have pointed out, the isocenter dose is simply not meaningful in IMRT. Thus, in addition to the dose–volume histograms of all relevant structures, it is prudent to find an alternate dose point that can be used for the plan reporting purposes. In light of the variation shown in figure 1 of our article, we suggested that the median dose could be a relevant dose to report in plan comparisons until a better parameter is supported by the clinical findings and advocated by national and/or international guidelines.
We thank Deye et al. and Galvin et al. for their comments and note that we acknowledged the contribution of the RTOG to IMRT, as clearly stated on page 305 in our article (1). Rather than providing the entire dose–volume histogram for 803 patients, we chose only the maximum, minimum, medium, and isocenter doses for our analysis. Picking a maximum dose point at 100%, 99%, 95%, or 90% and a minimum dose point at 10%, 5%, 1%, or 0% of the target volume from the dose–volume histogram is purely arbitrary unless defined and recommended by certain guidelines, as is clearly stated on pages 302–303 in our article (1) and as is also pointed out by Galvin et al. We are aware of various abstracts, a letter to the editor (3), and the contributions of the Advanced Technology Consortium (http://atc.wustl.edu) concerning the variability in clinical trials. It is obvious that there are no concrete guidelines that could be applied to IMRT, as stated by Engler and Rivard (4) and acknowledged by Deye et al. We disagree with Galvin et al. that 3D-CRT and IMRT provide similar dose inhomogeneity. As a result of the inverse planning process, dose variability in IMRT is rather large and dependent on dose–volume constraints, the grid size, the treatment planner, the planning system, and the algorithms used. Certainly, it is another topic for research, and Galvin et al. could provide such data through the RTOG.
Chien et al. point out several issues that require clarification regarding the planning target volume (PTV), the clinical target volume (CTV), and the impact of set-up error in fractionated radiation therapy. Our data did not specify PTV or CTV; instead, we reported the dose deviation from prescription either to PTV or CTV depending on the specification of physicians in each institution. We did not alter the dose reporting solely to PTV, and hence, the interpretation by Chien et al. is not applicable to our study. The second question about set-up error and its impact is also irrelevant in the context of our study. It is well known that additional smearing of the dose–volume histogram takes place with fractionated therapy and increases the dose variation. The RTOG-0615 protocol specifies that at least 95% of the planning target volume receiving 70 Gy (PTV70) is covered by the 70-Gy isodose surface, that no less than 20% of PTV70 receives 
77 Gy, that no more than 5% of PTV70 receives 80.5 Gy, and that no more than 1% of any distinct PTV70 receives 93% of the prescribed dose, which directly supports our findings in terms of dose variability. Following these criteria for PTV coverage may lead to even wider variation in the maximum and minimum dose to the PTV than what we have reported.
In conclusion, we thank the authors of the correspondences on our article for expressing their concerns, comments, and views about the imperfections associated with IMRT. We believe that figures 1–3 in our article (1) appropriately display the central problem in IMRT plan comparison; certainly guidelines to fix the problem may be forthcoming.
NOTES
Present address: Department of Radiation Oncology, Director of Medical Physics, Indiana University School of Medicine, Indianapolis, IN (I. J. Das).
REFERENCES
1. Das IJ, Cheng CW, Chopra KL, Mitra RK, Srivastava SP, Glatstein E. Intensity-modulated radiation therapy dose prescription, recording delivery: patterns of variability among institutions and planning systems. J Natl Cancer Inst (2008) 100(5):300–307.
2. ICRU 50. Prescribing, Recording, and reporting Photon Beam Therapy. (1993) Bethesda, MD: International Commission on Radiation Units and Measurements. ICRU Report 50.
3. Palta JR, Deye JA, Ibbott GS, Purdy JA, Urie MM. Credentialing of institutions for IMRT in clinical trials. Int J Radiat Oncol Biol Phys. (2004) 59(4):1257–1259.[CrossRef][Web of Science][Medline]
4. Engler MJ, Rivard MJ. Credentialing of institutions for IMRT in clinical trials; in response to Palta et al. Int J Radiat Oncol Biol Phys. (2004) 59(4):1259–1261.[CrossRef][Web of Science]
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