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):1263; doi:10.1093/jnci/djn234
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© The Author 2008. Published by Oxford University Press.
CORRESPONDENCE |
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, Childrens Hospital Los Angeles, and University of Southern California, Los Angeles, CA
Correspondence to: Arthur Olch, PhD, Childrens Hospital Los Angeles and University of Southern California, Department of Radiation Oncology, 4650 Sunset Blvd, MS #54, Los Angeles, CA 90027 (e-mail: aolch{at}chla.usc.edu).
The recent article by Das et al. (1) questions the validity of comparing outcome data for patients treated with intensity-modulated radiation therapy (IMRT) treatments across multiple centers because of the inherent heterogeneity of the dose distribution. Their data supporting this assertion consist of the minimum, maximum, median, and isocenter point doses for 803 patients who were treated at five centers. We believe that the spread of the data shown in figures 1–3 of their article misrepresents the clinically relevant variations in IMRT treatments because the dose to any single point does not characterize the treatment dosimetry, as stated both by Das et al. (1) and in the accompanying editorial (2). It would have been better for Das et al. to have evaluated the IMRT treatment plans using the readily accessible dose–volume parameters provided by all of the treatment planning systems included in their study instead of point doses. Dose–volume parameters are widely used in radiotherapy clinical trials to ensure treatment consistency across patients and institutions and are routinely used in the radiotherapy community to assess radiation dose distributions.
As indicated in the editorial (2), a common characterization of the dose distribution is that "95% of the target volume received 100% of the prescribed dose." National Cancer Institute–sponsored clinical trials that employ IMRT are required to specify the dose in a similar manner. These cooperative group clinical trial protocols also limit the proportion of the target volume that may receive a dose that is substantially greater than that prescribed. The Radiation Therapy Oncology Group protocol 0435 for head and neck cancer (available at: http://www.rtog.org/members/protocols/0435/0435.pdf), for example, specifies that no more than 5% of the planning target volume should receive more than 79 Gy. The protocols also require that the treatment plan meet designated dose–volume criteria for all critical structures. Taken together, these dose specification requirements in clinical trials that employ IMRT permit patient treatments and study outcomes to be evaluated and compared. Certainly, it would be undesirable to follow Das et al.'s suggestion that these dose–volume specifications be replaced with the median dose.
Both clinical significance and achievability were carefully considered in developing the dose–volume specifications used in current radiotherapy clinical trials. Radiotherapy centers in the United States that employ IMRT are generally aware of and follow the treatment site–specific dose–volume guidelines that have been developed by leading treatment centers and listed in clinical trial protocols rather than any of the point dose parameters used by Das et al. This is made feasible by the dose–volume analysis tools provided in all commercial treatment planning systems.
We agree with Das et al. that nationwide guidelines for IMRT dose prescription and reporting are important. However, the guidelines should consist of multiple dose–volume parameters, such as the percentage of the target volume receiving the prescribed dose, the volume receiving a specified dose above the prescribed dose, and the percentage of critical structure volumes receiving a certain dose, rather than Das et al.'s suggestion of median dose. Pooled data analyses can readily assess the mean and standard deviation of these parameters, permitting the generation of highly meaningful radiotherapy dose variability and outcomes data. We believe that the variability found would have been considerably less if Das et al. had analyzed their data using dose–volume parameters instead of point doses.
REFERENCES
1. Das IJ, Cheng C-W, Chopra KL, et al. Intensity-modulated radiation therapy dose prescription, recording, and delivery: patterns of variability among institutions and treatment planning systems. J Natl Cancer Inst. (2008) 100(5):300–307.
2. Willins J, Kachnic L. Clinically relevant standards for intensity-modulated radiation therapy dose prescription. J Natl Cancer Inst. (2008) 100(5):288–290.
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J Natl Cancer Inst 2008 100: 1265-1266.
J Natl Cancer Inst 2008 100: 1266-1267.
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