Re: Intensity-modulated radiation therapy dose prescription, recording, and delivery: patterns of variability among institutions and treatment planning systems.

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 het-erogeneity 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 for example, specifi es 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 specifi cation 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 specifi cations be replaced with the median dose. Both clinical signifi cance and achiev-ability were carefully considered in developing the dose – volume specifi cations used in current radiotherapy clinical trials. Radiotherapy centers in the United States that employ IMRT are generally aware of and follow the treatment site – specifi c …