Underestimation of Low-Dose Radiation in Treatment Planning of Intensity-Modulated Radiotherapy
Received 22 September 2007; received in revised form 26 March 2008; accepted 7 April 2008. published online 30 May 2008.
Purpose
To investigate potential dose calculation errors in the low-dose regions and identify causes of such errors for intensity-modulated radiotherapy (IMRT).
Methods and Materials
The IMRT treatment plans of 23 patients with lung cancer and mesothelioma were reviewed. Of these patients, 15 had severe pulmonary complications after radiotherapy. Two commercial treatment-planning systems (TPSs) and a Monte Carlo system were used to calculate and compare dose distributions and dose–volume parameters of the target volumes and critical structures. The effect of tissue heterogeneity, multileaf collimator (MLC) modeling, beam modeling, and other factors that could contribute to the differences in IMRT dose calculations were analyzed.
Results
In the commercial TPS-generated IMRT plans, dose calculation errors primarily occurred in the low-dose regions of IMRT plans (<50% of the radiation dose prescribed for the tumor). Although errors in the dose–volume histograms of the normal lung were small (<5%) above 10 Gy, underestimation of dose <10 Gy was found to be up to 25% in patients with mesothelioma or large target volumes. These errors were found to be caused by inadequate modeling of MLC transmission and leaf scatter in commercial TPSs. The degree of low-dose errors depends on the target volumes and the degree of intensity modulation.
Conclusions
Secondary radiation from MLCs contributes a significant portion of low dose in IMRT plans. Dose underestimation could occur in conventional IMRT dose calculations if such low-dose radiation is not properly accounted for.
Department of Radiation Physics, The University of Texas M. D. Anderson Cancer Center, Houston, TX
Reprint requests to: H. Helen Liu, Ph.D., Department of Radiation Physics, Unit 94, The University of Texas M. D. Anderson Cancer Center, 1515 Holcombe Blvd., Houston, TX 77030. Tel: (713) 563-2546; Fax: (713) 563-2545
Supported in part by National Cancer Institute grant R01-CA074043-08A2.
Presented at the 49th Annual Meeting of the American Association of Physicists in Medicine, July 22–26, 2007, Minneapolis, MN.
ABSTRACT