Risk assessment of radiation-induced malignancies based on whole-body equivalent dose estimates for IMRT treatment in the head and neck region

Background and purpose: Intensity modulated radiation therapy (IMRT) has be en introduced in our department for treatment of the head and neck region w ith the intention of reducing complications without compromising treatment outcome. However, these new treatment modalities inevitably require a subst antial increase in monitor units per target dose yielding an increased risk of secondary malignancies induced by the treatment. This study aims at ass essing the increased risk by means of in vivo measurements of the whole-bod y equivalent dose of both the conventional and the IMRT treatment technique s for head and neck lesions. Material and methods. A conventional technique using parallel opposed, wedg ed treatment fields has been compared with a slice-by-slice are rotation te chnique for IMRT, Both techniques were used to treat head and neck lesions with a 6-MV photon beam. Thermoluminescent badges and neutron bubble detect ors designed for personnel monitoring have been applied to obtain the estim ated whole-body equivalent dose on three patients for each treatment techni que, The nominal probability coefficient for a lifetime risk of excess fata l cancer, recommended by the ICRP 60 has been used for risk estimates based on the estimated dose values. Results: An estimated whole-body equivalent dose per monitor unit equal to 1.2 x 10(-2) mSv/MU and 1.6 x 10(-2) mSv/MU have been obtained with the con ventional and IMRT technique, respectively. Applying the average amount of MU necessary to realize a 70 Gy target dose the estimated whole-body equiva lent dose for both treatment techniques becomes 242 mSv (conventional) and 1969 mSv (IMRT), yielding an increase in the risk for secondary malignancie s with a factor 8. Conclusions: Historically the risk of secondary malignancies has been accep ted to take advantage of the possible benefits of improved local control an d treatment outcome. However, the introduction of new and sophisticated tre atment techniques will also increase the risk of radiation induced malignan cies. Therefore, these risk estimates become important to assess whether th e benefits of the treatment technique outweigh the possible risks. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.