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 been introduced in our department for treatment of the head and neck region with the intention of reducing complications without compromising treatment outcome. However, these new treatment modalities inevitably require a substantial increase in monitor units per target dose yielding an increased risk of secondary malignancies induced by the treatment. This study aims at assessing the increased risk by means of in vivo measurements of the whole- body equivalent dose of both the conventional and the IMRT treatment techniques for head and neck lesions. Material and methods: A conventional technique using parallel opposed, wedged treatment fields has been compared with a slice-by-slice arc rotation technique for IMRT. Both techniques were used to treat head and neck lesions with a 6-MV photon beam. Thermoluminescent badges and neutron bubble detectors designed for personnel monitoring have been applied to obtain the estimated whole-body equivalent dose on three patients for each treatment technique. The nominal probability coefficient for a lifetime risk of excess fatal 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 conventional and IMRT technique, respectively. Applying the average amount of MU necessary to realize a 70 Gy target dose the estimated whole-body equivalent dose for both treatment techniques becomes 242 mSv (conventional) and 1969 mSv (IMRT), yielding an increase in the risk for secondary malignancies with a factor 8. Conclusions: Historically the risk of secondary malignancies has been accepted to take advantage of the possible benefits of improved local control and treatment outcome. However, the introduction of new and sophisticated treatment techniques will also increase the risk of radiation induced malignancies. Therefore, these risk estimates become important to assess whether the benefits of the treatment technique outweigh the possible risks.