Out-of-field doses from radiotherapy using photon beams: A comparative study for a pediatric renal treatment

International audiencePurpose: First, this experimental study aims at comparing the normal tissue doses delivered by three radiotherapy techniques using photon beams (3DCRT, VMAT (with two different accelerators), and tomotherapy) for a typical paediatric renal treatment. Secondly, the accuracy of commercial treatment planning systems (TPS) for the assessment of normal tissue doses is evaluated.Methods: EBT3 films rigorously calibrated were positioned in two anthropomorphic phantoms representing children aged 5 and 10 years old. Those phantoms were irradiated according to one 3DCRT plan (Clinac 2100CS, Varian), two VMAT plans (Clinac 2100CS and Halcyon, Varian) and one tomotherapy plan optimized for a same tumour volume. 3-D dose determination was performed with an in-house Matlab tool using linear interpolation of film measurements. The doses were compared between techniques in 1-D, 2-D and 3-D. Finally, measured doses were also compared to the Eclipse™ (Varian Medical System) and Tomotherapy (Accuray) TPS dose calculations. Results: Advanced radiotherapy techniques (VMATs and tomotherapy) deliver higher out-of-field doses compared to 3DCRT. The differences increase with distance to target and reach a factor of 3 between VMAT and 3DCRT. This increase in peripheral doses is due to increased beam-on time triggered by intensity modulation. Besides, tomotherapy delivers lower doses than VMAT. Indeed, although tomotherapy beam-on-time is higher than in VMAT, the additional shielding of the Hi-Art tomotherapy system reduces out-of-field doses. The Halcyon system, operating in FFF mode, proves to deliver lower peripheral doses than conventional accelerators. Regarding TPS calculation, the Tomotherapy TPS proves to be suitable for out-of-field dose determination up to 30 cm from field edge whereas Eclipse™ largely underestimates those doses.Conclusion: This experimental study shows that the high dose conformation allowed by advanced radiotherapy is done at the cost of higher doses delivered outside the treatment field border. In the context of treatment-related risk estimation, the consequence of this increase in peripheral dose might be significative. The modern systems require adapted head shielding to spare normal tissues and a particular attention has to be taken regarding on-board imaging dose. Finally, advanced dose calculation algorithms implemented in commercial TPS do not certify dose accuracy beyond treatment field edges and thus, those doses are not suitable for risk assessment