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Radiobiological Evaluation of the Three Dimensional Conformal Radiation Therapy, SmartArc, and Helical Tomotherapy Treatment Techniques for Pediatric Cranio-Spinal Axis Irradiation

P Myers*, S Stathakis, P Mavroidis, A Gutierrez, C Esquivel, W Jones, T Eng, C Ha, N Papanikolaou, University of Texas HSC SA, San Antonio, TX

SU-E-T-308 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose:
Currently, radiotherapy treatment planning acceptance is based primarily on dosimetric plan performance and concerns. However, using radiobiological analysis to assess the benefit of tumor control while limiting injury to normal tissues can be greatly advantageous. For pediatric CSI patients in particular, knowing the technique that will optimize the benefit versus injury probability can lead to greater long-term outcomes.

Methods:
Twenty-four CSI pediatric patients (median age 10) were retrospectively planned with three techniques: three-dimensional conformal radiation therapy (3D-CRT), SmartArc (SA), and helical tomotherapy (HT). Each plan was normalized to 95% of target volume (whole brain and spinal cord) receiving prescription dose 23.4 Gy in 13 fractions. Using an in-house Matlab code and the DVH data from each plan, the three techniques were evaluated based on biologically equivalent uniform dose (BEUD), probability of controlling the tumor without causing severe injury to normal tissues (P₊), and optimal BEUD and P₊.

Results:
Overall, 3D-CRT and SA plans had similar values of BEUD (24.1 and 24.2 Gy) while HT had a BEUD slightly lower (23.6 Gy). P₊ percentages were 64.7, 67.3, and 56.7% on average for 3D-CRT, SA, and HT plans respectively. The optimal values for BEUD were 28.5, 33.0, and 32.2 Gy for the 3D-CRT, SA, and HT plans, respectively. Although the actual P+ values were lower for HT, choosing a more optimal BEUD prescription level yields an optimal P₊ of 94.3, 99.6, and 99.4% for 3D-CRT, SA, and HT, respectively.

Conclusion:
If the optimal dose level is prescribed using a radiobiological calculation method as opposed to purely dosimetric, the two IMRT techniques, SA and HT, will yield the greatest overall benefit to the CSI patient by maximizing tumor control and limiting normal tissue injury. Using SA or HT, may provide these pediatric patients with better long-term outcomes after radiotherapy.

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