Poster — Thur Eve — 31: Optimum Frequency of Spatial Registration in Image Guided Radiation Therapy for TMI

Purpose: To assess the influence of the frequency of spatial registration in total marrow irradiation using Helical Tomotherapy to correct for intrafraction variations. Methods: The analysis was performed using TCP and NTCP models in a phantom study. Different cases were investigated: one treatment per fraction (no‐junction), and splitting the treatment along Y‐axis (SUP‐INF direction) into 2 or 3 independent sub‐treatments (one/two‐junction), which allows 2 or 3 spatial registrations per treatment fraction. Linearly increasing margins were added to expand the CTV (ribs and spine) and OARs (lungs) with increasing distance from the registration point (iPTV and iPRV). Margin increases of 5, 10 and 15 mm were analyzed. The prescription was for 95% of the iPTV to receive 20 Gy. Results: The dose to the iPRVs was reduced when the treatment was split; D20 to lung was 22, 16.2 and 13.4 Gy for no‐junction, one‐junction and two‐junction cases respectively. However the dose received by the iPTV also decreased; the volume of the iPTV receiving the prescription was 93.9%, 90.5% and 88.4% respectively. The probability of uncomplicated cure TCP(1‐NTCP) increased 3% from no‐junction to two‐junction cases for a maximum margin of 5 mm. The corresponding difference for maximum margin of 15 mm was 22%. Larger margins combined with splitting the treatment to perform additional patient registrations showed benefit, mainly due to lower NTCP. Conclusion: Increasing uncertainties can be reduced by splitting large treatments and realigning the patient prior to each sub‐treatment. This technique reduces the NTCP and marginal misses of the PTV.