BLUE RIBBON POSTER THERAPY: Dosimetric Comparison of IMRT and VMAT on Circulating Immune Cell Depletion In Glioblastoma Radiotherapy
Abstract
Purpose
Radiation-induced lymphopenia (RiL) worsens survival in glioblastoma by impairing anti-tumor immunity. Reducing dose to circulating lymphocytes (CLs) may help mitigate RiL. Because IMRT and VMAT differ in their dynamic delivery time, they may alter blood exposure and CL depletion. We compared their dosimetric impact on CLs during brain radiotherapy
Methods
We retrospectively analyzed 28 GBM patients treated with either IMRT or VMAT. The CTV volume was 296.4±150.3 cm³. For each patient treated with IMRT, an alternative VMAT plan was created, and vice versa for patients treated with VMAT. The Replanning used the identical dose constraints as the corresponding clinical plans. A 4D dosimetric blood-flow model simulated dynamic beam delivery (gantry rotation/segment timing), and blood circulation to estimate the cumulative dose to CLs for both the clinical and the alternative plan. A cerebrovascular model was used to simulate the trajectories of blood particles through the irradiated field in the brain. Primary endpoints were total blood volume receiving any non-zero dose and fraction of CLs depleted at end of each treatment. Paired comparisons were performed between IMRT and VMAT.
Results
In patients with smaller CTVs (100 cm³), IMRT irradiated 14.0% ± 5.4% more CLs volume. Our simulations indicated that by the third treatment fraction, IMRT had irradiated >90% of the total CLs versus 77% with VMAT. The estimated mean depletion of CLs was significantly higher for IMRT (13.9 ± 6.4%) than VMAT (12.8 ± 5.6%), with a mean difference of 1.0 ± 1.7%, and CI95% [0.3, 1.7], and a paired t-test, p = 0.003).
Conclusion
With respect to RiL, our model found that VMAT is preferable for GBM patients with CTV >100 cm³, while IMRT is more beneficial for smaller CTVs.