KEY POINTS
- The paired in-silico analysis included 28 glioma patients, predominantly with glioblastoma, with four astrocytomas and one oligodendroglioma. Twelve had been treated clinically with IMRT and 16 with VMAT; every patient was replanned using the alternative technique with identical planning objectives.
- All plans prescribed 60 Gy, with target requirements of D95 ≥57 Gy and D10 ≤63 Gy, and were judged clinically acceptable by two radiation oncologists. IMRT used 5–8 beams with up to 25 step-and-shoot segments per beam, whereas VMAT used 1–3 arcs.
- A four-dimensional circulation model propagated approximately 20.7 million blood particles per patient at 0.001-second temporal resolution while reproducing each technique's beam timing, dose rate, gantry motion and MLC delivery. This allowed dose to circulating cells to be accumulated rather than approximated from static brain DVHs.
- After one fraction, 49.9% ± 7.2% of circulating blood received some radiation with IMRT versus 38.6% ± 6.3% with VMAT, an absolute difference of 11.3 percentage points (P<.00001). IMRT exposed more blood in 24 of 28 paired plans.
- Exposure accumulated rapidly with fractionation: after three fractions, approximately 90% of circulating blood had been exposed with IMRT versus 77% with VMAT. The 95% exposure level was reached after about four fractions with IMRT and seven with VMAT.
- Despite the clear exposure difference, predicted cell depletion differed little: 13.9% with IMRT versus 12.8% with VMAT, a mean paired difference of only 1.0 percentage point (95% CI 0.3–1.7; P=.003). VMAT produced lower modeled depletion in 23/28 patients (82.1%).
- A possible interaction with target size was exploratory: the technique difference changed by approximately 0.41 percentage points per 100 cm³ of CTV (P≈.04), but the estimated crossover occurred below the observed target-volume range. Crucially, serial patient lymphocyte counts were unavailable, and the model does not include temozolomide, baseline blood counts or disease-related immune effects.
CLINICAL TAKEAWAY
VMAT's shorter continuous delivery exposed a smaller fraction of circulating blood than segmented IMRT, despite its wider low-dose bath. But the modeled difference in actual immune-cell depletion was only about one percentage point, so these data do not justify choosing VMAT over IMRT solely to prevent treatment-related lymphopenia.