Effect of Different Spot Spacings On the Dose Homogeneity and Penumbra in Proton Therapy
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PO-GePV-T-173 (Sunday, 7/10/2022) [Eastern Time (GMT-4)]
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Purpose: The literature has documented how dose uniformity is affected by spot spacing in pencil beam scanning proton therapy. Those studies relate the ideal spacing to the spot size. Within the TPS, beam models are calculated from user supplied in-air spot size measurements. The same measurements are used in setting spot spacing parameters during optimization. We investigate how spot spacing based on in-medium spot size at the Bragg peak affects dose uniformity in a simple one-dimensional (1D) dose profile.
Methods: We compared dose profiles with uniformly weighted spots by calculating their flatness (ICRU 78). Dose profiles were 200 mm in width and calculated for low (69.4 MeV), medium (140.8 MeV), and high (221.3 MeV) energy spots. We wrote an optimization algorithm to adjust spot weighting to achieve optimal dose uniformity and penumbra width. The optimization was run for progressively increasing spot spacings.
Results: The difference in spot size measured in-air versus in-water increased as a function spot energy. No difference was observed for 69.4 MeV and sigma differences of 1.0 mm and 4.8 mm were observed for 140.8 MeV and 221.3 MeV, respectively. Loss in profile flatness was observed between 4 mm and 5 mm spacing for 221.3 MeV based on in-air data (0.0063 and 0.0307, respectively), while similar values were held through 11 mm and 12 mm spacings for in-water data (0.0057 and 0.0076, respectively). Results from optimized weightings were heavily influenced by user chosen objective costs and inconclusive for this limited scope experiment.
Conclusion: Spot spacing should be chosen carefully to allow for uniform dose distributions without an excessive number of spots that could lead to undeliverable, low MU spots. Our results show that increased spacing at higher energies can achieve similar uniformities with fewer total spots. Analysis on three-dimensional dose distributions in treatment planning systems is warranted.
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