Radiobiological Comparison of 3D Conformal Proton Therapy and Intensity Modulated Proton Therapy (IMPT) for Metastatic Brain Cancer
M Sullivan1, H Jin2, S Ahmad1*, (1) University of Oklahoma-Health Sciences Center, Oklahoma City, OK, (2) Baylor Scott & White Health, Temple, TX,
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PO-GePV-T-122 (Sunday, 7/10/2022) [Eastern Time (GMT-4)]
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Purpose: To compare radiobiological characteristics of three proton therapy treatment approaches: 3D conformal proton therapy (3DCPT), static aperture intensity modulated proton therapy (IMPTs), and dynamic aperture IMPT (IMPTd).
Methods: Ten patients with brain malignancies were chosen, all of whom had treatment plans created for each of the mentioned modalities. The parameters used in this comparison were equivalent uniform dose (EUD), tumor control probability (TCP), and normal tissue complication probability (NTCP). Variables chosen and varied to observe their effect on these parameters were α/β ratio, SF2, and clonogenic cell density.
Results: Average EUD relative to prescription dose for 3DCPT plans: 1.024, for IMPTs: 1.039, and for IMPTd: 1.04. Average TCPs for each modality were found for 3DCPT: 99.5%, IMPTs: 99.6%, and IMPTd: 99.7%. Average NTCPs for each modality were found as follows, with α/β set at 2; for brainstem; 3DCPT: <1%, IMPTs <0.1%, IMPTd <0.01%. For left optic nerve (LON); 3DCPT: <0.0001%, IMPTs <0.0001%, IMPTd <0.001%. For right optic nerve (RON); 3DCPT: ~0%, IMPTs <1%, IMPTd ~0%. For optic chiasm (OC); 3DCPT: <0.001%, IMPTs: <0.01%, IMPTd: <0.01%. With α/β set at 6, for brainstem; 3DCPT: <1%, IMPTs: <0.1%, IMPTd: <0.01%. For LON; 3DCPT: ~0%, IMPTs: <1%, IMPTd: <0.0001%. For RON; 3DCPT: ~0%, IMPTs: <1%, IMPTd: <0.1%. For OC; 3DCPT: <0.0001%, IMPTs < 0.001%, IMPTd: <0.001%.
Conclusion: IMPT presents a nominal advantage in localized tumor control over the conformal alternative, however statistically these differences are negligible. Normal tissue complication was observed to be lower, on average, in numbers in conformal treatments when focusing on the optic chiasm and optic nerves. However, in the context of the brainstem, IMPT was observed to be superior. Making these comparisons of NTCP, though relative between modalities, was very difficult due to wide distributions and very low values of NTCP for all modalities.
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