Purpose Empirical relative biological effectiveness (RBE) models have been used to estimate the biological dose in proton therapy but do not adequately capture the factors influencing RBE values for treatment planning. We reformulate the McNamara RBE model such that it can be added as a linear biological dose fidelity term within our previously developed sensitivity‐regularized and heterogeneity‐weighted beam orientation optimization (SHBOO) framework. Methods Based on our SHBOO framework, we formulated the biological optimization problem to minimize total McNamara RBE dose to OARs. We solve this problem using two optimization algorithms: FISTA (McNam‐FISTA) and Chambolle‐Pock (McNam‐CP). We compare their performances with a physical dose optimizer assuming RBE = 1.1 in all structures (PHYS‐FISTA) and an LET‐weighted dose model (LET‐FISTA). Three head and neck patients were planned with the four techniques and compared on dosimetry and robustness. Results Compared to Phys‐FISTA, McNam‐CP was able to match CTV [HI, Dmax, D95%, D98%] by [0.00, 0.05%, 1.4%, 0.8%]. McNam‐FISTA and McNam‐CP were able to significantly improve overall OAR [Dmean, Dmax] by an average of [36.1%,26.4%] and [29.6%, 20.3%], respectively. Regarding CTV robustness, worst [Dmax, V95%, D95%, D98%] improvement of [−6.6%, 6.2%, 6.0%, 4.8%] was reported for McNam‐FISTA and [2.7%, 2.7%, 5.3%, −4.3%] for McNam‐CP under combinations of range and setup uncertainties. For OARs, worst [Dmax, Dmean] were improved by McNam‐FISTA and McNam‐CP by an average of [25.0%, 19.2%] and [29.5%, 36.5%], respectively. McNam‐FISTA considerably improved dosimetry and CTV robustness compared to LET‐FISTA, which achieved better worst‐case OAR doses. Conclusion The four optimization techniques deliver comparable biological doses for the head and neck cases. Besides modest CTV coverage and robustness improvement, OAR biological dose and robustness were substantially improved with both McNam‐FISTA and McNam‐CP, showing potential benefit for directly incorporating McNamara RBE in proton treatment planning.

Purpose The combined use of Bragg peak (BP) and shoot‐through (ST) beams has previously been shown to increase the normal tissue volume receiving FLASH dose rates while maintaining dose conformality compared to conventional intensity‐modulated proton therapy (IMPT) methods. However, the fixed beam optimization method has not considered the effects of beam orientation on the dose and dose rates. To maximize the proton FLASH effect, here, we incorporate dose rate objectives into our beam orientation optimization framework. Methods From our previously developed group‐sparsity dose objectives, we add upper and lower dose rate terms using a surrogate dose‐averaged dose rate definition and solve using the fast‐iterative shrinking threshold algorithm. We compare the dosimetry for three head‐and‐neck cases between four techniques: (1) spread‐out BP IMPT (BP), (2) dose rate optimization using BP beams only (BP‐DR), (3) dose rate optimization using ST beams only (ST‐DR), and (4) dose rate optimization using combined BP and ST (BPST‐DR), with the goal of sparing organs at risk without loss of tumor coverage and maintaining high dose rate within a 10 mm region of interest (ROI) surrounding the clinical target volume (CTV). Results For BP, BP‐DR, ST‐DR, and BPST‐DR, CTV homogeneity index and Dmax were found to be on average 0.886, 0.867, 0.687, and 0.936 and 107%, 109%, 135%, and 101% of prescription, respectively. Although ST‐DR plans were not able to meet dosimetric standards, BPST‐DR was able to match or improve either maximum or mean dose in the right submandibular gland, left and right parotids, constrictors, larynx, and spinal cord compared to BP plans. Volume of ROIs receiving greater than 40 Gy/s (Vγ0)${V_{\gamma 0}})$ was 51.0%, 91.4%, 95.5%, and 92.1% on average. Conclusions The dose rate techniques, particularly BPST‐DR, were able to significantly increase dose rate without compromising physical dose compared with BP. Our algorithm efficiently selects beams that are optimal for both dose and dose rate.

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PurposeThe main objective of this study was to develop a technique to accurately determine the air gap between the end of the proton beam compensator and the body of the patient in proton radiotherapy.MethodsOrthogonal x‐ray image‐based automatic coordinate reconstruction was used to determine the air gap between the patient body surface contour and the end of beam nozzle in proton radiotherapy. To be able to clearly identify the patient body surface contour on the orthogonal images, a radiopaque wire was placed on the skin surface of the patient as a surrogate. In order to validate this method, a Rando® head phantom was scanned and five proton plans were generated on a Mevion S250 Proton machine with various air gaps in Varian Eclipse Treatment Planning Systems (TPS). When setting up the phantom in a treatment room, a solder wire was placed on the surface of the phantom closest to the beam nozzle with the knowledge of the beam geometry in the plan. After the phantom positioning was verified using orthogonal kV imaging, the last pair of setup kV images was used to segment the solder wire and the in‐room coordinates of the wire were reconstructed using a back‐projection algorithm. Using the wire as a surrogate of the body surface, we calculated the air gaps by finding the minimum distance between the reconstructed wire and the end of the compensator. The methodology was also verified and validated on clinical cases.ResultsOn the phantom study, the air gap values derived with the automatic reconstruction method were found to be within 1.1 mm difference from the planned values for proton beams with air gaps of 85.0, 100.0, 150.0, 180.0, and 200.0 mm. The reconstruction technique determined air gaps for a patient in two clinical treatment sessions were 38.4 and 41.8 mm, respectively, for a 40 mm planned air gap, and confirmed by manual measurements. There was strong agreement between the calculated values and the automatically measured values, and between the automatically and manually measured values.ConclusionsAn image‐based automatic method has been developed to conveniently determine the air gap of a proton beam, directly using the orthogonal images for patient positioning without adding additional imaging dose to the patient. The method provides an objective, accurate, and efficient way to confirm the target depth at treatment to ensure desired target coverage and normal tissue sparing.