Objective  To explore the parameter setting of whole brain radiotherapy (WBRT) plan using a helical tomotherapy (TOMO) machine to protect the hippocampus. To meet the clinical evaluation index, multi parameter analysis and dosimetric research were conducted to obtain the optimal value and range of the basic parameter setting of a radiotherapy plan.

Methods  The clinical data of 10 brain tumor patients who underwent WBRT at Shanxi Provincial Cancer Hospital from January 2017 to November 2022 were retrospectively analyzed. Among them, four males and six females were identified with an average age of (60.0±8.1) years and an age range of 42–74 years. Radiotherapy plans for protecting the hippocampus were designed for all patients, within the limited range of the maximum dose (D_max) to the hippocampus, D_max of critical organs, gantry period, and beam on time. In accordance with the parameter requirements of TOMO machine, a modulation factor (MF) of 2.0, 2.3, 2.5, 2.7, 3.0, and 4.0; field widths (FW) of 2.5 and 5.0 cm; and pitch of 0.143, 0.287, and 0.430 were selected for planning. The effects of MF, FW, and pitch on the hippocampus sparing plans were analysised. A paired sample t-test was used to compare the corresponding indicators of FW, and analysis of variance was used to compare the corresponding indicators of MF and pitch.

Results  MF=2.7, and pitch=0.287, in addition, FW was set to 2.5 and 5.0 cm. Statistically significant differences in beam on time ((508.90±34.44) s vs. (281.47±19.77) s), planned target volume (PTV)-D_max ((32.90±1.28) Gy vs. (34.14±1.20) Gy), left hippocampus D_max ((15.21±0.53) Gy vs. (15.88±0.94) Gy), and brainstem D_max ((33.69±0.23) Gy vs. (35.09±0.43) Gy) were found (t=39.756, 11.226, 2.467, 11.269; all P<0.05). The FW=2.5 cm is better at protecting the hippocampus. FW=2.5 cm, and pitch=0.287, MF was set to 2.0, 2.3, 2.5, 2.7, 3.0, 4.0. Statistically significant differences in gantry period ((17.91±0.96), (20.18±0.70), (21.61±0.73), (23.28±0.63), (25.82±1.20), and (34.54±1.22) s) and beam on time ((376.77±26.53), (432.58±27.73), (466.64±31.42), (508.90±34.44), (554.10±44.70), and (750.63±50.91) s) of TOMO were found (F=677.482, 209.716; both P<0.001). MF was set to 2.0, 2.3, 2.5, 3.0, and 4.0. D_max of the right hippocampus (18.19, 18.35, 18.83, 18.75, and 18.80 Gy) of one patient was exceeded the limit. The gantry period was (34.54±1.22) s at MF=4.0, which exceeds the optimal gantry period of TOMO. At MF=2.0, the left and right lens D_max of another patient were 9.07 and 10.30 Gy, respectively, which exceeded the limit. Therefore, the best MF value is 2.7. MF=2.7, FW=2.5 cm, pitch=0.143, 0.287 and 0.430, the gantry period is (11.81±0.03), (23.28±0.63) and (29.59±0.47) s, the difference in corresponding gantry period is statistically significant (F=3 939.056, P<0.001). At pitch=0.143, the gantry period is (11.81±0.03) s ,which is not the best gantry period. The difference in beam on time ((518.80±26.09), (508.90±34.44), and (431.37±25.44) s) corresponding to different pitches was statistically significant (F=26.889, P<0.001). Some patients with pitch of 0.143 and 0.430 exceeded the limit of hippocampus D_max. Overall, pitch=0.287 is more appropriate for the TOMO hippocampus sparing plans.

Conclusion  The recommended parameters are MF=2.7, FW=2.5 cm, and pitch=0.287 in TOMO for hippocampus sparing.