Technical Delivery Parameters of 2000 Proton Treatment Courses

Abstract

Purpose: To summarize the technical delivery parameters of proton plans delivered at the Mayo Clinic in Rochester, Minnesota.

Materials and methods: The database of treated patient proton plans was queried to extract field parameters such as gantry angle, patient support angle, minimum and maximum water-equivalent depth (WED) treated, number of layers, field size, patient orientation, and monitor units. The plans were analyzed in aggregate, by disease site, and by fractionation.

Results: There were 2963 proton plans for 2023 distinct treatment sites delivered between June 2015 and September 2018. The mean number of fields per plan was 2.8. The mean number of energy layers per field was 51.9. The mean monitor unit per field was 117.4. The median maximum field dimension was 12.4 cm; 95% of the fields had a maximum dimension < 28.7 cm, and the maximum field dimension was 39.8 cm. The median maximum field WED was 16.4 cm; 95% of the fields reached a maximum WED of ≤ 26.4 cm, and the maximum field WED was 32.4 cm.

Conclusion: A large variety of disease sites were treated using the maximum field size (40 cm) and WED (32.4 cm) capabilities of our half-gantry system.

Keywords: gantry; pencil-beam scanning; proton therapy.

Figure 1.

(Left) Differential gantry angle, the percentage of fields treating an anatomic site with a given gantry angle, binned by 5°. (Right) Cumulative gantry angle, the percentage of fields treating an anatomic site with a gantry angle less than or equal to given value. In the coordinates of the Mayo Clinic half gantry, an AP field has gantry angle of 0°, whereas a PA field has gantry angle of 180°. Steep segments, such as 90° for pelvis and CNS, indicate frequent use of gantry angles. Abbreviations: AP, anterior-posterior; CNS, central nervous system; PA, posterior-anterior.

Figure 2.

Target WED, the maximum WED (corresponding to the highest energy) treated by each field on the x-axis and the minimum WED treated on the y-axis. The high density of points with min(WED) = 0 cm in the breast figure indicates most fields treat the skin surface. The pelvis figure shows a cluster of deep targets with max(WED) > 20 cm that are relatively small—max(WED) − min(WED) ≈ 10 cm, as would be expected for a plans treating prostate cancer. Abbreviation: WED, water-equivalent depth.

Figure 3.

(Left) Differential maximum WED, the percentage of fields treating an anatomic site with a given maximum WED, binned by 2 cm. (Right) Cumulative maximum WED, the percentage of fields treating an anatomic site with a maximum WED treated less than the given value. The WED was calculated from the maximum energy of the field with corrections for any range-shifting devices employed. Abbreviation: WED, water-equivalent depth.

Figure 4.

Field size 2D, the width and length of each field, with the largest dimension always on the x-axis of the figure. The clusters of points in the lower left of the CNS, breast, and pelvis figures indicate small fields that are almost square in the beam's eye view. The breast figure has a second grouping of points in its upper right, indicating large nearly square fields. The long, narrow, spinal craniospinal fields are grouped in the CNS figure in the lower right with max(dimension) ≈ 25 cm and min(dimension) < 10 cm. Abbreviations: 2D, 2-dimensional; CNS, central nervous system.

Figure 5.

(Left) Differential maximum field dimension, the percentage of fields treating an anatomic site with a given maximum dimension, binned by 2 cm. (Right) Cumulative maximum field dimension, the percentage of fields treating an anatomic site with a maximum dimension less than the given value. The maximum field dimension is 30 cm × 40 cm, although the field is limited to 25 cm × 30 cm with the use of the extended range shifter.