A single-field integrated boost treatment planning technique for spot scanning proton therapy

Abstract

Purpose: Intensity modulated proton therapy (IMPT) plans are normally generated utilizing multiple field optimization (MFO) techniques. Similar to photon based IMRT, MFO allows for the utilization of a simultaneous integrated boost in which multiple target volumes are treated to discrete doses simultaneously, potentially improving plan quality and streamlining quality assurance and treatment delivery. However, MFO may render plans more sensitive to the physical uncertainties inherent to particle therapy. Here we present clinical examples of a single-field integrated boost (SFIB) technique for spot scanning proton therapy based on single field optimization (SFO) treatment-planning techniques.

Methods and materials: We designed plans of each type for illustrative patients with central nervous system (brain and spine), prostate and head and neck malignancies. SFIB and IMPT plans were constructed to deliver multiple prescription dose levels to multiple targets using SFO or MFO, respectively. Dose and fractionation schemes were based on the current clinical practice using X-ray IMRT in our clinic. For inverse planning, dose constraints were employed to achieve the desired target coverage and normal tissue sparing. Conformality and inhomogeneity indices were calculated to quantify plan quality. We also compared the worst-case robustness of the SFIB, sequential boost SFUD, and IMPT plans.

Results: The SFIB technique produced more conformal dose distributions than plans generated by sequential boost using a SFUD technique (conformality index for prescription isodose levels; 0.585 ± 0.30 vs. 0.435 ± 0.24, SFIB vs. SFUD respectively, Wilcoxon matched-pair signed rank test, p < 0.01). There was no difference in the conformality index between SFIB and IMPT plans (0.638 ± 0.27 vs. 0.633 ± 0.26, SFIB vs. IMPT, respectively). Heterogeneity between techniques was not significantly different. With respect to clinical metrics, SFIB plans proved more robust than the corresponding IMPT plans.

Conclusions: SFIB technique for scanning beam proton therapy (SSPT) is now routinely employed in our clinic. The SFIB technique is a natural application of SFO and offers several advantages over SFUD, including more conformal plans, seamless treatment delivery and more efficient planning and QA. SFIB may be more robust than IMPT and has been the treatment planning technique of choice for some patients.

Figure 1

Representative isodose distributions on axial CT images for patient 1. (A) SFIB, (B) SFUD sequential boost and (C) IMPT treatment plans for a central glioma. Targets and normal tissues are represented as color washes. (D) Dose volume histogram for corresponding plans. Solid lines correspond to SFIB, dashed lines to SFUD, and dotted lines to IMPT. Exposure of right and left hippocampi to low and high doses is reduced for SFIB and IMPT plans. (E) Comparison of measured (data points) and calculated (blue lines) depth dose curves for each of three beams employed in the treatment of patient 1. Error bars are fixed values of 3% in the vertical axis and 2 mm in the horizontal axis to help visualizing the difference between measurements and calculations.

Figure 2

Representative isodose distributions on sagittal CT images for patient 2. (A) SFIB and (B) SFUD treatment plans for a spinal ependymoma. Targets and normal tissues are represented as color washes. (C) Dose volume histogram for corresponding plans (solid lines, SFIB; dashed lines, SFUD).

Figure 3

Representative axial isodose distributions for treatment of a prostatic adenocarcinoma. Targets and normal tissues are represented as color washes. Slices depicting treatment of CTV2 (seminal vesicles) as well as CTV1 (prostate) are shown in left and right panels for SFIB (A), SFUD with sequential boost (B) and IMPT (C) plans. The dose volume histogram is shown in (D) (solid lines, SFIB; dashed lines, SFUD and dotted lines, IMPT).

Figure 4

Representative axial, coronal and sagittal (left, middle, and right panels, respectively) CT images for a patient 4. (A) SFIB, (B) SFUD with sequential boost and (C) IMPT treatment plans. Targets volumes are represented as color washes. (D) Dose volume histogram (solid lines, SFIB; dashed lines, SFUD and dotted line, IMPT).

Figure 5

Worst case robustness comparison of SFUD, SFIB and IMPT/MFO plans. The left column (blue) represents SFUD, the middle (red), SFIB, and the right (green), IMPT plans. For target volumes, the cold plans are used to evaluate target coverage in terms of percentage of doses of D₉₅, D₉₈ and EUD relative to the prescription doses. For critical structures, the hot plans are used assess the tissue sparing. (A) and (B) for patient 1 (BS – brain stem, HC – hippocampus, OC – optical chiasm, PH – pituitary and hypothalamus, WB – whole brain); (C) and (D) for patient 2 (SC – spinal cord, LK – left kidney, RK – right kidney); (E) and (F) for patient 3 (BLD – bladder, RET – rectum, FH – femoral heads); and (G) and (H) for patient 4 (LNX- larynx, RS – right submandibular, RC – right cochlea, and MDB – mandible).

Figure 6

Example of dose distributions of SFIB and IMPT for patient 4 in color wash to demonstrate the dose inhomogeneity of individual fields of an IMPT plan. (A) SFIB and (B) IMPT for all fields; (C) SFIB and (D) IMPT for field one; and (E) SFIB and (F) IMPT for field two.