A proton beam therapy system dedicated to spot-scanning increases accuracy with moving tumors by real-time imaging and gating and reduces equipment size

Abstract

Purpose: A proton beam therapy (PBT) system has been designed which dedicates to spot-scanning and has a gating function employing the fluoroscopy-based real-time-imaging of internal fiducial markers near tumors. The dose distribution and treatment time of the newly designed real-time-image gated, spot-scanning proton beam therapy (RGPT) were compared with free-breathing spot-scanning proton beam therapy (FBPT) in a simulation.

Materials and methods: In-house simulation tools and treatment planning system VQA (Hitachi, Ltd., Japan) were used for estimating the dose distribution and treatment time. Simulations were performed for 48 motion parameters (including 8 respiratory patterns and 6 initial breathing timings) on CT data from two patients, A and B, with hepatocellular carcinoma and with clinical target volumes 14.6 cc and 63.1 cc. The respiratory patterns were derived from the actual trajectory of internal fiducial markers taken in X-ray real-time tumor-tracking radiotherapy (RTRT).

Results: With FBPT, 9/48 motion parameters achieved the criteria of successful delivery for patient A and 0/48 for B. With RGPT 48/48 and 42/48 achieved the criteria. Compared with FBPT, the mean liver dose was smaller with RGPT with statistical significance (p<0.001); it decreased from 27% to 13% and 28% to 23% of the prescribed doses for patients A and B, respectively. The relative lengthening of treatment time to administer 3 Gy (RBE) was estimated to be 1.22 (RGPT/FBPT: 138 s/113 s) and 1.72 (207 s/120 s) for patients A and B, respectively.

Conclusions: This simulation study demonstrated that the RGPT was able to improve the dose distribution markedly for moving tumors without very large treatment time extension. The proton beam therapy system dedicated to spot-scanning with a gating function for real-time imaging increases accuracy with moving tumors and reduces the physical size, and subsequently the cost of the equipment as well as of the building housing the equipment.

Figure 1. RGPT system at Hokkaido University.

(a) The gantry of the passive scattering PBT system. (b) The gantry of the RGPT system. (c) The footprint for one gantry and one fixed beam system with a linear accelerator and synchrotron, and (d) The actual RGPT system installed.

Figure 2. Design of a spot-scanning proton beam therapy-dedicated system with X-ray fluoroscopy.

Two orthogonal sets of X-ray fluoroscopic generators and flat panels can be mounted in the gantry.

Figure 3. CT images and target delineations.

Computed tomography for the patients with hepatocellular carcinoma included in this study. Gross tumor volumes (red), clinical target volumes (green), liver (blue), and planning target volumes for the RGPT (white) for patient A (left) and patient B (right).

Figure 4. Tumor motions obtained with RTRT system.

Eight patterns of tumor motion derived from actual data of internal fiducial markers near hepatocellular carcinomas in the X-ray RTRT of 8 patients.

Figure 5. Diagram of the (a) synchrotron operation and (b) beam waiting function.

The operation cycle of the synchrotron varies approximately from 2 to 7

Figure 6. Comparison of dose distribution between FBPT and RGPT.

Images of the dose distributions with FBPT for PTV_fb(left), FBPT for PTV_rg (center), and RGPT for PTV_rg (right) for the CT images of patient B with tumor motion ID of b.