Innovations in Radiotherapy Technology

Purpose
Cancer radiation therapy treatment is performed by delivering a 3D dose distribution to the tumor via the relative rotation between beam and patient. While most modern machines rotate the radiation beam around a still patient, the treatment can also be delivered by rotating the patient relative to a fixed beam. Fixed-beam, patient rotation radiotherapy machines show promise for reducing the size, surface area footprint, and shielding requirements compared with rotating gantry machines. In this Technical Note, we describe the development of a bespoke horizontal patient rotation system for the purposes of a fixed-beam cancer radiotherapy architecture.

Methods
A horizontal Patient Rotation System was designed in accordance with the appropriate standards pertaining to performance and safety of medical electrical equipment and medical linear accelerators (ISO 9001, IEC 60601-1, IEC 60601-2-1, ISO 14971, ISO 13485, 21CFR820, IEC 62304, Machinery Directive 98/37/EC). The principal criteria for the design were safety, patient comfort, real-time control and the ability to be integrated with other radiation therapy componentry (including a linear accelerator and kV imaging systems).

Results
A first of its kind device for securing, immobilizing, translating, and rotating patients has been designed and built and tested against 161 different design, safety, and usability specifications. The device has real-time control for all critical applications.

Conclusions
We designed and built a bespoke device which can translate and rotate patients 360° around a horizontal axis. The device meets all design and safety criteria with early usability tests indicating a high degree of comfort and utility. The system has been installed in a clinical bunker, integrated with a fixed-beam linear accelerator and is currently being commissioned for the purposes of cancer radiotherapy treatment.

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Technical Note: The design and function of a horizontal patient rotation system for the purposes of fixed-beam cancer radiotherapy.

Purpose Compared to conventional linacs with rotating gantries, a fixed-beam radiotherapy system could be smaller, more robust and more cost-effective. In this work, we developed and commissioned a prototype x-ray radiotherapy system utilizing a fixed vertical radiation beam and horizontal patient rotation. Methods The prototype system consists of an Elekta Synergy linac with gantry fixed at 0° and a custom-built patient rotation system (PRS). The PRS was designed to immobilize patients and safely rotate them about the horizontal axis. The interlocks and emergency stops of the linac and PRS were connected. Custom software was developed to monitor the system status, control the motion of the PRS and modify treatment plans for the fixed-beam configuration. Following installation, the prototype system was commissioned for three-dimensional (3D) conformal therapy based on guidelines specified in AAPM TG-45 and TG-142, with modifications for the fixed-beam geometry made where necessary. Results The system and control software was tested in a variety of machine states and executed motion, stop and beam gating commands as expected. Interlocks and emergency stops of the linac and PRS were found to correctly stop PRS motion and both kV and MV radiation beams when triggered. For 3D conformal treatments, the prototype system met all AAPM TG-45 and TG-142 specifications for geometric and dosimetric accuracy. Motion of the PRS was within 0.6 ± 0.3 mm and 0.10° ± 0.07° of input values for translation and rotation respectively. The axis of rotation of the PRS was coincident with the radiation beam axis to less than 1 mm. End-to-end treatment verification for 6 MV conformal treatments showed less than 2% difference between planned and delivered dose for all fields. Conclusion In this work, we have developed and commissioned a radiotherapy system that utilizes a fixed vertical radiation beam and horizontal patient rotation. This system is a proof-of-concept prototype for a fixed-beam treatment system without a rotating gantry. Fixed-beam systems that are smaller and more cost-effective could help in improving global access to radiotherapy.

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Development and commissioning of a full-size prototype fixed-beam radiotherapy system with horizontal patient rotation.

Fixed-beam radiotherapy systems with subjects rotating about a longitudinal (horizontal) axis are subject to gravity-induced motion. Limited reports on the degree of this motion, and any deformation, has been reported previously. The purpose of this study is to quantify the degree of anatomical motion caused by rotating a subject around a longitudinal axis, using cone-beam CT (CBCT). In the current study, a purpose-made longitudinal rotating was aligned to a Varian TrueBeam kV imaging system. CBCT images of three live rabbits were acquired at fixed rotational offsets of the cradle. Rigid and deformable image registrations back to the original position were used to quantify the motion experienced by the subjects under rotation. In the rotation offset CBCTs, the mean magnitude of rigid translations was 5.7  ±  2.7 mm across all rabbits and all rotations. The translation motion was reproducible between multiple rotations within 2.1 mm, 1.1 mm, and 2.8 mm difference for rabbit 1, 2, and 3, respectively. The magnitude of the mean and absolute maximum deformation vectors were 0.2  ±  0.1 mm and 5.4  ±  2.0 mm respectively, indicating small residual deformations after rigid registration. In the non-rotated rabbit 4DCBCT, respiratory diaphragm motion up to 5 mm was observed, and the variation in respiratory motion as measured from a series of 4DCBCT scans acquired at each rotation position was small. The principle motion of the rotated subjects was rigid translational motion. The deformation of the anatomy under rotation was found to be similar in scale to normal respiratory motion. This indicates imaging and treatment of rotated subjects with fixed-beam systems can use rigid registration as the primary mode of motion estimation. While the scaling of deformation from rabbits to humans is uncertain, these proof-of-principle results indicate promise for fixed-beam treatment systems.

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A CBCT study of the gravity-induced movement in rotating rabbits.

Purpose Fixed beam radiotherapy systems utilize couch movement and rotation instead of gantry rotation in order to simplify linear accelerator design. We investigate the ability to deliver fixed beam treatments with the same level of clinical accuracy as conventional (rotating beam) treatments using real-time image guidance to maintain this accuracy in the presence of rigid target motion. Methods A prototype fixed beam radiotherapy system was built using a standard linac with the beam fixed in the vertical position and a computer controlled rotation stage that rotated a rigid phantom about the superior-inferior axis. Kilovoltage Intrafraction Monitoring (KIM) and real-time beam adaptation with MLC tracking was applied to a five-field IMRT treatment plan with motion introduced to the phantom. The same IMRT treatment was also delivered with real-time adaptation using the conventional rotating beam geometry. Film dosimetry was used to measure the dose delivered with a fixed beam compared to a rotating beam, as well as to compare treatments delivered with and without real-time adaptation. Results The dose distributions were found to be equivalent between the fixed beam and rotating beam geometry for real-time adaptive radiotherapy using KIM and MLC tracking beam adaptation. Gamma analysis on the films showed agreement >98% using a 2%/2 mm criteria with adaptation for static shifts and periodic motion. Conclusions Fixed beam treatments with real-time beam adaptation are dosimetrically equivalent to conventional treatments with a rotating beam, even in the presence of rigid target motion. This suggests that, for a rigid target, the high clinical accuracy of real-time adaptive radiotherapy can be achieved with simpler beam geometry.

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Technical Note: Real-time image-guided adaptive radiotherapy of a rigid target for a prototype fixed beam radiotherapy system.

Purpose The purpose of this article is to present the first imaging experiments to demonstrate the functional equivalence between a conventional rotational gantry and a fixed-beam imaging geometry, and the feasibility of an iterative image-reconstruction technique under gravitational deformation. Methods and materials Experiments were performed using an Elekta Axesse with Agility MLC and XVI, a custom-built rotating phantom stage, a Catphan QA phantom, and a porcine heart. For the imaging equivalence, a conventional cone beam computed tomography (CBCT) of the Catphan was acquired, as well as a set of 660 x-ray projections with a static gantry and rotating Catphan. Both datasets were reconstructed with the Feldkamp-Davis-Kress (FDK) algorithm, and the resultant volumetric images were compared using standard metrics. For imaging under gravitational deformation, a conventional CBCT of the Catphan and a set of 660 x-ray projections with a static gantry and rotating Catphan were also acquired with a porcine heart. The conventional CBCT was reconstructed using FDK. The projections that were acquired with the heart rotating were sorted into angular bins and reconstructed with prior image constrained compressed sensing using a deformation-blurred FDK prior. Deformation was quantified with B-spline transformation-based deformable image registration. Results For imaging equivalence, the difference between the two Catphan images was consistent with Poisson noise. For imaging under gravitational deformation, the conventional CBCT porcine heart image (ground truth at 0 degrees) matched the static gantry, rotating heart reconstruction with a mean magnitude of Conclusions We have demonstrated imaging equivalence in cone beam CT reconstructions between rigid phantom images acquired with a conventional rotating gantry and with a fixed-gantry and rotating phantom. We have presented a method for image reconstruction under a fixed-beam imaging geometry using a deformable phantom.

Sandra Fisher

Papers

Functional imaging equivalence and proof of concept for image-guided adaptive radiotherapy with fixed gantry and rotating couch.