The present invention relates to radiotherapy systems, such as those using ions like protons, for the treatment of cancer and, in particular, to a system providing improved treatment speed and accuracy.
In external beam radiation therapy, tumors within a patient are treated by directing high-energy radiation in one or more beams toward the tumor. Highly sophisticated external beam radiation systems, for example, as manufactured by TomoTherapy Inc., employ intensity modulation techniques to improve the conformity of high dose regions to the tumor volume. With TomoTherapy, a tumor is treated with multiple x-ray fan beams directed at the patient over an angular range of 360°. Each of the beams is comprised of individually modulated rays whose intensities can be controlled so that the combined effect of the rays over the range of angles, is the delivery of highly conformal dose distributions to arbitrarily complex target volumes within the patient.
One of the drawbacks of external beam x-ray therapy is that x-rays irradiate tissue along the entire path of each ray, including healthy tissues both proximal and distal to the tumor volume. While judicious selection of the angles and intensities of the x-ray beams can minimize radiation applied to healthy tissue outside of the tumor, the inevitability with x-rays of irradiating healthy tissue along the path leading to and exiting from the tumor has led to a renewed interest in the use of ions, such as protons, as a substitute for x-rays in radiotherapy.
Unlike x-rays, protons and other charged particles can be range modulated and made to stop within the target volume; thereby eliminating exit dose to healthy tissue on the far side of the tumor. In addition, the dose deposited by a proton beam is not uniform along the path of the beam, but rather rises substantially near the protons end of range in a region known as the “Bragg peak”. These two features allow improved concentration of dose within the tumor.
Because the size of the proton beam extracted from a typical proton accelerator is generally too small for the treatment of most disease sites, current proton therapy systems adopt one of two general approaches to treat clinically observed target volumes. In the first approach, termed the “spread out Bragg peak” (SOBP) approach, the range of the distal end of a narrow proton pencil beam (the Bragg peak) is modulated using a spinning propeller of low atomic-number material with blades of varying thickness; allowing for a uniform dose to be delivered to a spread out region in depth. This beam is then broadened laterally using a series of lead scattering foils and shaped using field specific brass collimators. At this point, the depth of penetration of the broadened beam is shaped to conform to the distal side of the target volume from each beam angle using custom-built, 2-D range compensators before finally being delivered to the patient for treatment.
This technique can treat the entire tumor at once and therefore is fast. However, the use of the range modulating wheel makes it difficult to conform the dose to the tumor in regions proximal to the target volume, and the construction of special collimators and compensators are required for each treatment field. In addition, the use of high atomic number scattering foils and collimators result in neutron production—which can contribute unwanted dose to the patient during treatment.
In a second approach, termed the “magnetic spot scanning” (MSS) approach, the narrowly collimated proton “pencil beam” extracted from the proton accelerator is modulated in range and magnetically steered in angle to deposit the dose as a series of small spots within the target volume. The spots are positioned in successive exposures until an arbitrary tumor volume has been irradiated. This approach is potentially very accurate, but because the tumor is treated in many successive exposures, this approach is much slower than the SOBP approach. Furthermore, the use of many small, precisely overlapping beam spots creates the risk of “hot and cold spots” appearing in the target volume due to errors in spot placement. This risk is greatly exacerbated if there is any patient movement between spot exposures.