- Email: [email protected]
Helical Tomotherapy: A New Tool for Radiation Therapy
MAHADEVAPPA MAHESH, MS, PhD NICHOLAS DETORIE, PhD
TECHNOLOGY TALK
Helical Tomotherapy: A New Tool for Radiation Therapy Nicholas A. Detorie, PhD INTRODUCTION Recent advances in radiation therapy have accented the role of image-guided radiation therapy. The ability to see the target tissues in the form of an image before treatment permits greater precision and accuracy for the delivery of the radiation dose. Furthermore, using these types of images enables clinicians to assess accurately any changes in the target due either to morphology or, possibly, therapeutic response. Monitoring changes in a patient during the course of radiation therapy, which may be several weeks in duration, also enables a clinician to alter the radiation course to adapt to the changes that occur in the patient’s treatment area. In a broad sense, this interactive monitoring and adjustment is referred to as “adaptive therapy.” One of the relatively new tools that combine imaging capability as well as radiation treatment capability in a single unit is the helical tomotherapy (TomoTherapy Inc., Middleton, Wisconsin) machine developed by the medical physics group under the leadership of Professor T. “Rocke” Mackie at the University of Wisconsin–Madison. The intent of this article is to provide an overview of the physics and engineering of this new hybrid tool for radiation oncology, which consists of a combination of a computed tomography unit and a linear accelerator. For more detailed information, the reader is referred to the articles by John Balog et al [1,2].
MAIN COMPONENTS OF THE TOMOTHERAPY UNIT The tomotherapy unit has the appearance of a large computed tomography unit, because it is, in part, precisely that. In fact, the general operational concept of the unit is based on helical computed tomography. There is a slip-ring gantry that provides for continuous rotational operation of the unit for the angular range to accommodate most patients. As in helical computed tomography, there is a moving couch top, synchronized with the rotational gantry, that moves the patient into the open bore of the unit. However, unlike a conventional computed tomography unit, the x-ray fan beam for the unit is generated by a low-energy linear accelerator, producing megavoltage photons rather than kilovoltage photons. The megavoltage photons are used for a dual purpose: for creating computed tomographic (CT) images with the linear accelerator detuned to about 3.5 MV and for providing delivery of therapeutic doses at about 6 MV with the tuned linear accelerator. The detection system for the fan-beam photons exiting the patient consists of a pressurized array of ion chambers filled with xenon gas, a well-known detection system used in early fan-beam scanners manufactured by General Electric. As in computed tomography, there is a set of jaws used to define the x-ray beam width or slice “thickness.” How-
© 2008 American College of Radiology 0091-2182/08/$34.00 ● DOI 10.1016/j.jacr.2007.09.019
ever, unlike in computed tomography, and unique to this device, there is a multileaf collimator (MLC) with 64 leaves that spans the fan beam, which projects to a length of 40 cm at the center of gantry rotation. The MLC is pneumatically driven so that it acts as a binary collimator, with a particular leaf being either open or closed. It is the MLC that permits the modulation of the x-ray beam intensity when the device is operated in treatment mode. Figure 1 is a schematic drawing illustrating the main components of the tomotherapy device. OPERATIONAL PROCESS AND FEATURES As in conventional intensity-modulated radiation therapy planning, the 3-D patient data set used for treatment planning is computed tomography based, usually kilovoltage computed tomography. This particular data set, containing all patient immobilization device information, and associated with the physician-approved treatment plan, becomes the reference data set. In image-guided radiation therapy, the reference data set is used to judge patient setup reproducibility and provides the basis for determining the quantitative adjustments that must be made on the current patient setup to get the patient back into the original treatment position. Mathematically, the required adjustments span a 6-D space: 3 translational degrees of freedom (x, y, and z in Cartesian coordinates) and 3 rotational degrees of freedom (role, yaw, and pitch.) Clinically, these 6 63
64 Technology Talk
Fig 1. Schematic drawing illustrates the main components of the tomotherapy device: the megavoltage linear accelerator source, the primary collimator to define the slice thickness, the multileaf collimator (MLC) for treatment beam modulation, the gantry bore, and the fan-beam ion chamber detector array. FOV ⫽ field of view.
dimensions are sometimes referred to as left-right, in-out, updown, patient rotation about an axis head to foot, angular leftright tilt of the head-to-foot axis, and angular up-down tilt of the head-to-foot axis. Currently, the tomotherapy unit can adjust for 5 of the 6 degrees of freedom; it cannot correct for pitch. (It should be noted that most other treatment systems use couch motion to correct for only 3 of the 6 degrees of freedom, namely, x, y, and z.) Operationally, a patient is set up for tomotherapy treatment using reference patient marks and lasers, which define patient alignment rel-
ative to the treatment device. Next, a megavoltage CT image is acquired, resulting in a patient dose of a few centigrays. Application software is available on the unit that permits a comparison of the present patient position to that of the reference data set using standard tools such as checkerboarding in overlay images of the CT reference and megavoltage CT data sets. Quantitative values are calculated that permit adjustment of the current setup to the best registration with the original reference data set. A repeat megavoltage CT scan may be made to provide conformation that the adjustments were carried out correctly.
Technically, the megavoltage CT image acquisition is carried out in a helical fashion, with 3.5-MV x-rays using a narrow jaw width and a pitch equal to 1 or more over the range of slices identified by the operator. Note that pitch, sometimes referred to as the “tightness” of the helical spiral, is jointly defined by the period of rotation of the gantry and the speed of the table moving the patient into the bore. For a pitch of 1, a single gantry rotation results in a table translation equal to the slice thickness. In simpler terms, if the slice-defining jaws are set to 1 cm, the table moves continuously and synchronously 1 cm with every full gantry rotation of
Technology Talk 65
Fig 2. A color-wash illustration of the conformal tomotherapy dose distribution for a central nervous system tumor. Note also the long field length achievable with the device.
360°. A pitch of unity produces the helical CT data set of contiguous slices with a thickness defined by the primary jaws. Image acquisition time is usually no more than a few minutes. In treatment mode, the 64-leaf MLC also comes into play, making radiation delivery of the 6-MV photons a very dynamic process involving gantry motion, table motion, and leaf motions of the binary collimator. Precise mechanical and temporal synchrony is necessarily required of these major components to carry out the radiation treatment. However, this synchronization alone is not sufficient. Sufficiency is achieved when the radiation output of the device is constant and of the correct quantity to deliver the prescribed dose. Typical radiation output for the Tomo-
therapy unit is about 900 cGy/ min on the axis of gantry rotation 85 cm from the source for a 5 ⫻ 40 cm field. The high output results from the lack of a beam-flattening filter and the shorter rotational axis distance of 85 cm, compared with the 100-cm source-to-axis distance of conventional linear accelerators. In treatment delivery, a pitch of less than 1 is usually used; a typical value would be close to 0.3. The small pitch yields an overlap of the field width inside the patient’s targeted treatment volume; each voxel in the patient may be exposed several times during the treatment process to receive the prescribed dose. Taking into account the 64 possible leaf positions and the time that each leaf remains open or closed while the gantry rotates and
the table moves leads to the realization that treatment delivery may be carried out with tens of thousands of beamlets of radiation over the targeted volume. For this reason, tomotherapy plans and treatments have an extremely high degree of conformality in their dose distributions. The rotational treatment geometry may be considered to be modeled by 51 intensity-modulated radiation therapy treatment beams but carried out with only one treatment field for the patient. Figure 2 illustrates a highly conformal dose distribution surrounding the target tissue with excellent avoidance of tissues to be spared from irradiation. Two standard jaw widths are usual for patient treatments: 2.5 and 5 cm. On occasion, a 1 cm treatment width may also be com-
66 Technology Talk
missioned for use. For a given pitch, the larger treatment width produces the shortest treatment time. However, considering the beam divergence associated with the larger jaw settings, there is a small degradation in the slice-toslice dose uniformity. Such considerations suggest that the 1-cm jaw width be used for small fields only. It should also be noted that the couch travel permits the treatment of a field length of 160 cm, keeping in mind that long field lengths require longer treatment
times. Dosimetric considerations for the target provide guidance as to the choice of filed width for treatment.
device. Hopefully, the improved precision and accuracy will translate into higher local control and cure rates for disease managed with radiation therapy.
SUMMARY Helical tomotherapy is a relatively new hybrid tool that combines imaging and treatment capabilities in one unit. The 3-D data set obtained with megavoltage photons provides a powerful image guidance capability for aiming the highly conformal intensity-modulated radiation therapy treatment beam of the
REFERENCES 1. Balog J, Olivera G, Kapatoes J. Clinical helical tomotherapy commissioning dosimetry. Med Phys 2003;30:3097-106. 2. Balog J, Mackie TR, Pearson D, Hui S, Paliwal B, Jeraj R. Benchmarking beam alignment for a clinical helical tomotherapy device. Med Phys 2003;30:111827.
Nicholas A. Detorie, PhD, Radiation Management Associates, Physics Department, 7505 Greenway Center Drive, Suite 3, Greenbelt, MD 20770; e-mail: [email protected].
TECHNOLOGY TALK
Helical Tomotherapy: A New Tool for Radiation Therapy Nicholas A. Detorie, PhD INTRODUCTION Recent advances in radiation therapy have accented the role of image-guided radiation therapy. The ability to see the target tissues in the form of an image before treatment permits greater precision and accuracy for the delivery of the radiation dose. Furthermore, using these types of images enables clinicians to assess accurately any changes in the target due either to morphology or, possibly, therapeutic response. Monitoring changes in a patient during the course of radiation therapy, which may be several weeks in duration, also enables a clinician to alter the radiation course to adapt to the changes that occur in the patient’s treatment area. In a broad sense, this interactive monitoring and adjustment is referred to as “adaptive therapy.” One of the relatively new tools that combine imaging capability as well as radiation treatment capability in a single unit is the helical tomotherapy (TomoTherapy Inc., Middleton, Wisconsin) machine developed by the medical physics group under the leadership of Professor T. “Rocke” Mackie at the University of Wisconsin–Madison. The intent of this article is to provide an overview of the physics and engineering of this new hybrid tool for radiation oncology, which consists of a combination of a computed tomography unit and a linear accelerator. For more detailed information, the reader is referred to the articles by John Balog et al [1,2].
MAIN COMPONENTS OF THE TOMOTHERAPY UNIT The tomotherapy unit has the appearance of a large computed tomography unit, because it is, in part, precisely that. In fact, the general operational concept of the unit is based on helical computed tomography. There is a slip-ring gantry that provides for continuous rotational operation of the unit for the angular range to accommodate most patients. As in helical computed tomography, there is a moving couch top, synchronized with the rotational gantry, that moves the patient into the open bore of the unit. However, unlike a conventional computed tomography unit, the x-ray fan beam for the unit is generated by a low-energy linear accelerator, producing megavoltage photons rather than kilovoltage photons. The megavoltage photons are used for a dual purpose: for creating computed tomographic (CT) images with the linear accelerator detuned to about 3.5 MV and for providing delivery of therapeutic doses at about 6 MV with the tuned linear accelerator. The detection system for the fan-beam photons exiting the patient consists of a pressurized array of ion chambers filled with xenon gas, a well-known detection system used in early fan-beam scanners manufactured by General Electric. As in computed tomography, there is a set of jaws used to define the x-ray beam width or slice “thickness.” How-
© 2008 American College of Radiology 0091-2182/08/$34.00 ● DOI 10.1016/j.jacr.2007.09.019
ever, unlike in computed tomography, and unique to this device, there is a multileaf collimator (MLC) with 64 leaves that spans the fan beam, which projects to a length of 40 cm at the center of gantry rotation. The MLC is pneumatically driven so that it acts as a binary collimator, with a particular leaf being either open or closed. It is the MLC that permits the modulation of the x-ray beam intensity when the device is operated in treatment mode. Figure 1 is a schematic drawing illustrating the main components of the tomotherapy device. OPERATIONAL PROCESS AND FEATURES As in conventional intensity-modulated radiation therapy planning, the 3-D patient data set used for treatment planning is computed tomography based, usually kilovoltage computed tomography. This particular data set, containing all patient immobilization device information, and associated with the physician-approved treatment plan, becomes the reference data set. In image-guided radiation therapy, the reference data set is used to judge patient setup reproducibility and provides the basis for determining the quantitative adjustments that must be made on the current patient setup to get the patient back into the original treatment position. Mathematically, the required adjustments span a 6-D space: 3 translational degrees of freedom (x, y, and z in Cartesian coordinates) and 3 rotational degrees of freedom (role, yaw, and pitch.) Clinically, these 6 63
64 Technology Talk
Fig 1. Schematic drawing illustrates the main components of the tomotherapy device: the megavoltage linear accelerator source, the primary collimator to define the slice thickness, the multileaf collimator (MLC) for treatment beam modulation, the gantry bore, and the fan-beam ion chamber detector array. FOV ⫽ field of view.
dimensions are sometimes referred to as left-right, in-out, updown, patient rotation about an axis head to foot, angular leftright tilt of the head-to-foot axis, and angular up-down tilt of the head-to-foot axis. Currently, the tomotherapy unit can adjust for 5 of the 6 degrees of freedom; it cannot correct for pitch. (It should be noted that most other treatment systems use couch motion to correct for only 3 of the 6 degrees of freedom, namely, x, y, and z.) Operationally, a patient is set up for tomotherapy treatment using reference patient marks and lasers, which define patient alignment rel-
ative to the treatment device. Next, a megavoltage CT image is acquired, resulting in a patient dose of a few centigrays. Application software is available on the unit that permits a comparison of the present patient position to that of the reference data set using standard tools such as checkerboarding in overlay images of the CT reference and megavoltage CT data sets. Quantitative values are calculated that permit adjustment of the current setup to the best registration with the original reference data set. A repeat megavoltage CT scan may be made to provide conformation that the adjustments were carried out correctly.
Technically, the megavoltage CT image acquisition is carried out in a helical fashion, with 3.5-MV x-rays using a narrow jaw width and a pitch equal to 1 or more over the range of slices identified by the operator. Note that pitch, sometimes referred to as the “tightness” of the helical spiral, is jointly defined by the period of rotation of the gantry and the speed of the table moving the patient into the bore. For a pitch of 1, a single gantry rotation results in a table translation equal to the slice thickness. In simpler terms, if the slice-defining jaws are set to 1 cm, the table moves continuously and synchronously 1 cm with every full gantry rotation of
Technology Talk 65
Fig 2. A color-wash illustration of the conformal tomotherapy dose distribution for a central nervous system tumor. Note also the long field length achievable with the device.
360°. A pitch of unity produces the helical CT data set of contiguous slices with a thickness defined by the primary jaws. Image acquisition time is usually no more than a few minutes. In treatment mode, the 64-leaf MLC also comes into play, making radiation delivery of the 6-MV photons a very dynamic process involving gantry motion, table motion, and leaf motions of the binary collimator. Precise mechanical and temporal synchrony is necessarily required of these major components to carry out the radiation treatment. However, this synchronization alone is not sufficient. Sufficiency is achieved when the radiation output of the device is constant and of the correct quantity to deliver the prescribed dose. Typical radiation output for the Tomo-
therapy unit is about 900 cGy/ min on the axis of gantry rotation 85 cm from the source for a 5 ⫻ 40 cm field. The high output results from the lack of a beam-flattening filter and the shorter rotational axis distance of 85 cm, compared with the 100-cm source-to-axis distance of conventional linear accelerators. In treatment delivery, a pitch of less than 1 is usually used; a typical value would be close to 0.3. The small pitch yields an overlap of the field width inside the patient’s targeted treatment volume; each voxel in the patient may be exposed several times during the treatment process to receive the prescribed dose. Taking into account the 64 possible leaf positions and the time that each leaf remains open or closed while the gantry rotates and
the table moves leads to the realization that treatment delivery may be carried out with tens of thousands of beamlets of radiation over the targeted volume. For this reason, tomotherapy plans and treatments have an extremely high degree of conformality in their dose distributions. The rotational treatment geometry may be considered to be modeled by 51 intensity-modulated radiation therapy treatment beams but carried out with only one treatment field for the patient. Figure 2 illustrates a highly conformal dose distribution surrounding the target tissue with excellent avoidance of tissues to be spared from irradiation. Two standard jaw widths are usual for patient treatments: 2.5 and 5 cm. On occasion, a 1 cm treatment width may also be com-
66 Technology Talk
missioned for use. For a given pitch, the larger treatment width produces the shortest treatment time. However, considering the beam divergence associated with the larger jaw settings, there is a small degradation in the slice-toslice dose uniformity. Such considerations suggest that the 1-cm jaw width be used for small fields only. It should also be noted that the couch travel permits the treatment of a field length of 160 cm, keeping in mind that long field lengths require longer treatment
times. Dosimetric considerations for the target provide guidance as to the choice of filed width for treatment.
device. Hopefully, the improved precision and accuracy will translate into higher local control and cure rates for disease managed with radiation therapy.
SUMMARY Helical tomotherapy is a relatively new hybrid tool that combines imaging and treatment capabilities in one unit. The 3-D data set obtained with megavoltage photons provides a powerful image guidance capability for aiming the highly conformal intensity-modulated radiation therapy treatment beam of the
REFERENCES 1. Balog J, Olivera G, Kapatoes J. Clinical helical tomotherapy commissioning dosimetry. Med Phys 2003;30:3097-106. 2. Balog J, Mackie TR, Pearson D, Hui S, Paliwal B, Jeraj R. Benchmarking beam alignment for a clinical helical tomotherapy device. Med Phys 2003;30:111827.
Nicholas A. Detorie, PhD, Radiation Management Associates, Physics Department, 7505 Greenway Center Drive, Suite 3, Greenbelt, MD 20770; e-mail: [email protected].