Radiation characteristics of helical tomotherapy

A review of the impact of photon and proton external beam radiotherapy treatment modalities on the dose distribution in field and out-of-field; implications for the long-term morbidity of cancer survivors

The use of untraditional treatment modalities for external beam radiotherapy such as intensity modulated radiation therapy (IMRT) and proton beam therapy is increasing. This review focuses on the changes in the dose distribution and the impact on radiation related risks for long-term cancer survivors. We compare conventional radiotherapy, IMRT, and proton beam therapy based on published treatment planning studies as well as published measurements and Monte Carlo simulations of out-of-field dose distributions. Physical dose parameters describing the dose distribution in the target volume, the conformity index, the dose distribution in organs at risk, and the dose distribution in non-target tissue, respectively, are extracted from the treatment planning studies. Measured out-of-field dose distributions are presented as the dose equivalent as a function of distance from the treatment field. Data in the literature clearly shows that, compared with conventional radiotherapy, IMRT improves the dose distribution in the target volume, which may increase the probability of tumor control. IMRT also seems to increase the out-of-field dose distribution, as well as the irradiated non-target volume, although the data is not consistent, leading to a potentially increased risk of radiation induced secondary malignancies, while decreasing the dose to normal tissues close to the target volume, reducing the normal tissue complication probability. Protons show no or only minor advantage on the dose distribution in the target volume and the conformity index compared to IMRT. However, the data consistently shows that proton beam therapy substantially decreases the OAR average dose compared to the other two techniques. It is also clear that protons provide an improved dose distribution in non-target tissues compared to conventional radiotherapy and IMRT. IMRT and proton beam therapy may significantly improve tumor control for cancer patients and quality of life for long-term cancer survivors.

Megavoltage Computed Tomography Image-based Low-dose Rate Intracavitary Brachytherapy Planning for Cervical Carcinoma

Initial results of megavoltage computed tomography (MVCT) brachytherapy treatment planning are presented, using a commercially available helical tomotherapy treatment unit and standard low dose rate (LDR) brachytherapy applicators used for treatment of cervical carcinoma. The accuracy of MVCT imaging techniques, and dosimetric accuracy of the CT based plans were tested with in-house and commercially-available phantoms. Three dimensional (3D) dose distributions were computed and compared to the two dimensional (2D) dosimetry results. Minimal doses received by the 2 cm3 of bladder and rectum receiving the highest doses (DB2cc and DR2cc, respectively) were computed from dose-volume histograms and compared to the doses computed for the standard ICRU bladder and rectal reference dose points. Phantom test objects in MVCT image sets were localized with sub-millimetric accuracy, and the accuracy of the MVCT-based dose calculation was verified. Fifteen brachytherapy insertions were also analyzed. The ICRU rectal point dose did not differ significantly from DR2cc (p=0.749, mean difference was 24 cGy ± 283 cGy). The ICRU bladder point dose was significantly lower than the DB2cc (p=0.024, mean difference was 291 cGy ± 444 cGy). The median volumes of bladder and rectum receiving at least the corresponding ICRU reference point dose were 6.1 cm3 and 2.0 cm3, respectively. Our initial experience in using MVCT imaging for clinical LDR gynecological brachytherapy indicates that the MVCT images are of sufficient quality for use in 3D, MVCT-based dose planning.

Treatment planning and delivery of IMRT using 6 and 18MV photon beams without flattening filter

In light of the increasing use of intensity modulated radiation therapy (IMRT) in modern radiotherapy practice, the use of a flattening filter may no longer be necessary. Commissioning data have been measured for a Varian 23EX linear accelerator with 6 and 18 MV photon energies without a flattening filter. Measurements collected for the commissioning of the linac included percent depth dose curves and profiles for field sizes ranging from 2 x 2 to 40 x 40 cm(2) as defined by the jaws and multileaf collimator. Machine total scatter factors were measured and calculated. Measurements were used to model the unflattened beams with the Pinnacle(3) treatment planning system. IMRT plans for prostate, lung, brain and head and neck cancer cases were generated using the flattening filter and flattening filter-free beams. From our results, no difference in the quality of the treatment plans between the flat and unflattened photon beams was noted. There was however a significant decrease in the number of monitor units required for unflattened beam treatment plans due to the increase in linac output-approximately two times and four times higher for the 6 and 18 MV, respectively.

Multimodality image guided total marrow irradiation and verification of the dose delivered to the lung, PTV, and thoracic bone in a patient: a case study

This work reports our initial experience using multimodality image guidance to improve total marrow irradiation (TMI) using helical tomotherapy. We also monitored the details of the treatment delivery to glean information necessary for the implementation of future adaptive processes. A patient with metastatic Ewing's sarcoma underwent MRI, and bone scan imaging prior to TMI. A whole body kilovoltage CT (kVCT) scan was obtained for intensity modulated TMI treatment planning, including a boost treatment to areas of bony involvement. The delivered dose was estimated by using MVCT images from the helical tomotherapy treatment unit, compared to the expected dose distributions mapped onto the kVCT images. Clinical concerns regarding patient treatment and dosimetric uncertainties were also evaluated. A small fraction of thoracic bone volume received lower radiation dose than the prescribed dose. Reconstructed planned treatment volume (PTV) and the dose delivered to the lung were identical to planned dose. Bone scan imaging had a higher sensitivity for detecting skeletal metastasis compared to MR imaging. However the bone scan lacked sufficient specificity in three dimensions to be useful for planning conformal radiation boost treatments. Inclusion of appropriate imaging modalities improves detection of metastases, which allows the possibility of a radiation dose boost to metastases during TMI. Conformal intensity modulated radiation therapy via helical tomotherapy permitted radiation delivery to metastases in the skull with reduced dose to brain in conjunction with TMI. While TMI reduces irradiation to the lungs, onboard megavoltage computed tomography (MVCT) to verify accurate volumetric dose coverage to marrow-containing thoracic bones may be essential for successful conformal TMI treatment.

Consistency Check of Planned Adaptive® Option on Helical Tomotherapy

This study aims to evaluate a new Planned Adaptive® software (TomoTherapy Inc., Madison, WI) of the helical tomotherapy system by retrospective verification and adaptive re-planning of radiation treatment. Four patients with different disease sites (brain, nasal cavity, lungs, prostate) were planned in duplicate using the diagnostic planning kVCT data set and MVCT studies of the first treatment fraction with the same optimization parameters for both plan types. The dosimetric characteristics of minimum, maximum, and mean dose to the targets as well as to organs at risk were compared. Both sets of plans were used for calculation of dose distributions in a water-equivalent phantom. Corresponding measurements of these plans in phantom were carried out with the use of radiographic film and ion chamber. In the case of the lung and prostate cancer patients, changes in dosimetric parameters compared to data generated with the kVCT study alone were less than 2%. Certain changes for the nasal cavity and brain cancer patients were greater than 2%, but they were explained in part by anatomy changes that occurred during the time between kVCT and MVCT studies. The Planned Adaptive software allows for adaptive radiotherapy planning using the MVCT studies obtained by the helical tomotherapy imaging system.

Technological approaches to in-room CBCT imaging

The use of Cone-Beam Computed Tomography (CBCT) in Image-Guided Radiation Therapy (IGRT) has become increasingly feasible and popular in recent years. Advances and developments in Flat-Panel Imager (FPI) technology and image reconstruction software allow for linac-mounted 3D CBCT imaging. Taking CBCT images on a daily/weekly basis, offers the possibility to guide the treatment beam according to tumour motion and to apply changes to the treatment plan if necessary. This however raises the issue of additional imaging dose and thus increases in secondary cancer risk. The performance characteristics of kV-CBCT and MV-CBCT solutions currently offered by Elekta, Siemens and Varian are compared in this paper in terms of additional imaging dose and image quality. The review also outlines applications of CBCT for IGRT and Adaptive Radiotherapy (ART). As CBCT is not the only in-room IGRT platform, helical MV-CT (Tomotherapy) and in-room CT designs are also presented.

Surface Dose for Five Telecobalt Machines, 6MV Photon Beam from Four Linear Accelerators and a Hi-Art TomoTherapy

The purpose of this study was to estimate the surface dose for five telecobalt machines (four from Best Theratronics Limited, Canada, one from Panacea Medical Technologies, India), 6 MV photon beam (static) from four linear accelerators (three Varian linear accelerators and one Siemens) and Hi-Art Tomotherapy unit. The surface dose was measured with Thermoluminescent dosimeters in phantom slabs. For Tomotherapy 6 MV beam the surface dose was estimated as 32% while it was 35%, 33%, and 36% for Clinac 6EX, Clinac 2100CD, and Clinac 2100C linear accelerators, respectively. Similarly, the surface dose for 6 MV photon beam from Primus linear accelerator was estimated as 35%. Surface doses from telecobalt machines Equinox-80, Elite-80, Th-780C, Th-780, and Bhabhatron-II was found to be 30%, 29.1%, 27.8%, 29.3%, and 29.9% for 10 cm × 10 field size, respectively. Measured surface dose from all four linear accelerators were in good agreement with that of the Tomotherapy. The surface dose measurements were useful for Tomotherapy to predict the superficial dose during helical IMRT treatments.

A review of dosimetry studies on external-beam radiation treatment with respect to second cancer induction

It has been long known that patients treated with ionizing radiation carry a risk of developing a second cancer in their lifetimes. Factors contributing to the recently renewed concern about the second cancer include improved cancer survival rate, younger patient population as well as emerging treatment modalities such as intensity-modulated radiation treatment (IMRT) and proton therapy that can potentially elevate secondary exposures to healthy tissues distant from the target volume. In the past 30 years, external-beam treatment technologies have evolved significantly, and a large amount of data exist but appear to be difficult to comprehend and compare. This review article aims to provide readers with an understanding of the principles and methods related to scattered doses in radiation therapy by summarizing a large collection of dosimetry and clinical studies. Basic concepts and terminology are introduced at the beginning. That is followed by a comprehensive review of dosimetry studies for external-beam treatment modalities including classical radiation therapy, 3D-conformal x-ray therapy, intensity-modulated x-ray therapy (IMRT and tomotherapy) and proton therapy. Selected clinical data on second cancer induction among radiotherapy patients are also covered. Problems in past studies and controversial issues are discussed. The needs for future studies are presented at the end.

Comparison of fixed-beam IMRT, helical tomotherapy, and IMPT for selected cases

A growing number of advanced intensity modulated treatment techniques is becoming available. In this study, the specific strengths and weaknesses of four techniques, static and dynamic multileaf collimator (MLC), conventional linac-based IMRT, helical tomotherapy (HT), and spot-scanning proton therapy (IMPT) are investigated in the framework of biological, EUD-based dose optimization. All techniques were implemented in the same in-house dose optimization tool. Monte Carlo dose computation was used in all cases. All dose-limiting, normal tissue objectives were treated as hard constraints so as to facilitate comparability. Five patient cases were selected to offer each technique a chance to show its strengths: a deep-seated prostate case (for 15 MV linac-based IMRT), a pediatric case (for IMPT), an extensive head-and-neck case (for HT), a lung tumor (for HT), and an optical neurinoma (for noncoplanar linac-based IMRT with a miniMLC). The plans were compared by dose statistics and equivalent uniform dose metrics. All techniques delivered results that were comparable with respect to target coverage and the most dose-limiting normal tissues. Static MLC IMRT struggled to achieve sufficient target coverage at the same level of dose homogeneity in the lung case. IMPT gained the greatest advantage when lung sparing was important, but did not significantly reduce the risk of nearby organs. Tomotherapy and dynamic MLC IMRT showed mostly the same performance. Despite the apparent conceptual differences, all four techniques fare equally well for standard patient cases. The absence of relevant differences is in part due to biological optimization, which offers more freedom to shape the dose than do, e.g., dose volume histogram constraints. Each technique excels for certain classes of highly complex cases, and hence the various modalities should be viewed as complementary, rather than competing.

Evaluation of coplanar partial left breast irradiation using tomotherapy-based topotherapy

PURPOSE

To investigate the use of topotherapy for accelerated partial breast irradiation through field-design optimization and dosimetric comparison to linear accelerator-based three-dimensional conformal radiotherapy (3D-CRT) and intensity-modulated radiation therapy (IMRT).

METHODS AND MATERIALS

Hypothetical 3-cm lumpectomy sites were contoured in each quadrant of a left breast by using dosimetric guidelines from the National Surgical Adjuvant Breast and Bowel Project B-39/Radiation Therapy Oncology Group 0413 protocol. Coplanar intensity-modulated topotherapy treatment plans were optimized by using two-, three-, four-, five-, and seven-field arrangements for delivery by the tomotherapy unit with fixed gantry angles. Optimized noncoplanar five-field 3D-CRT and IMRT were compared with corresponding topotherapy plans.

RESULTS

On average, 99.5% +/- 0.5% of the target received 100% of the prescribed dose for all topotherapy plans. Average equivalent uniform doses ranged from 1.20-2.06, 0.79-1.76, and 0.10-0.29 Gy for heart, ipsilateral lung, and contralateral lung, respectively. Average volume of normal breast exceeding 90% of the prescription and average area of skin exceeding 35 Gy were lowest for five-field plans. Average uniformity indexes for five-field plans using 3D-CRT, IMRT, and topotherapy were 1.047, 1.050, and 1.040, respectively. Dose-volume histograms and calculated equivalent uniform doses of all three techniques illustrate clinically equivalent doses to ipsilateral breast, lung, and heart.

CONCLUSIONS

This dosimetric evaluation for a single patient shows that coplanar partial breast topotherapy provides good target coverage with exceptionally low dose to organs at risk. Use of more than five fields provided no additional dosimetric advantage. A comparison of five-field topotherapy to 3D-CRT and IMRT for accelerated partial breast irradiation illustrates equivalent target conformality and uniformity.

Helical tomotherapy quality assurance

Helical tomotherapy uses a dynamic delivery in which the gantry, treatment couch, and multileaf collimator leaves are all in motion during treatment. This results in highly conformal radiotherapy, but the complexity of the delivery is partially hidden from the end-user because of the extensive integration and automation of the tomotherapy control systems. This presents a challenge to the medical physicist who is expected to be both a system user and an expert, capable of verifying relevant aspects of treatment delivery. A related issue is that a clinical tomotherapy planning system arrives at a customer's site already commissioned by the manufacturer, not by the clinical physicist. The clinical physicist and the manufacturer's representative verify the commissioning at the customer site before acceptance. Theoretically, treatment could begin immediately after acceptance. However, the clinical physicist is responsible for the safe and proper use of the machine. In addition, the therapists and radiation oncologists need to understand the important machine characteristics before treatment can proceed. Typically, treatment begins about 2 weeks after acceptance. This report presents an overview of the tomotherapy system. Helical tomotherapy has unique dosimetry characteristics, and some of those features are emphasized. The integrated treatment planning, delivery, and patient-plan quality assurance process is described. A quality assurance protocol is proposed, with an emphasis on what a clinical medical physicist could and should check. Additionally, aspects of a tomotherapy quality assurance program that could be checked automatically and remotely because of its inherent imaging system and integrated database are discussed.

Can megavoltage computed tomography reduce proton range uncertainties in treatment plans for patients with large metal implants?

Treatment planning calculations for proton therapy require an accurate knowledge of radiological path length, or range, to the distal edge of the target volume. In most cases, the range may be calculated with sufficient accuracy using kilovoltage (kV) computed tomography (CT) images. However, metal implants such as hip prostheses can cause severe streak artifacts that lead to large uncertainties in proton range. The purposes of this study were to quantify streak-related range errors and to determine if they could be avoided by using artifact-free megavoltage (MV) CT images in treatment planning. Proton treatment plans were prepared for a rigid, heterogeneous phantom and for a prostate cancer patient with a metal hip prosthesis using corrected and uncorrected kVCT images alone, uncorrected MVCT images and a combination of registered MVCT and kVCT images (the hybrid approach). Streak-induced range errors of 5-12 mm were present in the uncorrected kVCT-based patient plan. Correcting the streaks by manually assigning estimated true Hounsfield units improved the range accuracy. In a rigid heterogeneous phantom, the implant-related range uncertainty was estimated at <3 mm for both the corrected kVCT-based plan and the uncorrected MVCT-based plan. The hybrid planning approach yielded the best overall result. In this approach, the kVCT images provided good delineation of soft tissues due to high-contrast resolution, and the streak-free MVCT images provided smaller range uncertainties because they did not require artifact correction.

A Monte-Carlo derived dual-source model for helical tomotherapy treatment planning

Full Monte Carlo radiation transport simulations of accelerator heads are impractical for routine treatment planning because of the excessive computational burden and memory requirements. To improve the accuracy and efficiency of treatment plans for helical tomotherapy, we have developed a dual-source model to characterize the radiation emitted from the head of a commercial helical tomotherapy accelerator. Percentage depth dose (PDD) and beam profiles computed using the dual-source model with the EGS/BEAMnrc Monte Carlo package agree within 2% of measurements for a wide range of field sizes, which suggests that the proposed dual-source model provides an adequate representation of the tomotherapy head for dose calculations in routine treatment planning.

Multibeam tomotherapy: a new treatment unit devised for multileaf collimation, intensity-modulated radiation therapy

A fully integrated system for treatment planning, application, and verification for automated multileaf collimator (MLC) based, intensity-modulated, image-guided, and adaptive radiation therapy (IMRT, IGRT and ART, respectively) is proposed. Patient comfort, which was the major development goal, will be achieved through a new unit design and short treatment times. Our device for photon beam therapy will consist of a new dual energy linac with five fixed treatment heads positioned evenly along one plane but one electron beam generator only. A minimum of moving parts increases technical reliability and reduces motion times to a minimum. Motion is allowed solely for the MLCs, the robotic patient table, and the small angle gantry rotation of +/- 36 degrees. Besides sophisticated electron beam guidance, this compact setup can be built using existing modules. The flattening-filter-free treatment heads are characterized by reduced beam-on time and contain apertures restricted in one dimension to the area of maximum primary fluence output. In the case of longer targets, this leads to a topographic intensity modulation, thanks to the combination of "step and shoot" MLC delivery and discrete patient couch motion. Owing to the limited number of beam directions, this multislice cone beam serial tomotherapy is referred to as "multibeam tomotherapy." Every patient slice is irradiated by one treatment head at any given moment but for one subfield only. The electron beam is then guided to the next head ready for delivery, while the other heads are preparing their leaves for the next segment. The "Multifocal MLC-positioning" algorithm was programmed to enable treatment planning and optimize treatment time. We developed an overlap strategy for the longitudinally adjacent fields of every beam direction, in doing so minimizing the field match problem and the effects of possible table step errors. Clinical case studies show for the same or better planning target volume coverage, better organ-at-risk sparing, and comparable mean integral dose to the normal tissue a reduction in treatment time by more than 50% to only a few minutes in comparison to high-quality 3-D conformal and IMRT treatments. As a result, it will be possible to incorporate features for better patient positioning and image guidance, while sustaining reasonable overall treatment times at the same time. The virtual multibeam tomotherapy design study TOM'5-CT contains a dedicated electron beam CT (TOM'AGE) and an objective optical topometric patient positioning system (TOPOS). Thanks to the wide gantry bore of 120 cm and slim gantry depths of 70 cm, patients can be treated very comfortably, in all cases tumor-isocentrically, as well as with noncoplanar beam arrangements as in stereotactic radiosurgery with a couch rotation of up to +/- 54 degrees. The TOM'5 treatment unit on which this theoretical concept is based has a stand-alone depth of 40 cm and an outer diameter of 245 cm; the focus-isocenter distance of the heads is 100 cm with a field size of 40 cm x 7 cm and 0.5 cm leaves, which operate perpendicular to the axis of table motion.

Dosimetric characteristics of unflattened 6MV photon beams of a clinical linear accelerator: A Monte Carlo study

The purpose of this study is to evaluate the dosimetric properties of a flattening filter free 6 MV photon beam. The 6 MV photon beam of a Varian Clinac 21 EX linac was modeled using the MCNP4C Monte Carlo (MC) code. Dosimetric features including central axis absorbed doses, beam profiles and photon energy spectra were calculated for flattened and unflattened 6 MV photon beams. A substantial increase in the dose rate was seen for the unflattened beam, which was decreased with field size and depth. The penumbra width was decreased less than 0.2 mm (about 5%) and a 25% decrease in out-of-field dose was observed for the unflattened beam. The photon energy spectra were softer for the unflattened beam and the mean energies of spectra were higher for smaller field size. Our study showed that increase in the dose rate and lower out-of-field dose could be considered as practical advantages for unflattened 6 MV beams.

Dosimetric properties of a flattening filter-free 6-MV photon beam: a Monte Carlo study

PURPOSE

The dosimetric features of an unflattened 6-MV photon beam of an Elekta SL-25 linac was calculated by the Monte Carlo (MC) method.

MATERIAL AND METHODS

The head of the Elekta SL-25 linac was simulated using the MCNP4C MC code. The accuracy of the model was evaluated using measured dosimetric features, including depth dose values and dose profiles in a water phantom. The flattening filter was then removed, and beam dosimetric properties were calculated by the MC method and compared with those of the flattened photon beam.

RESULTS

Our results showed a significant (twofold) increase in the dose rate for all field sizes. Also, the photon beam spectra for an unflattened beam were softer, which led to a steeper reduction in depth doses. The decrease in the out-of-field dose and increase in the contamination electrons and a buildup region dose were the other consequences of removing the flattening filter.

CONCLUSION

Our study revealed that, for recent radiotherapy techniques, the use of multileaf collimators for beam shaping removing the flattening filter could offer some advantages, including an increased dose rate and decreased out-of-field dose.

Optimization of photon beam flatness for radiation therapy

In this work, we investigate the relation between lateral fluence/dose distributions and photon beam uniformity, possibly identifying ways to improve these characteristics. The calculations included treatment head scatter properties associated with three common types of linear accelerators in order to study their impact on the results. For 6 and 18 MV photon beams the lateral fluence distributions were optimized with respect to the resulting calculated flatness, as defined by the International Electrotechnical Commission (IEC), at 10 cm depth in six different field sizes. The limits proposed by IEC for maximum dose ratios ('horns') at the depth of dose maximum have also been accounted for in the optimization procedure. The conclusion was that typical head scatter variations among different types of linear accelerators have a very limited effect on the optimized results, which implies that the existing differences in measured off-axis dose distributions are related to non-equivalent optimization objectives. Finally, a comparison between the theoretically optimized lateral dose distributions and corresponding dose measurements for the three investigated accelerator types was performed. Although the measured data generally fall within the IEC requirements the optimized distributions show better results overall for the evaluated uniformity parameters, indicating that there is room for improved flatness performance in clinical photon beams.

Tomotherapy as a tool in image-guided radiation therapy (IGRT): theoretical and technological aspects

Helical tomotherapy (HT) is a novel treatment approach that combines Intensity-Modulate Radiation Therapy (IMRT) delivery with in-built image guidance using megavoltage (MV) CT scanning. The technique utilises a 6 MV linear accelerator mounted on a CT type ring gantry. The beam is collimated to a fan beam, which is intensity modulated using a binary multileaf collimator (MLC). As the patient advances slowly through the ring gantry, the linac rotates around the patient with a leaf-opening pattern optimised to deliver a highly conformal dose distribution to the target in the helical beam trajectory. The unit also allows the acquisition of MVCT images using the same radiation source detuned to reduce its effective energy to 3.5 MV, making the dose required for imaging less than 3 cGy. This paper discusses the major features of HT and describes the advantages and disadvantages of this approach in the context of the commercial Hi-ART system.

Measurements of in-air output ratios for a linear accelerator with and without the flattening filter

The in-air output ratio (Sc) for photon beams from linear accelerators describes the change of in-air output as a function of the collimator settings. The physical origin of the Sc is mainly due to the change in scattered radiation that can reach the point of measurement as the geometry of the head changes. The flattening filter (FF) and primary collimator are the major sources of scattered radiation. The change in amount of backscattered radiation from the collimator into the beam-monitoring chamber also contributes to the variation of output. In this work, we measured the Sc and backscatter factors (Sb) into the beam-monitoring chamber for a linear accelerator with and without the FF. We measured the Sc with a Farmer-type chamber in a miniphantom at the depth of 10 g/cm2 for 6- and 18-MV x-ray beams from a Varian Clinac 2100EX linear accelerator. The Sb were measured with a universal pulse counter and a diode array with build-in counting hardware and software. The head scatter component (Sh) was then derived from the relationship Sc= Sh x Sb, where Sb was the linear fit of measured results. Significant differences were observed for Sc with and without the FF. Within the range of experimental uncertainty, the Sb was similar with and without the FF. The variations in Sh differed significantly over the range of field sizes of 3 X 3 to 40 X 40 cm2 with and without the FF; for the 6-MV beam, it was 8% vs 3%, and for the 18-MV beam, 7% vs 1%. By analyzing the contributions of backscatter factor and total in-air output ratios with and without the FF, we directly gained insight into the contributions of different components to the total variations in Sc of a linear accelerator. Sc, Sb, and Sh are basic and useful dosimetric quantities for delivery of intensity-modulated radiation therapy using a linear accelerator operating in a mode without the FF.

Radiation shielding design of a new tomotherapy facility

It is expected that intensity modulated radiation therapy (IMRT) and image guided radiation therapy (IGRT) will replace a large portion of radiation therapy treatments currently performed with conventional MLC-based 3D conformal techniques. IGRT may become the standard of treatment in the future for prostate and head and neck cancer. Many established facilities may convert existing vaults to perform this treatment method using new or upgraded equipment. In the future, more facilities undoubtedly will be considering de novo designs for their treatment vaults. A reevaluation of the design principles used in conventional vault design is of benefit to those considering this approach with a new tomotherapy facility. This is made more imperative as the design of the TomoTherapy system is unique in several aspects and does not fit well into the formalism of NCRP 49 for a conventional linear accelerator.

Helical tomotherapy dynamic quality assurance

A multifaceted tomotherapy quality assurance procedure has been developed. This procedure tests most of the features inherent in the tomotherapy Hi-Art device. This includes the megavoltage imaging quality, spatial and temporal accuracy of the dynamic delivery properties, as well as more traditional beam output characteristics. This is accomplished with a specialized multichannel electrometer that measures collected charge every 100 ms and a Virtual Water cylindrical phantom that holds many ion chambers and differing density insert plugs. Both devices are offered with the Hi-Art product. These tests are presented as well as their sensitivity to beam and delivery variations.

Properties of unflattened photon beams shaped by a multileaf collimator

Several studies have shown that removal of the flattening filter from the treatment head of a clinical accelerator increases the dose rate and changes the lateral profile in radiation therapy with photons. However, the multileaf collimator (MLC) used to shape the field was not taken into consideration in these studies. We therefore investigated the effect of the MLC on flattened and unflattened beams. To do this, we performed measurements on a Varian Clinac 21EX and MCNPX Monte Carlo simulations to analyze the physical properties of the photon beam. We compared lateral profiles, depth dose curves, MLC leakages, and total scatter factors for two energies (6 and 18 MV) of MLC-shaped fields and jaw-shaped fields. Our study showed that flattening filter-free beams shaped by a MLC differ from the jaw-shaped beams. Similar differences were also observed for flattened beams. Although both collimating methods produced identical depth dose curves, the penumbra size and the MLC leakage were reduced in the softer, unflattened beam and the total scatter factors showed a smaller field size dependence.

A flattening filter free photon treatment concept evaluation with Monte Carlo

In principle, the concept of flat initial radiation-dose distribution across the beam is unnecessary for intensity modulated radiation therapy. Dynamic leaf positioning during irradiation could appropriately adjust the fluence distribution of an unflattened beam that is peaked in the center and deliver the desired uniform or nonuniform dose distribution. Removing the flattening filter could lead to reduced treatment time through higher dose rates and reduced scatter, because there would be substantially less material in the beam; and possibly other dosimetric and clinical advantages. This work aims to evaluate the properties of a flattening filter free clinical accelerator and to investigate its possible advantages in clinical intensity modulated radiation therapy applications by simulating a Varian 2100-based treatment delivery system with Monte Carlo techniques. Several depth-dose curves and lateral dose distribution profiles have been created for various field sizes, with and without the flattening filter. Data computed with this model were used to evaluate the overall quality of such a system in terms of changes in dose rate, photon and electron fluence, and reduction in out-of-field stray dose from the scattered components and were compared to the corresponding data for a standard treatment head with a flattening filter. The results of the simulations of the flattening filter free system show that a substantial increase in dose rate can be achieved, which would reduce the beam on time and decrease the out-of-field dose for patients due to reduced head-leakage dose. Also close to the treatment field edge, a significant improvement in out-of-field dose could be observed for small fields, which can be attributed to the change in the photon spectra, when the flattening filter is removed from the beamline.

Le système de tomothérapie hélicoïdale pour la radiothérapie modulée en intensité et guidée par l'image : développements récents et applications cliniques

The advent of 3D conformal radiotherapy and intensity modulated radiation therapy (IMRT) make possible the dose optimization to complex target volumes close to sane organs at risk. IMRT's introduction of numerous small radiation fields inherently increases delivery inaccuracies. As a consequence, the use of IMRT without precise localization of the tumor and sensitive structures, at both the planning and delivery stages, and the absence of continuous verification represent the most significant challenges to the implementation of IMRT in routine clinical use. Intensity modulated (or not) conformal radiotherapy delivery requires better precision in the definition of treatment volume, frequently if necessary. Helical tomotherapy has been designed to use CT imaging technology to plan, deliver, and verify that the delivery has been carried out as planned. The image-guided and intensity modulations processes of helical tomotherapy that enable this goal are described.

Out-of-field dosimetry measurements for a helical tomotherapy system

Helical tomotherapy is a rotational delivery technique that uses intensity‐modulated fan beams to deliver highly conformal intensity‐modulated radiation therapy (IMRT). The beam‐on time needed to deliver a given prescribed dose can be up to 15 times longer than that needed using conventional treatment delivery. As such, there is concern that this delivery technique has the potential to increase the whole body dose due to increased leakage. The purpose of this work is to directly measure out‐of‐field doses for a clinical tomotherapy system. Peripheral doses were measured in‐phantom using static fields and rotational intensity‐modulated delivery. In‐air scatter and leakage doses were also measured at multiple locations around the treatment room. At 20 cm, the tomotherapy peripheral dose dropped to 0.4% of the prescribed dose. Leakage accounted for 94% of the in‐air dose at distances greater than 60 cm from the machine's isocenter. The largest measured dose equivalent rate was 1×10−10 Sv/s in the plane of gantry rotation due to head leakage and primary beam transmission through the system's beam stopper. The dose equivalent rate dropped to 1×10−10 Sv/s at the end of the treatment couch. Even though helical tomotherapy treatment delivery requires beam‐on times that are 5 to 15 times longer than those used by conventional accelerators, the delivery system was designed to maximize shielding for radiation leakage. As such, the peripheral doses are equal to or less than the published peripheral doses for IMRT delivery on other linear accelerators. In addition, the shielding requirements are also similar to conventional linear accelerators.

PACS number: 87.53.Dq

Monte Carlo study of photon fields from a flattening filter-free clinical accelerator

In conventional clinical linear accelerators, the flattening filter scatters and absorbs a large fraction of primary photons. Increasing the beam-on time, which also increases the out-of-field exposure to patients, compensates for the reduction in photon fluence. In recent years, intensity modulated radiation therapy has been introduced, yielding better dose distributions than conventional three-dimensional conformal therapy. The drawback of this method is the further increase in beam-on time. An accelerator with the flattening filter removed, which would increase photon fluence greatly, could deliver considerably higher dose rates. The objective of the present study is to investigate the dosimetric properties of 6 and 18 MV photon beams from an accelerator without a flattening filter. The dosimetric data were generated using the Monte Carlo programs BEAMnrc and DOSXYZnrc. The accelerator model was based on the Varian Clinac 2100 design. We compared depth doses, dose rates, lateral profiles, doses outside collimation, total and collimator scatter factors for an accelerator with and without a flatteneing filter. The study showed that removing the filter increased the dose rate on the central axis by a factor of 2.31 (6 MV) and 5.45 (18 MV) at a given target current. Because the flattening filter is a major source of head scatter photons, its removal from the beam line could reduce the out-of-field dose.

History of tomotherapy

Tomotherapy is the delivery of intensity modulated radiation therapy using rotational delivery of a fan beam in the manner of a CT scanner. In helical tomotherapy the couch and gantry are in continuous motion akin to a helical CT scanner. Helical tomotherapy is inherently capable of acquiring CT images of the patient in treatment position and using this information for image guidance. This review documents technological advancements of the field concentrating on the conceptual beginnings through to its first clinical implementation. The history of helical tomotherapy is also a story of technology migration from academic research to a university-industrial partnership, and finally to commercialization and widespread clinical use.

Dosimetric properties of photon beams from a flattening filter free clinical accelerator

Basic dosimetric properties of 6 MV and 18 MV photon beams from a Varian Clinac 21EX accelerator operating without the flattening filter have been measured. These include dose rate data, depth dose dependencies and lateral profiles in a water phantom, total scatter factors and transmission factors of a multileaf collimator. The data are reviewed and compared with measurements for the flattened beams. The unflattened beams have the following: a higher dose rate by factors of 2.3 (6 MV) and 5.5 (18 MV) on the central axis; lower out-of-field dose due to reduced head scatter and softer spectra; less variation of the total scatter factor with field size; and less variation of the shape of lateral dose profiles with depth. The findings suggest that with a flattening filter free accelerator better radiation treatments can be developed, with shorter delivery times and lower doses to normal tissues and organs.

Integral radiation dose to normal structures with conformal external beam radiation

BACKGROUND

This study was designed to evaluate the integral dose (ID) received by normal tissue from intensity-modulated radiotherapy (IMRT) for prostate cancer.

METHODS AND MATERIALS

Twenty-five radiation treatment plans including IMRT using a conventional linac with both 6 MV (6MV-IMRT) and 20 MV (20MV-IMRT), as well as three-dimensional conformal radiotherapy (3DCRT) using 6 MV (6MV-3DCRT) and 20 MV (20MV-3DCRT) and IMRT using tomotherapy (6MV) (Tomo-IMRT), were created for 5 patients with localized prostate cancer. The ID (mean dose x tissue volume) received by normal tissue (NTID) was calculated from dose-volume histograms.

RESULTS

The 6MV-IMRT resulted in 5.0% lower NTID than 6MV-3DCRT; 20 MV beam plans resulted in 7.7%-11.2% lower NTID than 6MV-3DCRT. Tomo-IMRT NTID was comparable to 6MV-IMRT. Compared with 6MV-3DCRT, 6MV-IMRT reduced IDs to the rectal wall and penile bulb by 6.1% and 2.7%, respectively. Tomo-IMRT further reduced these IDs by 11.9% and 16.5%, respectively. The 20 MV did not reduce IDs to those structures.

CONCLUSIONS

The difference in NTID between 3DCRT and IMRT is small. The 20 MV plans somewhat reduced NTID compared with 6 MV plans. The advantage of tomotherapy over conventional IMRT and 3DCRT for localized prostate cancer was demonstrated in regard to dose sparing of rectal wall and penile bulb while slightly decreasing NTID as compared with 6MV-3DCRT.

Evaluation of image-guided helical tomotherapy for the retreatment of spinal metastasis

INTRODUCTION

Patients with vertebral metastasis that receive radiation therapy are typically treated to the spinal cord tolerance dose. As such, it is difficult to successfully deliver a second course of radiation therapy for patients with overlapping treatment volumes. In this study, an image-guided helical tomotherapy system was evaluated for the retreatment of previously irradiated vertebral metastasis.

METHODS AND MATERIALS

Helical tomotherapy dose gradients and maximum cord doses were measured in a cylindrical phantom for geometric test cases with separations between the planning target volume (PTV) and the spinal cord organ at risk (OAR) of 2 mm, 4 mm, 6 mm, 8 mm, and 10 mm. Megavoltage computed tomography (CT) images were examined for their ability to localize spinal anatomy for positioning purposes by repeat imaging of the cervical spine in an anthropomorphic phantom. In addition to the phantom studies, 8 patients with cord compressions that had received previous radiation therapy were retreated to a mean dose of 28 Gy using conventional fractionation.

RESULTS AND DISCUSSION

Megavoltage CT images were capable of positioning an anthropomorphic phantom to within +/-1.2 mm (2sigma) superior-inferiorly and within +/-0.6 mm (2sigma) anterior-posteriorly and laterally. Dose gradients of 10% per mm were measured in phantom while PTV uniformity indices of less than 11% were maintained. The calculated maximum cord dose was 25% of the prescribed dose for a 10-mm PTV-to-OAR separation and 71% of the prescribed dose for a PTV-to-OAR separation of 2 mm. Eight patients total have been treated without radiation-induced myelopathy or any other adverse effects from treatment.

CONCLUSIONS

A technique has been evaluated for the retreatment of vertebral metastasis using image-guided helical tomotherapy. Phantom and patient studies indicated that a tomotherapy system is capable of delivering dose gradients of 10% per mm and positioning the patient within 1.2 mm without the use of special stereotactic immobilization.

The use of megavoltage CT (MVCT) images for dose recomputations

Megavoltage CT (MVCT) images of patients are acquired daily on a helical tomotherapy unit (TomoTherapy, Inc., Madison, WI). While these images are used primarily for patient alignment, they can also be used to recalculate the treatment plan for the patient anatomy of the day. The use of MVCT images for dose computations requires a reliable CT number to electron density calibration curve. In this work, we tested the stability of the MVCT numbers by determining the variation of this calibration with spatial arrangement of the phantom, time and MVCT acquisition parameters. The two calibration curves that represent the largest variations were applied to six clinical MVCT images for recalculations to test for dosimetric uncertainties. Among the six cases tested, the largest difference in any of the dosimetric endpoints was 3.1% but more typically the dosimetric endpoints varied by less than 2%. Using an average CT to electron density calibration and a thorax phantom, a series of end-to-end tests were run. Using a rigid phantom, recalculated dose volume histograms (DVHs) were compared with plan DVHs. Using a deformed phantom, recalculated point dose variations were compared with measurements. The MVCT field of view is limited and the image space outside this field of view can be filled in with information from the planning kVCT. This merging technique was tested for a rigid phantom. Finally, the influence of the MVCT slice thickness on the dose recalculation was investigated. The dosimetric differences observed in all phantom tests were within the range of dosimetric uncertainties observed due to variations in the calibration curve. The use of MVCT images allows the assessment of daily dose distributions with an accuracy that is similar to that of the initial kVCT dose calculation.

Initial experience with megavoltage (MV) CT guidance for daily prostate alignments

PURPOSE

The on-board megavoltage (MV) computed tomography (CT) capabilities of a TomoTherapy Hi*ART unit were used to obtain daily MVCT images of prostate cancer patients. For patient alignment the daily MVCT image needs to be registered with the planning CT image to calculate couch shifts. Three manual techniques of registering the MVCT images with the planning kilovoltage (kV) CT images were evaluated. The techniques are based on visual alignment of (1) fiducial prostate markers, (2) CT anatomy, and (3) kVCT contours.

METHODS AND MATERIALS

One hundred and twelve alignments from 3 patients were available for analysis. The radiation therapists visually registered the MVCT images with the planning kVCT images based on fiducial markers for actual patient alignment. Retrospectively, the therapists registered each image set using anatomy and contour-based techniques. In addition to the therapists, a physician retrospectively registered each image set based on each of the three techniques. For each MVCT to kVCT image pair a reference alignment was computed from the center-of-mass (COM) of the three fiducial markers. All registration results were compared with these reference alignments. The physician's image registrations were compared with the radiation therapists' registrations to assess the user variability of the different techniques.

RESULTS

The marker-based registration results agree best with the reference alignments, while the contour-based registrations show the least degree of agreement. Using anatomy and contour-based registrations, the radiation therapist's alignments differed by > or = 3 mm from the reference alignments in 24%, 33%, and 3% and 55%, 48%, and 21% of all registrations in the anterior-posterior, superior-inferior, and lateral directions, respectively. The respective values for the marker-based alignments were 3%, 6%, and 3%. The physician's registrations showed the same general trend. The marker-based registrations showed the least amount of inter-user variability while the contour-based ones showed the most.

CONCLUSION

The use of fiducial markers for MVCT image guidance is advantageous to reduce the inter-user variability of the image registration. If fiducial markers are not used, anatomy-based registrations outperform contour-based registrations in terms of (1) agreement with a reference alignment and (2) inter-user variability.

Micro ionization chamber dosimetry in IMRT verification: Clinical implications of dosimetric errors in the PTV

BACKGROUND AND PURPOSE

Absolute dose measurements for Intensity Modulated Radiotherapy (IMRT) beamlets is difficult due to the lack of lateral electron equilibrium. Recently we found that the absolute dosimetry in the penumbra region of the IMRT beamlet, can suffer from significant errors (Capote et al., Med Phys 31 (2004) 2416-2422). This work has the goal to estimate the error made when measuring the Planning Target Volume's (PTV) absolute dose by a micro ion chamber (microIC) in typical IMRT treatment. The dose error comes from the assumption that the dosimetric parameters determining the absolute dose are the same as for the reference conditions.

MATERIALS AND METHODS

Two IMRT treatment plans for common prostate carcinoma case, derived by forward and inverse optimisation, were considered. Detailed geometrical simulation of the microIC and the dose verification set-up was performed. The Monte Carlo (MC) simulation allows us to calculate the delivered dose to water and the dose delivered to the active volume of the ion chamber. However, the measured dose in water is usually derived from chamber readings assuming reference conditions. The MC simulation provides needed correction factors for ion chamber dosimetry in non reference conditions.

RESULTS

Dose calculations were carried out for some representative beamlets, a combination of segments and for the delivered IMRT treatments. We observe that the largest dose errors (i.e. the largest correction factors) correspond to the smaller contribution of the corresponding IMRT beamlets to the total dose delivered in the ionization chamber within PTV.

CONCLUSION

The clinical impact of the calculated dose error in PTV measured dose was found to be negligible for studied IMRT treatments.

Investigation of dose homogeneity for loose helical tomotherapy delivery in the context of breath-hold radiation therapy

Loose helical delivery is a potential solution to account for respiration-driven tumour motion in helical tomotherapy (HT). In this approach, a treatment is divided into a set of interlaced 'loose' helices commencing at different gantry angles. Each loose helix covers the entire target length in one gantry rotation during a single breath-hold. The dosimetric characteristics of loose helical delivery were investigated by delivering a 6 MV photon beam in a HT-like manner. Multiple scenarios of conventional 'tight' HT and loose helical deliveries were modelled in treatment planning software, and carried out experimentally with Kodak EDR2 film. The advantage of loose helical delivery lies in its ability to produce a more homogeneous dose distribution by eliminating the 'thread' effect-an inherent characteristic of HT, which results in dose modulations away from the axis of gantry rotation. However, loose helical delivery was also subjected to undesirable dose modulations in the direction of couch motion (termed 'beating' effect), when the ratio between the number of beam projections per gantry rotation (n) and pitch factor (p) was a non-integer. The magnitude of dose modulations decreased with an increasing n/p ratio. The results suggest that for the current HT unit (n = 51), dose modulations could be kept under 5% by selecting a pitch factor smaller than 7. A pitch factor of this magnitude should be able to treat a target up to 30 cm in length. Loose helical delivery should increase the total session time only by a factor of 2, while the planning time should stay the same since the total number of beam projections remains unchanged. Considering its dosimetric advantage and clinical practicality, loose helical delivery is a promising solution for the future HT treatments of respiration-driven targets.

A Monte Carlo derived TG-51 equivalent calibration for helical tomotherapy

Helical tomotherapy (HT) requires a method of accurately determining the absorbed dose under reference conditions. In the AAPM's TG-51 external beam dosimetry protocol, the quality conversion factor, kQ, is presented as a function of the photon component of the percentage depth-dose at 10 cm depth, %dd(10)x, measured under the reference conditions of a 10 x 10 cm2 field size and a source-to-surface distance (SSD) of 100 cm. The value of %dd(10)x from HT cannot be used for the determination of kQ because the design of the HT does not meet the following TG-51 reference conditions: (i) the field size and the practical SSD required by TG-51 are not obtainable and (ii) the absence of the flattening filter changes the beam quality thus affecting some components of kQ. The stopping power ratio is not affected because of its direct relationship to %dd(10)x. We derive a relationship for the Exradin A1SL ion chamber converting the %dd(10)x measured under HT "reference conditions" of SSD=85 cm and a 5 x 10 cm2 field-size [%dd(10)x[HT Ref]], to the dosimetric equivalent value under for TG-51 reference conditions [%dd(10)x[HT TG-51]] for HT. This allows the determination of kQ under the HT reference conditions. The conversion results in changes of 0.1% in the value of kQ for our particular unit. The conversion relationship should also apply to other ion chambers with possible errors on the order of 0.1%.

Dose calibration of nonconventional treatment systems applied to helical tomotherapy

Current dosimetric protocols based on the absorbed dose (AAPM TG-51 and IAEA TRS-398 protocols) require calibration measurements under reference conditions. For some radiotherapy systems, this requirement cannot be met, and calibration has to be performed under nonreference experimental conditions. In order to solve this problem, both protocols can be extended by inclusion of the measured-to-reference conversion factor, k(mr). In order to determine this factor, basic dosimetric quantities, like stopping power ratios, mass attenuation coefficients and chamber correction factors have to be calculated. If measurements are not feasible, accurate Monte Carlo modeling is required. The extension of the protocols is illustrated using the case of the helical tomotherapy radiation unit, where the typical calibration measurement conditions are the 10 x 5 cm2 field size and the 85 cm surface source distance, limited by the system design. It was calculated that the k(mr) factor for this conditions is close to unity (0.997+/-0.001). In addition, the deviation of the measurement conditions from the reference conditions results in the change of the quality conversion factor (approximately 0.995-0.998, depending on the ionization chamber used). This change is the same regardless of the used calibration protocol. For smaller field sizes the corrections become more significant, resulting in the total correction factor compared to the reference conditions of up to 1.5% for the smallest considered field size of 2 x 2 cm2.