Tomotherapy: a new concept for the delivery of dynamic conformal radiotherapy

Doses to critical organs following radiotherapy and concomitant imaging of the larynx and breast

The development of conformal radiotherapy carries with it the implication of an increased number of imaging procedures at various stages throughout the overall treatment, principally for verification at some, or all, of the treatment fractions. This raises the issue of the balance between the benefit of these additional imaging exposures and the associated risk of radiocarcinogenesis arising from them. As such, it is necessary to appreciate the doses to critical organs for which individual carcinogenic risks have been estimated. In this study, doses to these organs have been measured with lithium fluoride thermoluminescence dosimetry loaded in anthropomorphic phantoms and subjected to realistic radiotherapy treatments of the larynx and breast, including concomitant CT and electronic portal imaging exposures associated with localization and verification of these treatments. Even for large numbers of concomitant images of either modality, arising from imaging at every fraction, the leakage and scatter from the radiotherapy itself is shown to dominate the overall organ dose, with imaging procedures generally contributing 5-20% of the total organ dose.

Advanced image-guided external beam radiotherapy

The goal of radiation therapy is to eradicate tumor stem cells while sparing healthy tissue. Therefore, the first aim must be to delineate tumor from healthy tissue. Advanced imaging techniques will enable one to reduce the uncertainty of microscopic extension of disease. Ultimately, advanced functional imaging systems correlated with image-registered pathological specimens will allow one to delineate disease extent from normal tissue at the tumor periphery. When it is not possible to determine the CTV margin with reasonable certainty, the margins must remain generous and conformal avoidance methodology could and should be deployed to spare critical normal structures. Of equal importance to defining the CTV is the need to guarantee that this target is indeed treated. For this purpose, image guidance using a variety of systems including portal images, ultrasound devices, and CT scanners at the time of treatment has been implemented. Some image-guided methods, portal images for instance, are more amenable for use with rigid structures such as encountered in the sinus whereas others like ultrasound or CT scanners are able to account for nonrigid setup variations. Several strategies for preventing organ motion from degrading the precision that radiotherapy offers have been described. In particular, a CT scan at the time of treatment delivery can also be used as the basis to reconstruct the dose received by the patient. Dose reconstruction will allow the dose just delivered to be superimposed on the pretreatment CT scan and will allow one to compare the reconstructed delivered dose distribution with the planned dose distribution to assess discrepancies between these. Furthermore, reconstruction of the delivered dose distributions holds the promise of allowing one to accumulate dose delivered to the tumor and normal structures on a fraction per fraction basis. This will ultimately allow for the determination of treatment-specific tumor control probabilities and normal tissue complication probabilities.

Intensity-modulated radiation therapy and helical tomotherapy: its origin, benefits, and potential applications in veterinary medicine

Intensity-modulated radiation therapy (IMRT), especially image-guided IMRT as represented by helical tomotherapy, is a novel approach to therapy and is rapidly evolving. Both of these forms of therapy aim to allow targeted radiation delivery to the tumor volume while minimizing dose to the surrounding normal tissues. Adaptive radiation therapy and conformal avoidance are possible with intensity-modulated therapy and helical tomotherapy, which offer opportunities for improved local tumor control, decreased normal tissue toxicity, and improved survival and quality of life. Human and veterinary patients are likely to benefit from the continued development of this radiation delivery technique, and data over the next several years should be crucial in determining its true benefit.

The impact of linac output variations on dose distributions in helical tomotherapy

It has been suggested for quality assurance purposes that linac output variations for helical tomotherapy (HT) be within +/-2% of the long-term average. Due to cancellation of systematic uncertainty and averaging of random uncertainty over multiple beam directions, relative uncertainties in the dose distribution can be significantly lower than those in linac output. The sensitivity of four HT cases with respect to linac output uncertainties was assessed by scaling both modeled and measured systematic and random linac output uncertainties until a dose uncertainty acceptance criterion failed. The dose uncertainty acceptance criterion required the delivered dose to have at least a 95% chance of being within 2% of the planned dose in all of the voxels in the treatment volume. For a random linac output uncertainty of 5% of the long-term mean, the maximum acceptable amplitude of the modeled, sinusoidal, systematic component of the linac output uncertainty for the four cases was 1.8%. Although the measured linac output variations represented values that were outside of the +/-2% tolerance, the acceptance criterion did not fail for any of the four cases until the measured linac output variations were scaled by a factor of almost three. Thus, the +/-2% tolerance in linac output variations for HT is a more conservative tolerance than necessary.

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.

The management of imaging dose during image-guided radiotherapy: report of the AAPM Task Group 75

Radiographic image guidance has emerged as the new paradigm for patient positioning, target localization, and external beam alignment in radiotherapy. Although widely varied in modality and method, all radiographic guidance techniques have one thing in common--they can give a significant radiation dose to the patient. As with all medical uses of ionizing radiation, the general view is that this exposure should be carefully managed. The philosophy for dose management adopted by the diagnostic imaging community is summarized by the acronym ALARA, i.e., as low as reasonably achievable. But unlike the general situation with diagnostic imaging and image-guided surgery, image-guided radiotherapy (IGRT) adds the imaging dose to an already high level of therapeutic radiation. There is furthermore an interplay between increased imaging and improved therapeutic dose conformity that suggests the possibility of optimizing rather than simply minimizing the imaging dose. For this reason, the management of imaging dose during radiotherapy is a different problem than its management during routine diagnostic or image-guided surgical procedures. The imaging dose received as part of a radiotherapy treatment has long been regarded as negligible and thus has been quantified in a fairly loose manner. On the other hand, radiation oncologists examine the therapy dose distribution in minute detail. The introduction of more intensive imaging procedures for IGRT now obligates the clinician to evaluate therapeutic and imaging doses in a more balanced manner. This task group is charged with addressing the issue of radiation dose delivered via image guidance techniques during radiotherapy. The group has developed this charge into three objectives: (1) Compile an overview of image-guidance techniques and their associated radiation dose levels, to provide the clinician using a particular set of image guidance techniques with enough data to estimate the total diagnostic dose for a specific treatment scenario, (2) identify ways to reduce the total imaging dose without sacrificing essential imaging information, and (3) recommend optimization strategies to trade off imaging dose with improvements in therapeutic dose delivery. The end goal is to enable the design of image guidance regimens that are as effective and efficient as possible.

Innovations in image-guided radiotherapy

The limited ability to control for the location of a tumour compromises the accuracy with which radiation can be delivered to tumour-bearing tissue. The resultant requirement for larger treatment volumes to accommodate target uncertainty restricts the radiation dose because more surrounding normal tissue is exposed. With image-guided radiotherapy (IGRT) these volumes can be optimized and tumoricidal doses can be delivered, achieving maximal tumour control with minimal complications. Moreover, with the ability of high-precision dose delivery and real-time knowledge of the target volume location, IGRT has initiated the exploration of new indications for radiotherapy, some of which were previously considered infeasible.

A quantitative image quality comparison of four different image guided radiotherapy devices

PURPOSE

A study to quantitatively compare the image quality of four different image guided radiotherapy (IGRT) devices based on phantom measurements with respect to the additional dose delivered to the patient.

METHODS

Images of three different head-sized phantoms (diameter 16-18 cm) were acquired with the following four IGRT-CT solutions: (i) the Siemens Primatom single slice fan beam computed tomography (CT) scanner with an acceleration voltage of 130 kV, (ii) a Tomotherapy HI-ART II unit using a fan beam scanner with an energy of 3.5 MeV and (iii) the Siemens Artíste prototype, providing the possibility to perform kV (121 kV) and MV (6 MV) cone beam (CB) CTs. For each device three scan protocols (named low, normal, high) were selected to yield the same weighted computed tomography dose index (CTDI(w)). Based on the individual inserts of the different phantoms the image quality achieved with each device at a certain dose level was characterized in terms of homogeneity, spatial resolution, signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR) and electron density-to-CT-number conversion.

RESULTS

Based on the current findings for head-sized phantoms all devices show an electron density-to-CT-number conversion almost independent of the imaging parameters and hence can be suited for treatment planning purposes. The evaluation of the image quality, however, points out clear differences due to the different energies and geometries. The Primatom standard CT scanner shows throughout the best performance, especially for soft tissue contrast and spatial resolution with low imaging doses. Reasonable soft tissue contrast can be obtained with slightly higher doses compared to the CT scanner with the kVCB and the Tomotherapy unit. In order to get similar results with the MVCB system a much higher dose needs to be applied to the patient.

CONCLUSION

Considering the entire investigations, especially in terms of contrast and spatial resolution, a rough tendency for decreasing image quality can be given: Primatom, Artíste prototype kVCB, Tomotherapy, Artíste prototype MVCB.

Whole brain radiotherapy with hippocampal avoidance and simultaneously integrated brain metastases boost: a planning study

PURPOSE

To evaluate the feasibility of using tomotherapy to deliver whole brain radiotherapy with hippocampal avoidance, hypothesized to reduce the risk of memory function decline, and simultaneously integrated boost to brain metastases to improve intracranial tumor control.

METHODS AND MATERIALS

Ten patients treated with radiosurgery and whole brain radiotherapy underwent repeat planning using tomotherapy with the original computed tomography scans and magnetic resonance imaging-computed tomography fusion-defined target and normal structure contours. The individually contoured hippocampus was used as a dose-limiting structure (<6 Gy); the whole brain dose was prescribed at 32.25 Gy to 95% in 15 fractions, and the simultaneous boost doses to individual brain metastases were 63 Gy to lesions >or=2.0 cm in the maximal diameter and 70.8 Gy to lesions <2.0 cm. The plans were generated with a field width (FW) of 2.5 cm and, in 5 patients, with a FW of 1.0 cm. The plans were compared regarding conformation number, prescription isodose/target volume ratio, target coverage, homogeneity index, and mean normalized total dose.

RESULTS

A 1.0-cm FW compared with a 2.5-cm FW significantly improved the dose distribution. The mean conformation number improved from 0.55 +/- 0.16 to 0.60 +/- 0.13. Whole brain homogeneity improved by 32% (p <0.001). The mean normalized total dose to the hippocampus was 5.9 +/- 1.3 Gy(2) and 5.8 +/- 1.9 Gy(2) for 2.5- and 1.0-cm FW, respectively. The mean treatment delivery time for the 2.5- and 1.0-cm FW plans was 10.2 +/- 1.0 and 21.8 +/- 1.8 min, respectively.

CONCLUSION

Composite tomotherapy plans achieved three objectives: homogeneous whole brain dose distribution equivalent to conventional whole brain radiotherapy; conformal hippocampal avoidance; and radiosurgically equivalent dose distributions to individual metastases.

On the making of sharp longitudinal dose profiles with helical tomotherapy

Since the beam width on the helical tomotherapy machine produced by TomoTherapy Inc., is typically a few centimeters in the longitudinal direction (into the bore), the optimizer must choose to have a relatively high intensity local to the inside edge of a tumor or planning treatment volume (PTV) when avoiding an immediately adjacent organ at risk (OAR), either superior or inferior. By using a standalone version of the TomoTherapy dose calculator, a realistic beam is applied to idealized deconvolution schemes including the MATLAB Optimizer Toolbox for a simple one-dimensional PTV with adjacent OARs. The results are compared to a clinical example on the TomoTherapy planning station. It is learned that a Gibbs phenomenon type of oscillation in the dose within the tumor under these special circumstances is not unique to TomoTherapy, but is related to the attempt to form a sharp dose gradient-sharper than the beam profile with typical optimization constraints set to achieve a uniform dose as close as possible to the prescription. The clinical implication is that the Gibbs-induced cold spots force the dose to increase in the PTV if a typical PTV dose-volume constraint is used. It is recommended that the dose prescription be smoothed prior to optimization or the dosimetric goals for an OAR adjacent to the PTV are such that a sharp dose falloff is not demanded, especially if the user reduces the requirements that such an OAR be of both high importance and immediately adjacent to the PTV edge.

Helical tomotherapy for radiotherapy in esophageal cancer: a preferred plan with better conformal target coverage and more homogeneous dose distribution

We compare different radiotherapy techniques-helical tomotherapy (tomotherapy), step-and-shoot IMRT (IMRT), and 3-dimensional conformal radiotherapy (3DCRT)-for patients with mid-distal esophageal carcinoma on the basis of dosimetric analysis. Six patients with locally advanced mid-distal esophageal carcinoma were treated with neoadjuvant chemoradiation followed by surgery. Radiotherapy included 50 Gy to gross planning target volume (PTV) and 45 Gy to elective PTV in 25 fractions. Tomotherapy, IMRT, and 3DCRT plans were generated. Dose-volume histograms (DVHs), homogeneity index (HI), volumes of lung receiving more than 10, 15, or 20 Gy (V(10), V(15), V(20)), and volumes of heart receiving more than 30 or 45 Gy (V(30), V(45)) were determined. Statistical analysis was performed by paired t-tests. By isodose distributions and DVHs, tomotherapy plans showed sharper dose gradients, more conformal coverage, and better HI for both gross and elective PTVs compared with IMRT or 3DCRT plans. Mean V(20) of lung was significantly reduced in tomotherapy plans. However, tomotherapy and IMRT plans resulted in larger V(10) of lung compared to 3DCRT plans. The heart was significantly spared in tomotherapy and IMRT plans compared to 3DCRT plans in terms of V(30) and V(45). We conclude that tomotherapy plans are superior in terms of target conformity, dose homogeneity, and V(20) of lung.

Retrospective evaluation of pediatric cranio-spinal axis irradiation plans with the Hi-ART tomotherapy system

Helical tomotherapy (HT) can be used for the delivery of cranio-spinal axis irradiation (CSAI) without the need for beam matching of conventional linac-based external beam irradiation. The aim of this study is to retrospectively evaluate HT plans used for treatment in nine patients treated with CSAI. Helical tomotherapy cranio-spinal axis irradiation (HT-CSAI) plans were created for each patient. Average length along the cranio-spinal axis of the PTV was 65.6 cm with a range between 53 and 74 cm. Treatment planning optimization and plan evaluation parameters were obtained from the HT planning station for each of the nine patients. PTV coverage by the 95% isodose surface ranged between 98.0 to 100.0% for all nine patients. The clinically acceptable dose variation within the PTV or tolerance range was between 0.7 and 2.5% for all nine patients. Doses to the organs at risk were clinically acceptable. An increasing length along the longitudinal axis of the PTV did not consistently increase the beam-on time indicating that using a larger jaw width had a greater impact on treatment time. With a larger jaw width it is possible to substantially reduce the normalized beam-on treatment time without compromising plan quality and sparing of organs at risk. By using a larger jaw width or lower modulation factor or both, normalized beam-on times were decreased by up to 61% as compared to the other evaluated treatment plans. From the nine cases reported in this study the minimum beam-on time was achieved with a jaw width of 5.0 cm, pitch of 0.287 and a modulation factor of 2.0. Large and long cylindrical volumes can be effectively treated with helical tomotherapy with both clinically acceptable dose distribution and beam-on time.

Comparison of intensity modulated x-ray therapy and intensity modulated proton therapy for selective subvolume boosting: a phantom study

Selective subvolume boosting can theoretically improve tumour control probability while maintaining normal tissue complication probabilities similar to those of uniform dose distributions. In this work the abilities of intensity-modulated x-ray therapy (IMXT) and intensity-modulated proton therapy (IMPT) to deliver boosts to multiple subvolumes of varying size and proximities are compared in a thorough phantom study. IMXT plans were created using the step-and-shoot (IMXT-SAS) and helical tomotherapy (IMXT-HT) methods. IMPT plans were created with the spot scanning (IMPT-SS) and distal gradient tracking (IMPT-DGT) methods. IMPT-DGT is a generalization of the distal edge tracking method designed to reduce the number of proton beam spots required to deliver non-uniform dose distributions relative to IMPT-SS. The IMPT methods were delivered over both 180 degrees and 360 degrees arcs. The IMXT-SAS and IMPT-SS methods optimally satisfied the non-uniform dose prescriptions the least and the most, respectively. The IMPT delivery methods reduced the normal tissue integral dose by a factor of about 2 relative to the IMXT delivery methods, regardless of the delivery arc. The IMPT-DGT method reduced the number of proton beam spots by a factor of about 3 relative to the IMPT-SS method.

Multicentre quality assurance of intensity-modulated radiation therapy plans: A precursor to clinical trials

A multicentre planning study comparing intensity-modulated radiation therapy (IMRT) plans for the treatment of a head and neck cancer has been carried out. Three Australian radiotherapy centres, each with a different planning system, were supplied a fully contoured CT dataset and requested to generate an IMRT plan in accordance with the requirements of an IMRT-based radiation therapy oncology group clinical trial. Plan analysis was carried out using software developed specifically for reviewing multicentre clinical trial data. Two out of the three plans failed to meet the prescription requirements with one misinterpreting the prescription and the third failed to meet one of the constraints. Only one plan achieved all of the dose objectives for the critical structures and normal tissues. Although each centre used very similar planning parameters and beam arrangements the resulting plans were quite different. The subjective interpretation and application of the prescription and planning objectives emphasize one of the many difficulties in carrying out multicentre IMRT planning studies. The treatment prescription protocol in a clinical trial must be both lucid and unequivocally stated to avoid misinterpretation. Australian radiotherapy centres must show that they can produce a quality IMRT plan and that they can adhere to protocols for IMRT planning before using it in a clinical trial.

Skin-sparing radiation using intensity-modulated radiotherapy after conservative surgery in early-stage breast cancer: a planning study

PURPOSE

To evaluate the feasibility of skin-sparing by configuring it as an organ-at-risk (OAR) while delivering whole-breast intensity-modulated radiotherapy (IMRT) in early breast cancer.

METHODS AND MATERIALS

Archival computed tomography scan images of 14 left-sided early-breast tumor patients who had undergone lumpectomy were selected for this study. Skin was contoured as a 4- to 5-mm strip extending from the patient outline to anterior margin of the breast planning target volume (PTV). Two IMRT plans were generated by the helical tomotherapy approach to deliver 50 Gy in 25 fractions to the breast alone: one with skin dose constraints (skin-sparing plan) and the other without (non-skin-sparing plan). Comparison of the plans was done using a two-sided paired Student t test.

RESULTS

The mean skin dose and volume of skin receiving 50 Gy were significantly less with the skin-sparing plan compared with non-skin-sparing plan (42.3 Gy vs. 47.7 Gy and 12.2% vs. 57.8% respectively; p < 0.001). The reduction in skin dose was confirmed by TLD measurements in anthropomorphic phantom using the same plans. Dose-volume analyses for other OARs were similar in both plans.

CONCLUSIONS

By configuring the skin as an OAR, it is possible to achieve skin dose reduction while delivering whole-breast IMRT without compromising dose profiles to PTV and OARs.

Novel application of helical tomotherapy in whole skull palliative radiotherapy

Helical tomotherapy (HT) is a radiation planning/delivery platform that combines inversely planned IMRT with on-board megavoltage imaging. A unique HT radiotherapy whole skull brain sparing technique is described in a patient with metastatic prostate cancer. An inverse HT plan and an accompanying back-up conventional lateral 6-MV parallel opposed pair (POP) plan with corresponding isodose distributions and dose-volume histograms (DVH) were created and assessed prior to initiation of therapy. Plans conforming to the planning treatment volume (PTV) with significant sparing of brain, optic nerve, and eye were created. Dose heterogeneity to the PTV target was slightly higher in the HT plan compared to the back-up POP plan. Conformal sparing of brain, optic nerve, and eye was achieved by the HT plan. Similar lens and brain stem/spinal cord doses were seen with both plans. Prospective clinical evaluation with relevant end points (quality of life, symptom relief) are required to confirm the potential benefits of highly conformal therapies applied to palliative situations such as this case.

A study of prostate delineation referenced against a gold standard created from the visible human data

PURPOSE

To measure inter- and intra-observer variation and systematic error in CT based prostate delineation, where individual delineations are referenced against a gold standard produced from photographic anatomical images from the Visible Human Project (VHP).

MATERIALS AND METHODS

The CT and anatomical images of the VHP male form the basic data set for this study. The gold standard was established based on 1mm thick anatomical photographic images. These were registered against the 3mm thick CT images that were used for target delineation. A total of 120 organ delineations were performed by six radiation oncologists.

RESULTS

The physician delineated prostate volume was on average 30% larger than the "true" prostate volume, but on average included only 84% of the gold standard volume. Our study found a systematic delineation error such that posterior portions of the prostate were always missed while anteriorly some normal tissue was always defined as target.

CONCLUSIONS

Our data suggest that radiation oncologists are more concerned with the unintentional inclusion of rectal tissue than they are in missing prostate volume. In contrast, they are likely to overextend the anterior boundary of the prostate to encompass normal tissue such as the bladder.

Possible fractionated regimens for image-guided intensity-modulated radiation therapy of large arteriovenous malformations

The aim of this study was to estimate a plausible alpha/beta ratio for arteriovenous malformations (AVMs) based on reported clinical data, and to design possible fractionation regimens suitable for image-guided intensity-modulated radiation therapy (IG-IMRT) for large AVMs based on the newly obtained alpha/beta ratio. The commonly used obliteration rate (OR) for AVMs with a three year angiographic follow-up from many institutes was fitted to linear-quadratic (LQ) formalism and the Poisson OR model. The determined parameters were then used to calculate possible fractionation regimens for IG-IMRT based on the concept of a biologically effective dose (BED) and an equivalent uniform dose (EUD). The radiobiological analysis yields a alpha/beta ratio of 2.2 +/- 1.6 Gy for AVMs. Three sets of possible fractionated schemes were designed to achieve equal or better biological effectiveness than the single-fraction treatments while maintaining the same probability of normal brain complications. A plausible alpha/beta ratio was derived for AVMs and possible fractionation regimens that may be suitable for IG-IMRT for large AVM treatment are proposed. The sensitivity of parameters on the calculation was also studied. The information may be useful to design new clinical trials that use IG-IMRT for the treatment of large AVMs.

Comparison of Plan Quality Provided by Intensity-Modulated Arc Therapy and Helical Tomotherapy

PURPOSE

Intensity-modulated arc therapy (IMAT) is an arc-based approach to intensity-modulated radiotherapy (IMRT) that can be delivered on a conventional linear accelerator using a conventional multileaf collimator. In a previous work, we demonstrated that our arc-sequencing algorithm can produce highly conformal IMAT plans. Through plan comparisons, we explored the ability of IMAT to serve as an alternative to helical tomotherapy.

METHODS AND MATERIALS

The IMAT plans were created for 10 patients previously treated with helical tomotherapy. Treatment plan comparisons, according to the target dose coverage and critical structure sparing, were performed to determine whether similar plan quality could be achieved using IMAT.

RESULTS

In 8 of 10 patient cases, IMAT was able to provide plan quality comparable to that of helical tomotherapy. In 2 of these 8 cases, the use of non-axial coplanar or non-coplanar arcs in IMAT planning led to significant improvements in normal tissue sparing. The remaining 2 cases posed particular dosimetric challenges. In 1 case, the target was immediately adjacent to a spinal cord that had received previous irradiation. The second case involved multiple target volumes and multiple prescription levels. Both IMAT and tomotherapy were able to produce clinically acceptable plans. Tomotherapy, however, provided a more uniform target dose and improved critical structure sparing.

CONCLUSIONS

For most cases, IMAT can provide plan qualities comparable to that of helical tomotherapy. For some intracranial tumors, IMAT's ability to deliver non-coplanar arcs led to significant dosimetric improvements. Helical tomotherapy, however, can provide improved dosimetric results in the most complex cases.

Shielding requirements in helical tomotherapy

Helical tomotherapy is a relatively new intensity-modulated radiation therapy (IMRT) treatment for which room shielding has to be reassessed for the following reasons. The beam-on-time needed to deliver a given target dose is increased and leads to a weekly workload of typically one order of magnitude higher than that for conventional radiation therapy. The special configuration of tomotherapy units does not allow the use of standard shielding calculation methods. A conventional linear accelerator must be shielded for primary, leakage and scatter photon radiations. For tomotherapy, primary radiation is no longer the main shielding issue since a beam stop is mounted on the gantry directly opposite the source. On the other hand, due to the longer irradiation time, the accelerator head leakage becomes a major concern. An analytical model based on geometric considerations has been developed to determine leakage radiation levels throughout the room for continuous gantry rotation. Compared to leakage radiation, scatter radiation is a minor contribution. Since tomotherapy units operate at a nominal energy of 6 MV, neutron production is negligible. This work proposes a synthetic and conservative model for calculating shielding requirements for the Hi-Art II TomoTherapy unit. Finally, the required concrete shielding thickness is given for different positions of interest.

Effective beam directions using radiobiologically optimized IMRT of node positive breast cancer

The purpose of this study was to investigate the optimal coplanar beam directions when treating an early breast cancer with locoregional lymphatic spread with a few radiobiologically optimized intensity modulated beams. Also to determine the increase in the probability of complication-free cure with the number of beam portals and the smallest number required to perform a close to optimal treatment for this tumour site. Four test patients with stage II left-sided breast cancer were studied with heart, lung and contralateral breast as principal organs at risk. The clinical target volume consisted of the breast tissue remaining after surgery, the axillary, the internal mammary as well as the supraclavicular lymph nodes. Through an exhaustive search of all possible beam directions the most effective coplanar beams with one to four intensity modulated photon beam portals were investigated. Comparisons with uniform beam treatment techniques and up to 12 intensity modulated beams were also made. The different plans were optimized using the probability of complication-free tumour cure, P(+), as biological objective function. When using two intensity modulated beam directions three major sets of suitable directions were identified denoted by A, P and T. A corresponds to an anterior oblique pair of beams around 25 degrees and 325 degrees , P is a perpendicular lateral pair at around 50 degrees and 130 degrees whereas T is a more conventional tangential pair at around 155 degrees and 300 degrees . Interestingly, these configurations identify simply three major effective beam directions namely at 30 degrees +/-20 degrees , 145 degrees +/-20 degrees and 310 degrees +/-15 degrees . For the three intensity modulated beam technique a combination of these three effective beam directions generally covered the global maximum of the probability of complication-free tumour control. The improvement in complication-free cure probability with two optimally selected intensity modulated beams is around 10% when compared to a uniform beam technique with three to four beam portals. This increase is mainly due to a reduction by almost 1% in the probability of injury to the heart and an increase of 6% in the probability of local tumour control. When three or four biologically optimized beam portals are used a further increase in the probability of complication-free cure of about 6% can often be obtained. This improvement is caused by a small decrease in the probability of injury to the heart, left lung and other surrounding normal tissue, as well as a slight further increase in the probability of tumour control. The increase in the treatment outcome is minimal when more than four intensity modulated beams are employed. A small increase in dose homogeneity in the target volume and a slight decrease in the normal tissue volume receiving high dose may be seen, but without appreciably improving the complication-free cure probability. For a stage II breast cancer, three and in more complex cases four optimally oriented beams are sufficient to reach close to the maximum probability of complication-free tumour control when biologically optimized intensity modulated dose delivery is used. Angle of incidence optimization may then be advantageous starting from the given most effective three beam directions.

Significant reduction of acute toxicity following pelvic irradiation with Helical Tomotherapy in patients with localized prostate cancer

PURPOSE

To assess and quantify the possible benefit deriving from IMRT with Helical Tomotherapy (HTT) delivery to the pelvic nodal area in patients with prostate cancer in terms of reduction of acute and late toxicities.

METHODS AND MATERIALS

Thirty-five patients candidate to radical or postoperative RT on whole pelvis (WPRT) were treated with HTT, while receiving a concomitant boost to the prostate or the prostatic bed (median 74.2 and 72 Gy, respectively) within a moderately hypofractionated (28-33 fractions; median HTT duration 44 days) regimen. Median and mean doses to whole pelvis were 52 and 54 Gy, respectively. One of the major goals of planning optimisation was to minimize the dose received by the intestinal cavity (IC) outside the nodal PTV.

RESULTS

HTT resulted to be very efficient in sparing the IC even at dose levels below 30-35 Gy and guaranteed a significant sparing of bladder and rectum even at intermediate-low doses (V20-V40). No acute Grade 3 RTOG toxicity was recorded. Eighteen G1 and two G2 GU acute toxicities, 13 G1 upper GI acute toxicities, 8 G1 and 1 G2 acute proctitis were observed; no patient experienced G2 upper GI toxicity. After a median FU of 11.5 months (>10 in 18 patients) one case of late G3 GU toxicity was reported in one post-prostatectomy treated patient; no G2 late rectal bleeding or other GI toxicity was recorded.

CONCLUSIONS

WPRT with HTT resulted in a very low incidence of acute Grade 2 and in the disappearance of acute Grade 3 toxicities.

Helical tomotherapy superficial dose measurements

Helical tomotherapy is a treatment technique that is delivered from a 6 MV fan beam that traces a helical path while the couch moves linearly into the bore. In order to increase the treatment delivery dose rate, helical tomotherapy systems do not have a flattening filter. As such, the dose distributions near the surface of the patient may be considerably different from other forms of intensity-modulated delivery. The purpose of this study was to measure the dose distributions near the surface for helical tomotherapy plans with a varying separation between the target volume and the surface of an anthropomorphic phantom. A hypothetical planning target volume (PTV) was defined on an anthropomorphic head phantom to simulate a 2.0 Gy per fraction IMRT parotid-sparing head and neck treatment of the upper neck nodes. A total of six target volumes were created with 0, 1, 2, 3, 4, and 5 mm of separation between the surface of the phantom and the outer edge of the PTV. Superficial doses were measured for each of the treatment deliveries using film placed in the head phantom and thermoluminescent dosimeters (TLDs) placed on the phantom's surface underneath an immobilization mask. In the 0 mm test case where the PTV extends to the phantom surface, the mean TLD dose was 1.73 +/- 0.10 Gy (or 86.6 +/- 5.1% of the prescribed dose). The measured superficial dose decreases to 1.23 +/- 0.10 Gy (61.5 +/- 5.1% of the prescribed dose) for a PTV-surface separation of 5 mm. The doses measured by the TLDs indicated that the tomotherapy treatment planning system overestimates superficial doses by 8.9 +/- 3.2%. The radiographic film dose for the 0 mm test case was 1.73 +/- 0.07 Gy, as compared to the calculated dose of 1.78 +/- 0.05 Gy. Given the results of the TLD and film measurements, the superficial calculated doses are overestimated between 3% and 13%. Without the use of bolus, tumor volumes that extend to the surface may be underdosed. As such, it is recommended that bolus be added for these clinical cases. For cases where the target volume is located 1 to 5 mm below the surface, the tumor volume coverage can be achieved with surface doses ranging from 56% to 93% of the prescribed dose.

Verification of helical tomotherapy delivery using autoassociative kernel regressiona)

Quality assurance (QA) is a topic of major concern in the field of intensity modulated radiation therapy (IMRT). The standard of practice for IMRT is to perform QA testing for individual patients to verify that the dose distribution will be delivered to the patient. The purpose of this study was to develop a new technique that could eventually be used to automatically evaluate helical tomotherapy treatments during delivery using exit detector data. This technique uses an autoassociative kernel regression (AAKR) model to detect errors in tomotherapy delivery. AAKR is a novel nonparametric model that is known to predict a group of correct sensor values when supplied a group of sensor values that is usually corrupted or contains faults such as machine failure. This modeling scheme is especially suited for the problem of monitoring the fluence values found in the exit detector data because it is able to learn the complex detector data relationships. This scheme still applies when detector data are summed over many frames with a low temporal resolution and a variable beam attenuation resulting from patient movement. Delivery sequences from three archived patients (prostate, lung, and head and neck) were used in this study. Each delivery sequence was modified by reducing the opening time for random individual multileaf collimator (MLC) leaves by random amounts. The errof and error-free treatments were delivered with different phantoms in the path of the beam. Multiple autoassociative kernel regression (AAKR) models were developed and tested by the investigators using combinations of the stored exit detector data sets from each delivery. The models proved robust and were able to predict the correct or error-free values for a projection, which had a single MLC leaf decrease its opening time by less than 10 msec. The model also was able to determine machine output errors. The average uncertainty value for the unfaulted projections ranged from 0.4% to 1.8% of the detector signal. The low model uncertainty indicates that the AAKR model is extremely accurate in its predictions and also suggests that the model may be able to detect errors that cause the fluence to change by less than 2%. However, additional evaluation of the AAKR technique is needed to determine the minimum detectable error threshold from the compressed helical tomotherapy detector data. Further research also needs to explore applying this technique to electronic portal imaging detector data.

Image-guided helical tomotherapy for localized prostate cancer: technique and initial clinical observations

The purpose of the present study was to implement a technique for daily computed tomography (CT)-based image-guided radiation therapy and to report observations on treatment planning, imaging, and delivery based on the first 2 years of clinical experience. Patients with previously untreated stage T1-T3 biopsy-proven adenocarcinoma of the prostate were considered eligible for treatment with daily CT-guided helical tomotherapy. The prostate was targeted daily using megavoltage CT (MVCT) images that were fused with treatment-planning CT images based on anatomic alignments. All patients were treated at 2 Gy per fraction to 76-78 Gy (mean: 76.7 Gy). As part of this study, 33 prostate patients were planned, imaged, and treated with a total of 1266 CT-guided fractions. The prostate, rectum, bladder, femoral heads, and pubis symphysis were visible in one or more slices for all 1266 MVCT image sets. The typical range of measured prostate displacement relative to a 3-point external laser setup in this study was 2-10 mm [3.4 mm standard deviation (SD)] in the anterior-posterior direction, 2-8 mm (3.7 mm SD) in the lateral direction, and 1-6 mm (2.4 mm SD) in the superior-inferior direction. The obese patients in this study had a substantially larger lateral variation (8.2 mm SD) attributable to mobility of skin marks. The prostate, seminal vesicles, rectum, and bladder anatomy were used to position the patient relative to the desired treatment position without the use of implanted markers. Acute toxicities were within the expected range given the number of patients treated and the dose level.

Distribution of Brain Metastases in Relation to the Hippocampus: Implications for Neurocognitive Functional Preservation

PURPOSE

With the advent of intensity-modulated radiotherapy, the ability to limit the radiation dose to normal tissue offers an avenue to limit side effects. This study attempted to delineate the distribution of brain metastases with relation to the hippocampus for the purpose of exploring the viability of tomotherapy-guided hippocampal sparing therapy potentially to reduce neurocognitive deficits from radiation.

METHODS AND MATERIALS

The pre-radiotherapy T1-weighted, postcontrast axial MR images of 100 patients who received whole brain radiotherapy, stereotactic radiosurgery, or a radiosurgical boost following whole brain radiotherapy between 2002 and 2006 were examined. We contoured brain metastases as well as hippocampi with 5-, 10-, and 15-mm expansion envelopes.

RESULTS

Of the 272 identified metastases, 3.3% (n = 9) were within 5 mm of the hippocampus, and 86.4% of metastases were greater than 15 mm from the hippocampus (n = 235). The most common location for metastatic disease was the frontal lobe (31.6%, n = 86). This was followed by the cerebellum (24.3%, n = 66), parietal lobe (16.9%, n = 46), temporal lobe (12.9%, n = 35), occipital lobe (7.7%, n = 21), deep brain nuclei (4.0%, n = 11), and brainstem (2.6%, n = 7).

CONCLUSIONS

Of the 100 patients, 8 had metastases within 5 mm of the hippocampus. Hence, a 5-mm margin around the hippocampus for conformal avoidance whole brain radiotherapy represents an acceptable risk, especially because these patients in the absence of any other intracranial disease could be salvaged using stereotactic radiosurgery. Moreover, we developed a hippocampal sparing tomotherapy plan as proof of principle to verify the feasibility of this therapy in the setting of brain metastases.

Breathing-synchronized delivery: a potential four-dimensional tomotherapy treatment technique

PURPOSE

To introduce a four-dimensional (4D) tomotherapy treatment technique with improved motion control and patient tolerance.

METHODS AND MATERIALS

Computed tomographic images at 10 breathing phases were acquired for treatment planning. The full exhalation phase was chosen as the planning phase, and the CT images at this phase were used as treatment-planning images. Region of interest delineation was the same as in traditional treatment planning, except that no breathing motion margin was used in clinical target volume-planning target volume expansion. The correlation between delivery and breathing phases was set assuming a constant gantry speed and a fixed breathing period. Deformable image registration yielded the deformation fields at each phase relative to the planning phase. With the delivery/breathing phase correlation and voxel displacements at each breathing phase, a 4D tomotherapy plan was obtained by incorporating the motion into inverse treatment plan optimization. A combined laser/spirometer breathing tracking system has been developed to monitor patient breathing. This system is able to produce stable and reproducible breathing signals representing tidal volume.

RESULTS

We compared the 4D tomotherapy treatment planning method with conventional tomotherapy on a static target. The results showed that 4D tomotherapy can achieve dose distributions on a moving target similar to those obtained with conventional delivery on a stationary target. Regular breathing motion is fully compensated by motion-incorporated breathing-synchronized delivery planning. Four-dimensional tomotherapy also has close to 100% duty cycle and does not prolong treatment time.

CONCLUSION

Breathing-synchronized delivery is a feasible 4D tomotherapy treatment technique with improved motion control and patient tolerance.

Development of an optimization concept for arc-modulated cone beam therapy

In this paper, we propose an optimization concept for a rotation therapy technique which is referred to as arc-modulated cone beam therapy (AMCBT). The aim is a reduction of the treatment time while achieving a treatment plan quality equal to or better than that of IMRT. Therefore, the complete dose is delivered in one single gantry rotation and the beam is modulated by a multileaf collimator. The degrees of freedom are the field shapes and weights for a predefined number of beam directions. In the new optimization loop, the beam weights are determined by a gradient algorithm and the field shapes by a tabu search algorithm. We present treatment plans for AMCBT for two clinical cases. In comparison to step-and-shoot IMRT treatment plans, it was possible by AMCBT to achieve dose distributions with a better dose conformity to the target and a lower mean dose for the most relevant organ at risk. Furthermore, the number of applied monitor units was reduced for AMCBT in comparison to IMRT treatment plans.

Adaptive biological image-guided IMRT with anatomic and functional imaging in pharyngo-laryngeal tumors: impact on target volume delineation and dose distribution using helical tomotherapy

BACKGROUND AND PURPOSE

Adaptive image-guided IMRT appears to be a promising approach for dose escalation in pharyngo-laryngeal tumors. In this framework, we assessed in a proof of concept study the impact of anatomic and functional imaging modalities acquired prior and during radiotherapy on the target volume delineation and the dose distribution using helical tomotherapy.

MATERIALS AND METHODS

Ten patients with pharyngo-laryngeal squamous cell carcinoma were treated by concomitant chemo-radiation delivered in 7 weeks. CT, T2-MRI, fat suppressed T2-MRI, and static and dynamic FDG-PET were acquired for each patient before the start of treatment and during radiotherapy, after mean prescribed doses of 14, 25, 35 and 45 Gy. GTVs were manually delineated on CT and MRI images while PET images were automatically segmented by means of a gradient-based method. From these volumes, CTVs and PTVs were derived using consistent guidelines. Simultaneous integrated boost IMRT planning was performed using helical tomotherapy.

RESULTS

GTVs significantly decreased throughout the course of RT for all imaging modalities (p<0.001). Clinically non-significant differences and high correlations were found between GTVs delineated on CT and MRI, irrespective of the sequence used. By contrast, FDG-PET-based GTVs segmented from pre- and per-treatment images were significantly smaller compared to anatomical imaging modalities, without any difference existing between static and dynamic acquisition. These differences in GTVs translated into parallel reductions of both prophylactic and therapeutic CTVs and PTVs. Resulting FDG-PET-based and adaptive IMRT planning reduced the irradiated volumes by 15-40% compared to pre-treatment CT planning (V(90), V(95) and V(100)), but did marginally impact on doses to the OAR such as the spinal cord and the parotid glands.

CONCLUSIONS

Adaptive IMRT with FDG-PET images has a significant impact on the delineation of TVs and on the dose distribution in pharyngo-laryngeal tumors. Such an approach might thus be considered for dose escalation strategies.

Treatment-planning study of prostate cancer intensity-modulated radiotherapy with a Varian Clinac operated without a flattening filter

PURPOSE

To assess the feasibility of intensity-modulated radiotherapy for prostate cancer using photon beams from an accelerator operated without a flattening filter; and to determine potential benefits and drawbacks of using unflattened beams for this type of treatment.

METHODS AND MATERIALS

Intensity-modulated radiotherapy plans were generated for 10 patients with early-stage prostate cancer. For each patient, four plans were generated: with and without the flattening filter, at 6 and 18 MV. The prescription dose was 75.6 Gy to 98% of the planning target volume. The number of beams, their orientations, and optimization constraints were the same for all plans. Plans were generated with Eclipse 8.0 (Varian Medical Systems).

RESULTS

All the plans developed with unflattened beams were clinically acceptable. In terms of patient dose distributions, plans with unflattened beams were similar to the corresponding plans with flattened beams. Plans with unflattened beams required fewer monitor units (MUs) per plan: on average, by a factor of 2.0 at 6 MV and 2.6 at 18 MV, assuming that removal of the flattening filter was not followed by recalibration of MUs.

CONCLUSIONS

Clinically acceptable intensity-modulated radiotherapy plans for prostate cancer can be developed with unflattened beams at both 6 and 18 MV. Dosimetrically, flattened and unflattened beams generated similar treatment plans. The plans with unflattened beams required substantially fewer MUs. The reduction in the number of MUs indicates corresponding reduction in beam-on time and in the amount of radiation outside the target.

Interfractional Variations in Patient Setup and Anatomic Change Assessed by Daily Computed Tomography

PURPOSE

To analyze the interfractional variations in patient setup and anatomic changes at seven anatomic sites observed in image-guided radiotherapy.

METHODS AND MATERIALS

A total of 152 patients treated at seven anatomic sites using a Hi-Art helical tomotherapy system were analyzed. Daily tomotherapy megavoltage computed tomography images acquired before each treatment were fused to the planning kilovoltage computed tomography images to determine the daily setup errors and organ motions and deformations. The setup errors were corrected before treatment and were used, along with the organ motions, to determine the clinical target volume/planning target volume margins. The organ motions and deformations for 3 representative patient cases (pancreas, uterus, and soft-tissue sarcoma) and for 14 kidneys of 7 patients are presented.

RESULTS

Interfractional setup errors in the skull, brain, and head and neck are significantly smaller than those in the chest, abdomen, pelvis, and extremities. These site-specific relationships are statistically significant. The margins required to account for these setup errors range from 3 to 8 mm for the seven sites. The margin to account for both setup errors and organ motions for kidney is 16 mm. Substantial interfractional anatomic changes were observed. For example, the pancreas moved up to +/-20 mm and volumes of the uterus and sarcoma varied <or=30% and 100%, respectively.

CONCLUSION

The interfractional variations in patient setup and in shapes, sizes, and positions of both targets and normal structures are site specific and may be used to determine the site-specific margins. The data presented in this work dealing with seven anatomic sites may be useful in developing adaptive radiotherapy.

Setup variations in radiotherapy of esophageal cancer: evaluation by daily megavoltage computed tomographic localization

PURPOSE

To use pretreatment megavoltage computed tomography (MVCT) scans to evaluate setup variations in anterior-posterior (AP), lateral, and superior-inferior (SI) directions and rotational variations, including pitch, roll, and yaw, for esophageal cancer patients treated with helical tomotherapy.

METHODS AND MATERIALS

Ten patients with locally advanced esophageal cancer treated by combined chemoradiation using helical tomotherapy were selected. After patients were positioned using their skin tattoos/marks, MVCT scans were performed before every treatment and automatically registered to planning kilovoltage CT scans according to bony landmarks. Image registration data were used to adjust patient setups before treatment. A total of 250 MVCT scans were analyzed. Correlations between setup variations and body habitus, including height, weight, relative weight change, body surface area, and patient age, were evaluated.

RESULTS

The standard deviations for systematic setup corrections in AP, lateral, and SI directions and pitch, roll, and yaw rotations were 1.5, 3.7, and 4.8 mm and 0.5 degrees, 1.2 degrees, and 0.8 degrees, respectively. The appropriate averages of random setup variations in AP, lateral, and SI directions and pitch, roll, and yaw rotations were 2.9, 5.2, and 4.4 mm, and 1.0 degrees, 1.2 degrees, and 1.1 degrees, respectively. Setup variations were stable throughout the entire course of radiotherapy in all three translational and three rotational displacements, with little change in magnitude. No significant correlations were found between setup variations and body habitus variables.

CONCLUSIONS

Daily MVCT scans before each treatment can effectively detect setup errors and thereby reduce planning target volume (PTV) margins. This will reduce radiation dose to critical organs and may translate into lower treatment-related toxicities.

Geometric uncertainty of 2D projection imaging in monitoring 3D tumor motion

The purpose of this study was to investigate the accuracy of two-dimensional (2D) projection imaging methods in three-dimensional (3D) tumor motion monitoring. Many commercial linear accelerator types have projection imaging capabilities, and tumor motion monitoring is useful for motion inclusive, respiratory gated or tumor tracking strategies. Since 2D projection imaging is limited in its ability to resolve the motion along the imaging beam axis, there is unresolved motion when monitoring 3D tumor motion. From the 3D tumor motion data of 160 treatment fractions for 46 thoracic and abdominal cancer patients, the unresolved motion due to the geometric limitation of 2D projection imaging was calculated as displacement in the imaging beam axis for different beam angles and time intervals. The geometric uncertainty to monitor 3D motion caused by the unresolved motion of 2D imaging was quantified using the root-mean-square (rms) metric. Geometric uncertainty showed interfractional and intrafractional variation. Patient-to-patient variation was much more significant than variation for different time intervals. For the patient cohort studied, as the time intervals increase, the rms, minimum and maximum values of the rms uncertainty show decreasing tendencies for the lung patients but increasing for the liver and retroperitoneal patients, which could be attributed to patient relaxation. Geometric uncertainty was smaller for coplanar treatments than non-coplanar treatments, as superior-inferior (SI) tumor motion, the predominant motion from patient respiration, could be always resolved for coplanar treatments. Overall rms of the rms uncertainty was 0.13 cm for all treatment fractions and 0.18 cm for the treatment fractions whose average breathing peak-trough ranges were more than 0.5 cm. The geometric uncertainty for 2D imaging varies depending on the tumor site, tumor motion range, time interval and beam angle as well as between patients, between fractions and within a fraction.

Cone-beam CT using a mobile C-arm: a registration solution for IGRT with an optical tracking system

A method for registering images acquired from a prototype flat panel mobile C-arm, capable of kilovoltage (kV) cone-beam computed tomography (CT), to a linear accelerator (LINAC) isocenter is presented. A calibration procedure is performed which involves locating reflective markers placed on the C-arm and a phantom in two coordinate systems. A commercial optical tracking system locates the markers relative to the LINAC isocenter (room coordinates). The cone-beam imaging capabilities of the C-arm provide the location of the markers on the calibration phantom in image coordinates. A singular value decomposition (SVD) algorithm is used to determine the relationship between the C-arm, image coordinates and room coordinates. Once the calibration is completed, the position of the C-arm at any arbitrary location is accurately determined from the tracking system. A final transformation is calculated capable of mapping voxels in the reconstructed image set to their corresponding position in room coordinates. An evaluation to determine the accuracy of this method was performed by locating markers on a phantom. The position of the phantom markers in room coordinates was obtained directly using the optical tracking system and compared with that using the described method above. A mean absolute distance of 1.4+/-0.5 was observed for a completely transformed image set. This is comparable to that of systems routinely used for image-guided radiation therapy (IGRT).

Segmentation and leaf sequencing for intensity modulated arc therapy

A common method in generating intensity modulated radiation therapy (IMRT) plans consists of a three step process: an optimized fluence intensity map (IM) for each beam is generated via inverse planning, this IM is then segmented into discrete levels, and finally, the segmented map is translated into a set of MLC apertures via a leaf sequencing algorithm. To date, limited work has been done on this approach as it pertains to intensity modulated arc therapy (IMAT), specifically in regards to the latter two steps. There are two determining factors that separate IMAT segmentation and leaf sequencing from their IMRT equivalents: (1) the intrinsic 3D nature of the intensity maps (standard 2D maps plus the angular component), and (2) that the dynamic multileaf collimator (MLC) constraints be met using a minimum number of arcs. In this work, we illustrate a technique to create an IMAT plan that replicates Tomotherapy deliveries by applying IMAT specific segmentation and leaf-sequencing algorithms to Tomotherapy output sinograms. We propose and compare two alternative segmentation techniques, a clustering method, and a bottom-up segmentation method (BUS). We also introduce a novel IMAT leaf-sequencing algorithm that explicitly takes leaf movement constraints into consideration. These algorithms were tested with 51 angular projections of the output leaf-open sinograms generated on the Hi-ART II treatment planning system (Tomotherapy Inc.). We present two geometric phantoms and 2 clinical scenarios as sample test cases. In each case 12 IMAT plans were created, ranging from 2 to 7 intensity levels. Half were generated using the BUS segmentation and half with the clustering method. We report on the number of arcs produced as well as differences between Tomotherapy output sinograms and segmented IMAT intensity maps. For each case one plan for each segmentation method is chosen for full Monte Carlo dose calculation (NumeriX LLC) and dose volume histograms (DVH) are calculated. In all cases, the BUS method outperformed the clustering, method. We recommend using the BUS algorithm and discuss potential improvements to the clustering algorithms.

A delivery transfer function (DTF) analysis for helical tomotherapy

The previous theoretical work of a delivery transfer function (DTF) in radiotherapy is expanded to include the unique intensity modulation method of helical tomotherapy. In addition to the collimation of each beamlet, and the Gaussian scatter convolution spreading of the dose that other radiotherapy units have, helical tomotherapy uses 51 small arcs of varying lengths to adjust the intensity. The blurring from these arcs is not taken into account during treatment planning. A theoretical DTF is constructed, and a calculation is performed which includes this unique source motion in relation to the other DTF components. Various typical delivery parameters are used to generate resolution maps for a constant intensity projection. Near the isocenter, the transverse (to a given beam direction) blurring is small but at larger radii (>6 cm), the source blurring dominates over leaf size. For most clinical situations, this inherent source motion blurring is expected to be negligible.