S.J. Thomas

Practical Aspects of Implementation of Helical Tomotherapy for Intensity-modulated and Image-guided Radiotherapy

AIMS Image-guided radiotherapy (IGRT) and intensity-modulated radiotherapy (IMRT) represent two important technical developments that will probably improve patient outcome. Helical tomotherapy, provided by the TomoTherapy HiArt system, provides an elegant integrated solution providing both technologies, although others are available. Here we report our experience of clinical implementation of daily online IGRT and IMRT using helical tomotherapy. MATERIALS AND METHODS Methods were needed to select patients who would probably benefit. Machine-specific commissioning, a quality assurance programme and patient-specific delivery quality assurance were also needed. The planning target volume dose was prescribed as the median dose, with the added criterion that the 95% isodose should cover 99% of the target volume. Although back-up plans, for delivery on conventional linear accelerators, were initially prepared, this practice was abandoned because they were used very rarely. RESULTS In the first 12 months, 114 patients were accepted for treatment, and 3343 fractions delivered. New starts averaged 2.6 per week, with an average of 17.5 fractions treated per day, and the total number capped at 22. This has subsequently been raised to 24. Of the first 100 patients, 96 were treated with radical intent. Five were considered to have been untreatable on our standard equipment. IGRT is radiographer led and all patients were imaged daily, with positional correction made before treatment, using an action level of 1mm. A formal training programme was developed and implemented before installation. The in-room time fell significantly during the year, reflecting increasing experience and a software upgrade. More recently, after a couch upgrade in April 2009, the mean in-room time fell to 18.6 min. CONCLUSIONS Successful implementation of tomotherapy was the result of careful planning and effective teamwork. Treatment, including daily image guidance, positional correction and intensity-modulated delivery, is fast and efficient, and can be integrated into routine service. This should encourage the adoption of these technologies.

Defining the tumour and target volumes for radiotherapy.

Radiotherapy is a localised treatment. The definition of tumour and target volumes for radiotherapy is vital to its successful execution. This requires the best possible characterisation of the location and extent of tumour. Diagnostic imaging, including help and advice from diagnostic specialists, is therefore essential for radiotherapy planning. There are three main volumes in radiotherapy planning. The first is the position and extent of gross tumour, i.e. what can be seen, palpated or imaged; this is known as the gross tumour volume (GTV). Developments in imaging have contributed to the definition of the GTV. The second volume contains the GTV, plus a margin for sub-clinical disease spread which therefore cannot be fully imaged; this is known as the clinical target volume (CTV). It is the most difficult because it cannot be accurately defined for an individual patient, but future developments in imaging, especially towards the molecular level, should allow more specific delineation of the CTV. The CTV is important because this volume must be adequately treated to achieve cure. The third volume, the planning target volume (PTV), allows for uncertainties in planning or treatment delivery. It is a geometric concept designed to ensure that the radiotherapy dose is actually delivered to the CTV. Radiotherapy planning must always consider critical normal tissue structures, known as organs at risk (ORs). In some specific circumstances, it is necessary to add a margin analogous to the PTV margin around an OR to ensure that the organ cannot receive a higher-than-safe dose; this gives a planning organ at risk volume. This applies to an organ such as the spinal cord, where damage to a small amount of normal tissue would produce a severe clinical manifestation. The concepts of GTV, CTV and PTV have been enormously helpful in developing modern radiotherapy. Attention to detail in radiotherapy planning is vital, and does affect outcomes: ‘the devil is in the detail’. Radiotherapy planning is also dependent on high quality imaging, and the better the imaging the better will be the outcomes from radiotherapy.

A modified power-law formula for inhomogeneity corrections in beams of high-energy x rays.

The Batho power-law formula is in common use in many treatment planning systems to correct for the presence of lungs and other inhomogeneities. While giving excellent agreement with measurement for Cobalt-60 radiation, it tends to underestimate the lung correction required for higher energy x rays and is undefined for distances beyond an interface less than the buildup distance. This paper suggests a simple modification that greatly improves the agreement with measured data and gives a continuously defined function at all depths. Measurements have been made in a polystyrene and cork phantom to simulate the effects of lung; data are presented for beams of 8-MV x rays, 16-MV x rays, and Cobalt-60 gamma rays.

Margins for treatment planning of proton therapy

For protons and other charged particles, the effect of set-up errors on the position of isodoses is considerably less in the direction of the incident beam than it is laterally. Therefore, the margins required between the clinical target volume (CTV) and planning target volume (PTV) can be less in the direction of the incident beam than laterally. Margins have been calculated for a typical head plan and a typical prostate plan, for a single field, a parallel opposed and a four-field arrangement of protons, and compared with margins calculated for photons, assuming identical geometrical uncertainties for each modality. In the head plan, where internal motion was assumed negligible, the CTV-PTV margin reduced from approximately 10 mm to 3 mm in the axial direction for the single field and parallel opposed plans. For a prostate plan, where internal motion cannot be ignored, the corresponding reduction in margin was from 11 mm to 7 mm. The planning organ at risk (PRV) margin in the axial direction reduced from 6 mm to 2 mm for the head plan, and from 7 mm to 4 mm for the prostate plan. No reduction was seen on the other axes, or for any axis of the four-field plans. Owing to the shape of proton dose distributions, there are many clinical cases in which good dose distributions can be obtained with one or two fields. When this is done, it is possible to use smaller PTV and PRV margins. This has the potential to convert untreatable cases, in which the PTV and PRV overlap, into cases with a gap between PTV and PRV of adequate size for treatment planning.

Dose calculation software for helical tomotherapy, utilizing patient CT data to calculate an independent three-dimensional dose cube.

PURPOSE Treatment plans for the TomoTherapy unit are produced with a planning system that is integral to the unit. The authors have produced an independent dose calculation system, to enable plans to be recalculated in three dimensions, using the patients CT data. METHODS Software has been written using MATLAB. The DICOM-RT plan object is used to determine the treatment parameters used, including the treatment sinogram. Each projection of the sinogram is segmented and used to calculate dose at multiple calculation points in a three-dimensional grid using tables of measured beam data. A fast ray-trace algorithm is used to determine effective depth for each projection angle at each calculation point. Calculations were performed on a standard desktop personal computer, with a 2.6 GHz Pentium, running Windows XP. RESULTS The time to perform a calculation, for 3375 points averaged 1 min 23 s for prostate plans and 3 min 40 s for head and neck plans. The mean dose within the 50% isodose was calculated and compared with the predictions of the TomoTherapy planning system. When the modified CT (which includes the TomoTherapy couch) was used, the mean difference for ten prostate patients, was -0.4% (range -0.9% to +0.3%). With the original CT (which included the CT couch), the mean difference was -1.0% (range -1.7% to 0.0%). The number of points agreeing with a gamma 3%∕3 mm averaged 99.2% with the modified CT, 96.3% with the original CT. For ten head and neck patients, for the modified and original CT, respectively, the mean difference was +1.1% (range -0.4% to +3.1%) and 1.1% (range -0.4% to +3.0%) with 94.4% and 95.4% passing a gamma 4%∕4 mm. The ability of the program to detect a variety of simulated errors has been tested. CONCLUSIONS By using the patients CT data, the independent dose calculation performs checks that are not performed by a measurement in a cylindrical phantom. This enables it to be used either as an additional check or to replace phantom measurements for some patients. The software has potential to be used in any application where one wishes to model changes to patient conditions.

Defining robustness protocols: a method to include and evaluate robustness in clinical plans.

We aim to define a site-specific robustness protocol to be used during the clinical plan evaluation process. Plan robustness of 16 skull base IMPT plans to systematic range and random set-up errors have been retrospectively and systematically analysed. This was determined by calculating the error-bar dose distribution (ebDD) for all the plans and by defining some metrics used to define protocols aiding the plan assessment. Additionally, an example of how to clinically use the defined robustness database is given whereby a plan with sub-optimal brainstem robustness was identified. The advantage of using different beam arrangements to improve the plan robustness was analysed. Using the ebDD it was found range errors had a smaller effect on dose distribution than the corresponding set-up error in a single fraction, and that organs at risk were most robust to the range errors, whereas the target was more robust to set-up errors. A database was created to aid planners in terms of plan robustness aims in these volumes. This resulted in the definition of site-specific robustness protocols. The use of robustness constraints allowed for the identification of a specific patient that may have benefited from a treatment of greater individuality. A new beam arrangement showed to be preferential when balancing conformality and robustness for this case. The ebDD and error-bar volume histogram proved effective in analysing plan robustness. The process of retrospective analysis could be used to establish site-specific robustness planning protocols in proton therapy. These protocols allow the planner to determine plans that, although delivering a dosimetrically adequate dose distribution, have resulted in sub-optimal robustness to these uncertainties. For these cases the use of different beam start conditions may improve the plan robustness to set-up and range uncertainties.

Intra-fraction motion of the prostate during treatment with helical tomotherapy

BACKGROUND AND PURPOSE To measure the geometric uncertainty resulting from intra-fraction motion and intra-observer image matching, for patients having image-guided prostate radiotherapy on TomoTherapy. MATERIAL AND METHODS All patients had already been selected for prostate radiotherapy on TomoTherapy, with daily MV-CT imaging. The study involved performing an additional MV-CT image at the end of treatment, on 5 occasions during the course of 37 treatments. 54 patients were recruited to the study. A new formula was derived to calculate the PTV margin for intra-fraction motion. RESULTS The mean values of the intra-fraction differences were 0.0mm, 0.5mm, 0.5mm and 0.0° for LR, SI, AP and roll, respectively. The corresponding standard deviations were 1.1mm, 0.8mm, 0.8mm and 0.6° for systematic uncertainties (Σ), 1.3mm, 2.0mm, 2.2mm and 0.3° for random uncertainties (σ). This intra-fraction motion requires margins of 2.2mm in LR, 2.1mm in SI and 2.1mm in AP directions. Inclusion of estimates of the effect of rotations and matching errors increases these margins to approximately 4mm in LR and 5mm in SI and AP directions. CONCLUSIONS A new margin recipe has been developed to calculate margins for intra-fraction motion. This recipe is applicable to any measurement technique that is based on the difference between images taken before and after treatment.

A comparison of four indices for combining distance and dose differences.

PURPOSE When one is comparing two dose distributions, a number of methods have been published to combine dose difference and distance to agreement into a single measure. Some have been defined as pass/fail indices and some as numeric indices. We show that the pass/fail indices can all be used to derive numeric indices, and we compare the results of using these indices to evaluate one-dimensional (1D) and three-dimensional (3D) dose distributions, with the aim of selecting the most appropriate index for use in different circumstances. METHODS AND MATERIALS The indices compared are the gamma index, the kappa index, the index in International Commission on Radiation Units & Measurements Report 83, and a box index. Comparisons are made for 1D and 3D distributions. The 1D distribution is chosen to have a variety of dose gradients. The 3D distribution is taken from a clinical treatment plan. The effect of offsetting distributions by known distances and doses is studied. RESULTS The International Commission on Radiation Units & Measurements Report 83 index causes large discontinuities unless the dose gradient cutoff is set to equal the ratio of the dose tolerance to the distance tolerance. If it is so set, it returns identical results to the kappa index. Where the gradient is very high or very low, all the indices studied in this article give similar results for the same tolerance values. For moderate gradients, they differ, with the box index being the least strict, followed by the gamma index, and with the kappa index being the most strict. CONCLUSIONS If the clinical tolerances are much greater than the uncertainties of the measuring system, the kappa index should be used, with tolerance values determined by the clinical tolerances. In cases where the uncertainties of the measuring system dominate, the box index will be best able to determine errors in the delivery system.

Reference dosimetry on TomoTherapy: an addendum to the 1990 UK MV dosimetry code of practice

The current UK code of practice for high-energy photon therapy dosimetry (Lillicrap et al 1990 Phys. Med. Biol. 35 1355-60) gives instructions for measuring absorbed dose to water under reference conditions for megavoltage photons. The reference conditions and the index used to specify beam quality require that a machine be able to set a 10 cm × 10 cm field at the point of measurement. TomoTherapy machines have a maximum collimator setting of 5 cm × 40 cm at a source to axis distance of 85 cm, making it impossible for users of these machines to follow the code. This addendum addresses the specification of reference irradiation geometries, the choice of ionization chambers and the determination of dosimetry corrections, the derivation of absorbed dose to water calibration factors and choice of appropriate chamber correction factors, for carrying out reference dosimetry measurements on TomoTherapy machines. The preferred secondary standard chamber remains the NE2611 chamber, which with its associated secondary standard electrometer, is calibrated at the NPL through the standard calibration service for MV photon beams produced on linear accelerators with conventional flattening filters. Procedures are given for the derivation of a beam quality index specific to the TomoTherapy beam that can be used in the determination of a calibration coefficient for the secondary standard chamber from its calibration certificate provided by the NPL. The recommended method of transfer from secondary standard to field instrument is in a static beam, at a depth of 5 cm, by sequential substitution or by simultaneous side by side irradiation in either a water phantom or a water-equivalent solid phantom. Guidance is given on the use of a field instrument in reference fields.

Evaluating Competing and Emerging Technologies for Stereotactic Body Radiotherapy and Other Advanced Radiotherapy Techniques

Stereotactic body radiotherapy (SBRT) refers to the precise irradiation of an image-defined extracranial lesion, using a high total radiation dose delivered in a small number of fractions. A significant proportion of SBRT treatment has been successfully delivered using conventional gantry-based linear accelerators with appropriate image guidance and motion management techniques, although a number of specialist systems are also available. Evaluating the competing SBRT technologies is difficult due to frequent refinements to all major platforms. Comparison of geometric accuracy or treatment planning performance can be hard to interpret and may not provide much useful information. Nevertheless, a general specification overview can provide information that may help radiotherapy providers decide on an appropriate system for their centre. A number of UK randomised controlled trials (RCTs) have shown that better radiotherapy techniques yield better results. RCTs should play an important part in the future evaluation of SBRT, especially where there is a smaller volume of existing data, and where outcomes from conventional radiotherapy are very good. RCT comparison of SBRT with surgery is more difficult due to the radically different treatment arms, although successful recruitment can be possible if the lessons from previous failed trials are learned. The evaluation of new technology poses a number of challenges to the conventional RCT methodology, and there may be situations where it is genuinely not possible, with careful observational studies or decision modelling being more appropriate. Further development in trial design may have an important role in providing clinical evidence in a more timely manner.