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Steve Webb

Publications and source records attributed to Steve Webb.

At least 19 recordsLinked to original sources
PubMed2006

Comparison of inverse-planned three-dimensional conformal radiotherapy and intensity-modulated radiotherapy for non-small-cell lung cancer.

PURPOSE: Lungs are the major dose-limiting organ during radiotherapy (RT) for non-small-cell lung cancer owing to the development of pneumonitis. This study compared intensity-modulated RT (IMRT) with three-dimensional conformal RT (3D-CRT) in reducing the dose to the lungs. METHODS: Ten patients with localized non-small-cell lung cancer underwent computed tomography (CT). The planning target volume (PTV) was defined and the organs at risk were outlined. An inverse-planning program, AutoPlan, was used to design the beam angle-optimized six-field noncoplanar 3D-CRT plans. Each 3D-CRT plan was compared with a series of five IMRT plans per patient. The IMRT plans were created using a commercial algorithm and consisted of a series of three, five, seven, and nine equidistant coplanar field arrangements and one six-field noncoplanar plan. The planning objectives were to minimize the lung dose while maintaining the dose to the PTV. The percentage of lung volume receiving >20 Gy (V20) and the percentage of the PTV covered by the 90% isodose (PTV90) were the primary endpoints. The PTV90/V20 ratio was used as the parameter accounting for both the reduction in lung volume treated and the PTV coverage. RESULTS: All IMRT plans, except for the three-field coplanar plans, improved the PTV90/V20 ratio significantly compared with the optimized 3D-CRT plan. Nine coplanar IMRT beams were significantly better than five or seven coplanar IMRT beams, with an improved PTV90/V20 ratio. CONCLUSION: The results of our study have shown that IMRT can reduce the dose to the lungs compared with 3D-CRT by improving the conformity of the plan.

Carcinoma, Non-Small-Cell Lung↗
PubMed2006

Evaluating the relationship between erectile dysfunction and dose received by the penile bulb: using data from a randomised controlled trial of conformal radiotherapy in prostate cancer (MRC RT01, ISRCTN47772397).

AIM: To evaluate the relationship between erectile function and the radiation dose to the penile bulb and other proximal penile structures in men receiving conformal radiotherapy (CFRT) for prostate cancer (PCa). METHODS: The Medical Research Council (MRC) RT01 trial randomised 843 men who had localised PCa to receive either 64 or 74 Gy after 3 - 6 months neoadjuvant hormonal treatment. Fifty-one men were selected who were potent prior to hormonal treatment, having completed both pre-hormone and 2-year post-CFRT Quality of Life assessments, and on whom dose volume data were available for analysis. The men were divided into three groups according to 2-year follow-up: potent, reduced potency, and impotent. The bulb of the penis together with the crura, were outlined on restored treatment plans. Dose - volume histograms were generated and compared between the three groups. An ordered logistic regression model was used to calculate the odds ratio of a range of dose - volume parameters to the penile bulb and effect on erectile dysfunction. The dose to the penile bulb was correlated to the dose received by the crura. RESULTS: Of the 51 patients, 12 remained potent, 22 had reduced potency, and 17 were impotent at 2 years. No differences were seen in mean dose to the penile bulb by allocated treatment (t test = 1.61, p = 0.11). The mean doses to the penile bulb received by the potent, reduced potency, and impotent groups were 45.5 Gy (SD 17.1), 48 Gy (SD 16.1), and 59.2 Gy (SD 13.8), respectively. There was a strong correlation between the mean dose received by the penile bulb and dose to the crura (r = 0.82, p < 0.0001). 83.3% of impotent patients received a D90 > or = 50 Gy to the penile bulb compared with 29.4% of patients who maintained potency at 2 years (p = 0.006). CONCLUSION: There is evidence from this study to suggest a dose volume effect on the penile bulb and erectile dysfunction. A D90 > or = 50 Gy is associated with a significant risk of erectile dysfunction and this should form a basis for selecting dose constraints in future dose escalation studies.

Adenocarcinoma↗
PubMed2006

Quantifying the effect of respiratory motion on lung tumour dosimetry with the aid of a breathing phantom with deforming lungs.

The contribution of organ and tumour motion to the degradation of planned dose distributions during radiotherapy to the breathing lung has been experimentally investigated and quantified. An anthropomorphic, tissue-equivalent breathing phantom with deformable lungs has been built, in which the lung tumour can be driven in any arbitrary 3D trajectory. The trajectory is programmed into a motion controller connected to a high-precision moving platform that is connected to the tumour. The motion controller is connected to the accelerator's dose counter and the speed of motion is scaled to the dose rate. This ensures consistent delivery despite variation in either the dose rate or inter-segment timing. For this study, the phantom was made to breathe by a set of periodic equations representing respiratory motion by an asymmetric, trigonometric function. Several motion amplitudes were selected to be applied in the primary axis of motion. Five three-dimensional, geometrically conformal (3DCRT) fractions with different starting phases (spaced uniformly in the breathing cycle) were delivered to the phantom and compared to a delivery where the phantom was static at the end-expiration position. A set of intensity-modulated radiotherapy plans (IMRT) was subsequently delivered in the same manner. Bigger amplitudes of motion resulted in a higher degree of dose blurring. Severe underdosages were observed when deliberately selecting the PTV wrongly, their extent being correlated with the degree of margin error. IMRT motion-averaged dose distributions exhibited areas of high dose in the gross tumour volume (GTV) which were not present in the static irradiations, arising from booster segments that the optimizer was creating to achieve planning target volume (PTV) homogeneity during the inverse-planning process. 3DCRT, on the other hand, did not demonstrate such effects. It has been concluded that care should be taken to control the delivered fluence when delivering IMRT to the breathing lung, even when the PTV margin has been adequately chosen to include the extent of the breathing motion.

Humans↗
PubMed2006

Constrained segment shapes in direct-aperture optimization for step-and-shoot IMRT.

Previous studies have shown that, by optimizing segment shapes and weights directly, without explicitly optimizing fluence profiles, effective IMRT plans can be generated with fewer segments. This study proposes a method of direct-aperture optimization with aperture shape constraints, which is designed to provide segmental IMRT plans using a minimum of simple, regular segments. The method uses a cubic function to create smoothly curving multileaf collimator shapes. Constraints on segment dimension and equivalent square are applied, and each segment can be constrained to lie within the previous one, for easy generation of fluence profiles with a single maximum. To simply optimize the segment shapes and reject any shapes which violate the constraints is too inefficient, so an innovative method of feedback optimization is used to ensure in advance that viable aperture shapes are generated. The algorithm is demonstrated using a simple cylindrical phantom consisting of a hemi-annular planning target volume and a central cylindrical organ-at-risk. A simple IMRT rectum case is presented, where segments are used to replace a wedge. More complex cases of prostate and seminal vesicles and prostate and pelvic nodes are also shown. The algorithm produces effective plans in each case with three to five segments per beam. For the simple plans, the constraint that each segment should be contained within the previous one adds additional simplicity to the plan, for a small reduction in plan quality. This study confirms that direct-aperture optimization gives efficient solutions to the segmental IMRT inverse problem and provides a method for generating simple apertures. By using such a method, the workload of IMRT verification may be reduced and simplified, as verification of fluence profiles from individual beams may be eliminated.

Algorithms↗
PubMed2006

The susceptibility of IMRT dose distributions to intrafraction organ motion: an investigation into smoothing filters derived from four dimensional computed tomography data.

This study investigated the sensitivity of static planning of intensity-modulated beams (IMBs) to intrafraction deformable organ motion and assessed whether smoothing of the IMBs at the treatment-planning stage can reduce this sensitivity. The study was performed with a 4D computed tomography (CT) data set for an IMRT treatment of a patient with liver cancer. Fluence profiles obtained from inverse-planning calculations on a standard reference CT scan were redelivered on a CT scan from the 4D data set at a different part of the breathing cycle. The use of a nonrigid registration model on the 4D data set additionally enabled detailed analysis of the overall intrafraction motion effects on the IMRT delivery during free breathing. Smoothing filters were then applied to the beam profiles within the optimization process to investigate whether this could reduce the sensitivity of IMBs to intrafraction organ motion. In addition, optimal fluence profiles from calculations on each individual phase of the breathing cycle were averaged to mimic the convolution of a static dose distribution with a motion probability kernel and assess its usefulness. Results from nonrigid registrations of the CT scan data showed a maximum liver motion of 7 mm in superior-inferior direction for this patient. Dose-volume histogram (DVH) comparison indicated a systematic shift when planning treatment on a motion-frozen, standard CT scan but delivering over a full breathing cycle. The ratio of the dose to 50% of the normal liver to 50% of the planning target volume (PTV) changed up to 28% between different phases. Smoothing beam profiles with a median-window filter did not overcome the substantial shift in dose due to a difference in breathing phase between planning and delivery of treatment. Averaging of optimal beam profiles at different phases of the breathing cycle mainly resulted in an increase in dose to the organs at risk (OAR) and did not seem beneficial to compensate for organ motion compared with using a large margin. Additionally, the results emphasized the need for 4D CT scans when aiming to reduce the internal margin (IM). Using only a single planning scan introduces a systematic shift in the dose distribution during delivery. Smoothing beam profiles either based on a single scan or over the different breathing phases was not beneficial for reducing this shift.

Body Burden↗
PubMed2005

Pleistocene human footprints from the Willandra Lakes, Southeastern Australia.

Human and other hominid fossil footprints provide rare but important insights into anatomy and behavior. Here we report recently discovered fossil trackways of human footprints from the Willandra Lakes region of western New South Wales, Australia. Optically dated to between 19-23 ka and consisting of at least 124 prints, the trackways form the largest collection of Pleistocene human footprints in the world. The prints were made by adults, adolescents, and children traversing the moist surface of an ephemeral soak. This site offers a unique glimpse of humans living in the arid inland of Australia at the height of the last glacial period.

Adolescent↗
PubMed2005

Prediction of radiation-induced normal tissue complications in radiotherapy using functional image data.

The aim of this study has been to explicitly include the functional heterogeneity of an organ as a factor that contributes to the probability of complication of normal tissues following radiotherapy. Situations for which the inclusion of this information can be advantageous to the design of treatment plans are then investigated. A Java program has been implemented for this purpose. This makes use of a voxelated model of a patient, which is based on registered anatomical and functional data in order to enable functional voxel weighting. Using this model, the functional dose-volume histogram (fDVH) and the functional normal tissue complication probability (fNTCP) are then introduced as extensions to the conventional dose-volume histogram (DVH) and normal tissue complication probability (NTCP). In the presence of functional heterogeneity, these tools are physically more meaningful for plan evaluation than the traditional indices, as they incorporate additional information and are anticipated to show a better correlation with outcome. New parameters m(f), n(f) and TD(50f) are required to replace the m, n and TD(50) parameters. A range of plausible values was investigated, awaiting fitting of these new parameters to patient outcomes where functional data have been measured. As an example, the model is applied to two lung datasets utilizing accurately registered computed tomography (CT) and single photon emission computed tomography (SPECT) perfusion scans. Assuming a linear perfusion-function relationship, the biological index mean perfusion weighted lung dose (MPWLD) has been extracted from integration over outlined regions of interest. In agreement with the MPWLD ranking, the fNTCP predictions reveal that incorporation of functional imaging in radiotherapy treatment planning is most beneficial for organs with a large volume effect and large focal areas of dysfunction. There is, however, no additional advantage in cases presenting with homogeneous function. Although presented for lung radiotherapy, this model is general. It can also be applied to positron emission tomography (PET)-CT or functional magnetic resonance imaging (fMRI)-CT registered data and extended to the functional description of tumour control probability.

Algorithms↗
PubMed2004

Analysis of stochastic noise in intensity-modulated beams.

Inverse planning techniques are known to produce intensity-modulated beams (IMBs) that are highly modulated. They are characterized by the fact that they contain high-frequency modulations that are absent in the profiles that are easier to deliver. For the purpose of this study these clinically unwanted fluctuations are being defined as 'noise'. Although these highly modulated solutions are also optimal solutions, as soon as the profiles are being delivered, they become unfavourable with respect to delivery efficiency and the analysis and verification of treatment. The aim of this work was therefore to understand the origins of the structure and complexity of IMBs. Ultimately, if one can characterize the essential features in optimum beam profiles, it might be possible to control the frequency distribution of IMBs and simplify the IMRT planning and delivery process. The study was based on two common optimization techniques: simulated annealing (SA) and gradient-descent (GD). The assumptions made at the start of this work were that the stochastic noise caused by the SA optimization technique is dominant over other sources of noise and that it could be separated out from the essential modulation after convergence of the cost function by averaging minimum-cost fluence profiles. The results indicate that there are three possible sources of stochastic noise in IMBs, i.e. the optimization technique, the cost function and the definition of convergence of that cost function. In terms of the optimization technique itself, it was confirmed that the gradient-descent technique does not introduce stochastic noise in the IMBs. The SA technique does introduce stochastic noise but averaging of minimum-cost fluence profiles does not result in smoother beam profiles. This originates from the fact that this type of noise is not the dominant factor in the optimization, but rather the curvature of the cost function close to the global minimum. It is shown that the choice of initial temperature in the SA optimization technique is crucial for the convergence of the cost function and the frequency distribution of the fluence profiles. If the initial temperature is too small the stochastic noise will get frozen into the fluence profiles and become the dominant component of noise, resulting in very random-looking and difficult to deliver patterns.

Algorithms↗
PubMed2004

Use of artificial neural networks to predict biological outcomes for patients receiving radical radiotherapy of the prostate.

BACKGROUND AND PURPOSE: This paper discusses the application of artificial neural networks (ANN) in predicting biological outcomes following prostate radiotherapy. A number of model-based methods have been developed to correlate the dose distributions calculated for a patient receiving radiotherapy and the radiobiological effect this will produce. Most widely used are the normal tissue complication probability and tumour control probability models. An alternative method for predicting specific examples of tumour control and normal tissue complications is to use an ANN. One of the advantages of this method is that there is no need for a priori information regarding the relationship between the data being correlated. PATIENTS AND METHODS: A set of retrospective clinical data from patients who received radical prostate radiotherapy was used to train ANNs to predict specific biological outcomes by learning the relationship between the treatment plan prescription, dose distribution and the corresponding biological effect. The dose and volume were included as a differential dose-volume histogram in order to provide a holistic description of the available data. RESULTS: It was shown that the ANNs were able to predict biochemical control and specific bladder and rectum complications with sensitivity and specificity of above 55% when the outcomes were dichotomised. It was also possible to analyse information from the ANNs to investigate the effect of individual treatment parameters on the outcome. CONCLUSION: ANNs have been shown to learn something of the complex relationship between treatment parameters and outcome which, if developed further, may prove to be a useful tool in predicting biological outcomes.

Gastrointestinal Hemorrhage↗
PubMed2004

Clinical implementation of dynamic and step-and-shoot IMRT to treat prostate cancer with high risk of pelvic lymph node involvement.

BACKGROUND AND PURPOSE: Two systems have been developed for treating patients with locally advanced prostate cancer using intensity-modulated radiotherapy (IMRT): one using dynamic multi-leaf collimator delivery and the other using step-and-shoot. This paper describes the clinical implementation of these two techniques, and presents results from the first 14 patients treated in a clinical setting (nine dynamic, five step-and-shoot). PATIENTS AND METHODS: Dynamic treatments were planned using Corvus, and step-and-shoot using Helax-TMS; all were delivered using Elekta accelerators. Prior to the first clinical treatments, validation measurements were carried out for each system, including measurements for a complete IMRT treatment. The reproducibility of dynamic delivery and the characteristics of the accelerator for low-monitor-unit (MU) deliveries were also assessed. An extensive quality assurance (QA) program was performed for each of the patients. Additionally, timing measurements were carried out to assess the practicalities of the technique. RESULTS: The planning objectives were met in most cases. Absolute doses for complete IMRT treatments were within 2%, on average, with dose distributions generally showing agreement within 3% or 3 mm. Beam modulation measurements made throughout each patient's treatment indicated that both delivery methods were reproducible. The dynamic plans required an average of 765 MU per beam, with a treatment delivery time of 14 min; corresponding results for step-and-shoot plans were 105 MU and 10 min. CONCLUSIONS: Two IMRT techniques for this group of patients have been successfully implemented in the clinic. The more complex dynamic treatments showed no advantages over the step-and-shoot approach. QA results have shown accurate and reproducible delivery for both techniques, giving increased confidence in the techniques and allowing a reduction in the QA program.

Humans↗
PubMed2004

Addition of 18F-FDG-PET scans to radiotherapy planning of thoracic lymphoma.

BACKGROUND AND PURPOSE: FDG-PET possesses greater sensitivity and accuracy than computed tomography (CT) in detecting diseased lymph nodes. Though the FDG-PET scans are acquired for similar diagnostic reasons as CT, they are not used in the radiotherapy (RT) planning process. Successful tumourcidal dose is usually delivered but large volumes of normal and non-malignant tissues are irradiated due to the nature of lymphoma and also to the subjectivity of the field determining process. This study tries to lessen the subjectivity of the field determining process by the addition of currently acquired PET to the conventional thoracic lymphoma RT. The differences between retrospectively delineated volumes from CT and FDG-PET were compared and the effect of this additional information was evaluated. MATERIALS AND METHODS: Seventeen FDG-PET scans were registered to corresponding CT scans using rigid-body registration with negligible intra-observer variability. Comparisons were made between the volumes, lateral extensions and the most inferior point of the delineated gross tumour volumes (GTVs). RESULTS: For 1/17 patient data, no diseased volumes were delineated and in 6/17, no volumes were delineated on PET and yet in CT, masses up to 367.2 cm3 were outlined. From the 10 positive-CT and PET data, the GTV(PET) were smaller than GTV(CT) in six cases. Greater than 3.0 cm lateral disease extension differences were observed in 4/10 cases. Inferior tumour extents were confirmed in 6/10 cases whereas in 2/10 patients GTV(CT) was greater than 12.0 cm inferior compared to GTV(PET). CONCLUSIONS: FDG-PET data can be introduced to current thoracic lymphoma RT planning protocol with minimal intervention and changes. The subjectivity in the RT planning of thoracic lymphoma would be decreased with the addition of currently acquired FDG-PET data.

Adolescent↗
PubMed2004

Segmentation of IMRT plans for radical lung radiotherapy delivery with the step-and-shoot technique.

The purpose of this work was to determine a segmentation protocol for the treatment of localized non-small-cell lung cancer (NSCLC) with intensity-modulated radiotherapy (IMRT) that is as effective as possible while practically simple and hence robust to known practical inaccuracies. This study focused on the stratification of continuous profiles into a discrete number of intensity levels. The selection of the segmentation parameters for the delivery of the fluence profiles using multiple static fields has been considered. Five-field equispaced IMRT treatment plans of five patients with NSCLC were selected. The study comprised nine treatment plans for each patient, starting from a conformal plan, optimizing it for IMRT and then segmenting it utilizing different numbers of segments in each case and optimizing for segment weights separately. A conformal plan, optimized for beam directions, collimator and wedge angles, was also used for comparison with the IMRT plans, so as to consider the best coplanar conformal case. A dose objective for the PTV and the organs-at-risk plus a constraint for the spinal cord were set for all inverse plans. All stages were compared with the aid of dose-volume histograms, dose distributions at the plane of the isocenter, intensity maps for key beams and plots of PTV homogeneity and overall conformality versus complexity. The unsegmented IMRT plans gave the best results but cannot be realized in practice with an MLC. They were best approximated by plans that needed 106-167 segments to deliver, but did not deteriorate significantly when approximated by plans which required 26-40 segments in total. All segmented IMRT plans gave a better lung sparing than the conformal plans, indicating that the deterioration of IMRT plans following segmentation is not equivalent to that of unmodulated, conformal plans. However, optimized conformal plans have the potential to approach the lung sparing achieved by segmented IMRT plans. Among the IMRT situations examined, five-field treatment plans for the lung, utilizing a maximum of 40 segments in total, have proven to give a good approximation of the IMRT plans with continuous modulation.

Algorithms↗
PubMed2003

Patient-specific planning for prevention of mechanical collisions during radiotherapy.

A common unwanted difficulty in treatment planning, especially in non-coplanar radiotherapy set-ups, is the potential collision of the rotating gantry with the couch and/or the patient's body. A technique and computer program that detects these and signals avoidance of such beam directions is presented. The problem was approached using analytical geometry. The separate components within the treatment room have either been measured and modelled for an Elekta linear accelerator, or read out from a Pinnacle3 treatment planning system and are represented as an integer grid of points in three-dimensional (3D) space. The module is attached to the treatment planning system and can provide rejection or acceptance of unwanted beam directions in a plan. In contrast to previous work that has only used patient models, each individual patient's outlines are considered here in their actual treatment position inclusive of any immobilization device. The extremities of the patient superiorly and inferiorly to the scanned region are simulated by an expanded version of the RANDO phantom. In this way, 'potential' collisions can be detected in addition to the certain ones. Patient position is not a limiting factor for the accuracy of the collision detection anymore, as each set-up is always created around the isocentre. Maps of allowed and forbidden zones within the treatment suite have been created by running the code for all possible gantry and couch angles for three commonly arising cases: a head and neck plan utilizing a small stereotactic collimator, a prostate plan with multileaf collimators and an abdominal plan with the lead tray attached. In the last case, the 3D map permitted significantly fewer set-up combinations. Good agreement between prediction and experiment confirmed the capability of the program and introduces a promising add-on for treatment planning.

Humans↗
PubMed2003

Combinational use of conformal and intensity-modulated beams in radiotherapy planning.

Intensity-modulated (IM) beam profiles computed by inverse-planning systems tend to be complex and may have multiple spatial minima and maxima. In addition to the structure originating from the treatment objectives, beam profiles might contain stochastic structure or noise and numerical artefacts, which present certain practical difficulties. The combinational use of conformal and intensity-modulated beams could be a different method of making the total fluence distribution less noisy and deliverable without compromising the advantages of IMRT. The investigation of this possibility provided the basis for this paper. A treatment-planning study was performed to compare plans combining modulated and unmodulated beams with a 5-field, equally spaced, full IMRT plan for treating the prostate and seminal vesicles in three patients. Beam angles for this study were 0 degrees, 72 degrees, 144 degrees, 216 degrees and 288 degrees. Additionally, a study was performed on a patient with a different beam arrangement (36 degrees, 108 degrees, 180 degrees, 252 degrees, 324 degrees) from the first study to test the obtained results. This study has demonstrated that it is possible to substitute up to two conformal beams in the originally full IMRT plan when carefully selecting the conformal beam angles. Making the anterior beam (0 degrees) and an anterior oblique beam (between 0 degrees and 90 degrees) conformal leads to a reduction in the total number of monitor units and segments of about 15% and 39%, respectively. Additionally, these two open fields can be used for simpler treatment verification.

Biometry↗
PubMed2003

Class solutions for conformal external beam prostate radiotherapy.

PURPOSE: To determine a class solution coplanar plan from comparisons of three-field (3F), four-field (4F), and six-field (6F) plans in conformal non-intensity-modulated prostate radiotherapy. METHODS AND MATERIALS: Doses to two clinical target volumes, prostate only (PO) and prostate plus seminal vesicles (PSV) were evaluated in each of 10 patients using a variety of 3F, 4F, and 6F plans with a planning target volume margin of 10 mm. All plans were prescribed to 64 and 74 Gy. The class solution plan for each of 3F, 4F, and 6F was chosen from a variety of symmetrical and asymmetrical field arrangements that had been previously assessed. The class solution plans, 3F (0, 90, 270 degrees ), 4F (35, 90, 270, 325 degrees ), and 6F (50/lat/25) were compared with reference plans: 3F (0, 120, 240 degrees ), 4F (0, 90, 180, 270 degrees ), and 6F (55, 90, 125, 235, 270, 305 degrees ). Rectal volumes irradiated to greater than 50% (V(50)), 80% (V(80)), and 90% (V(90)) of the prescribed dose, normal tissue complication probabilities (NTCP) for rectum, bladder, and femoral heads (FH), and tumor control probabilities (TCP) were assessed. FH tolerance was set at 52 Gy to 10% volume. RESULTS: The field arrangement that gave the lowest irradiated rectal volume with acceptable bladder and FH doses was a 3F (0, 90, 270 degrees ) class solution plan. This plan gave a reduction in rectal V(80) of 1.2-12.4% for the PO group and 2.3-23.8% for the PSV group compared with the other plans. The reduction in rectal V(90) was 0.2-11.9% for the PO group and 1.5-23.3% for the PSV group using the 3F (0, 90, 270 degrees ) plan. This plan provided one of the lowest rectal NTCPs, but the difference was not significant when compared with the 4F class solution plan. When target volumes with 10-mm margins remain unchanged to 74 Gy, the irradiated rectal volumes for all plans were higher and rectal NTCPs can be trebled. CONCLUSION: The use of appropriate beam arrangements can provide a class solution plan using only 3 fields compared with 4 or 6 fields for the parameters considered. Both 3F (0, 90, 270 degrees ) and 4F (35, 90, 270, 325 degrees ) plans can be used as a class solution plan. Other practical issues that may influence the choice of class solution include delivery time with smaller number of fields, ease of verification, the use of 10-mm multileaf collimation vs. conformal blocks, and field shape fitting limitations when using dynamic wedges.

Adenocarcinoma↗
PubMed2003

Radiotherapy treatment planning of prostate cancer using magnetic resonance imaging alone.

PURPOSE: Accurate anatomical delineation of the gross tumour volume (GTV) is crucial for effective radiotherapy (RT) treatment of prostate cancers. Although reference to pelvic magnetic resonance (MR) for improved delineation of the prostate is a regular practice in some clinics, MR has not replaced CT due to its geometrical distortions and lack of electron-density information. The possibility and practicality of using MR only for RT treatment planning were studied. MATERIALS AND METHODS: The addition of electron-density information to MR images for conformal radiotherapy (CRT) planning of the prostate was quantified by comparing dose distributions created on the homogeneous density- and bulk-density assigned images to original CT for four patients. To quantify the MR geometrical distortions measurements of a phantom imaged in CT (Siemens Somatom Plus 4) and FLASH 3D T1-weighted MR (1.5 T whole body Siemens Magnetom Vision) were compared. Dose statistics from CRT treatment plans made on CT and MR for five patient data were compared to determine if MR-only treatment plans can be made. RESULTS: The differences between dose-plans on bulk-density assigned images when compared to CT were less than 2% when water and bone values were assigned. Dose differences greater than 2% were observed when images of homogeneous-density assignment were compared to the CT. Phantom measurements showed that the distortions in the FLASH 3D T1-weighted MR averaged 2 mm in the volume of interest for prostate RT planning. For the CT and MR prostate planning study, doses delivered to the planning target volume (PTV) in CT and MR were always inside a 93-107% dose range normalised to the isocentre. Also, the doses to the organs-at-risk in the MR images were similar to the doses delivered to the volumes in the registered CT image when the organ volumes between the two images were similar. CONCLUSIONS: Negligible differences were observed in dose distribution between CRT plans using bone+water CT number bulk-assigned image and original CT. Also, the MR distortions were reduced to negligible amounts using large bandwidth MR sequence for prostate CRT planning. MR treatment planning was demonstrated using a large bandwidth sequence and bulk-assigned images. The development of higher quality, low distortion MR sequence will allow regular practice of this technique.

Feasibility Studies↗