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Biomedical subjects

Steve Webb

Publications and source records attributed to Steve Webb.

9 recordsLinked to original sources
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↗
PubMed2003

Elimination of importance factors for clinically accurate selection of beam orientations, beam weights and wedge angles in conformal radiation therapy.

A method of simultaneously optimizing beam orientations, beam weights, and wedge angles for conformal radiotherapy is presented. This method removes the need for importance factors by optimizing one objective only, subject to a set of rigid constraints. This facilitates the production of inverse solutions which, without trial-and-error modification of importance factors, precisely satisfy the specified constraints. The algorithm minimizes an objective function which is based upon the single objective to be optimized, but which is forced to an artificially high value when the constraints are not met, so that only satisfactory solutions are allowed. Due to the complex nature of the objective function space, including multiple local minima separated by large regions of plateau, a random search technique equivalent to fast simulated annealing is used for producing inverse plans. To illustrate the novel features of the new algorithm, a simulation is first presented, for the case of a cylindrical phantom. The morphology of the objective function space is shown to be significantly different for the new algorithm, compared to that for a conventional quadratic objective function. Clinical cases for prostate and craniopharyngioma are then presented. For the prostate case, the objective is to reduce irradiated rectal volume. Three-field, four-field, and six-field optimizations, with or without orientation optimization, are shown to provide solutions which are consistent with previously reported plans and class solutions. For the craniopharyngioma case, which involves the use of a high-precision stereotactic conformal technique, the objective is to reduce the irradiated volume of normal brain. Practically feasible beam angles are produced which, compared to a standard plan, provide a small but worthwhile sparing of normal brain. The algorithm is thereby shown to be robust and suitable for clinical application.

Algorithms↗
PubMed2002

Fewer segments for IMRT generated by modulation splitting.

For intensity-modulated radiation therapy (IMRT), a simple technique is described which splits any N x N matrix I of fluences into two mathematically equivalent matrices IA and IB summing to 21, each of which requires considerably fewer segments to deliver IMRT using the jaws-plus-mask technique (Webb 2002b). When IA and IB are delivered alternately at fractions throughout the treatment, the total number of field segments required is between 1/2 and 2/3 that of delivering matrix I when unadjusted.

Algorithms↗
PubMed2002

Intensity-modulated radiation therapy using only jaws and a mask: II. A simplified concept of relocatable single-bixel attenuators.

Intensity-modulated radiation therapy (IMRT) can be delivered inefficiently using the movable jaws of a linac. The efficiency can be much improved by delivering IMRT with the movable jaws together with a relocatable mask below the jaws (Webb 2002 Phys. Med. Biol. 47 257-75). This paper extends the modelling work by showing that a much simpler relocatable mask than previously conceptualized can lead to very similar improvements in monitor-unit efficiency and a decrease in the number of field components compared with the use of jaws only (JO). The new concept comprises a set of relocatable single-bixel attenuators (SBAs) which can be moved into the field components otherwise collimated by jaws only. Typically for a 15 x 15 bixel2 2D matrix of fluence with a peak value of Ip = 10 MUs (or equivalently ten stratified fluence levels) and using just four SBAs the MU-efficiency is nearly three times that of the JO technique and the number of field components is reduced to about 0.6 the number required by the JO technique. These gains become greater by using more SBAs or for larger Ip values. Component reordering was achieved to minimize the total delivery time including intersegment deadtimes. Practicalities are discussed.

Algorithms↗
PubMed2002

Intensity-modulated radiation therapy using only jaws and a mask.

Intensity-modulated radiation therapy (IMRT) generally requires complex equipment for delivery. Just one study has investigated the use of 'jaws-only' IMRT with not discouraging conclusions. However, the monitor-unit efficiency is still considered to be too low compared with the use of a multileaf collimator (MLC). In this paper a new IMRT delivery technique is proposed which does not require the MLC and is only moderately more complex than the use of jaws alone. In this method a secondary collimator (mask) is employed together with the jaws. This mask may translate parallel to the jaw axes. Two types of mask have been investigated. One is a regular binary-attenuation pattern and the other is a random binary-attenuation pattern. Studies show that the monitor-unit efficiency of this 'jaws-plus-mask' technique, with a random binary mask, is more than double that of the jaws-only technique for typical two-dimensional intensity-modulated beams of size 10 x 10 bixels2 and with a peak value of 10 MU (or quantized into 10 fluence increments). For two-dimensional intensity-modulated beams of size 15 x 15 bixels2 with a peak value of 10 MU (or quantized into 10 fluence increments), the monitor-unit efficiency of the 'jaws-plus-mask' technique with a random binary mask is almost triple that of the jaws-only technique. Some further extensions to this concept are presented showing that some more practical mask arrangements are possible but with somewhat compromised monitor-unit efficiency. Some comments are provided on practicalities and on delivery times.

Algorithms↗
PubMed2002

Configuration space analysis of common cost functions in radiotherapy beam-weight optimization algorithms.

The successful implementation of downhill search engines in radiotherapy optimization algorithms depends on the absence of local minima in the search space. Such techniques are much faster than stochastic optimization methods but may become trapped in local minima if they exist. A technique known as 'configuration space analysis' was applied to examine the search space of cost functions used in radiotherapy beam-weight optimization algorithms. A downhill-simplex beam-weight optimization algorithm was run repeatedly to produce a frequency distribution of final cost values. By plotting the frequency distribution as a function of final cost, the existence of local minima can be determined. Common cost functions such as the quadratic deviation of dose to the planning target volume (PTV), integral dose to organs-at-risk (OARs), dose-threshold and dose-volume constraints for OARs were studied. Combinations of the cost functions were also considered. The simple cost function terms such as the quadratic PTV dose and integral dose to OAR cost function terms are not susceptible to local minima. In contrast, dose-threshold and dose-volume OAR constraint cost function terms are able to produce local minima in the example case studied.

Algorithms↗