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Biomedical subjects

Steve Webb

Publications and source records attributed to Steve Webb.

25 records · Page 2Linked to original sources
PubMed2003

A systematic study of techniques for elective cervical nodal irradiation with anterior or opposed anterior and posterior beams.

PURPOSE: To assess target coverage and dose homogeneity using conventional radiotherapy (RT) and intensity-modulated RT (IMRT) with anterior and posterior beams for elective irradiation of the cervical lymph nodes in patients with head and neck cancer. MATERIALS AND METHODS: A planning study was performed in six patients who had undergone radical RT for head and neck cancer. RT plans to irradiate the cervical lymph nodes using a single anterior field, or opposed anterior and posterior fields, with 6 or 10 MV photons were compared. Plans using IMRT for missing-tissue compensation were also studied. An algorithm was developed to guide clinicians to the most appropriate treatment technique depending on the nodal groups to be irradiated. RESULTS: With 6 MV single field (SF) irradiation significant under-dose (minimum dose <70% of prescription dose) was seen in nodal groups II and V, due to their posterior position. With SF 10 MV the mean dose to level II was higher (p<0.001) and dose homogeneity to levels Ib and II was improved. Using opposed fields (OF), minimum doses to the nodes in levels II and V were improved. OF using 10 MV showed significant advantage over 6 MV with reduction of maximum doses to levels II, III and V. SF 10 MV IMRT improved maximum doses to levels Ib and II compared to SF 6 MV IMRT. OF IMRT gave the best dose distributions with optimal mean dose and dose homogeneity. Beam energy made no difference with OF IMRT. CONCLUSIONS: The optimal technique for elective cervical node irradiation depends on the lymph node levels within the PTV. If irradiation of the level II or V nodes is required, then the OF IMRT technique with either 6 or 10 MV gives the best dose distributions. In the absence of IMRT, then OF conventional techniques are best. If the aim is to irradiate levels III and IV or level IV only, then 6 MV SF non-IMRT is the simplest technique.

Algorithms↗
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↗