A computed tomography-based protocol vs conventional clinical mark-up for breast electron boost.

AIMS: Computed tomography planning of whole breast radiotherapy (WBRT) improves breast coverage and reduces the normal tissue dose. Computed tomography planning may increase tumour bed boost treatment accuracy. The aims of this investigation were (1) to compare the breast boost volume treated with clinical mark-up with the volume delineated with computed tomography planning and (2) to study tumour bed volume changes between the initial planning computed tomography scan and a second computed tomography scan at the time of breast boost mark-up. MATERIALS AND METHODS: Women receiving adjuvant WBRT and an electron boost after breast-conserving surgery were eligible. As per standard practice, WBRT was computed tomography planned while the boost electron portal was clinically defined. Electron field borders were then traced with wire and a second computed tomography scan was carried out in the boost treatment position. Post-surgical radiological abnormalities were contoured to create a tumour bed clinical target volume (CTV) on both scans (CTV1 and CTV2). A 1cm margin to CTV2 defined the planning target volume (PTV). The proportions of the CTV2 and PTV receiving 90% (V90) and 80% (V80) of the dose were calculated. Changes in volume between CTV1 and CTV2 were analysed. RESULTS: Data from 47 eligible patients were analysed. The mean V90 for the PTV was 61%. Lower electron energy (P<0.001) and small field sizes (P=0.004) were associated with a low V90. The mean CTV decreased by 4.3 cm3 (P=0.014) and was smaller in those with a long surgery to computed tomography interval (P=0.008). On average, the 90% isodose covered 61 cm3 of normal tissue. CONCLUSIONS: Conventional clinical breast boost planning is inaccurate. Electron boost computed tomography planning together with appropriate surgical clip placement and the use of mammograms and pathological information should provide optimal coverage of the tumour site. The boost could usually be planned from the initial computed tomography scan.

A practical method for the calculation of multileaf collimator shaped fields output factors.

Output factors of multileaf-collimator (MLC) shaped radiation fields were measured for a commercial linear accelerator whose MLC leaves form parts of the upper collimator system. The approach of taking into account the reduced phantom scatter due to the MLC shaping on the output factor has previously been shown to be inadequate for this type of machine because of the effect of the MLC leaves on the collimator factor [Palta et al., Med. Phys. 23, 1219-1224(1996)]. In this article, we present two forms of the collimator factor that give satisfactory agreement with measured values of the output factors of MLC-shaped fields. The present method should be directly applicable to other linacs of similar MLC configuration. For clinical treatment planning, we believe the method is practical and accurate enough to be satisfactory. The equation for calculating the output factor requires only peak scatter and output factors of the machine. These are normally measured during machine commissioning.

A control method for a nonlinear multivariable system: application to interstitial laser hyperthermia.

An original adaptive control method is presented for controlling a nonlinear multivariable system. The method, which could be described as a modified quasi-linear approach, involves dividing the source excitation into a series of pulsing rounds and is implemented as a control algorithm on a computer. The theory underlying the method is developed with reference to an application involving temperature control in interstitial laser hyperthermia. In this application, the method is both successful and necessary to achieve optimally uniform elevated temperatures in a ground beef phantom. Apart from variable and parameter definitions, the method is otherwise general and might be useful for controlling a nonlinear system in which no prior exact characterization of the system is possible. Simulations were conducted to assess the effectiveness of the method in systems for which the unit excitation response changes by factors ranging from zero to three over the total period of excitation. In each case the method has proven stable.