Irregular surface compensation for radiotherapy of the breast: correlating depth of the compensation surface with breast size and resultant dose distribution.

Irregular surface compensation uses dynamic multileaf collimators to modify the fluence to an irregular surface along the cranio-caudal axis. The depth of the compensation surface can be varied by specifying a user-defined parameter called the transmission penetration depth (TPD). In our institution, a review has been carried out of 60 breast patients treated using irregular surface compensation of the tangent fields. The effect of changes in the TPD on the dose distribution was investigated, and the optimum TPD was correlated with the maximum field separation (S(max)) along the posterior border. Reducing the TPD below 50% pushes the dose towards the front of the breast. This reduces hot spots at the medial and lateral regions next to the posterior border of the tangential fields, particularly for patients with large separation. In 23/60 patients, with a mean S(max) of 23.9 +/- 1.6 cm, a TPD between 35% and 45% was used to reduce the proportion of the planning target volume receiving more than 107% of the prescribed dose by 3.4% +/- 2.8%. Our department protocol states that, subject to an acceptable dose distribution, a TPD of 40% is used if S(max) is greater than 24 cm; for smaller separations, a TPD of 50% is used.

Practical experience with intensity-modulated radiotherapy.

At the Ipswich Hospital implementation of intensity-modulated radiotherapy (IMRT) commenced in February 2001 based on an established 3D conformal radiotherapy (3D CRT) service. This paper describes our experiences as we commissioned a fully-integrated IMRT planning and delivery system, and established IMRT within the department. Commissioning measurements incorporated a series of tests to ensure the integrity of the system and form the basis of routine quality assurance (QA) procedures. Potential IMRT patients proceeded through pre-treatment in the same way as standard 3D CRT patients. All were dual-planned for IMRT and 3D CRT with no change in established fractionation regimen, and the resulting plans evaluated. IMRT was selected for treatment where it offered a significant advantage by improving dose homogeneity and conformity within the target volume and/or reducing dose to organs at risk. Extensive pre-treatment verification was undertaken on all plans to check dynamic multileaf collimator (MLC) delivery and monitor unit calculation. Patients were monitored throughout treatment with amorphous silicon electronic portal imaging to ensure reproducibility of set-up. Between June 2001 and June 2003 21 patients were treated with inverse-planned IMRT to sites within the head and neck and lung. IMRT has enabled precise delivery to irregular shaped target volumes, avoiding organs at risk and enabling doses to be increased to radical levels in some cases. Additionally over 200 CT scanned breast patients were treated with forward-planned electronic compensation delivered by dynamic MLC, improving dose homogeneity within the breast volume compared with standard wedged plans. The IMRT programme will continue at the Ipswich Hospital with the introduction of further clinical sites and adoption of more aggressive fractionation regimens within the confines of multicentre clinical trials.

Leaf position verification during dynamic beam delivery: a comparison of three applications using electronic portal imaging.

The use of a dynamic multileaf collimator (MLC) to deliver intensity-modulated beams presents a problem for conventional verification techniques. The use of electronic portal imaging to track MLC leaves during beam delivery has been shown to provide a solution to this problem. An experimental comparison of three different verification systems, each using a different electronic portal imaging technology, is presented. Two of the systems presented are commercially available imagers with in-house modifications, with the third system being an in-house built experimental system. The random and systematic errors present in each of the verifications systems are measured and presented, together with the study of the effects of varying dose rate and leaf speed on verification system performance. The performance of the three systems is demonstrated to be very similar, with an overall accuracy in comparing measured and prescribed collimator trajectories of approximately +/-1.0 mm. Systematic errors in the percentage delivered dose signal provided by the accelerator are significant and must be corrected for good performance of the current systems. It is demonstrated that, with suitable modifications, commercially available portal imaging systems can be used to verify dynamic MLC beam delivery.

Verification of dynamic multileaf collimation using an electronic portal imaging device.

High standards of treatment verification are necessary where complex new delivery techniques, such as intensity modulated radiation therapy using dynamic multileaf collimation, are being developed. This paper describes the use of a fluoroscopic electronic portal imaging device (EPID) to provide real-time qualitative verification of leaf position during delivery of a dynamic MLC prescription in addition to off-line quantitative verification. A custom-built circuit triggers the EPID to capture a series of snap-shot images at equally spaced dose points during a dynamic MLC prescription. Real-time verification is achieved by overlaying a template of expected leaf positions onto the images as they are acquired. Quantitative off-line verification is achieved using a maximum gradient edge detection algorithm to measure individual leaf positions for comparison with required leaf positions. Investigations have been undertaken to optimize image acquisition and assess the edge detection algorithm for variations in machine dose rate, leaf velocity and beam attenuation. On-line verification enables the operator to monitor the progress of a dynamic delivery and has been used for independent confirmation of accurate dynamic delivery during intensity modulated treatments. Off-line verification allows measurement of leaf position with a precision of 1 mm although image acquisition times must be less than or equal to 140 ms to ensure coincidence of the maximum gradient in the image with the 50% dose level.

Design of a summary screen for an ICU patient data management system.

The paper describes a used-centred design for the summary screen of a computerised ICU patient data management system (PDMS). The screen also forms the resting state display, or default screen, and provides the principal navigation tool to other functionality within the system. The design process identified the most frequent potential users of this screen to be the nurses. Their tasks and the information resources required to perform them were analysed. The analysis identified that the nurses' main task of planning and implementing patient care required an awareness of a set of physiological parameters which provided an overview of the patient's general condition. Novel formats are proposed for displaying the trends in physiological parameters and these have been incorporated into a proposed screen design. These display formats have been evaluated by ICU nurses; they were adjudged to be clear, relevant, easy to learn and simple to use. Nurses considered the content of the screen, and the display formats used, to be suitable for maintaining an awareness of a patient's state during routine patient management.

Assessment of effective dose to staff in brachytherapy.

The aim of this paper is to investigate the problem of monitoring effective dose to hospital staff who are involved in the treatment of tumors using sealed sources placed inside the body (brachytherapy). In addition, the use of an unsealed source to treat the thyroid was also considered. Radiation distributions produced by both sealed sources commonly used in brachytherapy (192I, 137Cs, 226Ra) and an unsealed source used in the treatment of the thyroid (131I) were used to irradiate a Rando phantom. The brachytherapy treatments of esophageal and gynecological carcinoma were simulated. The Rando phantom was loaded with lithium fluoride thermoluminescent dosimeters at positions corresponding to a number of radiosensitive organs. Film badges and electronic personal dosimeters were attached to the Rando phantom at various anatomical sites. The Rando phantom was positioned adjacent to the patient at an angle of 90 degrees to the longitudinal axis of the patient. Irradiations were performed with and without a portable lead screen used on the radiotherapy wards. Effective dose was estimated for each simulated radiotherapy treatment and compared with the personal monitor readings. The data were used as a basis for the provision of advice on the wearing of the film badge dosimeters and the design of portable lead screens. The data also permitted a comparison between the two types of dosimeter when used for personal monitoring in brachytherapy.