Treatment of the internal mammary nodes in early breast cancer: back to the future

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.

Recommendations for clinical electron beam dosimetry: supplement to the recommendations of Task Group 25

The goal of Task Group 25 (TG-25) of the Radiation Therapy Committee of the American Association of.Physicists in Medicine (AAPM) was to provide a methodology and set of procedures for a medical physicist performing clinical electron beam dosimetry in the nominal energy range of 5-25 MeV. Specifically, the task group recommended procedures for acquiring basic information required for acceptance testing and treatment planning of new accelerators with therapeutic electron beams. Since the publication of the TG-25 report, significant advances have taken place in the field of electron beam dosimetry, the most significant being that primary standards laboratories around the world have shifted from calibration standards based on exposure or air kerma to standards based on absorbed dose to water. The AAPM has published a new calibration protocol, TG-51, for the calibration of high-energy photon and electron beams. The formalism and dosimetry procedures recommended in this protocol are based on the absorbed dose to water calibration coefficient of an ionization chamber at 60Co energy, N60Co(D,w), together with the theoretical beam quality conversion coefficient k(Q) for the determination of absorbed dose to water in high-energy photon and electron beams. Task Group 70 was charged to reassess and update the recommendations in TG-25 to bring them into alignment with report TG-51 and to recommend new methodologies and procedures that would allow the practicing medical physicist to initiate and continue a high quality program in clinical electron beam dosimetry. This TG-70 report is a supplement to the TG-25 report and enhances the TG-25 report by including new topics and topics that were not covered in depth in the TG-25 report. These topics include procedures for obtaining data to commission a treatment planning computer, determining dose in irregularly shaped electron fields, and commissioning of sophisticated special procedures using high-energy electron beams. The use of radiochromic film for electrons is addressed, and radiographic film that is no longer available has been replaced by film that is available. Realistic stopping-power data are incorporated when appropriate along with enhanced tables of electron fluence data. A larger list of clinical applications of electron beams is included in the full TG-70 report available at http://www.aapm.org/pubs/reports. Descriptions of the techniques in the clinical sections are not exhaustive but do describe key elements of the procedures and how to initiate these programs in the clinic. There have been no major changes since the TG-25 report relating to flatness and symmetry, surface dose, use of thermoluminescent dosimeters or diodes, virtual source position designation, air gap corrections, oblique incidence, or corrections for inhomogeneities. Thus these topics are not addressed in the TG-70 report.

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.