Inadequacy of conventional computerized tomography scans for treatment planning of tangential breast (chest wall) fields

Can virtual simulation of breast tangential portals accurately predict lung and heart volumes?

A treatment portal or simulator image has traditionally been used to demonstrate the lung and heart coverage of the breast tangential portal. In many cases, these images were acquired as a planning session on the linear accelerator. The patients were also CT scanned to assess the lung/heart volume and to determine the surgical site depth for the electron-boost energy. A study using 50 consecutive patients was performed comparing the digitally reconstructed radiograph (DRR) from the virtual simulation with treatment portal images. Modification to the patient's arm position is required when performing the planning CT scans due to the aperture size of the CT scanner. Virtual simulation was used to assess the potential variation of lung and heart measurements. The average difference in lung volume between the DRR and portal image was less than 2 mm, with a range of 0-5 mm. Arm position did not have a significant impact on field deviation; however, great care was taken to minimize any changes in arm position. The modification of the arm position for CT scanning did not lead to significant variations between the DRRs and portal images. The Advantage Sim software has proven capable of producing good quality DRR images, providing a realistic representation of the lung and heart volume included in the treatment portal.

On the use of virtual simulation in radiotherapy of the intact breast

In this paper a method of breast cancer treatment planning using virtual simulation implemented at the Department of Human Oncology at the University of Wisconsin is described. All patients in this procedure are placed in a custom vacuum mold in treatment position with both arms up to avoid collision with the CT scanner aperture. For all patients a CT scan of 5-mm-slice thickness is acquired. The ipsilateral and contralateral breast, the ipsilateral lung and the heart are delineated and a three-dimensional plan is generated that tries to minimize the dose to the ipsilateral lung and heart while ensuring adequate coverage of the affected breast. Digitally reconstructed radiographs are used to verify the patient setup on the treatment machine.

Measuring irradiated lung and heart area in breast tangential fields using a simulator-based computerized tomography device

PURPOSE

To illustrate the use of a simulator based computerized tomography system (SIMCT) in the simulation and planning of tangential breast fields.

METHODS AND MATERIALS

Forty-five consecutive patients underwent treatment planning using a radiotherapy simulator with computerized tomography attachment. One to three scans were obtained for each patient, calculations were made on the central axis scan. Due to the wide aperture of this system all patients were able to be scanned in the desired treatment position with arm abducted 90 degrees. Using available software tools the area of lung and/or heart included within the tangential fields was calculated. The greatest perpendicular distance (GPD) from the chest wall to posterior field edge was also measured.

RESULTS

The mean GPD for the group was 25.40 mm with 71% of patients having GPDs of < or = 30 mm. The mean area of irradiated lung was 1780 sq mm which represented 18.0% of the total ipsilateral lung area seen in the central axis. Seven of the patients with left sided tumors had an average 1314 sq mm heart irradiated in the central axis. This represented 11.9% of total heart area in these patients.

CONCLUSIONS

Measurements of irradiated lung and heart area can be easily and accurately made using a SIMCT device. Such measurements may help identify those patients potentially at risk for lung or heart toxicity as a consequence of their treatment. A major advantage of this device is the ability to scan patients in the actual treatment position.

Three-field isocentric breast irradiation using asymmetric jaws and a tilt board

Perfect abutment of medial and lateral tangential breast portals with the adjacent supraclavicular field may be achieved with ease. A simple and safe approach was developed using a tilt board and new technology that is standard on a popular linear accelerator. The patient is secured on a tilt board as a means to level the chest wall. Isocenter is placed at depth on the matchline, where asymmetric jaws are used to produce non-divergent field edges and a perfect abutment. This is done without the need for table or collimator rotations, beam-splitters, or vertical cephalad blocks. The dorsal beam edge of the tangents is made coplanar by rotating the gantry more than 180 degrees. This procedure produces a dosimetrically sharp field edge and eliminates concern about block transmission and excess dose to the contralateral breast. Set-up is fast, and the steps involved are simple and few. Advantages and limitations of this technique are presented.