New equations for matching a low neck field to oblique upper neck fields with collimator rotation in a 3-field monoisocentric setup for head-and-neck cancers.

With a monoisocentric 3-field technique for treating head-and-neck cancer, collimator rotation may be needed for the upper lateral fields to avoid cephalad-positioned shoulders or to avoid unnecessary arytenoid irradiation while maintaining more anterior coverage. For patients with unilateral lymphadenopathy, lateral oblique-opposed boost fields can be used to encompass the primary tumor and ipsilateral lymph nodes without junctioning through gross disease. When initial collimated lateral fields are also rotated with a gantry angle to produce oblique boost fields, however, the resulting matchline with a low anterior neck (LAN) boost field is no longer nondivergent. This can be corrected by manual adjustment of collimator and gantry angles for the LAN field using 3D treatment planning software. The goal of this study was to derive mathematical formulas to simplify this process. We used a transformation matrix to define formulas that could predict the appropriate modifications to the LAN boost field. Output from the formulas was (1) visually tested within 3D treatment planning software and (2) verified using a solid water head-and-neck phantom and radiographic film dosimetry to confirm that a nondivergent matchline was obtained in several clinical scenarios. When evaluated with 3D treatment planning software, the formulas accurately predicted the appropriate gantry and collimator angles of the LAN boost field for a variety of possible beam combinations. When evaluated with film dosimetry, the formulas were shown to accurately predict the appropriate gantry and collimator angles of the LAN boost field to within the +/- 2 mm/1 degrees tolerance specifications of the linear accelerator and acceptable for routine clinical use. The presented formulas are simple and geometrically precise. They predict the necessary manipulations of the LAN boost field to maintain a geometrically precise matchline, as verified by 3D treatment planning software, phantom dosimetry, and actual patient setups.

Dosage along the matchline between upper head-and-neck IMRT fields and conventional supraclavicular fields.

There is concern about dose along the matchline when upper head-and-neck fields are treated with IMRT and the lower (supraclavicular) field is treated with a conventional fixed beam. An anatomical phantom was scanned and planned with IMRT upper head-and-neck fields and a fixed-beam supraclavicular field. The plans were fused and dose was calculated to points along the matchline. These doses were compared with thermolaminescent dosimetry measurements in the phantom, as the phantom was treated according to the plans.

Design, fabrication, and testing of a fiduciary tray to be used with the multileaf intensity-modulating collimator for portal film localization.

The purpose of this study was to design, fabricate, and test a fiduciary tray to be used for portal film localization using the multileaf intensity-modulating collimator (MIMiC) developed by NOMOS Corporation of Sewickley, PA. The MIMiC is a tertiary collimator that is utilized in intensity-modulated radiation therapy (IMRT) treatments. At this time, no MIMiC fiduciary tray exists for commercial use. Due to the escalation of dose in IMRT treatments, the need for accurate localization is of utmost importance. A slight error in interpretation could lead to significant errors in dosimetric coverage of the target volume and possible overdosing of normal tissues. A fiduciary tray that fits onto the MIMiC was designed. Special attention was given to ensure that treatment could not be accidentally given with the tray in place. Various localization tests were administered to ensure accuracy for brain and pelvic treatments. The fiduciary tray should directly impact the daily operations for not only the radiation therapists but also the radiation oncologists by aiding in the direct comparison of setup films to weekly port films.