The Beamlet

This is the last in a series on the attributes of CTrue™ technology. Here, we consider a key feature that sets the TomoTherapy^® Hi·Art System^® apart from other radiotherapy devices—its helical treatment delivery pattern.

Unlike conventional linear accelerators, which were designed for 2D treatments, updated for 3D conformal therapy capabilities, and further adapted for IMRT, the Hi·Art System was designed from inception for efficient, full-time IMRT delivery. And the difference starts at the source, where the 6MV beam is collimated using a total of 23 cm of tungsten to keep leakage dose outside of the treatment area very low, even for high monitor unit treatments.

The beam is shaped and modulated by 64 10 cm deep, binary MLC leaves which allow only a fraction of the transmission seen on conventional systems (less than 0.3% intra-leaf transmission and less than 0.5% inter-leaf transmission). The MLC leaves, which project a width of 0.6 cm at the isocenter, move in or out of the field in 20 ms, using a pneumatic system with no electrical motors. Radiation is either blocked or unblocked by a leaf, meaning there are no leaf tip effect issues as there can be with conventional MLCs.

The beam and collimation apparatus is mounted on a ring gantry that provides unmatched stability (isocenter position is stable to within 0.75 mm), efficiency (through the use of rotation speeds matched to required dose per degree) and accuracy (via a common imaging and treatment isocenter).

The beam emanates from the collimation system in a “fan” shape of variable width, and is used to both image and treat the patient in helical fashion. CT images with 2, 4 or 6 mm slice width are acquired helically (just like your modern diagnostic CT scanner or CT simulator) using a conventional xenon single-row CT detector. As the dose from imaging is very low (typically 1–2.5 cGy), and the time it takes to acquire and register images with planning CT scans is quick (typically 2-4 minutes), daily CT guidance is made practical for any clinical environment. The resulting 3D volumetric image allows for the matching of boney and soft tissue anatomy, and gives the clinician the confidence needed to deliver intricate dose patterns.

After the patient’s position is adjusted to sub-mm accuracy based on the registration of the daily CT to the plan CT, the treatment begins. The couch moves the patient into the bore of the system as the gantry and MLC move around the patient in 360º rotations. With helical delivery, the continuously-modulating fan beam overlaps itself on successive rotations, much like wrapping the tape on a hockey stick! The overlapping beamlets have a user-selectable length of 1, 2.5, or 5 cm (in the direction along the patient) and a width (across the patient) equal to the 0.6 cm MLC leaf projection. With multiple 360º gantry rotations, there are tens of thousands of individually-optimized beams used in a treatment. And, unlike treatments on conventional machines, the Hi·Art System allows for the use of a single patient set-up for all lesions to be treated—there's no need to shift the patient to align each tumor with the isocenter. With a maximum treatment field length of 160 cm, the extent and dosimetric quality of dose delivered in a single automated sequence is unmatched. All this is performed in the same amount of time, or less, when compared to conventional linear accelerators attempting similar treatments.

Upcoming Events Spotlight

Just announced...

Join us Thursday, March 15, 2007, from 6–9pm, at the Westin Michigan Avenue Hotel in Chicago, for the TomoTherapy Hi·Art System Regional Meeting - Midwest.

This event, held in conjunction with the ASTRO 2007 Spring Refresher Course, will feature Midwest area physicians and physicists who will share their experiences and expertise using the TomoTherapy Hi·Art System.

Scheduled speakers include: