Tomotherapy

Abstract

Tomotherapy is delivery of intensity-modulated, rotational radiation therapy using a fan-beam delivery. The NOMOS (Sewickley, PA) Peacock system is an example of sequential (or serial) tomotherapy that uses a fast-moving, actuator-driven multileaf collimator attached to a conventional C-arm gantry to modulate the beam intensity. In helical tomotherapy, the patient is continuously translated through a ring gantry as the fan beam rotates. The beam delivery geometry is similar to that of helical computed tomography (CT) and requires the use of slip rings to transmit power and data. A ring gantry provides a stable and accurate platform to perform tomographic verification using an unmodulated megavoltage beam. Moreover, megavoltage tomograms have adequate tissue contrast and resolution to provide setup verification. Assuming only translational and rotational offset errors, it is also possible to determine the offsets directly from tomographic projections, avoiding the time-consuming image reconstruction operation. The offsets can be used to modify the leaf delivery pattern to match the beam to the patient's anatomy on each day of a course of treatment. If tomographic representations of the patient are generated, this information can also be used to perform dose reconstruction. In this way, the actual dose distribution delivered can be superimposed onto the tomographic representation of the patient obtained at the time of treatment. The results can be compared with the planned isodose on the planning CT. This comparison may be used as an accurate basis for adaptive radiotherapy whereby the optimized delivery is modified before subsequent fractions. The verification afforded tomotherapy allows more precise conformal therapy. It also enables conformal avoidance radiotherapy, the complement to conformal therapy, for cases in which the tumor volume is ill-defined, but the locations of sensitive structures are adequately determined. A clinical tomotherapy unit is under construction at the University of Wisconsin.

Dosimetric verification of RapidArc treatment delivery

“…This programme is intended to verify the correct interplay of gantry rotation with the leaf sequence, and of combinations of dose rate, gantry speed and MLC speed in the RapidArc mode, using designed RapidArc delivery plans [2].…”

“…The RapidArc approach can be considered an extension to dynamic MLC (DMLC) IMRT, and the machine commissioning and QA should validate standard DMLC delivery and the combination with dose rate variations and gantry rotation with variable speed which is specific to RapidArc. A recent paper describes a programme for RapidArc commissioning and QA using designed delivery plans to test accuracy in DMLC position, precise dose rate control during gantry rotation, and accurate control of gantry speed [2]. We here present the quality assurance programme and measurements at the patient specific level in the introduction of RapidArc into clinical practice at our institution.…”

RapidArc volumetric modulated therapy planning for prostate cancer patients

“…Following another path, the development has included the introduction of intensity-modulated radiotherapy (IMRT) which has the ability to tailor the dose distribution around targets with complex shapes, facilitated by inverse optimization to the desired dose distribution often using many (5Á9) beam angles. In the most advanced approaches, the intensity modulated and arc delivery approaches are combined to techniques such as intensity-modulated radiosurgery [1], intensity-modulated arc therapy [3], and helical tomotherapy [4].…”

Robotics and its applications in stereotactic radiosurgery

“…This method of imaging the body during treatment was first described by NOMOS Corporation 10,26,30 in 1997 but was successfully applied only later by Mackie 26 and colleagues 27 and Fitchard and associates 14 after the introduction of spiral CT. The result was highly integrated adaptive radiotherapy, a procedure that provides a combination of real-time image guidance and IMRT by using a binary multileaf collimator.…”