Megavoltage Imaging

The major advantage of MV-based imaging is the fact that the actual treatment beam is used for patient set-up (or verification). There is a direct alignment of the target and treatment beams, avoiding the need for additional calibration procedures of the IGRT system. As the treatment beam is used, it allows for verification of the beam-shaping and assessment of the transmission dose. The latter could be used for dose reconstruction strategies, allowing verification of the ‘dose-of-the-day’.9–11 The application of portal films (radiographic film) as a tool for treatment set-up verification could be considered as the first step in MV-based IGRT. This was a cumbersome method requiring off-line strategies based on populationbased statistics to deduce clinically relevant treatment margins. EPIDs have been introduced to replace conventional film-cassette combinations.6 With the introduction of EPIDs, two classes of strategies have been introduced: the so-called ‘off-line’6,12–14 and ‘on-line’ approaches.6–8 Off-line monitors the position of the individual patient during a limited number of fractions and adapts the safety margins and/or treatment plan accordingly. This approach does not allow for decreasing the treatment margins sufficiently for aggressive CRT and is based on the notion that the systematic component is more important than the random component. The approach should not be generalised, as an example of obese patients clearly illustrates that the random component can be more prominent.15 The on-line approach offers the possibility of reducing both systematic and random uncertainties, yet it was considered to be time-consuming and requiring automated control of the treatment couch to make it efficient in clinical routine.16–18 The use of EPIDs is limited in that it is a planar imaging technique requiring at least two images to assess 3-D information on patient set-up. The image quality from MV beams is inferior to that obtained from kV, and often surrogates such as bony structures or implanted radio-opaque markers are required to locate the target volume.6

The next step in the evolution process was the introduction of the cone beam MV computed tomography (MV-CBCT).10 The advantages to this were: volumetric imaging with sufficient image quality for soft-tissue contrast; no additional hardware needed as the same flat panel detector introduced for EPI could be used; and a stable and linear relationship between Hounsfield Units (HU) and electron density with a potential use for dose calculation, and no high-Z artefacts in the images. This allows for accurate target delineation and dose calculation in the presence of high-Z prostheses in patients. An important advantage of CT-based imaging is that it facilitates comparison between the CT-of-the-day volumetric image data set with the reference CT data set that has been acquired for treatment planning. It allows for 3-D localisation, as well as assessment of volumetric changes during the course of treatment, potentially enabling the adaptation of the treatment based on this information.

A completely novel approach is presented with helical TomoTherapy (Hi- Art®, TomoTherapy Inc.), which is the fusion of a linear accelerator (linac) with a helical CT scanner (see Figure 1). This system uses a fan beam to acquire an MV-CT (the energy of the treatment beam is de-tuned from the initial 6MV to 3.5MV in imaging mode, with a lower dose rate of 11cGy/min, opposed to approximately 850cGy/min for treatment mode) of the patient prior to and, if necessary, during treatment.9,19 For treatment, a dedicated binary multileaf collimator (MLC) is used to modulate the fan beam to provide rotational IMRT.5,19 The beam rotation is synchronised with continuous longitudinal movement of the couch through the bore of the gantry, forming a helical beam pattern. When operating as a helical MV-CT system, the leaves are fully retracted to an open state. The on-board MV-CT option offers a number of verification processes. The MV-CT scan can be fused with the planning CT scan for automated target localisation and positioning prior to treatment. Verification of the automated fusion routine on an anthropomorphic phantom showed correct translations and rotations to an accuracy of less than 1mm or 1º.19 The set-up correction (involving rotations and translations) can be implemented either by moving the patient or, in principle, by modifying the IMRT delivery to account for the patient’s actual geometric offset. It is possible to superpose the prescribed dose distribution on the ‘CT-of-the-day’ images to align the patient’s anatomy with the dose (see Figure 2).

The CT detector system can be operated during treatment to compare the detector signal with the expected signal and, as such, detect deviations or, alternatively, reconstruct the dose delivered to the patient from exit dose measurements on the ‘CT-of-the-day’. This reconstructed dose distribution represents the dose the patient actually received, representing a new form of in vivo dosimetry. A concern with MV-CT imaging is patient dose. Measurements from both MV-CBCT (approx. 20–90mSv per scan) and MV-CT (approx. 20mSv per scan) show patient doses comparable to other IGRT techniques based on kV or MV.10