Alternative Solutions

A typical example of high-precision radiotherapy can be found in stereotactic radiosurgery of intra-cranial lesions, which uses an invasive frame that is attached to the patient’s head and used as a reference frame for imaging, localisation and treatment. Following this idea, one might argue that this approach could be extrapolated to high-precision radiotherapy for extra-cranial locations.3 This brings us to the concept of immobilisation. Opposed to intra-cranial locations, where tumour motion with respect to the skull can be assumed to be negligible, this assumption no longer holds for the extra-cranial situation. Wulf et al. investigated the use of stereotactic body frames, observing deviations of up to 12mm (with a safety margin of 5mm) where 12–16% of the target might be partially missed.29–31 These investigators concluded that, even with the use of immobilisation techniques, IGRT should be applied for safe margin reduction. It can be argued that patient comfort is to be preferred in combination with IGRT, as opposed to forcing a patient into an uncomfortable position (using immobilisation devices) to maintain a reproducible position during treatment. This is illustrated in Figure 1, where a customised combination of positioning aids (AIO Solution, Orfit Industries) is used to help the patient maintain a stable position during MV-CT scanning, positioning and treatment on the helical TomoTherapy machine at the AZ-VUB.

As most IGRT techniques currently used in a clinical set-up are based on ionising radiation (either kV or MV), one might challenge this approach in view of the ‘as low as reasonably achievable’ (ALARA) principle. Using daily imaging for 38 treatment fractions, patient doses of approximately 2340mSv, 1950mSv, 780mSv, 1950mSv and 40mSv will be obtained from MV EPID, MV-CBCT, MV-CT, kV-CBCT and a ceiling-mounted dual kV source-detector system, respectively.10,17,27 Some alternative solutions are currently being investigated, avoiding the use of ionising radiation in patient set-up. Ultrasound-based solutions are aimed at visualising soft tissue and, in particular, the target volume prior to treatment. The device is typically a portable system situated adjacent to the treatment couch. The import of patient-specific CT structures is required, as well as the isocentre localisation from the treatment planning system, prior to target volume alignment. A system to track the ultrasound probe position in space is introduced (i.e. a mechanical arm or an optical tracking such as infrared (IR) lightemitting devices (LEDs) or IR reflectors that are monitored by stereoscopically mounted IR cameras).25,32,33 Initial studies reported that ultrasound realtime positioning of the prostate showed promising results.25 Recent studies comparing these initial ultrasound devices with EPIDs, in combination with implanted radio-opaque markers or daily CT scans, revealed some drawbacks for prostate localisation.32,33 The ultrasound-based alignments were systematically different from the marker-based alignments in some directions (depending on the study) and the remaining random variability of the prostate position, after the ultrasound-based alignment, was similar to the initial variability without the use of any alignment other than room lasers. A promising solution is the introduction of magnetic resonance imaging (MRI) in the treatment room, such as the combination linac–MRI that is being investigated at the Universitair Medisch Centrum (UMC) Utrecht in The Netherlands or three rotating sources in combination with a 0.5 Tesla MRI scanner (Viewray Inc).34

Conclusion

Radiation therapy has gone through a series of (r)evolutions in the last few decades and it is now possible to produce highly conformal dose distributions. This improved dose conformity, together with its sharp dose gradients, have necessitated enhanced beam-targeting techniques for radiotherapy treatments. Components affecting the reproducibility of target position during and between subsequent treatment fractions include displacement of internal organs between fractions and internal organ motion within a fraction. IGRT uses advanced imaging technology to better localise the target volume during treatment or for treatment set-up, and is the key to reducing and ultimately eliminating the uncertainties. Volumetric IGRT solutions allow for assessment of changes in the tumour volume in time and, as such, the potential of adaptive radiotherapy. In this review, some of the clinically implemented developments have been discussed; however, as technology is evolving, many new solutions are being developed and there is always the danger that some developments have been neglected here. A consequence of these fast-moving developments and emerging technologies will be the challenge of deciding which technology is optimal or clinically relevant for treatment objectives. An important logistical question needs to be asked, “Who is going to take the final decision as to whether to correct and then implement the intervention in (near) realtime?”