Image guidance in radiation therapy: techniques and applications

Abstract

In modern day radiotherapy, the emphasis on reduction on volume exposed to high radiotherapy doses, improving treatment precision as well as reducing radiation-related normal tissue toxicity has increased, and thus there is greater importance given to accurate position verification and correction before delivering radiotherapy. At present, several techniques that accomplish these goals impeccably have been developed, though all of them have their limitations. There is no single method available that eliminates treatment-related uncertainties without considerably adding to the cost. However, delivering "high precision radiotherapy" without periodic image guidance would do more harm than treating large volumes to compensate for setup errors. In the present review, we discuss the concept of image guidance in radiotherapy, the current techniques available, and their expected benefits and pitfalls.

Figure 1

Use of MV EPID for online correction using orthogonal 2D images (anteroposterior and lateral). Both the field and the bony anatomy are matched sequentially to give an estimate of error. The comparison of live image with reference DRR helps assess and correct translational shifts but does not estimate rotational errors. (a) Right parietal glioma. (b) Head and neck cancer. MV: Megavoltage; EPID: Electronic portal imaging device; 2D: two-dimensional; DRR: Digital reconstructed radiograph.

Figure 2

KV CBCT volumetric imaging. Both translational and rotational errors may be estimated. Translational errors are easily corrected whereas few systems have provisions for correcting rotational errors with couch rotations. (a) CBCT compared with reference scan before and after correction of setup error in a case of Carcinoma right breast, post-mastectomy. (b) CBCT correction in a case of Carcinoma larynx. (c) CBCT in a case of Carcinoma prostate not only corrects for setup errors, but also provides an estimate of reproducibility of prostate position with respect to bladder filling. In this particular case, the live image shows negligible bladder filling and treatment was delayed to allow for optimum bladder position for obtaining a reproducible position of prostate as well as moving the bowel out of treatment field. KV: kilovoltage; CBCT: cone beam computed tomography.

Figure 3

CyberKnife console showing the tumor tracking options in a case of head and neck malignancy. (a) 6D skull for skull base lesions. (b) Spine tracking for paravertebral tumors. (c) Fiducial tracking for all other lesions whose motion is independent of skull or spine position, such as base of tongue or neck nodes.