Clinical impact of new radiation therapy techniques

Full text: The cornerstone of external beam radiation therapy is the delivery of a therapeutic dose to the target tissues. Traditional '2D' techniques broadly rely on the physician to assimilate the clinical information (history, examination, and radiographs) to define the target volume which is then localized via fluoroscopy on a traditional simulator. This approach required an in-depth understanding of the relationship between surface anatomy, radiographically visible anatomy (i.e. bones on simulator films) and three dimensional soft tissue anatomy. Radiation therapy treatment beams were generally limited to orientations wherein the physician/planner could understand the three dimensional relationship between the internal structures and their projection onto a simulator film. Three dimensional treatment planning allowed the more direct incorporation of three dimensional imaging information into the planning process. The 3D relationship between internal targets and normal tissues seen on 3D imaging (e.g. computed tomography - CT), were therefore more accurately known. This facilitated the use of 'non-standard' beam orientations, and more conformal shaping of the treatment beams. Software allows incorporation of multi-modality three dimensional imaging with, for example, positron emission tomography (PET), magnetic resonance imaging (MRI), and single photon emission computed tomography (SPECT). Therefore, the vast anatomic/functional information from multiple three dimensional imaging modalities can be used in concert to facilitate accurate treatment delivery. Software allows such three dimensional information to be displayed and viewed from any orientation. Beam orientations and shapes are then chosen to encompass the target yet minimize, as possible, normal tissue exposure. Thus, 3D tools allow three dimensional anatomic information to be more accurately incorporated into the planning process. In almost all instances, the target is fully encompassed within each of the 3D planned treatment beams. Further, each RT beam typically delivers a similar intensity of radiation (i.e. dose) to each part of the target. Thus, significant RT doses are typically delivered to all tissues in the 'shadow' of the target, as seen in the 'beams eye view' (BEV). Selection of the beam orientation is therefore critical with 3D planning. Compensators (such as wedges) can be added to the beam to modify the intensity profile of the beam. However, such compensators are relatively simple and provide only uniform and monotonic modulation of the beams intensity (e.g. the entire anterior part of a lateral photon beam given less intensity than the posterior aspect of that same field). With intensity modulated radiation therapy (IMRT), each portion of the beam, or 'beamlet', is modulated to provide a unique intensity. Thus each beam can deliver highly variable doses to each region of the tumour. The purposely non-uniform doses from several beam orientations are combined to deliver the desired dose in a three dimensional space. It is typically not practical for a planner to 'forwardly design' the necessary non-uniform intensity profiles that will yield the desired dose distribution. Rather, the physician defines the desired three dimensional dose distribution and software is used to compute the necessary beam intensity or profiles (i.e. the amount of modulation necessary). Since this process of defining dose, and then beam intensities, is the reverse order from conventional planning, this process as been termed inverse planning. In general, multiple radiation beams (>5-7) are needed to yield an acceptable dose distribution. IMRT appears to be superior to 3D conformal therapy for irregularly shaped tumours, particularly those with concavities. IMRT can be delivered by linear accelerator, or other machines such as the CyberKnife or Tomotherapy unit. With all of these advanced technologies, care must be taken to assure adequate target coverage. Treatment fields and dose distributions need to be designed such that they account for inter- and intra-frac tion motion of tumour. These technological advances require increased efforts on the part of physicians, dosimetrists, therapists, and physicists. An integrated team approach is required to assure that these technologies are applied in a clinically logical and effective manner. (author)