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Both RS and SFS have gained eminent positions in radiation oncology and have become established modalities in the treatment of cranial lesions. Most leading radiation departments offer this technique and their numbers have grown significantly in the past decade.

The LINAC RS and gamma knife RS are equivalent techniques; however, technological and physical differences between these two methods have led to some confusion. Considering the RS, comparative clinical studies have documented that both therapeutic methodologies can be used with similar results. In comparison with gamma knife, the use of LINAC technology offers the possibility of dose fractionation, which has substantial clinical implications. The quality control of the complex LINAC is higher than of gamma knife and requires a specialized team of medical physicists and radiation oncologists. On the other hand, it is undisputed that stereotactic radiation therapy with isocentric LINAC has a high potential for further developments. Examples in this direction are the introduction of computer-guided micro-multileaf collimators which allows the delivery of a conform dose distribution with only one isocenter, using static fields or dynamic arcs and the implementation of the stereotactic intensity-modulated radiotherapy. These new technologies amplify substantially the potential of the stereotactic modality.

Stereotactic radiation treatment of extracranial targets shows promising initial results. The techniques are getting increasingly more specialized, especially in the treatment of small lung tumors; however, many questions remain unanswered. More experimental work and clinical trials are underway which should answer these question and should strengthen this promising approach.

Target volume definition is an interactive process. Based on radiological (and biological) imaging, the radiation oncologist has to outline the GTV, CTV, ITV, and PTV and BTV. In this process, a lot of medical and technological aspects have to be considered. The criteria for GTV, CTV, etc. definition are often not exactly standardised, and this leads, in many cases to variability between clinicians; however, exactly defined imaging criteria, imaging with high sensitivity and specificity for tumour tissue and special training could lead to a higher consensus in target volume delineation and, consequently, to lower differences between clinicians. It must be emphasised, however, that further verification studies and cost-benefit analyses are needed before biological target definition can become a stably integrated part of target volume definition.

The ICRU report 50 from 1993 and the ICRU report 62 from 1999 defining the anatomically based terms CTV, GTV and PTV must still be considered as the gold standard in radiation treatment planning; however, further advances in technology concerning signal resolution and development of new tracers with higher sensitivity and specificity will induce a shift of paradigms away from the anatomically based target volume definition towards biologically based treatment strategies. New concept and treatment strategies should be defined based on these new investigation methods, and the standards in radiation treatment planning — in a continuous, evolutionary process — will have to integrate new imaging methods in an attempt to finally achieve the ultimate goal of cancer cure.

For continued clinical gains in the practice of radiotherapy, management of breathing motion is essential. The problem of organ motion in radiotherapy is complex; thus, interventions to reduce organ-motion-related uncertainties require effort, expertise, and collaboration from many disciplines. The application of image-guidance techniques, i.e., imageguided radiotherapy, will play an increasing important role in developing new and improved delivery techniques, i.e., adaptive radiotherapy. With some anecdotal clinical evidence and many potentially beneficial but unproven technologies under development and on the horizon, it is essential to place equal emphasis on the planning and implementation of prospective clinical trials.

Adaptive radiotherapy system is designed to systematically manage treatment feedback, planning, and adjustment in response to temporal variations occurring during the radiotherapy course. A temporal variation process, as well as its subprocess, can be classified as a stationary random process or a nonstationary random process. Image feedback is normally designed based on this classification, and the imaging mode can be selected as radiographic imaging, fluoroscopic imaging, and/or 3D/4D CT imaging, with regard to the feature and frequency of a patient anatomical variation, such as rigid body motion and/ or organ deformation induced by treatment setup,organ filling, patient respiration, and/or dose response. Parameters of a temporal variation process, as well as treatment dose in organs of interest, can be estimated using image observations. The estimations are then used to select the planning/adjustment parameters and the schedules of imaging, delivery, and planning/adjustment. Based on the selected parameters and schedules, 4D adaptive planning/adjustment are performed accordingly.

Adaptive radiotherapy represents a new standard of radiotherapy, where a “pre-designed adaptive treatment strategy” a priori treatment delivery will replace the “pre-designed treatment plan” by considering the efficiency, optima, and also clinical practice and cost.

Local recurrences after conservative surgery and WBRT are most likely to occur in the immediate vicinity of the lumpectomy site. This fact has prompted the investigation of new approach of limited-field RT. Brachytherapy using either low or high dose rates delivering the total dose during a few days after surgery is advocated by several teams. While with interstitial brachytherapy the first results at 5 years are promising, the results with the MammoSite balloon device are still immature with a relatively short follow-up. The balloon catheter applicator has been developed in North America because of the theoretical disadvantages reported after the standard catheter-based interstitial brachytherapy. In the U.S. very few clinicians are familiar with the technique: many patients and health care find the placement, appearance, and the numerous puncture sites disturbing. If a simpler, safer, and quicker technique for the delivery of radiation could be offered to patients with early-stage breast cancer, such an approach could theoretically increase the breast-conserving therapy option to more women and improve their quality of life.

Accelerated PBI is logistically simpler and a more practical method for breast-conserving therapy, but it has to be demonstrated in randomized phase-III trials that it is at least equivalent to WBRT before its routine use.

Optimized inverse planning can yield superior treatment plans, especially in complex situations with convex-concave target volumes and nearby critical structures; however,the optimization criteria must be carefully chosen. Determining appropriate optimization criteria is not straightforward and requires some trial and error in a “human iteration loop.” Using current commercial inverse planning systems this process can be quite time-consuming. Experienced treatment planners know how to steer an IMRT plan in the desired direction by appropriately changing the optimization criteria. Also, class solutions can help to avoid or reduce the “human iteration loop” in cases that do not vary too much between individuals, such as prostate treatments, because optimization criteria can be re-used. Nevertheless, plan optimization leaves something to be desired. The main problem is that it may not be possible to come up with a quantitative, complete optimization formulation for radiotherapy planning in the near future; however, an achievable alternative is to design optimization systems that let the physicians exercise their experienced clinical judgment or intuition in the most direct interactive way. Therefore, some future developments aim at a more interactive approach towards inverse planning. Multicriteria optimization and navigating a treatment plan database have been described as promising approaches in this context.

Several biological models have been developed. Although these models give a correct description of the main characteristics of the radiation response, great caution has to be taken if these models are to be applied to patients.

While the linear-quadratic model provides a good description of experimental settings, a larger uncertainty is involved in the prediction of iso-effects for clinical applications. The more advanced NTCP and TCP models should only be applied for relative, rather than absolute, predictions of effect probabilities. When using relative values, the uncertainty of the predictions should be considered to decide whether a detected difference is really significant. As TCP/NTCP models are currently not completely validated, integration of these models into the cost function of the dose optimisation algorithm is not warranted. Whether it is possible to arrive at fully biologically optimised treatment plans for photon therapy has to be investigated by further research.

In this context, the clinical application of heavy charged particles plays an exceptional role as biological optimisation is routinely performed and an adequate RBE model is an essential prerequisite. The applied RBE model may still contain some degree of uncertainty which has to be considered carefully at treatment plan assessment and dose prescription.

MRSI has the potential to provide metabolic evidence of tumor activity that may be an important guide for therapeutic decisions. The treatment planning process and treatment planning systems should therefore have the ability to incorporate both metabolic and anatomic data in order to determine appropriate target volumes. Many problems need to be addressed and much work needs to be done in order to determine the optimal way to incorporate indices of metabolic activity, especially in light of newer treatment techniques such as IMRT; however, it is the present author’s belief that strong consideration should be given to the incorporation of functional imaging into the treatment process for focal or boost treatments for brain and prostate tumors. Given the discrepancies that have been found between MRI and MRSI determinants of target volumes, the results of controlled dose escalation studies for malignant tumors of the brain that have used MRI-derived target volumes should also be reevaluated given the possibility that these volumes may have been suboptimally defined.

MRSI has the potential to provide metabolic evidence of tumor activity that may be an important guide for therapeutic decisions. The treatment planning process and treatment planning systems should therefore have the ability to incorporate both metabolic and anatomic data in order to determine appropriate target volumes. Many problems need to be addressed and much work needs to be done in order to determine the optimal way to incorporate indices of metabolic activity, especially in light of newer treatment techniques such as IMRT; however, it is the present author’s belief that strong consideration should be given to the incorporation of functional imaging into the treatment process for focal or boost treatments for brain and prostate tumors. Given the discrepancies that have been found between MRI and MRSI determinants of target volumes, the results of controlled dose escalation studies for malignant tumors of the brain that have used MRI-derived target volumes should also be reevaluated given the possibility that these volumes may have been suboptimally defined.