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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.

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.

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.

Dose calculation algorithms play a central role for the clinical practice of radiation therapy. They form the basis for any treatment plan optimization, a feature which becomes increasingly important with the development of complex treatment techniques such as IMRT. The role of highly accurate and therefore mostly time-consuming dose algorithms, such as superposition algorithms or Monte Carlo simulations, in clinical radiation therapy is still under investigation. Their increased accuracy offers substantial advantages for clinical cases which involve intricate tissue inhomogeneities.

Even if in many radiotherapy centers the treatment plans are still based on the pencil-beam method, its general applicability to inhomogeneous clinical cases has to be questioned. On the other hand, inside quite homogeneous regions, as in the central head region or the abdomen, the pencil beam generates dose distributions with excellent precision and provides the best trade-off between accuracy and calculation times.

In the case of severe tissue inhomogeneities the superposition method produces dose distributions which fairly cover the target region, even if minor differences are observed in comparison with Monte Carlo calculations. These offer the best prediction of the deposited dose inside arbitrary tissue types. Some Monte Carlo-based programs already offer computation times comparable to those of superposition algorithms, and therefore their applicability in clinical practice will probably further increase for a small and special class of clinical cases.

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.

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.