A primary goal of radiation therapy is to deliver radiation doses that conform to targets, while minimizing dose to surrounding normal tissue. Intensity-modulated radiation therapy (IMRT) deliverd with a multileaf collimator (MLC) is a promising resource to achieve conformal therapy. A medical linear accelerator (linac) equipped with an IMRT system can increase dose to the target and decrease dose to normal tissue. These capabilities may be significantly enhanced with dose optimization techniques. IMRT integrates rapidly evolving imaging, computer, optimization, and treatment delivery technologies. Resulting systems fundamentally distinguish themselves from prior technology. Thus, new quality assurance (QA) procedures are needed.

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The CyberKnife is an image-guided robotic system designed for stereotactic radiosurgery. This system uses a lightweight, x-band linear accelerator, computer-controlled robotic arm, a pair of orthogonal x-ray imagers (TLS: Target Locating System), and a computer workstation. During the treatment, the TLS determines the location of the lesion and communicates these coordinates to the robot. The robot adjusts the position of the beam to the target. The accuracy of this system is 0.7 mm (median) at Osaka University.
The CyberKnife system offers new options for radiosurgery/therapy. Stereotactic fractionated radiotherapy can now be performed with the same accuracy as singlefraction stereotactic radiosurgery. The frameless nature of CyberKnife allows tumors in the chest and abdomen to be treated as well. The real time tracking system option enables one to treat tumors that move with respiration, such as lesions in lung. Tumors in the lower spine, pancreas, and lung have already been treated in the USA.
A description of the components, accuracy, and future of the CyberKnife will be presented.

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Recent external radiotherapy requires precise localization of the target because advance in diagnostic imaging has made it possible to visualize a tiny tumor which would be curable with focused high dose irradiation. However, tumors in respiratory and bowel organs have been difficult to be given the high dose because of 1 to 3 cm movement during delivery of irradiation. Respiratory-gating techniques have been used with medical linear accelerators and particle therapy machines. Real-time tumortracking radiotherapy has been realized using fluoroscopic x-rays, internal gold-markers, and pattern recognition technology. Advantage and disadvantage of each gating technique have been realized. Active breath control method would be a cost-effective way of precise treatment without gating. More work is required to find the relationship between abdominal wall and internal movement of the tumor in many respiratorygating radiotherapy and between the internal markers and target volume in real-time tracking radiotherapy.

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This paper was reviewed to evaluate the feasibility of three-dimensional (3-D) conformal radiotherapy for extracranial tumors, especially for solitary lung tumors using a stereotactic body frame.
To extend the te chnique of stereotactic irradiation for intracranial tumors, accurate body fixation and regulation of internal target motion are essential. In our study, a stereotactic body frame was used, and daily setup accuracy was verified. As a result, its setup accuracy was maintained within 0-8.5 mm (Ave = 2.5mm). In our initial clinical experiences for thirty-two patients with 6-10 non-coplanar static beams, forty or 48 Gy was irradiated. During the follow-up of 4-27 (Average =11) months, twentynine (94%) tumors were locally controlled without any symptomatic complications. Recently, respiratory-gated irradiation systems, CT-linac systems, a real-time tumor tracking system, Cyber-knife, and C-arm linac were developed. With all these techniques, stereotactic irradiation for extracranial tumors are future direction of threedimensional conformal radiotherapy.

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In radiation therapy, it is very important to concentrate the high dose to the clinical target volume. For this purpose, S. Takahashi developed the dynamic conformal radiotherapy in 1960. In the first half of 1990' this conformal technique with multileaf collimator (MLC) was gradually becoming popular, because the three dimensional extent of the clinical target volume and the surrounding normal tissues was accurately recognized by the use of the serial CT-images, and the 3D-treatment planning was considerably easily obtained by the development of computer-technique. The 3D-CRT (conformal radiation therapy) based on the 3D-treatment planning in a broad sense means the coplanar or non-coplanar irradiation with multi-leaf collimator, and at present it is one of the most reasonable treatment techniques. In order to correspond the high dose region to the clinical target volume as possible, IM (intensity modulation)technique was introduced to the 3D-CRT using MLC in the second half of 1990'. Therefore, the static CRT is routinely used not only in Europe/ USA but also in Japan. But, it is often very difficult to obtain the concave high dose region in static CRT with MLC, even though the IM-technique is applied. At this occasion, the concave high dose region is easily obtained by the use of dynamic CRT such as the hollowed -out technique developed by S. Takahashi in Japan. The dynamic CRT technique is also useful with the static IM-CRT technique.
There are two main aims of co nformal radiotherapy: First, the protection of the surrounding normal tissues, and second, improvement of the local control rate. Usually the second aim can't be easily achieved, because the tumor control probability curves are often flat.
The DVH concept is usually valuable to choose the optimal treatment planning. But, it is difficult to find the optimal dose-distribution, when two DVH-curves cross each other on the middle range of irradiated dose. On that occasion, the NTCP-concept is useful. From the standpoint of the tolerable dose, normal organs are classified into two types: parallel and serial. Therefore, two types of the NTCP-formula should be prepared to apply the NTCP-concept clinically.

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In order to confirm quality assurance and quality control in radiation therapy, it is common to perform a monitor unit verification calculation independent on RTP system. Generally, this verification calculation is often to perform at manual calculation level separately from RTP system. Characteristics of output factors included in this calculation were explained on the basis of the situation in the world. Khan et al. had proposed the concept of collimator scatter factor and phantom scatter factor. The validity of these factors was addressed in of role played in dose monitor unit calculation. If we will utilize SMR, which was derived from zero-area phantom scatter factor, and zero-area TMR in dosimetry, a simple and accurate Clarkson integration is possible. These scatter factors and Clarkson integration is able to use not only for monitor unit calculation in a simple treatment technique, but also in an excellent technique as IMRT. These factors had been also used in the convolution method to improve an accuracy of dose calculation. These factors are not family in Japan, but it is expected to be got an agreement of the usage in future.

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Committee on Radiological Protection of Japan Radiological. Society (JRS) is making a database on patient doses for typical radiographies for introducing the guidance levels in Japan. The Committee reported necessity of a protocol on dosimetry in diagnostic radiology for introducing the guidance levels. Although a protocol on dosimetry in radiation therapy is established, there is no protocol on dosimetry in diagnostic radiology in Japan. Then, as a first step to establish a protocol on dosimetry in diagnostic radiology, annual changes in a calibration constant of an ionization chamber as a practical standard dosimeter has been investigated for 4 years. Changes in calibration constants for 4 years are within 2% for X-rays with effective energies than 32keV. From these results, practical frequencies of calibrations for practical standard dosimeters are discussed for maintain of their precision. If an accuracy of practical standard dosimeters is 5% for diagnostic radiology, frequencies of their calibrations may be once in a year or two years.

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