SIlO

426

A P P L I C A T I O N O F ICRU R E P O R T 50 C O N C E P T S T O S T E R E O T A C T I C R A D I O T H E R A P Y

E. Thomson, Norfolk & Norwich Care NHS Trust, Norwich, England.

Linear Accelerator based stereotactic radiotherapy has undergone a rapid increase in populari ty since its first introduction in the late 1980's and the number of new centres using stereotactic techniques continues to rise. The fundamental concept is to use a stereotactic frame, originally a neurosurgical device, to improve the geometric accuracy of intracranial irradiation. Based on this concept, however, there are now a number variations including use of static irregular fields. In this approach, the concept of using 3-dimensional coordinate systems defined by a stereotactic frame are retained but conformal fields are used to reduce the ratio of Treated Volume to PTV. ICRU report 50 is a general document setting down recommended standards for 'Prescribing, Recording and Reporting Photon Beam Therapy'. Its stated intention is as follows : 'To define clear, well defined and unambiguous concepts and parameters for use in the report ing process to ensure a common language between different centres'. This stated intention has par t icular s ignif icance to stereotact ic radiotherapy (SRT) and radiosurgery (SRS). Both are still new techniques in the majority of radiotherapy departments and further, technical and clinical aspects of treatment delivery are rapidly changing. In addition, expertise is currently concentrated in a small number of centres. The purpose of this presentation is to assess the applicability of ICRU Report 50 concep t s to l inear acce le ra tor based s tereotac t ic radiotherapy. The relevance of terminology such as GTV, CTV, PTV and Treated Volume is analysed using practical examples. Possible recommendations for dose reporting are suggested.

428

M U L T I - S L I C E BREAST C O N T O U R I N G USING AN O P T I C A L C O N T O U R I N G SYSTEM

I-I. Brown, A.J.Moloney, D.A.L.Morgan. Nottingham City Hospital NHS Trust, Nottingham, England.

The latest ICKU recommendations (report no.50) state that the maximum dose heterogeneity across the planning Target Volume (PTV) should be kept within +7% to -5% of the prescribed dose at the ICRU reference point. Conservative management o f breast malignancy with radiotherapy requires the highest standard o f delivery techniques at all stages and especially in treatment planning. Unfortunately the three-dimensional variation in surface contour is often simplified to a two-dimensional model Although attempts to satisfy the ICRU 50 criteria are successfully accomplished in this plane (usually the central plane), little or no regard for dose inhomogeneity is made off-axis. Obtaining accurate off-axis contours with their correct geometric relationship is often prohibitively expensive for a busy cancer centre ( eg CT). The OSIRIS optical contouring system fulfils both o f these requirements and is relatively inexpensive. This system has been successfully used to satisfy the dosimetric requirements o f the EORTC breast trial giving an accurate reconstruction o f designated planes or indeed the complete breast.

427

A LOCALIZATION TOOl. ON THE BASIS OF DIGITAl. TOMOSYNTHESIS

M. Kosta. G. Messaris. C. Badea. Z. Kolitsi ttni~, of Patras. Dept. of Medical Physics. Patras. Greece

Accurate tumor localization and delineation is still one of the great challenges, facing radiotherapy today. Often. CT treatmem planning data needs to be augmented by a second imagmg sodal i ty , such as angiography in the case of intracranial vascular lesions, by merging procedures based on stereotactic association. We have developed a localization tool on the basis of a DTS system. The technique reconstructs planes of variable orientation and is used in conjunction with isocentric fluoroscopic units such as DSA or radiotherapy simulator. The system combines tomographic with projection images. The availability of projection images allows for verification of the user's perception of spatial relationships and results in precise localization with respect to the isocenter, through interactive steps that involve the correlation of the location o1" a Latget point in the reconstruction image with it', location in two selected projection images. The system provides localization of the AVM nidus in the Ircalnlenl geonletries with an accuracy o1 the order of I m s . Ii1 addilion, it provides precise 3-D determinamm of the actual nidus extent by discerning between Ihe nidus complex and its feeding and dranllng vessels in relation with tile cranial anatomy. The technique can al,,o be effecli~eI.~ used in brachytherapy to facilitate accurate determination of the s~tlrce posilionr, based on a single set of prt+jcctitm data acquired o1+ the widel) u,,ed and broadl', accessihlc I adiolhclapy Mlllulatof

429

THE HIGH RISK OF STANDARD FIELDS IN ADVANCED T4 HEAD AND NECK CANCER

A. Rovirosa, J. Casals, F. Casas, A. Sanchez-Reyes, N. CarnY, A. Biete Radiation Oncology Dpt., Hospital Clinic i Universitari, Barcelona, Spain.

Background and purpose : The radiotherapy in head and neck cancers requires a perfect knowledge of the clinical tumoral volume. We were able to treat 24 patients with advanced T4 head and neck neoplasms upon knowing the clinical tumoral volume, and we were able to establish the limits of the field using CT and/or MRI. We will now present the comparison between the limits obtained by the image techniques and those standard limits referred to in the literature; we also report the usefulness of the standard fields to cover the tumor volume. Patients and methods : Twenty-four patients with T4 head and neck cancer were treated at the Hospital Clinic i Universitari of Barcelona from January 1994 to January 1996; sites of neoplasma treated were: 4 paranasal sinuses, 12 nasopharynx (7N0, 5N+), 8 pyriform sinus (3N0, 5N+). Procedure : the head of the patient was immobilized with a thermoplastic mask in the same position, whenever it was possible, as was done with the CT or MRI. After obtaining an x-ray, we transferred the tumoral area from each CT and/or MR1 slices to the x-ray. We were thus able to locate the tumoral area in the x-ray which permitted us to adapt the shrinking fields to the tumor. Results : (I) The standard limits would cut the tumor in the following way: 75% paranasal sinuses, 60% nasopharynx, 25% pyriform sinus. (2) Not enough margins to treat the tumor using standard limits were thus found in the following cases: 25% of paranasal sinuses, 50% of nasopharyx, 25% of pyriform sinus. (3) In the nasopharynx 100% of the cases were needed to be modified by the limits of the fields to cover the tumor, but in the case of paranasal sinuses and pyriform sinus the modification was done in 75% and 50% of the cases, respectively. (4) The size oftbe field in the nasopharynx tumors was increased by a mean of 3.4cm ( 1-7), with a median of 3cm. The 73% of cases survived with a mean follow-up of 18 months (5-27); the 73% of the cases achieved a complete response. Conclusions : (I) In our study, standard fields would have caused an incorrect treatment of the tumor in 100% of the cases of T4 nasopharynx cancer and in the 58% of the rest of the cases. (2) CT and/or MR/ are absolutely necessary to define the clinical tumoral volume and to obtain a correct treatment.