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Disseminating the knowledge and lessons learned from accidental exposures is crucial in preventing re-occurrence. This is particularly important in radiation therapy; the only application of radiation in which very high radiation doses are deliberately given to patients to achieve cure or palliation of disease. Lessons from accidental exposures are, therefore, an invaluable resource for revealing vulnerable aspects of the practice of radiotherapy, and for providing guidance for the prevention of future occurrences. These lessons have successfully been applied to avoid catastrophic events with conventional technologies and techniques. Recommendations, for example, include the independent verification of beam calibration and independent calculation of the treatment times and monitor units for external beam radiotherapy, and the monitoring of patients and their clothes immediately after brachytherapy. New technologies are meant to bring substantial improvement to radiation therapy. However, this is often achieved with a considerable increase in complexity, which in turn brings opportunities for new types of human error and problems with equipment. Dissemination of information on these errors or mistakes as soon as it becomes available is crucial in radiation therapy with new technologies. In addition, information on circumstances that almost resulted in serious consequences (near-misses) is also important, as the same type of events may occur elsewhere. Sharing information about near-misses is thus a complementary important aspect of prevention. Lessons from retrospective information are provided in Sections 2 and 4 of this report. Disseminating lessons learned for serious incidents is necessary but not sufficient when dealing with new technologies. It is of utmost importance to be proactive and continually strive to answer questions such as 'What else can go wrong', 'How likely is it?' and 'What kind of cost-effective choices do I have for prevention?'. These questions are addressed in Sections 3 and 5 of this report. Section 6 contains the conclusions and recommendations. This report is expected to be a valuable resource for radiation oncologists, hospital administrators, medical physicists, technologists, dosimetrists, maintenance engineers, radiation safety specialists, and regulators. While the report applies specifically to new external beam therapies, the general principles for prevention are applicable to the broad range of radiotherapy practices where mistakes could result in serious consequences for the patient and practitioner.

En radiotherapie, il est indispensable d'avoir une connaissance precise de la dose delivree dans le volume cible et dans les organes critiques avoisinants. Pour etre utilisables cliniquement, les modeles de calcul doivent tenir compte des caracteristiques exactes des faisceaux utilises et des densites des tissus. Une precision de l'ordre de 2 % dans les regions a faible gradient de dose, et de 2 mm dans les regions a fort gradient est necessaire tout en conservant un temps de calcul compatible avec une approche interactive. Les modeles de calcul utilises sont ici succintement decrits et commentes.

Disseminating the knowledge and lessons learned from accidental exposures is crucial in preventing re-occurrence. This is particularly important in radiation therapy; the only application of radiation in which very high radiation doses are deliberately given to patients to achieve cure or palliation of disease. Lessons from accidental exposures are, therefore, an invaluable resource for revealing vulnerable aspects of the practice of radiotherapy, and for providing guidance for the prevention of future occurrences. These lessons have successfully been applied to avoid catastrophic events with conventional technologies and techniques. Recommendations, for example, include the independent verification of beam calibration and independent calculation of the treatment times and monitor units for external beam radiotherapy, and the monitoring of patients and their clothes immediately after brachytherapy. New technologies are meant to bring substantial improvement to radiation therapy. However, this is often achieved with a considerable increase in complexity, which in turn brings opportunities for new types of human error and problems with equipment. Dissemination of information on these errors or mistakes as soon as it becomes available is crucial in radiation therapy with new technologies. In addition, information on circumstances that almost resulted in serious consequences (near-misses) is also important, as the same type of events may occur elsewhere. Sharing information about near-misses is thus a complementary important aspect of prevention. Lessons from retrospective information are provided in Sections 2 and 4 of this report. Disseminating lessons learned for serious incidents is necessary but not sufficient when dealing with new technologies. It is of utmost importance to be proactive and continually strive to answer questions such as 'What else can go wrong', 'How likely is it?' and 'What kind of cost-effective choices do I have for prevention?'. These questions are addressed in Sections 3 and 5 of this report. Section 6 contains the conclusions and recommendations. This report is expected to be a valuable resource for radiation oncologists, hospital administrators, medical physicists, technologists, dosimetrists, maintenance engineers, radiation safety specialists, and regulators. While the report applies specifically to new external beam therapies, the general principles for prevention are applicable to the broad range of radiotherapy practices where mistakes could result in serious consequences for the patient and practitioner.

This study assessed and compared various image quality indices in order to manage the dose of pediatric abdominal MDCT protocols and to provide guidance on dose reduction. PMMA phantoms representing average body diameters at birth, 1 year, 5 years, 10 years and 15 years of age were scanned in a four-channel MDCT with a standard pediatric abdominal CT protocol. Image noise (SD, standard deviation of CT number), noise derivative (ND, derivative of the function of noise with respect to dose) and contrast-to-noise ratio (CNR) were measured. The 'relative' low-contrast detectability (rLCD) was introduced as a new quantity to adjust LCD to the various phantom diameters on the basis of the LCD(1%) assessed in a Catphan phantom and a constant central absorbed dose. The required variations of CTDIvol(16) with respect to phantom size were analyzed in order to maintain each image quality index constant. The use of a fixed SD or CNR level leads to major dose ratios between extreme patient sizes (factor 22.7 to 44 for SD, 31.7 to 51.5 for CNR(2.8%)), whereas fixed ND and rLCD result in acceptable dose ratios ranging between factors of 2.9 and 3.9 between extreme phantom diameters. For a 5-9 mm rLCD1(%), adjusted ND values range between -0.84 and -0.11 HU mGy(-1). Our data provide guidance on dose reduction on the basis of patient dimensions and the required rLCD (e.g., to get a constant 7 mm rLCD(1%) for abdominal diameters of 10, 13, 16, 20 and 25 cm, tube current-time product should be adjusted in order to obtain CTDIvol(16) values of 6.2, 7.2, 8.8, 11.6 and 17.7 mGy, respectively).

Respiration-gated radiotherapy offers a significant potential for improvement in the irradiation of tumor sites affected by respiratory motion such as lung, breast and liver tumors. An increased conformality of irradiation fields leading to decreased complications rates of organs at risk (lung, heart...) is expected. Respiratory gating is in line with the need for improved precision required by radiotherapy techniques such as 3D conformal radiotherapy or intensity modulated radiotherapy. Reduction of respiratory motion can be achieved by using either breath-hold techniques or respiration synchronized gating techniques. Breath-hold techniques can be achieved with active techniques, in which airflow of the patient is temporarily blocked by a valve, or passive techniques, in which the patient voluntarily holds his/her breath. Synchronized gating techniques use external devices to predict the phase of the respiration cycle while the patient breaths freely. This work summarizes the different experiences of the centers of the STIC 2003 project. It describes the different techniques, gives an overview of the literature and proposes a practice based on our experience.

Whereas it has been proposed almost one century ago, brachytherapy for prostate cancer has only recently emerged, especially thanks to endorectal echography, allowing to visualize seed implantation, to the development of seeds for permanent delivery and of micro-sources for high-dose rate delivery, and also to the development of three-dimension dosimetry programs allowing real-time implantations. For selected patients with localized prostate cancer (PSA10, Gleason7, no extracapsular extension, volume50-60 g), prostate brachytherapy with permanent implants (iodine 125 or, less frequently, palladium 103) gives results which appear at 10-15 years comparable to those of surgery. Incontinence and impotence rates appear lower than those of classical surgery. However, the first post-implant months are usually accompanied by urinary toxicity that should not be minimized. High-dose rate brachytherapy (HDR) could find its indications, in combination with conformal radiotherapy, in the treatment of more advanced forms, presenting an intermediate risk. It could also be an alternative to brachytherapy with permanent implants for the low-risk forms mentioned above, especially in developing countries where the cost of radioactive seeds slows down the use of this technique. Brachytherapy for prostate cancer should, therefore, find more and more indications, because of the increased incidence of prostate cancer, due to population ageing, of the increased proportion of localized forms, due to better detection, of the patient's request for less toxicity, and of the expecting lowering of the costs, which are now equivalent to those of surgery and should further lower.

To assess the frequency of the PSA "bouncing" phenomenon after a significant follow-up in a series of patients treated by permanent implant brachytherapy for a prostate cancer. To look for the clinical and dosimetric parameters possibly linked to this transitory secondary PSA increase. To evaluate in which percentage of cases this bouncing could have mimicked a biochemical relapse according to the ASTRO consensus criteria.From January 1999, to December 2001, 295 patients were treated by a permanent prostate implantation (real-time technique, with free (125)I seeds- Isoseed Bebig-) by the Institut Curie-Hôpital Cochin-Hôpital Necker Paris group. The mean follow-up is 40.3 months (9-66 months). The PSA level was regularly checked, at least every 6 months. We defined as a "bouncing" all increase in PSA, starting at 0.1 ng/ml, subsequently followed by a spontaneous (without any treatment) decrease, with return to the previous level or lower. We particularly focused on the patients fulfilling the criteria for a biochemical relapse according to the ASTRO consensus (Three successive increases in PSA). A multivariate analysis tried to identify independent factors among the usual clinical and dosimetric parameters.In our series, 161 patients (55%) showed a transitory PSA increase (bouncing) of at least 0.1 ng/ml; 145 patients (49%) a bouncing of 0.2 ng/ml, 93 patients (32%) a bouncing of 0.4 ng/ml and 43 patients (15%) a bouncing of at least 1 ng/ml. Mean PSA bounce was 0.8 ng/ml (0.1-4.1), and mean time to bounce was 19 months. Thirty-two patients (11% of the total number) presented three successive PSA increases with a significant (3 months) interval between the dosages, and therefore were to be considered as being in biochemical relapse according to the ASTRO consensus criteria. Actually, among those 32 patients, 18 (56%) subsequently showed a complete normalization of their PSA, without any treatment. Ten patients went on increasing their PSA, and were considered to be really in biochemical relapse. For the last 4 patients, the situation still remains ambiguous. In multivariate analysis, age70 years (P0.00001) and D90200 Gy (P0.003) were identified as independent factors for a PSA bouncing of at least 0.4 ng/ml.The observed rate of 32% of patients showing a PSA bouncing of at least 0.4 ng/ml in our series is in good agreement with what has been previously reported in the literature. Age70 years and D90200 Gy were found to be independent factors predicting for such a secondary transitory increase in PSA. Interestingly, among 32 patients fulfilling the classical criteria of the ASTRO for a biochemical relapse, 18 (56%) subsequently showed a spontaneous PSA decrease, demonstrating that the ASTRO consensus is not well adapted to the biochemical follow-up of our patients undergoing permanent implant prostate Brachytherapy.

Intensity modulated radiation therapy (IMRT) is based on a methodology called inverse planning. Starting from dosimetric objectives, constraints of optimization are fixed and given to the inverse planning system, which in turn calculates the modulated intensity to apply to each beam. Since the algorithms allow the constraints to be violated, the results of optimization may differ from the initial dosimetric objectives. Consequently, the user is compelled to adapt the choice of the constraints according to the type of modulation and until satisfactory results are found. The purpose of this work is to present our experience in the choice of these constraints for prostate cancer treatments, as we moved from conformal radiotherapy to IMRT. Treatments were performed with a Varian 23EX linac and calculations were realized with the Varian CadPlan-Helios planning system.The approach used for the first 12 patients treated at institut Curie with IMRT from June 2002 was analysed. The treatment always consisted of a combination of conformal radiotherapy with and without intensity modulation.Results showed that, a larger fraction of the treatment performed with IMRT induced a better sparing of the organs at risk for the same homogeneous dose distribution to the target volume. Apart from the dose-volume constraint for the rectum, a fixed set of constraints, slightly more restrictive than the dosimetric objectives, could be used for all patients. Compared with conformal radiotherapy, the conformation factor for IMRT increased up to 16%. A specific study was undertaken in view of treatments completely performed with IMRT. The optimal technique consisted in performing separated IMRT plans for the two target volumes, the prostate volume and the prostate plus seminal vesicles volume respectively. Another satisfactory possibility was to define new constraints on two separated planning target volumes, prostate and seminal vesicles. This last approach is now routinely implemented for our IMRT patients.

To study the number of migrating seeds, the anatomical site of migration and possible predictive parameters of migration, after prostate cancer brachytherapy using a loose-seed (I125) implantation technique.The charts of the 170 patients consecutively treated by the Institut Curie/Hospital Cochin/Hospital Necker Group between September 1, 2001 and August 31, 2002, were analysed. All seeds having migrated to the lungs and seen on the chest X-ray systematically performed at 2 months, have been recorded, as well as the seeds lost by the urines (after sieving) or in the sperm (condom).Among 12,179 implanted seeds, 44 were found to have migrated (0.36%). Most of the migrating seeds (32/44; 73%), were found in the lungs. Overall, one or several seed migrations were observed in 35 patients (21% of the total number of patients in this series). In the majority of cases (77%), only one seed migrated. A significant relationship (P = 0.04) was found between the number of migrating seeds and the number of implanted ones (or with the prostate volume, but those two parameters were closely linked in our series). More specifically, a significant relationship (P = 0.02) could be demonstrated between the number of seeds implanted at the periphery of the prostate and the number of seeds migrating to the lungs.The percentage of migrating seeds observed in this series is low, actually one of the lowest found in the literature when using the loose-seed technique. There was no clinical consequences and the loss of-usually-only one seed is very unlikely to alter the quality of the dose distribution. However, the predominance of pulmonary migrations in our series led us to slightly modify our implantation technique. We now try to avoid too "peripheral" seed implantations, due to the risk of migration towards the periprostatic veins, and subsequently to the lungs.

This retrospective study compared the rate of local recurrence after irradiation of uveal melanoma treated with iodine 125 plaques or proton beam therapy.Iodine 125 plaques were used to treat all uveal melanomas between the end of 1989 and 1991. Since 1991, we have used iodine plaques for small anterior tumors and proton beam for other tumors. We use a plaque with a larger diameter than the tumor diameter (2-4mm) with a dose of 90Gy at the apex. Proton beam therapy is used for all tumors at the equator or posterior to the equator not thicker than 12mm. The dose given is 60Gy cobalt equivalent in four fractions. For each patient, the initial size and location of the tumor were noted as well as the follow-up each year: the outcome for the eye (local recurrence, ocular conservation, and functional results), the occurrence of metastasis, and survival. A statistical analysis was performed.Between December 1989 and September 1998, 1272 patients were treated: 926 (72.8%) were treated with proton beam irradiation and 346 (27.8%) with iodine 125 plaques. The median follow-up was 5 years (60 months). For the patients treated with proton beam therapy, the mean age was 58 years, the tumor location was anterior to the equator for 3.8%, at the equator for 43.6%, and posterior to the equator for 52.6%. The mean tumor diameter was 13.4mm and the mean tumor thickness was 5.69mm. For the patients treated with iodine 125 plaques, the mean age was 61.5 years. The location of the tumor was anterior to the equator for 34.4%, at the equator for 46.5%, and posterior to the equator for 19.1%. The mean tumor diameter was 11.5mm and the mean tumor thickness was 5.12mm. The recurrence rate was 4% for the proton beam treatment and 3.75% for iodine plaques. There was no statistical difference.In the literature, the rate of local recurrence is usually higher with iodine 125 plaques than proton beam therapy. We discuss the risk factors for local recurrence after iodine 125 plaques: tumor diameter, lower dose to the tumor apex and lower dose rate, and posterior location of the tumor. We found a higher mortality rate in patients who presented local recurrence.When we use iodine 125 plaques for anterior tumors with the proper dose and dose rate to the apex of the tumor, we do not find more recurrence than with proton beam therapy.

To study different methods of CT and MR images fusion in patient treated by brachytherapy for localized prostate cancer. To compare the results of the dosimetric study realized on CT slices and images fusion.Fourteen cases of patients treated by I125 were retrospectively studied. The CT examinations were realized with contiguous section of 5mm thickness, and MR images were obtained with a surface coil with contiguous section of 3 mm thickness. For the images fusion process, only the T2 weighted MR sequence was used. Two processes of images fusion were realized for each patient, using as reference marks the bones of the pelvis and the implanted seeds. A quantitative and qualitative appreciation was made by the operators, for each patient and both methods of images fusion. The dosimetric study obtained by a dedicated software was realized on CT images and all types of images fusion. The usual dosimetric indexes (D90, V100 and V150) were compared for each type of image.The quantitative results given by the software of images fusion showed a superior accuracy to the one obtained by the pelvic bony reference marks. Conversely, qualitative and quantitative results obtained by the operators showed a better accuracy of the images fusion based on iodine seeds. For two patients out of three presenting a D90 inferior to 145 Gy on CT examination, the D90 was superior to this norm when the dosimetry was based on images fusion, whatever the method used.The images fusion method based on implanted seed matching seems to be more precise than the one using bony reference marks. The dosimetric study realized on images fusion could allow to rectify possible errors, mainly due to difficulties in surrounding prostate contour delimitation on CT images.

The conformal radiotherapy approach, three-dimensional conformal radiotherapy (3DCRT) or intensity-modulated radiotherapy (IMRT), is based on modern imaging modalities, efficient 3D treatment planning systems, sophisticated immobilization systems and rigorous quality assurance and treatment verification. The central objective of conformal radiotherapy is to ensure a high dose distribution tailored to the limits of the target volume while reducing exposure of normal tissues. These techniques would then allow further tumor dose escalation. Head-and-neck tumors are some of the most attractive localizations to test conformal radiotherapy. They combine ballistic difficulties due to particularly complex shapes (nasopharynx, ethmoid) and problems due to the number and low tolerance of neighbouring organs like parotids, eyes, brainstem and spinal cord. The therapeutic irradiation of head-and-neck tumors thus remains a challenge for the radiation oncologist. Conformal radiotherapy does have a significant potential for improving local control and reducing toxicity when compared to standard radiotherapy. However, in the absence of prospective randomized trials, it is somewhat difficult at present to evaluate the real benefits drawn from 3DCRT and IMRT. The published clinical reports on the use of conformal radiotherapy are essentially dealing with dosimetric comparisons on relatively small numbers of patients. Recently, a few publications have emphasized the clinical experience of several precursor teams with a suitable follow-up. This paper describes the current state-of-the-art of 3DCRT and IMRT in order to evaluate the impact of these techniques on head-and-neck cancers irradiation.

The conformal radiotherapy approach, three-dimensional conformal radiotherapy (3DCRT) and intensity-modulated radiotherapy (IMRT), is based on modern imaging modalities, efficient 3D treatment planning systems, sophisticated immobilization devices and demanding quality assurance and treatment verification. The main goal of conformal radiotherapy is to ensure a high dose distribution tailored to the limits of the target volume while reducing exposure of healthy tissues. These techniques would then allow a further dose escalation increasing local control and survival. Non-small cell lung cancer (NSCLC) is one of the most difficult malignant tumors to be treated. It combines geometrical difficulties due to respiratory motion, and number of low tolerance neighboring organs, and dosimetric difficulties because of the presence of huge inhomogeneities. This localization is an attractive and ambitious example for the evaluation of new techniques. However, the published clinical reports in the last years described very heterogeneous techniques and, in the absence of prospective randomized trials, it is somewhat difficult at present to evaluate the real benefits drawn from those conformal radiotherapy techniques. After reviewing the rationale for 3DCRT for NSCLC, this paper will describe the main studies of 3DCRT, in order to evaluate its impact on lung cancer treatment. Then, the current state-of-the-art of IMRT and the last technical and therapeutic innovations in NSCLC will be discussed.

The development of computerized systems in radiotherapy opens new challenging possibilities. There is however a major risk of specific errors if the use of such systems is not properly controlled. On the other hand, the software sophistication, such as encountered in modern treatment planning systems (TPS), makes it very difficult to set up efficient quality assurance programs compatible with the workload of users in charge of daily routine. From the analysis of previous accidents involving TPS, we can conclude that the major risks are erroneous calculations of monitor units resulting from a poor understanding of the software capabilities combined to a lack of vigilance with respect to the data obtained from a computer. To reduce the risks, it is necessary to setup a comprehensive quality assurance program covering all steps of the process, including commissioning of a new system and systematic checks of individual treatment plans. Such a program offers the users the opportunity to acquire a deeper understanding of the system and avoid potential pitfalls. There is presently a lack of consensus for an international protocol related to quality assurance of computerized systems in radiotherapy. It is therefore important to define protocols at the national level, with special consideration of the aspects which are more likely to generate significant errors.

In the last decade, brachytherapy emerged as a particularly appealing new way ot treating localized prostate cancer. Recently published 10-12 years biochemical control results appear to be superimposable to the best percentages achieved by surgery or conformal radiotherapy, with a small percentage of complications. This applied to severely patients. Only patients with T1/T2, PSA10 ng/mL, and Gleason score7 should be proposed such a treatment. The potential benefit of exploring patients with a endorectal coil MRI is being evaluated. The number of positive biopsies is also a parameter which should probably be considered in the therapeutic choice. Moreover, a prostate volume60 g, hip mobility limitations, a urinary obstructive syndrome and previous transurethral resection lead to difficulties in technical implantation and therefore must be taken into account when discussing brachytherapy. In conclusion, for adequately selected patients, brachytherapy offers a particularly applied alternative to surgery and external radiotherapy, with satisfactory long term biochemical control rates and limited complications.

Conformal irradiation of non-small cell lung carcinoma (NSCLC) is largely based on a precise definition of the nodal clinical target volume (CTVn). The reduction of the number of nodal stations to be irradiated would render tumor dose escalation more achievable. The aim of this work was to design an mathematical tool based on documented data, that would predict the risk of metastatic involvement for each nodal station.From the large surgical series published in the literature we looked at the main pre-treatment parameters that modify the risk of nodal invasion. The probability of involvement for the 17 nodal stations described by the American Thoracic Society (ATS) was computed from all these publications and then weighted according to the French epidemiological data. Starting from the primitive location of the tumour as the main characteristic, we built a probabilistic tree for each nodal station representing the risk distribution as a function of each tumor feature. From the statistical point of view, we used the inversion of probability trees method described by Weinstein and Feinberg.Taking into account all the different parameters of the pre-treatment staging relative to each level of the ATS map brings up to 20,000 different combinations. The first chosen parameters in the tree were, depending on the tumour location, the histological classification, the metastatic stage, the nodal stage weighted in function of the sensitivity and specificity of the diagnostic examination used (PET scan, CAT scan) and the tumoral stage. A software is proposed to compute a predicted probability of involvement of each nodal station for any given clinical presentation.To better define the CTVn in NSCLC 3DRT, we propose a software that evaluates the mediastinal nodal involvement risk from easily accessible individual pre-treatment parameters.

Technical problems often arise during irradiation to the breast, chest wall, and regional lymph nodes. The following are among the most frequently encountered problems: avoidance of normal tissues (heart and lungs) during chest wall, internal mammary nodes, and large breast irradiations; dose heterogeneity in large breasts; under- or overdosage at field junctions (breast medial tangent and internal mammary fields in particular). Various technical solutions have been offered: modified treatment positions, field inclinations, and conformal irradiation. Many are currently under evaluation. These new technical approaches in breast cancer irradiation require modern facilities for imaging, simulation, and dosimetry, which help to individually design treatment planning.

Radical prostatectomy remains the 'golden standard' therapy for localized prostate carcinoma for patients with a survival rate of more than ten years. However, because of the complications inherent in this surgical procedure, prostatectomy is presently increasingly challenged by various radiotherapy procedures. In the last decade, more sophisticated conformal therapy techniques have been proposed for prostate cancer patients. In parallel, for highly selected patients, brachytherapy is being promoted by an increasing number of medical centers. In fact, brachytherapy techniques for prostate cancers can be traced back to 1911, but recently developed techniques offer reliability and reproducibility, with satisfactory results in terms of tumor control and reduced toxicity, in selected patients. We present here the different techniques that are available today in prostate cancer brachytherapy.

'Conformal radiotherapy' is the name fixed by usage and given to a new form of radiotherapy resulting from the technological improvements observed during, the last ten years. While this terminology is now widely used, no precise definition can be found in the literature. Conformal radiotherapy refers to an approach in which the dose distribution is more closely 'conformed' or adapted to the actual shape of the target volume. However, the achievement of a consensus on a more specific definition is hampered by various difficulties, namely in characterizing the degree of 'conformality'. We have therefore suggested a classification scheme be established on the basis of the tools and the procedures actually used for all steps of the process, i.e., from prescription to treatment completion. Our classification consists of four levels: schematically, at level 0, there is no conformation (rectangular fields); at level 1, a simple conformation takes place, on the basis of conventional 2D imaging; at level 2, a 3D reconstruction of the structures is used for a more accurate conformation; and level 3 includes research and advanced dynamic techniques. We have used our personal experience, contacts with colleagues and data from the literature to analyze all the steps of the planning process, and to define the tools and procedures relevant to a given level. The corresponding tables have been discussed and approved at the European level within the Dynarad concerted action. It is proposed that the term 'conformal radiotherapy' be restricted to procedures where all steps are at least at level 2.

The determination of the various volumes (GTV: gross target volume, CTV: clinical target volume, PTV: planned target volume) recommended by the ICRU 50 report is a critical step in conformal treatment planning, since treatment optimisation procedures and documentation rely on accurate dose-volume histograms. The shape and the size of the CTV vary with the computer algorithm, the patient image acquisition parameters, the definition of the GTV and the margins surrounding it. The automatic expansion programs included in commercially available treatment planning system require careful validation and control before and during their routine use by the clinicians. Significant differences have been observed between 2D- and 3D-based expansions, with a usual underestimation of the PTV by 2D algorithms.

In external radiotherapy, the use of intensity modulated fields has been proposed for tissue and non-homogeneity compensation or for the generation of conformal dose distributions. Multileaf collimators can be employed dynamically for the modulation of the X-ray field in two dimensions. Efficient dynamic collimation became possible due to advances in computer and linear accelerator technology. It presents a number of advantages over conventional methods such as the use of compensators. We have developed a program which calculates, from a given intensity distribution, the motion of the MLC leaves as a function of monitor units, and we have applied it on a Varian linear accelerator with a 40 pair multileaf collimator. The analysis of the experimental results demonstrates the feasibility and the potential of the method.

To calibrate a megavoltage therapy beam using an ionization chamber, it is necessary to know the fraction of the ionization arising in the chamber wall when this is made of a material different than the medium. A method for measuring the ionization fraction produced by electrons arising in the chamber wall (alpha) is presented here. The method uses three measurements at the same point in a medium in order to calculate alpha. These measurements are made using the examined chamber with and without a buildup cap and one reference chamber of wall material equivalent to the medium (i.e., in our case, A1 and A-150 were used as wall materials for the examined and the reference chamber, respectively). Using this method, it is possible to calculate alpha in the medium for a series of irradiation conditions and assess its characteristics. Two main conclusions came out of this assessment. The first one is the independence of alpha from the wall material, even if this is aluminum (alpha is only dependent on wall thickness expressed in g cm-2). The second one is that alpha depends on the irradiation conditions; it increases with field size and depth.

The effect of reversing the voltage polarity applied to an ionization chamber has been investigated in electron beams for several types of chambers and several irradiation conditions. It has been found that differences in readings can be significant for cylindrical chambers (about 10%) as well as for plane parallel chambers (20%). The effect is larger for large field sizes than small ones. It generally includes an appreciable stem and cable effect. Differences in readings with both polarities are related to the energy distribution of the electron beam and are greater for lower electron energies than higher. Polarity effect and charge deposit within the chamber wall material appear to be closely connected. This charge deposit, expressed as a proportion of the total collected charge, can be directly derived from double polarities measurements. Careful investigation of the effect should be made to avoid significant error (over 5%) in the determination of the absorbed dose.

Using proton beams for treatment of malignant tumors of the eye allows high dose irradiation of the tumor volume with negligible irradiation of the surrounding critical structures. Actually, the clinical results reported from the Boston or Villigen groups which have experience of such a technique, are impressive. In France, it is planned to use soon 200 MeV proton beams from the Orsay synchrocyclotron 20 km from Paris. Therefore, a feasibility study has been conducted during year 1988, consisting of a dosimetric analysis of the characteristics of the Orsay proton beams. It was concluded that only simple modifications were necessary to make the beam clinically available for protontherapy and more specifically for treatment of uveal melanomas.

The localization of sources used for brachytherapy has become more difficult since radium sources are replaced by flexible wires such as Iridium 192 wires. We describe the COUREP program which automatically reconstructs the real 3-D arrangement of curved wires from two orthogonal radiographs of the implant. The data used by the program consist mainly of the projections of the wires on the radiographs expressed as the co-ordinates of points selected on these projections individually on each radiograph. The output consists of the 3-D co-ordinates of points located on the wires with optionally a graphic output including the plotting of a perspective view of the implant.

Electron treatments give skin reactions far more severe than high energy X-ray treatments at equal doses. Sometimes these reactions may become even too severe for the treatment to be continued at the dose initially planned. This paper describes how the surface dose is modified according to the different treatment parameters and what can be done to keep it as low as possible.