[Biological mechanisms of late effects of ionizing radiations]

[Management of cerebral radionecrosis: experience in the Department of Neurology at the Mohammed V Military Teaching Hospital]

La radionécrose cérébrale est une complication rare mais redoutable de la radiothérapie externe pour les cancers ORL en particulier du nasopharynx et pour les tumeurs cérébrales. Peu de travaux se sont intéressés à cette complication aussi bien dans la population maghrébine qu'africaine. L'objectif de notre étude est donc de décrire les aspects démographiques, cliniques, paracliniques, thérapeutiques et évolutifs de la radionécrose cérébrale au sein du Service de Neurologie de l'Hôpital Militaire d'Instruction Mohammed V de Rabat (HMIMV) sur une durée de 18 ans (2000-2017). Il s'agissait de 04 femmes et 13 hommes âgés en moyenne de 50 ans. Le délai moyen entre la fin de la radiothérapie et le début des signes neurologiques était de 28 mois. La réalisation systématique d'une IRM cérébrale parfois complété d'une spectro-IRM avait permis de faire le diagnostic chez 100% des cas. Sur le plan étiologique, cette complication survenait après radiothérapie pour cancer du cavum et du larynx chez tous les cas. 15 patients ont été traités par l'association: corticothérapie en bolus, antiagrégants plaquettaires, associée à l'oxygénation hyperbare (OHB) avec une bonne évolution, et 2 patients, chez qui l'oxygénothérapie était contre-indiquée pour problème pulmonaire, et ORL, ont reçu l'association corticothérapie en bolus et antiagrégants plaquettaires avec une évolution stationnaire. À la lumière de ces résultats, nous insistons sur l'intérêt du diagnostic précoce en raison de la gravité potentielle de certains tableaux surtout neuropsychiatriques ainsi que sur le traitement par association bolus de corticoïdes + OHB car il est le meilleure garant d'une évolution favorable de ces atteintes, sans omettre le rôle crucial des mesures de prévention.

[Dosimetry comparison of irradiation with conformal radiotherapy, intensity modulated radiotherapy, conformal radiotherapy in stereotactic conditions and robotic stereotactic radiotherapy for benign brain tumours]

PURPOSE

To compare several techniques in order to determine the best treatment for benign brain tumours.

METHODS AND PATIENTS

A retrospective study was performed for five patients who received 3D-conformal radiotherapy, intensity modulated radiotherapy or CyberKnife(®). These patients had a meningioma, a pituitary tumour, a craniopharyngioma or a neurinoma. In each case, these treatment plans were optimised and compared with the three other dosimetries. Radiobiological or positioning parameters were evaluated, as well as dosimetric parameters, in order to compare treatments with different characteristics.

RESULTS

The dosimetric parameters showed that the choice of treatment seemed to be determined mostly by tumour size, shape and proximity with organs at risk (not tumour localisation). Whereas the results showed no significant deviations with regards to the radiobiological parameters. Therefore, with these parameters, it was difficult to give priority to a treatment.

CONCLUSIONS

With regards to benign brain tumours of medium or large size, intensity modulated radiotherapy seemed the recommended treatment. It enabled to obtain a good ratio between efficacy and toxicity for tumours that are really close to organs at risk. Concerning small benign brain tumours, the CyberKnife(®) was probably the best treatment.

[Interest of blood markers in predicting radiation-induced toxicity]

The oncologic outcome and the total dose are highly correlated with the treatment by ionizing radiation. The dose increase (total or per fraction) may provoke late-side effects that are potentially irreversible. The radiation-induced CD8 lymphocyte apoptotic value and the molecular modifications within the lymphocyte are capable of predicting the level of risk of developing late-side effects after curative intent radiotherapy. In this review, we present the different blood assays in this setting and discuss the current possibilities of researches, namely those involving the proteomic process.

Identifying patients at risk for late radiation-induced toxicity

The impact of curative radiotherapy depends mainly on the total dose delivered in the targeted volume. Nevertheless, the dose delivered to the surrounding healthy tissues may reduce the therapeutic ratio of many treatments. Two different side effects (acute and late) can occur during and after radiotherapy. Of particular interest are the radiation-induced late complications (LC) due to their irreversibility and the potential impact on quality of life. In one population treated with the same technique, it appears that individual radiosensitivity clearly exists. In the hypothesis that genetic is involved in this area of research, low CD4 and CD8 lymphocyte apoptosis were shown to be correlated with high grade of LC. In addition, recent data suggest that patients with severe radiation-induced LC possess 4 or more single nucleotide polymorphisms (SNPs) in candidate genes and low radiation-induced CD8 lymphocyte apoptosis in vitro. On-going studies are being analyzing the entire genome using a genome-wide association study (GWAS).

[Probabilities of organs at risk damage: history and mathematical models]

The a priori evaluation of normal tissue complication probability is an important issue for the radiation oncologist looking for the best therapeutic index. The advances in radiobiological and technological knowledge provide a better understanding of the determinants of radiation effects. The amount of information required to optimize the treatment modalities justifies the use of mathematical models linking the treatment characteristics (dose, volume, treatment time...) to the likelihood of complications. The radiation oncologist needs a minimal understanding of the mathematical models and their limits to justify his prescriptions.

Radionécrose cérébrale chez les patients irradiés pour cancer du nasopharynx: à propos de neuf cas

PURPOSE

To study the clinical, radiological, therapeutic and progressive aspects of brain radionecrosis after treatment for nasopharyngeal carcinoma.

PATIENTS AND METHODS

Nine patients (seven men and two women) of mean age 47.7 years old (extremes: 18-57 years old) were treated for UCNT (undifferentiated carcinoma of the nasopharynx) between 1989 and 2003 and developed cerebral radionecrosis. All patients were treated with radical radiotherapy. The mean total dose was 73.5 Gy (70-75 Gy). Dose per fraction was 2 to 2.5 Gy, one fraction daily. One patient received adjuvant brachytherapy to the dose of 8 Gy and four patients also received chemotherapy.

RESULTS

Brain radionecrosis was authenticated by brain imaging (CT scan+/-MRI): the imaging was ordered in seven cases to elucidate non-specific neurological signs and two cases were discovered fortuitously. The time to the appearance of neurological signs was 40.3 months (10 to 108 months). The localization was temporal in six cases, parieto-occipital (one case) and bulbomedullar (two cases). After a mean follow-up period of 30.6 months (12-84 months), clinical outcomes were favorable in all cases receiving medical treatment (corticoids), with a stabilization of the radiological lesions in eight cases and complete radiological regression in one patient.

CONCLUSION

Brain radionecrosis is a late complication rarely occurring in patients irradiated for UCNT. Imaging techniques (CT scan but more so MRI) play a major role in the diagnosis. Corticotherapy resulted in a durable objective response in all patients and, in most cases, resulted in radiological stabilization.

Conformity index: a review

We present a critical analysis of the conformity indices described in the literature and an evaluation of their field of application. Three-dimensional conformal radiotherapy, with or without intensity modulation, is based on medical imaging techniques, three-dimensional dosimetry software, compression accessories, and verification procedures. It consists of delineating target volumes and critical healthy tissues to select the best combination of beams. This approach allows better adaptation of the isodose to the tumor volume, while limiting irradiation of healthy tissues. Tools must be developed to evaluate the quality of proposed treatment plans. Dosimetry software provides the dose distribution in each CT section and dose-volume histograms without really indicating the degree of conformity. The conformity index is a complementary tool that attributes a score to a treatment plan or that can compare several treatment plans for the same patient. The future of conformal index in everyday practice therefore remains unclear.

[Conformal index and radiotherapy]

Goal of radiotherapy is to treat patient with the best therapeutic ratio, i.e. the highest local control and the lowest toxicity rates. The conformal approach, three-dimensional conformal radiotherapy or intensity-modulated radiotherapy, is based on imageries, up-dated 3-D treatment planning systems, immobilization systems, restricted quality assurance and treatment verification. The aim is to ensure a high dose distribution tailored to the limits of the target volume, while reducing exposure of normal tissues. The evaluation tools used for optimizing treatment are the visual inspection of the dose distribution in various planes, and the dose-volume histograms, but they do not fully quantify the conformity of dose distributions. The conformal index is a tool for scoring a given plan or for evaluating different treatment plans for the same patient. This paper describes the onset and evolution of conformal index and his potential application field.

Effet volume en radiothérapie [II]. Deuxième partie : volume et tolérance des tissus sains

The first part of our work has focused on the relationship between tumor volume and tumor control. Indeed, it is well known that the importance of irradiated volume could be a main parameter of radiation-induced complications. Numerous mathematical models have described the correlation between the irradiated volume and the risk of adverse effects. These models should predict the complication rate of each treatment planning. At the present time late effects have been the most studied. In this report we firstly propose a review of different mathematical models described for volume effect. Secondly, we will discuss whether these theoretical considerations can influence our view of radiation treatment planning optimization.

[Biological basis of total body irradiation]

A comprehensive understanding of the radiobiological bases of total body irradiation (TBI) is made difficult by the large number of normal and malignant tissues that must be taken into account. In addition, tissue responses to irradiation are also sensitive to associated treatments, type of graft and a number of patient characteristics. Experimental studies have yielded a large body of data, the clinical relevance of which still requires definite validation through randomized trials. Fractionated TBI schemes are able to reduce late normal tissue toxicity, but the ultimate consequences of the fractional dose reduction do not appear to be equivocal. Thus, leukemia and lymphoma cells are probably more radiobiologically heterogeneous than previously thought, with several cell lines displaying relatively high radioresistance and repair capability patterns. The most primitive host-type hematopoietic stem cells are likely to be at least partly protected by TBI fractionation and may hamper late engraftment. Similarly, but with possibly conflicting consequences on the probability of engraftment, the persistence of a functional marrow stroma may also be fractionation-sensitive, while higher rejection rates have been reported after T-depletion grafts and fractionated TBI. In clinical practice (as for the performance of relevant clinical trials), the influence of these results are rather limited by the heavy logistic constraints created by a sophisticated and time-consuming procedure. Lastly, clinicians are now facing an increasing incidence of second cancers, at least partly induced by irradiation, which jeopardize the long-term prospects of otherwise cured patients.