A generalized model for monitor units determination in ocular proton therapy using machine learning: A proof-of-concept study

Objective.Determining and verifying the number of monitor units is crucial to achieving the desired dose distribution in radiotherapy and maintaining treatment efficacy. However, current commercial treatment planning system(s) dedicated to ocular passive eyelines in proton therapy do not provide the number of monitor units for patient-specific plan delivery. Performing specific pre-treatment field measurements, which is time and resource consuming, is usually gold-standard practice. This proof-of-concept study reports on the development of a multi-institutional-based generalized model for monitor units determination in proton therapy for eye melanoma treatments.Approach.To cope with the small number of patients being treated in proton centers, three European institutes participated in this study. Measurements data were collected to address output factor differences across the institutes, especially as function of field size, spread-out Bragg peak modulation width, residual range, and air gap. A generic model for monitor units prediction using a large number of 3748 patients and broad diversity in tumor patterns, was evaluated using six popular machine learning algorithms: (i) decision tree; (ii) random forest, (iii) extra trees, (iv) K-nearest neighbors, (v) gradient boosting, and (vi) the support vector regression. Features used as inputs into each machine learning pipeline were: Spread-out Bragg peak width, range, air gap, fraction and calibration doses. Performance measure was scored using the mean absolute error, which was the difference between predicted and real monitor units, as collected from institutional gold-standard methods.Main results.Predictions across algorithms were accurate within 3% uncertainty for up to 85.2% of the plans and within 10% uncertainty for up to 98.6% of the plans with the extra trees algorithm.Significance.A proof-of-concept of using machine learning-based generic monitor units determination in ocular proton therapy has been demonstrated. This could trigger the development of an independent monitor units calculation tool for clinical use.

Visual outcomes of macular melanocytic lesions after early or delayed proton beam therapy

PURPOSE

During their initial management, some macular melanocytic lesions can be closely monitored to wait for a documented growth before advocating a treatment by irradiation. However, the visual outcomes of this strategy have not yet been assessed. This study compares the visual outcomes of macular melanocytic lesions that underwent delayed proton beam therapy (PBT) after an initial observation to those treated early.

METHODS

A total of 162 patients with suspicious melanocytic lesions whose margins were located within 3 mm of the fovea were recruited from two French ocular oncology centers.

RESULTS

Overall, 82 patients treated with PBT within 4 months after the initial visit (early PBT group) were compared to 24 patients treated with delayed PBT (delayed PBT group) and 56 patients not treated with PBT (observation group). Visual acuity was not significantly different between baseline and last visit in the observation group (p = 0.325). Between baseline and last visit, the median [IQR] loss in visual acuity was significant in both the early (0.7 [0.2; 1.8], p < 0.001) and the delayed (0.5 [0.2; 1.5], p < 0.001) PBT groups. After irradiation, there was no significant difference between the early and delayed PBT groups for visual loss (p = 0.575), diameter reduction (p = 0.190), and thickness lowering (p = 0.892). In multivariate analysis, history of diabetes mellitus and Bruch's membrane rupture remained significantly associated with greater visual loss (p = 0.036 and p = 0.002, respectively).

CONCLUSION

For small lesions in which there is no clear diagnosis of malignant melanoma, an initial close monitoring to document tumor growth does not impact visual prognosis, despite the potential complications associated with the untreated tumor. However, the survival should remain the main outcome of the treatment of these lesions.

Ocular proton therapy, pencil beam scanning high energy proton therapy or stereotactic radiotherapy for uveal melanoma; an in silico study

PURPOSE

In radiotherapy, the dose and volumes of the irradiated normal tissues is correlated to the complication rate. We assessed the performances of low-energy proton therapy (ocular PT) with eye-dedicated equipment, high energy PT with pencil-beam scanning (PBS) or CyberKnife^R  -based stereotactic irradiation (SBRT).

MATERIAL AND METHODS

CT-based comparative dose distribution between external beam radiotherapy techniques was assessed using an anthropomorphic head phantom. The prescribed dose was 60Gy_RBE in 4 fractions to a typical posterior pole uveal melanoma. Clinically relevant structures were delineated, and doses were calculated using radiotherapy treatment planning softwares and measured using Gafchromic dosimetry films inserted at the ocular level.

RESULTS

Precision was significantly better with ocular PT than both PBS or SBRT in terms of beam penumbra (80%-20%: laterally 1.4 vs. ≥10mm, distally 0.8 vs. ≥2.5mm). Ocular PT duration was shorter, allowing eye gating and lid sparing more easily. Tumor was excellent with all modalities, but ocular PT resulted in more homogenous and conformal dose compared to PBS or SBRT. The maximal dose to ocular/orbital structures at risk was smaller and often null with ocular PT compared to other modalities. Mean dose to ocular/orbital structures was also lower with ocular PT. Structures like the lids and lacrimal punctum could be preserved with ocular PT using gaze orientation and lid retractors, which is easier to implement clinically than with the other modalities. The dose to distant organs was null with ocular PT and PBS, in contrast to SBRT.

CONCLUSIONS

ocular PT showed significantly improved beam penumbra, shorter treatment delivery time, better dose homogeneity, and reduced maximal/mean doses to critical ocular structures compared with other current external beam radiation modalities. Similar comparisons may be warranted for other tumor presentations.

Non-Cancer Effects following Ionizing Irradiation Involving the Eye and Orbit

Simple Summary

The irradiation of tumors involving the eye or orbit represents a complex therapeutic challenge due to the proximity between the tumor and organs that are susceptible to radiation. The challenges include tumor control, as it is often a surrogate for survival; organ (usually the eyeball) preservation; and the minimization of damage of sensitive tissues surrounding the tumor in order to preserve vision. Anticipation of the spectrum and severity of radiation-induced complications is crucial to the decision of which technique to use for a given tumor. The aim of the present review is to report the non-cancer effects that may occur following ionizing irradiation involving the eye and orbit and their specific patterns of toxicity for a given radiotherapy modality. The pros and cons of conventional and advanced forms of radiation techniques and their clinical implementation are provided with a clinical perspective.

Abstract

The eye is an exemplarily challenging organ to treat when considering ocular tumors. It is at the crossroads of several major aims in oncology: tumor control, organ preservation, and functional outcomes including vision and quality of life. The proximity between the tumor and organs that are susceptible to radiation damage explain these challenges. Given a high enough dose of radiation, virtually any cancer will be destroyed with radiotherapy. Yet, the doses inevitably absorbed by normal tissues may lead to complications, the likelihood of which increases with the radiation dose and volume of normal tissues irradiated. Precision radiotherapy allows personalized decision-making algorithms based on patient and tumor characteristics by exploiting the full knowledge of the physics, radiobiology, and the modifications made to the radiotherapy equipment to adapt to the various ocular tumors. Anticipation of the spectrum and severity of radiation-induced complications is crucial to the decision of which technique to use for a given tumor. Radiation can damage the lacrimal gland, eyelashes/eyelids, cornea, lens, macula/retina, optic nerves and chiasma, each having specific dose–response characteristics. The present review is a report of non-cancer effects that may occur following ionizing irradiation involving the eye and orbit and their specific patterns of toxicity for a given radiotherapy modality.

Characterization of the HollandPTC proton therapy beamline dedicated to uveal melanoma treatment and an interinstitutional comparison

PURPOSE

Eye-dedicated proton therapy (PT) facilities are used to treat malignant intraocular lesions, especially uveal melanoma (UM). The first commercial ocular PT beamline from Varian was installed in the Netherlands. In this work, the conceptual design of the new eyeline is presented. In addition, a comprehensive comparison against five PT centers with dedicated ocular beamlines is performed, and the clinical impact of the identified differences is analyzed.

MATERIAL/METHODS

The HollandPTC eyeline was characterized. Four centers in Europe and one in the United States joined the study. All centers use a cyclotron for proton beam generation and an eye-dedicated nozzle. Differences among the chosen ocular beamlines were in the design of the nozzle, nominal energy, and energy spectrum. The following parameters were collected for all centers: technical characteristics and a set of distal, proximal, and lateral region measurements. The measurements were performed with detectors available in-house at each institution. The institutions followed the International Atomic Energy Agency (IAEA) Technical Report Series (TRS)-398 Code of Practice for absolute dose measurement, and the IAEA TRS-398 Code of Practice, its modified version or International Commission on Radiation Units and Measurements Report No. 78 for spread-out Bragg peak normalization. Energy spreads of the pristine Bragg peaks were obtained with Monte Carlo simulations using Geant4. Seven tumor-specific case scenarios were simulated to evaluate the clinical impact among centers: small, medium, and large UM, located either anteriorly, at the equator, or posteriorly within the eye. Differences in the depth dose distributions were calculated.

RESULTS

A pristine Bragg peak of HollandPTC eyeline corresponded to the constant energy of 75 MeV (maximal range 3.97 g/cm2 in water) with an energy spread of 1.10 MeV. The pristine Bragg peaks for the five participating centers varied from 62.50 to 104.50 MeV with an energy spread variation between 0.10 and 0.70 MeV. Differences in the average distal fall-offs and lateral penumbrae (LPs) (over the complete set of clinically available beam modulations) among all centers were up to 0.25 g/cm2 , and 0.80 mm, respectively. Average distal fall-offs of the HollandPTC eyeline were 0.20 g/cm2 , and LPs were between 1.50 and 2.15 mm from proximal to distal regions, respectively. Treatment time, around 60 s, was comparable among all centers. The virtual source-to-axis distance of 120 cm at HollandPTC was shorter than for the five participating centers (range: 165-350 cm). Simulated depth dose distributions demonstrated the impact of the different beamline characteristics among institutions. The largest difference was observed for a small UM located at the posterior pole, where a proximal dose between two extreme centers was up to 20%.

CONCLUSIONS

HollandPTC eyeline specifications are in accordance with five other ocular PT beamlines. Similar clinical concepts can be applied to expect the same high local tumor control. Dosimetrical properties among the six institutions induce most likely differences in ocular radiation-related toxicities. This interinstitutional comparison could support further research on ocular post-PT complications. Finally, the findings reported in this study could be used to define dosimetrical guidelines for ocular PT to unify the concepts among institutions.

Three-dimensional MRI-based treatment planning approach for non-invasive ocular proton therapy

PURPOSE

To develop a high-resolution three-dimensional (3D) magnetic resonance imaging (MRI)-based treatment planning approach for uveal melanomas (UM) in proton therapy.

MATERIALS/METHODS

For eight patients with UM, a segmentation of the gross tumor volume (GTV) and organs-at-risk (OARs) was performed on T1- and T2-weighted 7 Tesla MRI image data to reconstruct the patient MR-eye. An extended contour was defined with a 2.5-mm isotropic margin derived from the GTV. A broad beam algorithm, which we have called πDose, was implemented to calculate relative proton absorbed doses to the ipsilateral OARs. Clinically favorable gazing angles of the treated eye were assessed by calculating a global weighted-sum objective function, which set penalties for OARs and extreme gazing angles. An optimizer, which we have named OPT'im-Eye-Tool, was developed to tune the parameters of the functions for sparing critical-OARs.

RESULTS

In total, 441 gazing angles were simulated for every patient. Target coverage including margins was achieved in all the cases (V95%  > 95%). Over the whole gazing angles solutions space, maximum dose (Dmax ) to the optic nerve and the macula, and mean doses (Dmean ) to the lens, the ciliary body and the sclera were calculated. A forward optimization was applied by OPT'im-Eye-Tool in three different prioritizations: iso-weighted, optic nerve prioritized, and macula prioritized. In each, the function values were depicted in a selection tool to select the optimal gazing angle(s). For example, patient 4 had a T2 equatorial tumor. The optimization applied for the straight gazing angle resulted in objective function values of 0.46 (iso-weighted situation), 0.90 (optic nerve prioritization) and 0.08 (macula prioritization) demonstrating the impact of that angle in different clinical approaches.

CONCLUSIONS

The feasibility and suitability of a 3D MRI-based treatment planning approach have been successfully tested on a cohort of eight patients diagnosed with UM. Moreover, a gaze-angle trade-off dose optimization with respect to OARs sparing has been developed. Further validation of the whole treatment process is the next step in the goal to achieve both a non-invasive and a personalized proton therapy treatment.

[Measurement of out-of-field dose to the uterus during proton therapy of the head and neck]

The decision to irradiate during pregnancy is based on a risk benefit compromise of two kinds: maternal risk and fetal risk. The aim of this work is to determine the foetal risk, and uterine dose measurement in proton therapy. Foetal exposure during treatment is linked to two sources: the treatment phase, and the repositioning phase. An Alderson-Rando anthropomorphic ghost (170cm, 74kg) was positioned on the table in the treatment position. A tissue-equivalent proportional counter (TEPC), adapted to the analysis of complex radiation fields (neutron and photonics), was used to determine the irradiation related to the treatment phase. An AT1123 radiation survey meter was used to measure photons generated by X-ray radiation. I dosimetry was proposed using radio-photoluminescent dosimeters, allowing for a daily check of the dose received in the uterus. The treatment phase produces higher uterine doses than the positioning phase, but these remain very low. The equivalent dose received in the uterus for the entire treatment is estimated at 840 μSv. Using a methodology for measuring the out-of-field dose with pencil beam scanning proton therapy, the foetal dose in the first trimester was well below the acceptance dose of 100 mGy determined by the International Commission on Radiological Protection.

The Lens Opacities Classification System III Grading in Irradiated Uveal Melanomas to Characterize Proton Therapy-Induced Cataracts

PURPOSE

To evaluate the use of the Lens Opacities Classification System III grading (LOCS III) for the characterization of radiation-induced cataract, and to correlate the proton beam projection onto the lens with cataract location and grade as defined by the LOCS III.

DESIGN

Prospective, interventional case series.

METHODS

Fifty-two consecutive patients with cataract following proton therapy were included. All cataracts were graded using LOCS III. Relationships between proton beam and cataract subtypes, as well as between dose, proportion of lens irradiated, and extent of cataracts, were assessed.

RESULTS

Tumor diameter, volume, stage, and equatorial tumor location were associated with extent of posterior subcapsular cataracts (PSC) that were diagnosed at a median (interquartile range) 36 months (22;83) after treatment. In multivariate analysis, the tumor volume (P < .01) and an equatorial tumor location (P = .01) were risk factors for extensive PSC. Lens irradiation was avoided in 10 patients. In the remaining 42 patients (81%), the extent of PSC significantly correlated with the dose to the lens receiving 10, 26, and 47 Gy (P = .03, P = .03, and P = .04, respectively), the dose to the lens periphery receiving 10 and 26 Gy (P = .02 and P = .02, respectively), and the dose to the ciliary body receiving 10 and 26 Gy (P = .03 and P = .02, respectively). Nuclear color significantly correlated with the dose to the ciliary body receiving 10 Gy (P = .03) and 26 Gy (P = .02). After adjustment of the results on tumor volume and tumor location, the volume of lens receiving 10 Gy (P = .04) and 26 Gy (P = .03) remained significantly associated with the extent of PSC.

CONCLUSIONS

Proton dose correlated with the occurrence of PSC and nuclear color cataracts as defined by LOCS III grading. Better characterization of cataracts with the LOCS III after irradiation may help to further fill gaps in the current understanding of the mechanisms of radiation-induced cataracts.

Optimum reduction of the dynamo threshold by a ferromagnetic layer located in the flow

We consider a fluid dynamo model generated by the flow on both sides of a moving layer. The magnetic permeability of the layer is larger than that of the flow. We show that there exists an optimum value of magnetic permeability for which the critical magnetic Reynolds number for dynamo onset is smaller than for a nonmagnetic material and also smaller than for a layer of infinite magnetic permeability. We present a mechanism that provides an explanation for recent experimental results. A similar effect occurs when the electrical conductivity of the layer is large.

Decay rates of magnetic modes below the threshold of a turbulent dynamo

We measure the decay rates of magnetic field modes in a turbulent flow of liquid sodium below the dynamo threshold. We observe that turbulent fluctuations induce energy transfers between modes with different symmetries (dipolar and quadrupolar). Using symmetry properties, we show how to measure the decay rate of each mode without being restricted to the one with the smallest damping rate. We observe that the respective values of the decay rates of these modes depend on the shape of the propellers driving the flow. Dynamical regimes, including field reversals, are observed only when the modes are both nearly marginal. This is in line with a recently proposed model.

Curvilinear component analysis: a self-organizing neural network for nonlinear mapping of data sets

We present a new strategy called "curvilinear component analysis" (CCA) for dimensionality reduction and representation of multidimensional data sets. The principle of CCA is a self-organized neural network performing two tasks: vector quantization (VQ) of the submanifold in the data set (input space); and nonlinear projection (P) of these quantizing vectors toward an output space, providing a revealing unfolding of the submanifold. After learning, the network has the ability to continuously map any new point from one space into another: forward mapping of new points in the input space, or backward mapping of an arbitrary position in the output space.

Periodic magnetorotational dynamo action as a prototype of nonlinear magnetic-field generation in shear flows

The nature of dynamo action in shear flows prone to magnetohydrodynamc instabilities is investigated using the magnetorotational dynamo in Keplerian shear flow as a prototype problem. Using direct numerical simulations and Newton's method, we compute an exact time-periodic magnetorotational dynamo solution to three-dimensional dissipative incompressible magnetohydrodynamic equations with rotation and shear. We discuss the physical mechanism behind the cycle and show that it results from a combination of linear and nonlinear interactions between a large-scale axisymmetric toroidal magnetic field and nonaxisymmetric perturbations amplified by the magnetorotational instability. We demonstrate that this large-scale dynamo mechanism is overall intrinsically nonlinear and not reducible to the standard mean-field dynamo formalism. Our results therefore provide clear evidence for a generic nonlinear generation mechanism of time-dependent coherent large-scale magnetic fields in shear flows and call for new theoretical dynamo models. These findings may offer important clues to understanding the transitional and statistical properties of subcritical magnetorotational turbulence.

Microdosimetric assessment of the radiation quality of a therapeutic proton beam: comparison between numerical simulation and experimental measurements

Using protons for the treatment of ocular melanoma (especially of posterior pole tumours), the radiation quality of the beam must be precisely assessed to preserve the vision and to minimise the damage to healthy tissue. The radiation quality of a therapeutic proton beam at the Centre Antoine Lacassagne in Nice (France) was measured using microdosimetric techniques, i.e. a miniaturised version of a tissue-equivalent proportional counter. Measurements were performed in a 1-µm site at different depths in a Lucite phantom. Experimental data showed a significant increase in the beam quality at the distal edge of the spread-out Bragg peak (SOBP). In this paper, the numerical simulation of the experimental setup is done with the FLUKA Monte Carlo radiation transport code. The calculated microdosimetric spectra are compared with the measured ones at different depths in tissue for a monoenergetic proton beam (E=62 MeV) and for a modulated SOBP. Numerically and experimentally predicted relative biological effectiveness values are in good agreement. The calculated frequency-averaged and dose-averaged lineal energy mean values are consistent with measured data.

[CyberKnife robotic stereotactic radiotherapy: technical aspects and medical indications]

In 2006, 3 sites have been selected by the Institut national of cancer (Lille, Nancy et Nice) to evaluate a radiotherapy robot, the CyberKnife. This machine, able to track mobile tumours in real time, gives new possibilities in the field of extra cranial stereotactic radiotherapy. Functionalities and medico economical issues of the machine will be evaluated during 2 years on the 3 sites.

Thermoluminescence properties of CVD diamond for clinical dosimetry use

The application of diamond to dosimetry is desirable because of its tissue equivalence, chemical inertness and small size, but this has not been commercially viable owing to the non-reproducible response of natural diamond. The chemical vapour deposition (CVD) of diamond permits controlled, reproducible and large-scale production of this material at potentially low cost. An investigation of some clinically relevant features like the depth-dose distribution as well as the absorbed dose profile, obtained using thermoluminescence (TL), is reported for several CVD diamond films. The TL characterisation presented here shows that CVD diamond films should be excellent TL-mode detectors in instances of radiotherapy and in vivo radiation dosimetry.

Radiobiological studies on the 65 MeV therapeutic proton beam at Nice using human tumour cells

PURPOSE

To determine the relative biological effectiveness (RBE) for initial and delayed inactivation of cells by a modulated proton beam suitable for the treatment of tumours of the eye, within the spread-out Bragg peak and in its distal declining edge.

MATERIALS AND METHODS

Human tumour SCC25 cells were irradiated with the 65 MeV proton beam at the Cyclotron Medicyc in Nice. Perspex plates of different thickness were used to simulate five positions along the beam line: 2mm corresponding to the entrance beam; 15.6 and 25 mm in the spread-out Bragg peak; 27.2 and 27.8mm for the distal edge. At each position clonogenic survival of the irradiated cells and of their progeny were determined at various dose values. 60Co gamma-rays were used as reference radiation.

RESULTS

RBE values evaluated at the survival level given by 2 Gy of gamma-rays increased with increasing depth from close to 1.0 at the proximal to about 1.2 at the distal part of the peak. Within the declining edge it reached the value of about 1.4 at 27.2 and about 2 at 27.8 mm. For the progeny of irradiated cells, the RBE value ranged from 1.0 to 1.1 within the spread-out Bragg peak and then increased up to a value of 2.0 at the last position. The dose-effect curves for the progeny always had a larger shoulder than for the irradiated progenitors, their alpha parameters being lower by a factor of about 4 and their beta parameters always being higher. The alpha/beta ratio was about 50 Gy for the progenitors and about 6 Gy for their progeny. The incidence of delayed effects increased with dose and with the depth within the beam.

CONCLUSIONS

RBE values for the inactivation of cells irradiated in the spread-out Bragg peak are compatible with the value currently assumed in clinical applications. In the distal declining edge of the beam, the RBE values increased significantly to an extent that may be of concern when the region of the treatment volume is close to sensitive tissues. The yield of delayed reproductive cell death was significant at each position along the beam line.

Effector protease receptor 1 mediates the mitogenic activity of factor Xa for vascular smooth muscle cells in vitro and in vivo

The binding of 125I-factor Xa to human aortic smooth muscle cell (SMC) monolayers was studied. At 4 degreesC, 125I-factor Xa bound to a single class of binding sites with a dissociation constant value of 3.6+/-0.7 nM and a binding site density of 11,720+/-1,240 sites/cell (n = 9). 125I-factor Xa binding was not affected by factor X, thrombin, or by DX9065, a direct inhibitor of factor Xa, but was inhibited by factor Xa (IC50 = 5.4+/-0.2 nM; n = 9) and by antibodies specific for the effector cell protease receptor 1 (EPR-1), a well-known receptor of factor Xa on various cell types. A factor X peptide duplicating the inter-EGF sequence Leu83-Leu88-(Gly) blocked the binding of 125I-factor Xa to these cells in a dose-dependent manner (IC50 = 110+/-21 nM). Factor Xa increased phosphoinositide turnover in SMCs and when added to SMCs in culture was a potent mitogen. These effects were inhibited by DX9065 and by antibodies directed against EPR-1 and PDGF. Increased expression of EPR-1 was identified immunohistochemically on SMCs growing in culture and in SMCs from the rabbit carotid artery after vascular injury. When applied locally to air-injured rabbit carotid arteries, antibodies directed against EPR-1 (100 mug/ artery) strongly reduced myointimal proliferation 14 d after vascular injury (65-71% inhibition, P < 0.01). DX9065 (10 mg/kg, subcutaneous) inhibited myointimal proliferation significantly (43% inhibition, P < 0.05). These findings indicate that SMCs express functional high affinity receptors for factor Xa related to EPR-1, which may be of importance in the regulation of homeostasis of the vascular wall and after vascular injury.

The Nice high-energy neutron facility: dosimetry intercomparisons

Neutron dosimetry intercomparison studies have been undertaken at the Nice neutrontherapy facility with the staff at Louvain-la-Neuve which has had wide experience in both dosimetric and radiobiological intercomparisons. Tissue equivalent (TE) ionization chambers were first calibrated in 60Co beams and then exposed in the neutron beam at different depths in a water phantom; the largest difference observed in neutron beam measurements with all the chambers tested was 0.89%, and most of them agreed to within less than 0.5%. The gamma component at four depths was derived from measurements with Geiger-Müller counters; the results obtained with the two counters (Nice and Louvain-la-Neuve), expressed as a percentage of the total dose (neutron + gamma), agreed to within less than 0.03% and the value increased from 1.4 to 4.2% between 2 and 20 cm in depth.

The RBE of fast neutrons for in vitro inactivation of human tumour cells determined by the ratio of mean inactivation doses

In an effort to clarify the relationship between sensitivity of human tumour cells to low-LET and to fast neutron irradiation, 10 human tumour cell lines were exposed to cobalt gamma-rays and to 60 MeV (p -> Be+) neutron beam. The data were pooled with results of 31 human tumour cell lines previously published. The analysis of date using the linear-quadratic model indicated that not only alpha values increased after neutron irradiation, but so did beta values too, although to a lesser extent. The mean inactivation dose (MID) was derived for each cell line from the linear-quadratic parameters after low-LET and high-LET exposure. MID values following neutron irradiation were closely correlated to those after gamma-ray irradiation. In these 41 cell lines, the extreme values of RBE derived by the ratio of MID varied by a factor of 3 among the cell lines. RBE was positively correlated to photon MID, meaning that intrinsically radiation resistant tumour cells have a higher neutron RBE, on average. Similar findings were observed if alpha ratios were used instead of MID ratios. In addition, the RBE/dose variations were more marked in cells with the higher RBE. Taken together, these data suggest that, although considerable variations exist among human tumour cell lines, intrinsically radioresistant cells are relatively more sensitized when exposed to high LET beams than radioresponsive tumours. An 'intrinsic gain factor' may thus be expected in irradiating radiation resistant tumours with fast neutrons, in addition to the hypoxic or kinetic gain factors. Because the quadratic component is still present after neutron irradiation, we suggest using MID ratio as a reference RBE when comparing survival curves of cells exposed to radiations of different qualities.

Determination by high-performance liquid chromatography with electrochemical detection of free and conjugated N-acetyldopamine excretion in urine of children with neuroblastoma and nephroblastoma

A simple method for the determination of N-acetyldopamine (NADA) (both free and conjugated) in children's urine by high-performance liquid chromatography with electrochemical detection has been developed. Conjugated NADA was measured as the free compound after enzymatic hydrolysis and purification on alumina. The total analysis time is 25 min. The results show a linearity of the whole assay from 0.005 to 20 mumol/l NADA; the sensitivity is 0.2 pmol per 20 microliters injected sample. Mean recoveries of 96.7 and 86.6% were obtained for free and total NADA, respectively. Modifications of the retention times (between 2 and 50 min) induced by changes in the eluent were determined. Conjugated NADA accounted for about 90% of the total excretion of NADA. These results suggest that this compound could play an important part in neuroblastoma; its concentration is thirteen times higher in children with neuroblastoma than in normal subjects.

Determination of urinary vanillactic acid and plasma dihydroxyphenylalanine as markers of non-secreting neuroblastoma by high-performance liquid chromatography with electrochemical detection

An accurate and precise isocratic high-performance liquid chromatographic technique for the analysis of urinary vanillactic acid (VLA) and plasma dihydroxyphenylalanine (DOPA), especially at low concentrations (pmol/l) for VLA and nmol/l for DOPA), is described. The compounds were purified in a single step, (on an anion exchanger for VLA and on aluminium oxide for DOPA), separated by ion-pair reversed-phase liquid chromatography, and detected electrochemically. A single analysis was complete within 18 min. Mean recoveries of 103 and 81% were obtained for VLA and DOPA, respectively, and the limits of detection were 42 and 76 pmol/l, respectively. The mean values of the intra-assay coefficient of variation were 14 and 7.1% for VLA and DOPA, respectively, and the mean values of the inter-assay coefficient of variation were 15.7 and 11.6%, respectively. Modifications of the retention times (between 2 and 42 min) induced by changes in the eluent were determined. Reference values for normal children and children with neuroblastoma or various tumours are given.