How Thick Should CT/MR Slices be to Plan Conformal Radiotherapy? A Study on the Accuracy of Three-Dimensional Volume Reconstruction

Aim and background

The aim of this experimental study was to correlate the thickness of acquired CT slices (2, 4 and 8 mm) or MR slices (4 and 7 mm) with the accuracy of three-dimensional volume reconstruction as performed by a commercially available radiation therapy planning system.

Methods

We used a cylindrical phantom, with a 15-cm diameter and 20-cm height, containing 5 spheres (12.7-31.8 mm diameter) of solid Plexiglas sunk in a 3% agar jelly solution. The phantom was scanned by the CT scan with 3 different slice thicknesses (2, 4 and 8 mm and a distance of 0 mm between the slices). Two different acquisition techniques (slice thickness of 4 and 7 mm with 0.8 and 1.4 mm slice distance, respectively) were compared in the MR study. The volume values calculated from measurements were compared with the known true volume values of the spheres.

Results

The average percentage volume difference between calculated and true values for the smaller spheres reconstructed with CT images 2 and 4 mm thick was generally less than 8%, whereas the error for volumes reconstructed with 8-mm-thick CT slices was more than 20%. For the larger spheres, the error was generally less than 5%. The data produced by MR acquisition agreed with those obtained using CT sections.

Conclusions

For targets less than 1.5 cm in diameter on our system it is reasonable to acquire CT images with the smallest thickness available. For targets between 1.5 and 3 cm, it seems sufficient to acquire the localization images with a slice thickness of 4 mm. For targets more than 4 cm in diameter, considering that with our radiation therapy planning system the time spent for manual contouring and for isodose calculation highly increased with the number of acquired images, we suggest that the acquisition of CT-MR slices 8-10-mm thick is totally adequate even for Conformal radiotherapy treatments.

Diagnostic Accuracy of Xeromammography

The analytical study of the case material has allowed a precise evaluation of the distribution by age groups of the different pathologic processes of the breast, of the localization and dimensions of malignant tumors, as well as the diagnostic accuracy for those cases histologically ascertained and those with a follow-up. The diagnostic accuracy and the consequent reliability of the xeroradiographic method for the diagnosis of breast cancer is distinctly superior to that reported in the literature for traditional mammography, whereas for the differential diagnosis between circumscribed dysplastic manifestations and benign tumors xeromammography is not sufficiently reliable. The authors critically discuss the results and in particular the problem of false positives, which also include diagnostic errors that cannot be avoided in that they directly derive from the pathologic morphology of the disease process (plasma cell mastitis and sclerosing adenosis). As regards the problem of false negatives, they can be reduced within certain limits by resorting to other instrumental investigations. However, there are cases (1% of the malignant neoplasias histologically ascertained) that present a completely negative xeroradiographic finding. These are the limits of the radiologic investigation which cannot in any way be surmounted.

The anatomical foundations of acquired reading disorders: a neuropsychological verification of the dual-route model of reading

In this study we investigated the neural correlates of acquired reading disorders through an anatomo-correlative procedure of the lesions of 59 focal brain damaged patients suffering from acquired surface, phonological, deep, undifferentiated dyslexia and pure alexia. Two reading tasks, one of words and nonwords and one of words with unpredictable stress position, were used for this study. We found that surface dyslexia was predominantly associated with left temporal lesions, while in phonological dyslexia the lesions overlapped in the left insula and the left inferior frontal gyrus (pars opercularis) and that pure alexia was associated with lesions in the left fusiform gyrus. A number of areas and white matter tracts, which seemed to involve processing along both the lexical and the sublexical routes, were identified for undifferentiated dyslexia. Two cases of deep dyslexia with relatively dissimilar anatomical correlates were studied, one compatible with Coltheart's right-hemisphere hypothesis (1980) whereas the other could be interpreted in the context of Morton and Patterson's (1980), multiply-damaged left-hemisphere hypothesis. In brief, the results of this study are only partially consistent with the current state of the art, and propose new and stimulating challenges; indeed, based on these results we suggest that different types of acquired dyslexia may ensue after different cortical damage, but white matter disconnection may play a crucial role in some cases.

Lexical and semantic factors influencing picture naming in aphasia

Picture naming requires early visual analysis, accessing stored structural knowledge, semantic activation, and lexical retrieval. We tested the effect of perceptual, lexical, and semantic variables on the performance of aphasics in picture naming and assessed prevalence of natural categories vs artifact dissociations. Forty-nine aphasics were asked to name 60 pictures, from three natural (animals, fruits, and vegetables) and three artificial categories (tools, furniture, and vehicles). For each item visual (drawing complexity, image agreement), semantic (prototypicality, concept familiarity) and lexical variables (word frequency, name agreement) were available. The effect of these variables showed individual differences; altogether, visual complexity had little influence, whereas lexical and semantic variables were more influential. Name agreement was most important, followed by word frequency. On a multiple single case analysis 10 patients (20%) showed a natural/artificial category dissociation. Five of the six subjects faring better with artifacts were males, and all of four patients faring better with natural categories were females. Interpretations of this finding are discussed.

Dosimetric verification of a commercial 3D treatment planning system for conformal radiotherapy with a dynamic multileaf collimator

The dosimetric accuracy of a 3D treatment planning system (TPS) for conformal radiotherapy with a computer-assisted dynamic multileaf collimator (DMLC) was evaluated. The DMLC and the TPS have been developed for clinical applications where dynamic fields not greater than 10 x 10 cm2 and multiple non-coplanar arcs are required. Dosimetric verifications were performed by simulating conformal treatments of irregularly shaped targets using several arcs of irradiation with 6 MV x-rays and a spherical-shaped, tissue-simulating phantom. The accuracy of the delivered dose at the isocentre was verified using an ionization chamber placed in the centre of the phantom. Isodose distributions in the axial and sagittal planes passing through the centre of the phantom were measured using double-layer radiochromic films. Measured dose at the isocentre as well as isodose distributions were compared to those calculated by the TPS. The maximum percentage difference between measured and prescribed dose was less than 2.5% for all the simulated treatment plans. The mean (+/-SD) displacement between measured and calculated isodoses was, in the axial planes, 1.0 +/- 0.6 mm, 1.2 +/- 0.7 mm and 1.5 +/- 1.1 mm for 80%, 50% and 20% isodose curves, respectively, whereas in the sagittal planes it was 2.0 +/- 1.2 mm and 2.2 +/- 2 mm for 80% and 50% isodose curves, respectively. The results indicate that the accuracy of the 3D treatment planning system used with the DMLC is reasonably acceptable in clinical applications which require treatments with several non-coplanar arcs and small dynamic fields.

Dosimetry of Gamma Knife and linac-based radiosurgery using radiochromic and diode detectors

In stereotactic radiosurgery the choice of appropriate detectors, whether for absolute or relative dosimetry, is very important due to the steep dose gradient and the incomplete lateral electronic equilibrium. For both linac-based and Leksell Gamma Knife radiosurgery units, we tested the use of calibrated radiochromic film to measure absolute doses and relative dose distributions. In addition a small diode was used to estimate the relative output factors. The data obtained using radiochromic and diode detectors were compared with measurements performed with other conventional methods of dosimetry, with calculated values by treatment planning systems and with data prestored in the treatment planning system supplied by the Leksell Gamma Knife (LGK) vendor. Two stereotactic radiosurgery techniques were considered: Leksell Gamma Knife (using gamma-rays from 60Co) and linac-based radiosurgery (LR) (6 MV x-rays). Different detectors were used for both relative and absolute dosimetry: relative output factors (OFs) were estimated by using radiochromic and radiographic films and a small diode; relative dose distributions in the axial and coronal planes of a spherical polystyrene phantom were measured using radiochromic film and calculated by two different treatment planning systems (TPSs). The absolute dose at the sphere centre was measured by radiochromic film and a small ionization chamber. An accurate selection of radiochromic film was made: samples of unexposed film showing a percentage standard deviation of less than 3% were used for relative dose profiles, and for absolute dose and OF evaluations this value was reduced to 1.5%. Moreover a proper calibration curve was made for each set of measurements. With regard to absolute doses, the results obtained with the ionization chamber are in good correlation with radiochromic film-generated data, for both LGK and LR, showing a dose difference of less than 1%. The output factor evaluations, performed using different methods, are in good agreement with a maximum difference of 1.5% for all field sizes considered (LGK and LR) except the 4 mm helmet used in the LGK unit. In this case, differences exist between diode and radiochromic film measurements and both detectors show data values larger than the prestored OF value of 0.80. Dose profiles measured by radiochromic film and calculated are in excellent agreement for both LGK and LR with a maximum deviation of less than 1.0 mm, when full widths of the dose profiles at 20%, 50%, 80% levels are considered. When external photon beams are used in stereotactic radiosurgery, the 'well selected' radiochromic films are very accurate detectors both for relative and absolute dosimetry. The experimental results, obtained using both radiochromic and diode detectors, show that the 4 mm helmet relative output factor could be underestimated.

[The absorbed dose to the gonads in adult patients undergoing defecographic study by digital or traditional radiographic imaging]

INTRODUCTION

Doses to the gonads and skin of adult people were evaluated during the study of functional anorectal disorders with defecography.

MATERIALS AND METHODS

The radiologic procedure relies heavily on fluoroscopy with lateral and anteroposterior projections for a mean time of 2.4 minutes; the stages of interest are recorded on a mean of 6 films. Absorbed doses to the gonads and to the skin were measured with LiF thermoluminescence dosimeters placed inside and outside the anthropomorphic "Rando" phantom. The administered doses during the different examination phases (anteroposterior and lateral fluoroscopy and radiography) were measured. Digital and conventional radiography were compared.

RESULTS

Gonadal absorbed doses in a standard DSI examination were: 5.0 +/- .1 mGy to the right ovary, 25.2 +/- .7 mGy to the left ovary and 2.7 +/- .1 mGy to testes when at the margin of the irradiation field. Doses to gonads and skin are about 15% higher when the examination is performed with conventional radiography.

DISCUSSION

The considerable radiation dose to ovaries shows that defecography can cause radiation-induced genetic effects in women younger than 40 years with a probability of the order of one thousandth of the natural incidence genetic defects.

CONCLUSIONS

Defecography must be correctly indicated in fertile women, because of the relatively high absorbed dose to ovaries. In fact, the ovaries absorb about twice as many radiations during defecography than radiologic examinations of the lower gastrointestinal tract.

[The evaluation of aphasic deficits for the definition of a targetted logotherapeutic treatment]

The Aachener Aphasie Test (AAT) is the major German test for the diagnosis of aphasic disorders. The test is easy to use and is valid and reliable for the diagnosis of aphasia and its severity and to evaluate the recovery of the aphasic disorder after language rehabilitation. The AAT is, however, not sufficient to define cognitively sound logotherapeutic treatment. The use of tasks which are based on cognitive functional models allows the identification of specific processing levels that have been damaged by a cerebral lesion, and the definition of a focussed rehabilitation plan. In this paper, we will discuss the results of a cognitive neurolinguistic treatment in a patient who suffered of Broca's aphasia with agrammatism and phonological dyslexia.

[Design, development, and dosage control of individual compensatory filters for 6 MV X-ray radiotherapy]

An automated system for the design and manufacturing of individual compensators has been implemented. The system based on computed tomography enables 3D compensation of missing tissue and tissue heterogeneities. The relationship between Hounsfield numbers and electron densities was obtained empirically. Compensator design is based on the calculation of the water equivalent thicknesses between the compensation plane and the patient surface. After calculation a styrofoam mould is cut by a computer driven machine and filled with bee's wax or tin granules. Compensator thickness is calculated by means of the conversion ratio tau, which is defined as t/x, where t is the compensator thickness equivalent to the missing tissue in the treatment geometry. Relations between tau and field size, depth of compensation plane and focus-compensation plane distance were assessed. The conversion ratio is a linear function of the missing tissue and depends markedly on field size; for a 10-cm-deep compensation plane at 1 m from the accelerator target the tau value, calculated for bee's wax, decreases by 25% from 7 x 7 cm2 to 23 x 23 cm2 field size. Conversion ratio rises by approximately 10% for a 3-cm increase in compensation plane depth and reduces by about 5% when increasing the focus-compensation plane distance from 100 cm to 140 cm. It must be stressed that a 10% variation of tau, for bee's wax, involves only a 2% dose variation in the compensation plane. Therefore, for compensator design it is enough to consider tau as depending on field size only. Compensation effectiveness has been tested by a film-densitometric technique using phantoms with tilted incident surfaces and heterogeneities. The results show that the compensators reduce the flatness of the beam profile below 4% and increase the relative dose uniformity on the compensation plane from 18% to 60%.

[Digital fluorography in gastroenterological radiology]

The clinical value of two digital fluoroscopy systems not connected to a PACS was investigated and compared with that of conventional radiology. Some critical variables were considered: image intensifier diameter, image definition, examination time, acquisition speed, patient exposure and finally film consumption. The main problems in the use of the digital techniques consisted in the limited size of the examination fields, which was not big enough to demonstrate the whole colon during double contrast enema, and in the difficult representation of the actual size on the image printed on the laser films. Definition was considered as sufficient for GI examinations. The main advantages consisted in acquisition speed, allowing detailed examinations of cervical esophagus, cardias and fistulous tracts, reduced patient dose and finally the real time visualization of the acquired image on the TV monitor.

[Comparison of acceptance tests of 5 linear electron accelerators for radiotherapy]

The acceptance tests of five electron linear accelerators have been analyzed to compare their completeness, the standards specified by the manufacturers and the measured deviations. Involved tests were relative to geometrical and mechanical parameters, photon and electron beam characteristics, dose monitoring systems, treatment table and safety systems. The comparison of acceptance tests has shown a significant nonuniformity, also due to the absence of a specific legislation; anyway, the measured deviations generally appeared to be within internationally recommended values.

[Use of fast electrons in oncologic therapy. More than a decade's experience]

Criteria of choice in the utilization of fast electrons in radiotherapy on the basis of 2400 cases and 13 years of experience in the radiotherapy department of Cancer Institute in Milan are discussed. In the opinion of the authors fast electrons represent the most rational treatment to homogenize or differentiate the dose distribution according to space as a complementary technique after first time treatments by different ionizing radiation. Authors demonstrate also that fast electron treatment is the most rational technique for metastases in the retina and for recurrence of rhinopharynx cancer. Radiotherapy by fast electrons allows to deliver to the patient's body on integral dose lesser than with ionizing radiation of other type. Sequelae are regarding in most of the cases soft tissues and are caused by an unrecognized distribution of dose delivered by the ionizing radiations of other type in the first time treatment more than by difficulty im dosimetry of post-electrons.

[Verification of 3 computerized systems for drafting a plan for treatment with electrons]

The electron beam treatment planning systems, with the availability of more and more developed hardware for complex calculation algorithms, need quality controls of accuracy and comparability of treatment plans from different systems. In this paper three systems have been evaluated; their similar computation algorithms (pencil beam) have been used but in some conditions (beam reconstruction, oblique incidence and dishomogeneity) different results have been drawn. The basic data (PDD, dose profile, etc.) were the same for every system, therefore it has been observed that different results are mainly due to differences in: a) input of parameter values, b) calculation algorithm approximations, c) image acquisitions and elaboration.

Imitating gesture learning in apraxic patients

The authors studied a group of patients with ideomotor apraxia (IMA) to verify if these patients had some difficulties in gesture learning task in respect to non-apraxic patients and normal control subjects. The results show that learning difficulties are present in patients with IMA. Apraxic patients are not able to perform the gestures in the testing situation and non in everyday life; this fact would demonstrate that the motor pattern still exists in these patients but is inaccessible unless elicited by a strong stimulation.

Quality control of a hyperthermia system

Characterization of the applicators, of the thermometers and of the cooling (bolus) system of hyperthermic apparatus must be made before starting hyperthermic treatment and also starting with a frequency depending on the technical features of equipments that make up the whole system. This data gives the radiotherapist some useful parameters for the selection of applicators and general knowledge of the accuracy of the system.

[Treatment technic and clinical dosimetry in whole-body irradiation]

At the Istituto Nazionale Tumori, Milan, total-body irradiation (TBI) is delivered by a 15 MV linear accelerator, with two lateral opposed beams. Maximum build-up at the skin is achieved by lateral slabs of perspex 3 cm thick. Attenuation filters or bolus are used for dose compensation, or reduction, to the head and lungs. The dose delivered to clinically relevant anatomic regions is determined by "in vivo" dosimetry. For this purpose, calibrated diodes are employed, which are positioned at the entrance and at the exit of the beams. "In vivo" dosimetry data show our TBI technique to allow an homogeneous irradiation of all body areas, with maximum deviation of the mean dose value from reference point dose of -11% in the posterior abdomen, at the spinal cord shielded by arms.

[3-dimensional automatic dosimetry in curietherapy]

Isodose curves can be viewed on multiple planes along with the radioactive implant and anatomical structures by projecting them on a video-display. The main options offered are the parallel projection of the implant selecting all the possible rotations, zooming and shifting, calculation of distance between two points of a selected plane, stereoscopic display.

[Computerized dosimetry in curie therapy]

Automatic treatment planning for calculation of absorbed dose in curietherapy has been performed at the Istituto Nazionale Tumori of Milano on a routine for some years. The radioactive implant is reconstructed from orthogonal or stereo-roentgenograms, a tabular listing of doses or dose rates can be obtained on the grid of points and the isodose curves are automatically drawn with the projection of sources onto the selected planes of calculation by an interactive plotter.

MEDIC: a language to process clinical data for elementary statistical purposes

A new language has been developed which is helpful for handling data, already collected and stored on disk, with the aim of producing reports of clinical interest. For each clinical data bank a dictionary is generated, which contains all information needed to fully identify each medical item (e.g., name, type, attributes, indexed/single value). Now the user can write programs in a very easy way using the statements of the MEDIC language which give him the possibility of powerfully filtering data, making calculations and producing reports. Data integrity is achieved by the fact that users can only accede to data from the data bank and cannot alter data or modify them on disk. A special emphasis has been given, when designing MEDIC, to the possibility of counting data which satisfy certain sets of conditions, and to special output functions, as histograms. Programs written in MEDIC language are processed by MEDIC generator, are immediately executable, and have the possibility of tracing/debugging (the tracing option is chosen when program is starting execution). At execution time the user chooses in a very easy way the rules of scanning data bank, for example, the whole data bank and single ranges of records.

Dosimetry of a Filter for Treating Carcinoma of the Uterine Cervix with Cobalt to Complement Intracavitary Curietherapy

A shaped filter has been designed for treating carcinoma of the uterine cervix with cobalt in cases previously treated with intracavitary curietherapy. The filter is designed so that the dose delivered by cobalt from the midline of the pelvis to the periphery in order to compensate rises the decreasing dose of intracavitary therapy. Since this method rules out the possibility of overdosed areas, the total dose delivered to the pelvis may be increased to levels not formerly allowed.