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.

[Pulmonary concentration of asbestos fibers in steel workers with pleural mesothelioma]

The asbestos fibre burden of the lung has been used in the past as a biological indicator of cumulative exposure to the mineral so much so that in 1997 reference limits even for non-occupationally exposed people have been proposed. This kind of analysis was performed on groups of workers of different type of industries and allowed to achieve a qualitative-quantitative estimate of past exposure to asbestos, even in absence of exposure estimates by environmental monitoring. An important example is the steel industry where asbestos was widely used in the past, but for which there are not available exposure estimates of workers. Among the mesothelioma cases collected by the Mesothelioma Registry of the Province of Brescia from 1980 to present there are 55 workers who spent at least 5 years in steel industry, on a total of 289 cases classified as asbestos exposed (19%). For 8 subjects who worked in steel mills and production of electrical steel pipes, of which 4 in the same plant, lung tissue samples were available for the asbestos fibres burden analysis (7 samples coming from autopsies and 1 from extra-pleural pneumonectomy). In all cases the diagnosis was given with histological analyses supplemented with immunohistochemistry. In 7 cases autopsied the diagnosis was confirmed. The work histories have been reconstructed in detail through the interview process, inclusive of details of duties performed. The asbestos fibre burden analyses showed a range of concentrations between 260,000 and 11,000,000 ff per grams of dry tissue; the concentration of amphiboles was much higher than that of chrysotile. The highest body burden was detected in the maintenance workers of the same plant in witch a cluster of malignant mesothelioma was observed. In conclusion, this study illustrates the results of asbestos fibres burden analyses in subjects where exposure to asbestos is sure but not quantifiable. The results showed also that these concentrations can reach values that overlap with those found in asbestos-cement workers and in asbestos-textile workers. These data suggest to consider the cases of mesothelioma occurred in the steel workers at least as "possible" exposure, even in the absence of adequate information on the circumstances of contact with asbestos. This study, although based on a small number of cases, is the only one ever held in Italy on workers in this sector.

[Recycle of jute bags; asbestos in agriculture, exposure and pathology ]

During the last four years, a deeper examination of malignant mesothelioma (MM) cases occurred within non asbestos textile industry highlighted asbestos past exposure in several textile industrial divisions. In spite of that, poor information about recycled textile bags previously containing asbestos fibres is available to the National Mesothelioma Registry, although holding a remarkable data bank on more than 3500 work histories and sources of asbestos exposures. Besides the analysis of the exposure circumstances and the registered health effects of the past exposure within the recycling activity, the aim of this research was to relate the possible involvement of the agricultural sector, where the use of recycled jute bags was very diffused. The MM cases were collected from the Mesothelioma Registry of Brescia, asbestosis, pleural plaques and lung cancer cases were collected from the Occupational Diseases Archive of the Local Public Occupational Health Service of the Province of Brescia. During the 1977-2006 period, 8 cases of MM, 4 cases of pulmonary asbestosis, 4 of isolated bilateral pleural plaques and I of lung cancer in pulmonary asbestosis, were observed among workers employed in bags recycling activity in 4 small companies, one of them still operating, employing about 50 workers. Even more, among the 65 MM cases classified by the Registry with "unknown asbestos exposure" (UAE), the most relevant frequency of working histories concerned the agriculture sector. Confirming a past signalling, the investigations underlined the cross linkage between this working activity and the diffusion of recycled bags in the agriculture sector. In the Province of Brescia, the activities of these small jute bags recycling plants were linked, even geographically, to the asbestos cement manufacture plant using a huge number of bags, roughly until mid seventies. Therefore, a large number of these recycled bags, previously containing asbestos, were generally used for harvesting and trading agricultural typical products of northern Italy. According to the 2003 National Mesothelioma Registry Guide Lines, MM in agricultural workers are still classified as UAE due to poor information available. In the light of these new findings, it looks reasonable to review the UAE within agriculturalists attributing a new classification of "possible" occupational asbestos exposure, although other exposure circumstances might have occurred in the past.

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.

Design and characterization of a dynamic multileaf collimator

The characteristics of a prototype computer-assisted dynamic multileaf collimator (DMLC), specifically designed for small-field conformal radiotherapy, were evaluated at the Istituto Nazionale Tumori of Milan. The collimating device consists of two opposing banks of 16 pairs of 8 cm thick, 3.6 mm wide tungsten leaves and allows shaping of a radiation field up to a size of 10 x 10 cm2 at the isocentre. The screening thickness of each leaf is 6.25 mm at the accelerator gantry isocentre. The leaves have a trapezoidal cross section and move along an arched path, thus providing a 'double focused' collimation system. The DMLC was installed on the head of a Varian Clinac 2100C linear accelerator. Mechanical and dosimetric evaluations were performed to test the stability of the mechanical isocentre and to determine leaf leakage, penumbra width, accuracy of leaf positions and uniformity of leaf speed. Displacement of the mechanical isocentre was less than 1 mm at all gantry angles. Standard radiographic films exposed to 6 MV x-ray radiation were used for dosimetric evaluations. Leakage between leaves was less than 2.5%, and leakage through abutted leaves was less than 5.5%. The penumbra width between 20% and 80% isodose at different positions of leaf banks was 2.7 mm in the direction of the leaf motion and 3.1 mm along the side of the leaf with a standard deviation of 0.2 mm in both directions. Accuracy in the positioning of the leaf was 0.3 mm, whereas the maximum repositioning error was less than 0.2 mm. Finally, during movement of the leaves at the maximum speed of 0.5 mm s(-1), the standard deviation of the leaf positioning error was 0.2 mm, proving an accurate uniformity of leaf speed.

Treatment planning for radiotherapy in northern Italy: a survey by the National AIFB-AIRO Committee on 3D-Treatment Planning. Italian Association for Biomedical Physics. Italian Association for Radiation Oncology

BACKGROUND AND PURPOSE

A survey was performed in 1996 to investigate the structures and the process of radiation therapy treatment planning in clinical practice within northern Italy, with particular emphasis on the current and future implementation of 3D equipment and techniques.

MATERIALS AND METHODS

Of 57 existing radiation therapy (RT) centres covering a population of 25 million people (45% of the total population of Italy) and treating over 58,000 cancer patients (70% of the cancer cases in Italy) each year, 46 centres were deemed eligible for the survey; a questionnaire was sent to a medical physicist working in each eligible RT centre, 40 of whom responded, making the basis for this report.

RESULTS

A dedicated CT scanner was available in 25% of the responding centres and a total of 49 radiation therapy planning systems (RTPS) were reported; none of the RTPS were able to perform 3D calculations, but 50% of the centres had an advanced 2D or 2.5D system. Connection between CT scan and RTPS was by tape or disk in 62% of centres. Immobilization devices were used frequently for head and neck patients (88% of centres), but not for lung (16%) or prostate cancer (24%) patients; the number of contoured slices was largely variable, exceeding 10 in only about 30% of the responding centres. The average working time per patient seemed to closely reflect the number of slices used and the number of critical organs contoured. Finally, the majority of the responding physicists did not favour the use of more than 20 CT slices for 3D treatment planning, nor did they forecast a general spread of this technique in the next 2-3 years.

CONCLUSIONS

This survey has shown (1) a heterogeneous picture, with 20% of centres ready to implement 3D techniques and 20% of centres lacking the possibility of planning treatments and (2) a general difficulty in coping with the workload represented by 3D treatment planning.

[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%.