Metal-polysiloxane shields for radiation therapy of maxillo-facial tumors

Assessment of different shielding materials for radiation therapy of maxillofacial tumors - An in vitro study

AIM

Since the advent of radiotherapy, the success rate of head and neck cancer treatment has increased significantly. However, when the tissue tolerance level is exceeded, unnecessary and uncontrolled exposure to radiation is considered detrimental. Such problems remain difficult to prevent and manage. The aim of the study to evaluate and compare the degree of attenuation of therapeutic radiation using four different radiation shielding materials of varying thickness.

STUDY SETTING AND DESIGN

In vitro experimental study.

MATERIALS AND METHODS

The samples were divided into four groups based on the different radiation shielding materials of thickness 3mm and 5mm. The materials are Lead (Pb), Silver-tin alloy (Ag-Sn) with Polyvinylsiloxane (PVS), Polymethylmethacrylate (PMMA) with Barium sulphate (BaSO4), and a combination of Ag-Sn with PVS and PMMA with BaSO4 which was exposed to radiation. The radiation dose measurements were recorded and the radiation attenuation properties of the shielding materials were evaluated. Among all of the shielding materials the most efficient material under consideration is determined.

STATISTICAL ANALYSIS USED

One-way ANOVA followed by post hoc test was used to compare the means of all four groups.

RESULTS

A statistically significant difference between groups was found by a one-way ANOVA with a P = 0.001. In the post hoc test, statistically significant findings were obtained with a P = 0.05 when comparing the variation values of 3mm and 5mm thickness between each group and other groups.

CONCLUSION

The shielding materials results in significant reductions in radiation dosage. It was concluded that the combinations of Ag-Sn alloy with PVS, and PMMA with BaSO4 of thickness 5 mm had a good shielding effect.

Impact of radiotherapy and shielding on the efficacy of the self-etch adhesive technique

Context:

The exposure to gamma radiation affects the enamel and dentin in teeth restored with composite restoration, but a little has been done to protect from the detrimental effect.

Aims:

The aim of this in vitro study was to evaluate the effect of gamma radiation, with or without shielding (0.5 mm thickness of lead), in Class V cavities prepared on teeth exposed before and after restoration using the self-etch adhesive technique.

Methods:

A total of 75 intact teeth were selected. The samples were divided into five groups: Group I (15 teeth) not exposed to gamma radiation. Group II and III exposed to gamma radiation as per the standardized radiation protocol (2 Gy/day for 5 days/week = 10 Gy/week for 6 weeks = 60 Gy). Group III shielded, using a “0.5 mm thickness of lead” molded into a “C” shaped tube. Standardized Class V cavities were prepared on the buccal surface of all teeth and were restored by composite. Groups IV (not shielded) and V (shielded) were then exposed to standardized radiation protocol. All the samples were evaluated for the assessment of microleakage under stereomicroscope.

Statistical Analysis Used:

Statistics were tabulated using the Kruskal–Wallis ANOVA test.

Results:

Samples showed a significant difference in dye penetration scores.

Conclusions:

The samples restored before being exposed to gamma radiations performed better. Shielding with 0.5 mm of lead has shown increased efficacy of self-etch adhesive system irrespective of the stage of exposure.

Radiation dose enhancement associated with contemporary dental materials

STATEMENT OF PROBLEM

Electron backscatter radiation from dental materials can contribute to soft tissue injury in patients undergoing head and neck radiation therapy.

PURPOSE

The dose enhancement from the materials used for prosthodontic restoration of the dentition has not been well quantified. This study reports the magnitude of backscatter dose from the contemporary dental materials lithium disilicate and zirconia as compared with high-noble alloy and investigates the role of a spacer material in mitigating this effect.

MATERIAL AND METHODS

Three flat slabs of dental materials high-noble alloy, lithium disilicate, and zirconia with thicknesses of 1.5 and 3 mm were irradiated with 6-MV photons from a clinical linear accelerator. Measurements were made using a thin-window parallel-plate ionization chamber placed at 0, 1, 3, and 5 mm from the material. One millimeter of poly(methyl methacrylate) or thermoplastic material was used to cover the dental material and measure the effect on the adjacent dose enhancement.

RESULTS

Dose enhancements between 8% and 50% were recorded adjacent to the dental restoration materials. The largest enhancements occurred for the material of the highest density, the high-noble alloy. Dose enhancement was substantially lower for lithium disilicate (8%) and zirconia (30%). The thickness of the restoration material did not significantly affect dose enhancement. The dose enhancement decreased with distance from the material, dropping to <10% for all materials at 3 mm.

CONCLUSIONS

Contemporary dental restorations enhance the backscatter dose. The presence of dental restorations may warrant the use of a stent to create separation from these materials as this can mitigate the effect.

Study of dental prostheses influence in radiation therapy

Dental prostheses made of high density material contribute to modify dose distribution in head and neck cancer treatment. Our objective is to quantify dose perturbation due to high density inhomogeneity with experimental measurements and Monte Carlo simulations. Firstly, measurements were carried in a phantom representing a human jaw with thermoluminescent detectors (GR200A) and EBT2 Gafchromic films in the vicinity of three samples: a healthy tooth, a tooth with amalgam and a Ni-Cr crown, irradiated in clinical configuration. Secondly, Monte Carlo simulations (BEAMnrc code) were assessed in an identical configuration. Experimental measurements and simulation results confirm the two well-known phenomena: firstly the passage from a low density medium to a high density medium induces backscattered electrons causing a dose increase at the interface, and secondly, the passage from a high density medium to a low density medium creates a dose decrease near the interface. So, the results show a 1.4% and 23.8% backscatter dose rise and attenuation after sample of 26.7% and 10.9% respectively for tooth with amalgam and crown compared to the healthy tooth. Although a tooth with amalgam has a density of about 12-13, the changes generated are not significant. However, the results for crown (density of 8) are very significant and the discordance observed may be due to calculation point size difference 0.8 mm and 0.25 mm respectively for TLD and Monte Carlo. The use of Monte Carlo simulations and experimental measurements provides objective evidence to evaluate treatment planning system results with metal dental prostheses.

The application of a computer-assisted thermoplastic membrane navigation system in screw fixation of the sacroiliac joint--a clinical study

BACKGROUND

We inserted iliosacral screws with the aid of a computer-assisted thermoplastic membrane navigation (CATMN) system which is widely used for the accurate and repeatable location tumour in radiation therapy. We hypothesised that application of the CATMN system on IS screws' insertion will provide a superior result to conventional fluoroscopic imaging with less operative time, more accuracy and lower complication rates.

METHODS

We prospectively evaluated 26 consecutive patients who suffered from sacroiliac joint fractures and dislocations (type C, Tile classification) from April 2007 to June 2010 in our hospital. Patients were randomised into two groups: 13 patients in control group and 13 patients in CATMN groups. After operation, inlet and outlet X-ray views and computed tomography (CT) scanning were performed to confirm and compare the screw positions. The operative time, blood loss and accuracy (measured with postoperative CT) were analysed between groups.

RESULTS

In the control group, 18 screws were placed in 13 patients with conventional fluoroscopic technique; two of 18 (11.1%) screws were misplaced. The average intra-operative blood loss was 145.4±112.0ml, and operation time was 619.2±199.5s. In the CATMN group, 21 screws were placed in 13 patients with the application of the CATMN system. All 21 screws were in safe zones. The average intra-operative blood loss was 46.2±24.3ml and the operation time was 353.8±111.2s. Operative time and blood loss were reduced significantly with the CATMN system (p<0.05).

CONCLUSION

Application of CATMN system has high accuracy in treating sacroiliac joint dislocations and provides a new alternative method for guidance of the IS screw placement.

Effect of dental restorations and prostheses on radiotherapy dose distribution: a Monte Carlo study

Dental restorations, fixed prosthodontics, and implants affect dose distribution in head and neck radiation therapy due to the high atomic number of the materials utilized. The backscatter of electrons from metallic materials due to the impinging treatment x‐ray results in localized dose enhancements. These dose enhancements cause localized mucositis in patients who have dental work, a significant clinical complication. We investigated the backscatter effect of 23 configurations of dental work using the EGS4nrc Monte Carlo (MC) simulation system. We found that all‐metal fixed partial dentures caused the highest amount of dose enhancement – up to 33% – while amalgam restorations did not cause a significant amount. Restorations with a ceramic veneer caused up to 8% enhancement. Between 3 mm and 5 mm of water‐equivalent material almost completely absorbed the backscatter. MC simulations provide an accurate estimate of backscatter dose, and may provide patient‐specific estimates in future.

Dosimetric evaluation of GAFCHROMIC XR type T and XR type R films

The high spatial resolution of radiochromic film makes it ideal for dosimetric measurements and dose distributions in regions of high dose gradient. Intensity‐modulated radiation therapy, intravascular brachytherapy, and eye‐plaque radiation therapy demand precise spatial dosimetric calculations. Such precision is not possible with conventional dosimeters, such as thermoluminescent dosimeters and ionization chambers. Recently, new GAFCHROMIC® XR type T and type R films have been developed for radiation dosimetry, specifically in interventional radiology procedures. Dosimetric characteristics (i.e., linearity, post‐exposure density growth, energy dependence, dose‐rate dependence, and UV light sensitivity) of these new films were investigated. To evaluate the clinical applications of these films, their characteristics were compared with other commercially available film models. GAFCHROMIC® XR type T and type R films were found to be more sensitive to low‐energy doses as compared with GAFCHROMIC® MD‐55 films.

PACS numbers: 87.66‐a, 87.53‐j

Validation of a precision radiochromic film dosimetry system for quantitative two-dimensional imaging of acute exposure dose distributions

We present an evaluation of the precision and accuracy of image-based radiochromic film (RCF) dosimetry performed using a commercial RCF product (Gafchromic MD-55-2, Nuclear Associates, Inc.) and a commercial high-spatial resolution (100 microm pixel size) He-Ne scanning-laser film-digitizer (Personal Densitometer, Molecular Dynamics, Inc.) as an optical density (OD) imaging system. The precision and accuracy of this dosimetry system are evaluated by performing RCF imaging dosimetry in well characterized conformal external beam and brachytherapy high dose-rate (HDR) radiation fields. Benchmarking of image-based RCF dosimetry is necessary due to many potential errors inherent to RCF dosimetry including: a temperature-dependent time evolution of RCF dose response; nonuniform response of RCF; and optical-polarization artifacts. In addition, laser-densitometer imaging artifacts can produce systematic OD measurement errors as large as 35% in the presence of high OD gradients. We present a RCF exposure and readout protocol that was developed for the accurate dosimetry of high dose rate (HDR) radiation sources. This protocol follows and expands upon the guidelines set forth by the American Association of Physicists in Medicine (AAPM) Task Group 55 report. Particular attention is focused on the OD imaging system, a scanning-laser film digitizer, modified to eliminate OD artifacts that were not addressed in the AAPM Task Group 55 report. RCF precision using this technique was evaluated with films given uniform 6 MV x-ray doses between 1 and 200 Gy. RCF absolute dose accuracy using this technique was evaluated by comparing RCF measurements to small volume ionization chamber measurements for conformal external-beam sources and an experimentally validated Monte Carlo photon-transport simulation code for a 192Ir brachytherapy source. Pixel-to-pixel standard deviations of uniformly irradiated films were less than 1% for doses between 10 and 150 Gy; between 1% and 5% for lower doses down to 1 Gy and 1% and 1.5% for higher doses up to 200 Gy. Pixel averaging to form 200-800 microm pixels reduces these standard deviations by a factor of 2 to 5. Comparisons of absolute dose show agreement within 1.5%-4% of dose benchmarks, consistent with a highly accurate dosimeter limited by its observed precision and the precision of the dose standards to which it is compared. These results provide a comprehensive benchmarking of RCF, enabling its use in the commissioning of novel HDR therapy sources.

A technique for shielding electron beams used in radiotherapeutic management of head and neck cancer

One of the major complications of radiation therapy in the management of maxillofacial tumors is the postradiation damage to the healthy tissues adjacent to the tumors. The success of radiation therapy is often limited by the treatment sequelae. The use of shielding prostheses reduces the number of potential treatment sequelae caused by the administration of therapeutic radiation for head and neck tumors. A technique for shielding uninvolved tissues from external electron beams used in therapeutic radiation treatment of head and neck cancer is described. The materials required are readily available and the shielding prosthesis can be easily fabricated by any dentist with a thorough understanding of radiation therapy, dosimetry, and the goals of treatment with radiation therapy.

A direct method for fabricating tongue-shielding stent

During oral cancer radiotherapy, a tongue-shielding radiation stent guides the patient's upper and lower jaws to a repeatable position, attenuates radiation doses, and protects the tongue and structures adjacent to the irradiated field. Conventionally, a tongue-shielding radiation stent is made of heat-cured polymethyl methacrylate resin in which a low-melting Pb-Bi-Sn alloy is embedded as a shielding layer. Its use involves multiple and lengthy clinical and laboratory procedures. An improved polyvinyl siloxane-metal composite shielding system for radioprotection has recently been developed. This two-component, base and catalyst, putty material offers a shielding effect similar to that of the conventional shielding alloys. Its major advantages are that it is simple to use, requires only one clinical appointment, and affords efficient collaboration between dental and medical teams during cancer treatment. This article describes a simplified direct method of fabricating a tongue-shielding stent with the use of a new polyvinylsiloxane-metal composite in conjunction with impression putty material.