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Biomedical subjects

G J Meijer

Publications and source records attributed to G J Meijer.

At least 19 recordsLinked to original sources
PubMed2006

Influence of the linac design on intensity-modulated radiotherapy of head-and-neck plans.

In this study, we quantify the impact of linac/MLC design parameters on IMRT treatment plans. The investigated parameters were leaf width in the MLC, leaf transmission, related to the thickness of the leaves, and penumbra related primarily to the source size. Seven head-and-neck patients with stage T1-T3N0-N2cM0 oropharyngeal cancer were studied. For each patient nine plans were made with a different set of linac/MLC parameters. The plans were optimized in Pinnacle(3) v7.6c and PLATO RTS v2.6.4, ITP v1.1.8. A hypothetical ideal linac/MLC was introduced to investigate the influence of one parameter at a time without interaction of other parameters. When any of the three parameters was increased from the ideal set-up values (leaf width 2.5 mm, transmission 0%, penumbra 3 mm), the mean dose to the parotid glands increased, given the same tumour coverage. The largest increase was found for increasing leaf transmission. The investigation showed that by changing more than one parameter of the ideal linac/MLC set-up, the increase in the mean dose was smaller than the sum of dose increments for each parameter separately. As a reference to clinical practice, we also optimized the plans of the seven patients with the clinically used Elekta SLi 15, equipped with a standard MLC with a leaf width of 10 mm. As compared to the ideal linac, this resulted in an increase of the average dose to the parotid glands of 5.8 Gy.

Dose-Response Relationship, Radiation↗
PubMed2005

[(Pain) complaints that are "not understood" after implant surgery. Diagnosis with CT/DentaScan].

Combining clinical examination and panoramic, periapical or lateral cephalometric radiographs is generally sufficient in the pre- and post-operative investigation of most implant patients. Sometimes there is a need for more information needing computer tomography (CT) as an additional diagnostic tool. The obtained CT data can be manipulated and reconstructed with software programs such as DentaScan. The resulting cross sections, that can be given in true proportions, are perpendicular to the course of the jaw. Interpretation of these processed images is much easier but not always more accurate than the original scans. Pre-operatively, CT/DentaScans can be used not only for the determination of bone quantity and quality, but also for the location of relevant anatomical structures. Post-operatively, the scans can be valuable in the diagnosis of unexplained problems that may be related to the placed implants. Three case reports illustrate the use and indication of CT/DentaScans in solving implant related maxillary pathology or neuralgic pain.

Adult↗
PubMed2004

Anterior tooth replacement with implants in grafted alveolar cleft sites: a case series.

When a residual edentulous space in the anterior region remains after surgical and orthodontic treatment of alveolar cleft patients, implant placement with or without additional grafting is a treatment option. We placed a total of 10 implants in nine consecutive alveolar cleft patients (five females, four males) with residual anterior edentulous spaces and non-restored neighbouring teeth. Patient age generally varied from 18 to 22 years. Additional (tertiary) bone grafting, implant placement after 3 months of graft consolidation and implant uncover after no less than 6 months was the favoured surgical protocol in the five most recent cases. Ample time was allowed to evaluate and guide the development of soft tissues, following second-stage surgery. For this purpose, temporary crowns were fabricated, adjusted and left in place for over 3-4 months in seven out of nine cases. No implants were lost after a mean observation period of 3.4 years and all implants function without objective (radiographic) or subjective problems. Aesthetics were considered acceptable in seven out of nine cases. Unsatisfactory aesthetic results originated from poor implant positioning or alignment in two patients who did not undergo tertiary bone grafting. We conclude that implant placement in alveolar cleft sites is a comprehensive but viable treatment option. Tertiary grafting of the site is recommended to achieve better ridge contour and bone height. This allows optimal implant placement and alignment.

Adolescent↗
PubMed2004

[Complications during and after dentoalveolar surgery].

In dentistry complications are undesirable adventures provoked by oral health care delivery. If there is any risk of a complication, the patient needs to be informed before treatment. Occurrence of complications can be minimized by adequate diagnostics and planning sufficient time for treatment. The most common complications during and after dentoalveolar surgery are discussed, such as complications during or after producing local anaesthesia, nerve injuries, complications during or after tooth extraction, aspiration, jaw luxation, disturbed wound healing, and complications during or after implantation procedures.

Alveolar Process↗
PubMed2002

Bone growth in biomimetic apatite coated porous Polyactive 1000PEGT70PBT30 implants.

We recently, developed a simple one-day one-step incubation method to obtain bone-like apatite coating on flexible and biodegradable Polyactive 1000PEGT70PBT30. The present study reports a preliminary biological evaluation on the coated polymer after implantation in rabbit femurs. The porous cylindrical implants were produced from a block fabricated by injection molding and salt leaching. This technique provided the block necessary mechanical integrity to make small cylinders (diameter 3.5 x 5 mm2) that were suitable for implantation in rabbits. The coating continuously covered the surface of the polymer, preserving the porous architecture of outer contour of the cylinders. Two defects with a diameter of 3.5 or 4 mm were drilled in the proximal and distal part of femur diaphysis. The implants were inserted as press-fit or undersized into the cortex as well as in the marrow cavity. The polymer swelled after implantation due to hydration, leading to a tight contact with the surrounding bone in both defects. The adherence of the coating on the polymer proved to be sufficient to endure a steam sterilization process as well as the 15% swelling of the polymer in vivo. The coated Polyactive 1000PEGT70PBT30 has a good osteoconductive property, as manifested by abundant bone growth into marrow cavity along the implant surface during 4-week implantation. A favorable bioactive effect of the coating with an intimate bone contact and extensive bone bonding with this polymer was qualitatively confirmed. Concerning the bone ingrowth into the porous implant in the defect of 4 mm diameter, only marginal bone formation was observed up to 8 weeks with a maximal penetration depth of about 1 mm. The pore interconnectivity is important not only for producing a coating inside the porous structure but also for bone ingrowth into this biodegradable material. This preliminary study provided promising evidence for a further study using a bigger animal model.

Animals↗
PubMed2001

Accurate in vivo dosimetry of a randomized trial of prostate cancer irradiation.

PURPOSE: To guarantee an accurate dose delivery, within +/- 2.5%, in a Phase III randomized trial of prostate cancer irradiation (68 vs. 78 Gy) by means of a comprehensive in vivo dosimetry program. METHODS AND MATERIALS: Prostate patients are generally treated in our clinic with a 3-field isocentric technique: an 8-MV anteroposterior beam and 2 18-MV wedged laterals. All fields are shaped conformally to the PTV. Patients were randomized between two dose levels of 68 Gy and 78 Gy. During treatment, the entrance and exit dose were measured for each patient with diodes. Special 2.5-mm thick steel build-up caps were applied to make the diodes appropriate for measurements in 18-MV photon beams as well. Portal images were used to verify the correct position of the diodes and to detect and correct for gas filling in the rectum that may influence the exit dose reading. Entrance and exit dose measurements were converted to midplane dose, which was used in combination with a depth dose correction to obtain the dose at the specification point. An action level of 2.5% was applied. RESULTS: The added build-up for the diodes in the 18-MV beams resulted in correction factors that were only slightly sensitive to changes in beam setup and comparable to the corrections used in the 8-MV beams for diodes without extra build-up. The calibration factor increased almost linearly with cumulative dose: 0.7%/kGy for the 8-MV and 1.2%/kGy for the 18-MV photon beams. The introduction of average correction factors made the analysis easier, while keeping the accuracy within acceptable limits. In a period of 3 years, 225 patients were analyzed, from which 8 patients needed to be corrected. The average ratio of measured and prescribed dose was 1.009 (standard deviation [SD] 0.012) for the total group treated on two linear accelerators. When the results were analyzed per accelerator, the ratios were 1.002 (SD, 0.001) for Accelerator A and 1.015 (SD, 0.001) for Accelerator B. This difference could be attributed to the cumulative effect of three small imperfections in the performance of Accelerator B that were well within the limits of our quality assurance program. CONCLUSION: Diodes can be used for accurate in vivo dosimetry during prostate irradiation in high-energy photon beams. The dose delivery in this randomized trial is guaranteed within the 2.5% limits on an individual patient basis. This could not be achieved without the in vivo dosimetry program, despite our high-standard quality assurance program of treatment delivery.

Algorithms↗
PubMed2000

A treatment planning method to correct dose distributions distorted by setup verification fields.

PURPOSE: Portal images of conformal treatment fields are often not suitable for setup verification purposes because they contain insufficient bony structures. Therefore, additional rectangular fields are frequently applied for setup verification purposes. It is the aim of this study to reduce the dose distortions induced by these extra fields by appropriately adjusting the beam weights and wedge angles of the treatment fields. METHODS AND MATERIALS: A second treatment plan for the setup verification session is generated, with an identical beam setup as the original plan, but which also includes two orthogonal setup verification fields. An algorithm has been developed, based on vector analysis methods, that adjusts the beam weights and wedge angles of the treatment fields in such a way that both the dose at the isocenter and the dose homogeneity over the planning target volume (PTV) are conserved. RESULTS: The algorithm has been applied to three clinical cases. The number of MUs for the setup verification fields, using a liquid-filled electronic portal imaging device, varied between 16 MU in the head and neck region up to 34 MU for lateral images in the pelvic region. In all cases, the method yielded a treatment plan including two orthogonal setup verification fields with a similar dose distribution over the PTV as the original treatment plan without the setup verification fields. CONCLUSION: The dose distortions resulting from the acquisition of orthogonal verification imaging can be neutralized by modifying the original beam weights and wedge angles of the treatment fields.

Algorithms↗
PubMed1999

Dose-wall histograms and normalized dose-surface histograms for the rectum: a new method to analyze the dose distribution over the rectum in conformal radiotherapy.

PURPOSE: To develop an accurate method to generate a dose-volume histogram (DVH) of the rectum wall, solely based on the outer contours of the rectum wall. METHODS AND MATERIALS: A mathematical model for the rectum wall is developed, incorporating the stretching of the rectum wall due to variable rectal filling and neighboring structures. The model is based on the assumption that the amount of intersected rectum wall tissue normal to the central axis of the rectum is constant. The main objective of the model is to determine the thickness of the rectum wall in each wall element. Two approaches are described, each yielding a DVH of the rectum wall, based only on the delineated outer contours of the rectum. In the first approach, the model is used to create a set of inner contours out of the axial outer contours. Both sets of contours are used to derive a dose-wall histogram (DWH) of the rectum. In the second approach, the model is used to generate a normalized 2D sampling space, which is subsequently binned into a normalized dose-surface histogram (NDSH). The model is verified using 20 sets of CT data (5 patients x 4 scans) in which both outer and inner contours of the rectum are carefully delineated. The DWHs and NDSHs are compared with DVHs of the rectum wall, which require contouring of the outer and inner surfaces of the rectum wall, and with DVHs of the total rectum (including rectal filling). The variation between DWHs, NDSHs, and DVHs is investigated using normal tissue complication probability (NTCP) calculations. RESULTS: The local wall thickness of the rectum as outlined on CT data was in conformity with the described rectum model. The amount of rectum wall tissue per unit length rectum varied considerably between patients (27%, 1 SD). In all analyzed patients, the DWHs and NDSHs corresponded well to the DVHs of the rectum wall. Much more discrepancies were observed between the DVHs of the total rectum and the DVHs of the rectum wall. CONCLUSION: The applied methods yield accurate dose distributions of the rectum wall, without delineating the inner surface of the rectum. This reduces both the workload and variations due to inaccurate delineation of the rectum wall. The DWH and NDSH are effective tools to evaluate 3D dose distributions of the rectum wall and to estimate the complication probability of the rectum in high-dose conformal radiotherapy.

Humans↗
PubMed1999

Comparison of prostate cancer treatment in two institutions: a quality control study.

PURPOSE: To minimize differences in the treatment planning procedure between two institutions within the context of a radiotherapy prostate cancer trial. PATIENTS AND METHODS: Twenty-two patients with N0 M0 prostate cancer underwent a computed tomography (CT) scan for radiotherapy treatment planning. For all patients, the tumor and organs at risk were delineated, and a treatment plan was generated for a three-field technique giving a dose of 78 Gy to the target volume. Ten of the 22 cases were delineated and planned in the other institution as well. The delineated volumes and dose distributions were compared. RESULTS: All treatments fulfilled the trial criteria. The mean volume ratio of the gross tumor volumes (GTVs) in both institutions was 1.01, while the mean volume ratio of the planning target volumes (PTVs) was 0.88. The three-dimensional (3D) PTV difference was 3 mm at the prostate apex and 6-8 mm at the seminal vesicles. This PTV difference was mainly caused by a difference in the method of 3D expansion, and disappeared when applying an improved algorithm in one institution. The treated volume (dose > or =95% of isocenter dose) reflects the size of the PTV and the conformity of the treatment technique. This volume was on average 66 cm3 smaller in institution A than in institution B; the effect of the PTV difference was 31 cm3 and the difference in technique accounted for 36 cm3. The mean delineated rectal volume including filling was 112 cm3 and 125 cm3 for institution A and B, respectively. This difference had a significant impact on the relative dose volume histogram (DVH) of the rectum. CONCLUSION: Differences in GTV delineation were small and comparable to earlier quantified differences between observers in one institution. Different expansion methods for generation of the PTV significantly influenced the amount of irradiated tissue. Strict definitions of target and normal structures are mandatory for reliable trial results.

Humans↗
PubMed1999

In vivo dosimetry during conformal radiotherapy: requirements for and findings of a routine procedure.

PURPOSE: Conformal radiotherapy requires accurate knowledge of the actual dose delivered to a patient. The impact of routine in vivo dosimetry, including its special requirements, clinical findings and resources, has been analysed for three conformal treatment techniques to evaluate its usefulness in daily clinical practice. MATERIALS AND METHODS: Based on pilot studies, routine in vivo dosimetry quality control (QC) protocols were implemented in the clinic. Entrance and exit diode dose measurements have been performed during two treatment sessions for 378 patients having prostate, bladder and parotid gland tumours. Dose calculations were performed with a CT-based three-dimensional treatment planning system. In our QC-protocol we applied action levels of 2.5% for the prostate and bladder tumour group and 4.0% for the parotid gland patients. When the difference between the measured dose at the dose specification point and the prescribed dose exceeded the action level the deviation was investigated and the number of monitor units (MUs) adjusted. Since an accurate dose measurement was necessary, some properties of the on-line high-precision diode measurement system and the long-term change in sensitivity of the diodes were investigated in detail. RESULTS: The sensitivity of all diodes decreased by approximately 7% after receiving an integrated dose of 10 kGy, for 4 and 8 MV beams. For 34 (9%) patients the difference between the measured and calculated dose was larger than the action level. Systematic errors in the use of a new software release of the monitor unit calculation program, limitations of the dose calculation algorithms, errors in the planning procedure and instability in the performance of the accelerator have been detected. CONCLUSIONS: Accurate in vivo dosimetry, using a diode measurement system, is a powerful tool to trace dosimetric errors during conformal radiotherapy in the range of 2.5-10%, provided that the system is carefully calibrated. The implementation of an intensive in vivo dosimetry programme requires additional staff for measurements and evaluation. The patient measurements add only a few minutes to the total treatment time per patient and guarantee an accurate dose delivery, which is a prerequisite for conformal radiotherapy.

Humans↗
PubMed1998

Consistency in quality control programmes for electron accelerators in radiotherapy centres.

BACKGROUND AND PURPOSE: To gain insight into the current practice of quality control (QC) of medical electron accelerators and to reduce possible variations in test frequencies and test procedures. MATERIALS AND METHODS: An extensive questionnaire on QC procedures of medical electron accelerators was distributed and completed by all (21) radiotherapy institutions in The Netherlands. The questions were related to safety systems, mechanical parameters, beam profiles, beam energy, absolute dosimetry, wedge filters, the dose monitor system and radiation leakage. The data of the questionnaire were compared with recommendations given in national and international reports on QC of electron accelerators. RESULTS: Large variations in time spent on QC exist, especially for accelerators having dual energy photon beams and several electron beam energies. This diversity is mainly due to differences in philosophy with regard to QC and the differences in resources and machine time available. Furthermore, large variations in test frequencies and test methodologies were observed. The staffing level involved in the QC measurements was evaluated and compared with recent recommendations provided by EFOMP-ESTRO. CONCLUSIONS: From these recommendations and the results of the questionnaire, a set of minimum guidelines for a QC programme could be formulated and implemented in all radiotherapy institutions in The Netherlands.

Guidelines as Topic↗
PubMed1997

Flexible (Polyactive) versus rigid (hydroxyapatite) dental implants.

In a beagle dog study, the peri-implant bone changes around flexible (Polyactive) and rigid hydroxyapatite (HA) implants were investigated radiographically by quantitative digital subtraction analysis and by assessment of marginal bone height, with the aid of a computerized method. A loss of approximately 1 mm of marginal bone height was observed for both the dense Polyactive and the HA implants, after 6 months of loading. This value appeared to be stable from 12 weeks of loading onward. Along the total length of the implant during the first 6 weeks of loading, both the flexible (dense Polyactive) and the rigid (HA) implants showed a decrease in density. However, after this 6-week period, the bone density around the implants increased, and after 18 weeks the original bone density was reached. The flexible Polyactive implants provoked less decrease in density than the rigid HA implants, although not to a statistically significant level. This finding sustains the hypothesis that flexible implant materials may transfer stresses to the surrounding bone more favorably.

Alveolar Bone Loss↗
PubMed1997

A comparative study of flexible (Polyactive) versus rigid (hydroxylapatite) permucosal dental implants. II. Histological aspects.

Of the many materials that are applied in dental implantology, most have a high Young's modulus. In the concept of osseointegration, which is generally preferred, chewing forces are therefore directly transmitted from rigid implant materials to the relatively flexible surrounding bone. Polyactive is an elastomeric polyethylene-oxide polybutylene-terephthalate (PEO:PBT) copolymer, with a low modulus of elasticity, that exhibits bone-bonding characteristics. Previous finite-element analysis emphasized the benefit of the application of flexible implant materials. To assess the validity of this finite element model, one dense and two porous types of flexible Polyactive permucosal dental implants and one rigid hydroxylapatite (HA) implant were clinically tested in a beagle dog study and were compared during 30 weeks of loading. In an earlier report, it was concluded that dense Polyactive implants function clinically adequately and resemble the mobility of natural teeth. In the current study, the amount of bone contact was quantitatively assessed. Polyactive implants showed a statistically significantly higher bone contact, as compared to the HA implants, probably due to a significant water uptake and a subsequent increase in the volume of the denser Polyactive. Morphological analysis, based on back-scatter electron microscopy and light microscopy, revealed little remodelling activity of the bone surrounding the Polyactive implants. Few remodelling lacunae were seen and de novo bone formation was rarely observed. At some locations, a continuity between calcification zones with the polymer surface and the surrounding interfacial bone was observed, indicating the occurrence of bone-bonding. Also, the bone surrounding the HA implants showed little remodelling activity, although large resorption lacunae were observed, in which HA particles were present. Based on the observations of this study, flexible bone bonding implants might be more capable of transferring stresses to the surrounding bone and are therefore promising alternatives to "routine' rigid implants.

Alveolar Process↗
PubMed1997

A comparative study of flexible (Polyactive) versus rigid (hydroxylapatite) permucosal dental implants. I. Clinical aspects.

Recently, an elastomeric polyethyleneoxide polybutylene-terephthalate (PEO:PBT) copolymer (Polyactive) was introduced, which exhibits bone-bonding characteristics. In contrast to ceramics, bioglass, titanium and other metals, PEO:PBT copolymers are flexible materials that could reduce undesirable peak stresses along the neck of a permucosal implant. The application of three types of Polyactive permucosal dental implants (one dense and two porous types) and one dense hydroxylapatite (HA) implant were clinically tested and compared during 30 weeks of loading in a dog experiment. With respect to the porous Polyactive implants, it was observed that the pore diameter had decreased to such an extent that optimal bone ingrowth was not achieved. Polyactive is known to expand due to water uptake, and, as a consequence, the sizes of the pores of the press-fit inserted implants had decreased to a large extent. This feature explained the high loss of the implants with the porous outer layer (6 out of 22). None of the dense Polyactive implants and only one of the HA implants were lost. Statistical analysis was performed solely on the dense Polyactive and the HA implants. At the lingual sites, plaque scores, gingiva indices, bleeding indices and corresponding pocket depths were statistically significantly lower for both implant types, when compared to the corresponding values at the buccal, mesial and distal sites, irrespective of the observation period. Differences between the dense Polyactive and the HA implants were not observed. A considerable difference in mobility was registered between the Polyactive and the hydroxylapatite implants, as measured by the Periotest. The dense Polyactive implants functioned adequately and had mobility resembling natural teeth. As such, these dense flexible materials showed a favourable clinical function and they seem promising for reducing undesired peak stresses when applied as a dental implant.

Analysis of Variance↗