Scintigraphic studies to evaluate stability of ceramics (hydroxyapatite) in bone replacement

In this study, the suitability of a radionuclide bone imaging technique was examined in cases of hydroxyapatite implants for segmental replacements of long bones. The radionuclide bone imaging technique, using [99mTc]MDP was applied to estimate osseous changes at the sites of the hydroxyapatite implants up to 2 yr after their implantation in an animal experimental model. The results were correlated with histological and radiographical findings at the same time. The radionuclide bone imaging with technetium proved to be a useful method to estimate the intensity of osseous changes after bone replacement by a hydroxyapatite implant. It proved to be more sensitive than radiography. The differences in elasticity of the implanted material and adjacent bone may cause an increase of radionuclide uptake at the site of the implant 2 yr after implantation. These osseous changes cannot be detected by standard histological and radiographical methods but could be detected by radionuclide bone imaging. It is submitted that the radionuclide bone imaging is a noninvasive, sensitive, and useful method for the estimation of osseous changes in sites of long term bone implants.     

The dose-rate effect in interstitial brachytherapy: a controversy resolved.

Low-dose-rate interstitial implants generally involve a range of dose rates varying from about 0.3 to 1.5 Gy/h. It has been a matter of some debate as to whether such a variation in dose rate has any impact on clinical results, i.e. whether implant treatment time needs to be taken into account when prescribing the treatment dose. In particular, Paterson and Ellis both published data based on clinical experience suggesting that the prescribed dose should vary with overall treatment time, whereas the Paris school suggested that isoeffect dose should not be varied for treatment times between 3 and 8 days. Experimental radiobiological data obtained in vitro and in vivo imply a significant change in biological effectiveness over this range of dose rates with a greater variation for normal-tissue late effects than for tumour control. We show, based on radiological considerations and model calculations, that the disagreement between the two schools in their recommendations for generating isoeffect doses for different treatment times was a result of two factors. First, the Paterson recommendation was unequivocally based on matching limiting late effects, whereas the Paris system recommendations were based on an analysis of a combination of late and early effects. Second, the Paterson recommendations were based on a dosimetry system in which the dose rate does not change greatly with tumour volume, whereas the Paris recommendations were based on a dosimetric system in which dose rate is correlated with tumour volume, which in turn is correlated with changes in both tumour control and, especially, necrosis.     

Antibody and radionuclide characteristics and the enhancement of the effectiveness of radioimmunotherapy by selective dose delivery to radiosensitive areas of tumour

PURPOSE: Estimating the absorbed dose to tumour relative to normal tissues has often been used in the assessment of the therapeutic efficacy of radiolabelled antibodies for radioimmunotherapy. Typically, the calculations assume a uniform dose deposition and response throughout the tumour. However, the heterogeneity of the dose delivery and response within tumours can lead to a radiobiological effect inconsistent with dose estimates. The aim was to assess the influence of antibody and radionuclide characteristics on the heterogeneity of dose deposition. MATERIALS AND METHODS: Quantitative images of the temporal and spatial heterogeneity of a range of antibodies in tumour were acquired using radioluminography. Subsequent registration with images of tumour morphology then allowed the delineation of viable and necrotic areas of tumour and the measurement of the antibody concentration in each area. A tumour dosimetry model then estimated the absorbed dose from 131I and 90Y in each area. RESULTS: Tumour-specific antibodies initially localized in the viable radiosensitive areas of tumour and then penetrated further into tumour with continued tumour accretion. Multivalent antibodies were retained longer and at higher concentrations in viable areas, while monovalent antibodies had greater mobility. In contrast, non-specific antibodies penetrated into necrotic regions regardless of their size. As a result, multivalent, specific antibodies delivered a significantly larger dose to viable cells compared with monovalent antibodies, while non-specific antibodies deposited most of the dose in necrotic areas. There was a significant difference in dose estimates when assuming a uniform dose deposition and accounting for heterogeneity. The dose to the viable and necrotic areas also depended on the properties of the radionuclide where antibodies labelled with 131I generally delivered a higher dose throughout the tumour even though the instantaneous dose-rate distribution for 90Y was more uniform. CONCLUSIONS: The extent of heterogeneity of dose deposition in tumour is highly dependent on the antibody characteristics and radionuclide properties, and can enhance therapeutic efficacy through the selective dose delivery to the radiosensitive areas of tumour.     

Antibody and radionuclide characteristics and the enhancement of the effectiveness of radioimmunotherapy by selective dose delivery to radiosensitive areas of tumour

Purpose : Estimating the absorbed dose to tumour relative to normal tissues has often been used in the assessment of the therapeutic efficacy of radiolabelled antibodies for radioimmunotherapy. Typically, the calculations assume a uniform dose deposition and response throughout the tumour. However, the heterogeneity of the dose delivery and response within tumours can lead to a radiobiological effect inconsistent with dose estimates. The aim was to assess the influence of antibody and radionuclide characteristics on the heterogeneity of dose deposition. Materials and methods : Quantitative images of the temporal and spatial heterogeneity of a range of antibodies in tumour were acquired using radioluminography. Subsequent registration with images of tumour morphology then allowed the delineation of viable and necrotic areas of tumour and the measurement of the antibody concentration in each area. A tumour dosimetry model then estimated the absorbed dose from 131 I and 90 Y in each area. Results : Tumour-specific antibodies initially localized in the viable radiosensitive areas of tumour and then penetrated further into tumour with continued tumour accretion. Multivalent antibodies were retained longer and at higher concentrations in viable areas, while monovalent antibodies had greater mobility. In contrast, non-specific antibodies penetrated into necrotic regions regardless of their size. As a result, multivalent, specific antibodies delivered a significantly larger dose to viable cells compared with monovalent antibodies, while non-specific antibodies deposited most of the dose in necrotic areas. There was a significant difference in dose estimates when assuming a unifrom dose deposition and accounting for heterogeneity. The dose to the viable and necrotic areas also depended on the properties of the radionuclide where antibodies labelled with 131 I generally delivered a higher dose throughout the tumour even though the instantaneous dose-rate distribution for 90 Y was more uniform. Conclusions : The extent of heterogeneity of dose deposition in tumour is highly dependent on the antibody characteristics and radionuclide properties, and can enhance therapeutic efficacy through the selective dose delivery to the radiosensitive areas of tumour.     

金纳米簇放射治疗增敏作用的研究进展

目前恶性肿瘤的放射治疗疗效仍欠满意,放疗增敏剂是提高放疗疗效的有效手段。金纳米材料因其高原子序数可有效增加肿瘤细胞的放疗敏感性。金纳米簇因其更小的尺寸有更加优良的放射生物学、放射物理学特性。本文综述了金纳米簇特殊的放射生物学、放射物理学特性,并详细地介绍了其对外照射放疗、放射性核素治疗、X射线诱导的光动力治疗的增敏作用。     

Pulsed brachytherapy: a modelled consideration of repair parameter uncertainties and their influence on treatment duration extension and daytime-only "block-schemes"

OBJECTIVES: The radiobiological modelling of all types of protracted brachytherapy is susceptible to uncertainties in the values of tissue repair parameters. Although this effect has been explored for many aspects of pulsed brachytherapy (PB), it is usually considered within the constraint of a fixed brachytherapy treatment time. Here the impact of repair parameter uncertainty is assessed for PB treatments of variable duration. The potential use of "block-schemes" (blocks of PB pulses separated by night-time gaps) is also investigated. METHODS: PB schedule constraints are based on the cervical cancer protocols of the Royal Marsden Hospital (RMH), but the methodology is applicable to any combination of starting schedule and treatment constraint. Calculations are performed using the biologically effective dose (BED) as a tissue-specific comparison metric. The ratio of normal tissue BED to tumour BED is considered for PB regimens with varying total pulse numbers and/or "block-schemes". Results: For matched brachytherapy duration, PB has a good "window of opportunity" relative to the existing RMH continuous low dose rate (CLDR) practice for all modelled repair half-times. The most clear-cut route to radiobiological optimisation of PB is via modest temporal extension of the PB regimen relative to the CLDR reference. This option may be practicable for those centres with scope to extend their relatively short CLDR treatment durations. Conclusion: Although daytime-only "block-scheme" PB for cervical cancer has not yet been employed clinically, the possibilities appear to be theoretically promising, providing the overall (external beam plus brachytherapy) treatment duration is not extended relative to current practice, such that additional tumour repopulation becomes a concern.     

Application of the linear-quadratic model with incomplete repair to radionuclide directed therapy

The linear-quadratic (LQ) model for fractionated external beam therapy has been modified by previous authors to include the effects due to an exponentially decaying dose rate. However, the LQ model has now been extended to include a general time varying dose rate profile, and the equations can be readily evaluated if an exponential radiation damage repair process is assumed. These equations are applicable to radionuclide directed therapy, including brachytherapy. Kinetic uptake data obtained during radionuclide directed therapy may therefore be used to determine the radiobiological dosimetry of the target and non-target tissues. Also, preliminary tracer studies may be used to pre-plan the radionuclide directed therapy, provided that tracer and therapeutic amounts of the radionuclide carrier are identically processed by the tissues. It is also shown that continuous radionuclide therapy will induce less damage in late-responding tissues than 2 Gy/fraction external beam therapy if the ratio of the maximum dose rate and the sublethal damage repair half-life in the tissue is less than 1.0 Gy. Similar inequalities may be derived for beta-particle radionuclide directed therapy. For example, it can be shown that radionuclide directed therapy will induce less damage to slowly repopulating tissue than 2 Gy/fraction external beam therapy for the same total dose if the maximum percentage initial uptake in tissue is less than 0.046%/g or 0.23%/g for an injected activity of 50 mCi of 90Y or 131I, respectively.     

The 4Rs in radiation therapy

It has been realized that the 4Rs (repair, repopulation, redistribution, and reoxygenation) would affect the result of cell irradiation, and thus radiation treatment. The 4Rs each occurs at different dose rates, usually very low dose rates. Depending on the dose rate used for treatment, the corresponding R should be included in the linear-quadratic equation (LQ) and biological effective dose (BED) calculation. For low dose rate brachytherapy(LDR) especially permanent implant, all the 4Rs should be included in LQ for BED calculation. The 4Rs, especially repair and repopulation, play a critical role in dose fractionation. Various dose fractionation schemes such as hyperfractionation and hypofractionation are determined in consideration of the 4Rs. Stereotactic radiation therapy uses hypofractionation with high fractional doses and combine with high accuracy target localization techniques to achieve high local control rates compared to conventional dose fractionation schemes. The 4Rs have been taken into account for LDR and permanent implant. Recently, LQ for permanent implant brachytherapy has been modified to include all the 4Rs for gynecological malignancy ¹³¹Cs permanent implants. Including the 4Rs in radiation therapy has significantly improved the effectiveness and efficiency of radiation therapy for cancer treatment.     

3D dosimetry in patients with early breast cancer undergoing Intraoperative Avidination for Radionuclide Therapy (IART?) combined with external beam radiation therapy

Purpose

Intraoperative Avidination for Radionuclide Therapy (IART?) is a novel targeted radionuclide therapy recently used in patients with early breast cancer. It is a radionuclide approach with ⁹⁰Y-biotin combined with external beam radiotherapy (EBRT) to release a boost of radiation in the tumour bed. Two previous clinical trials using dosimetry based on the calculation of mean absorbed dose values with the hypothesis of uniform activity distribution (MIRD 16 method) assessed the feasibility and safety of IART?. In the present retrospective study, a voxel dosimetry analysis was performed to investigate heterogeneity in distribution of the absorbed dose. The aim of this work was to compare dosimetric and radiobiological evaluations derived from average absorbed dose vs. voxel absorbed dose approaches.

Methods

We evaluated 14 patients who were injected with avidin into the tumour bed after conservative surgery and 1?day later received an intravenous injection of 3.7?GBq of ⁹⁰Y-biotin (together with 185?MBq ¹¹¹In-biotin for imaging). Sequential images were used to estimate the absorbed dose in the target region according to the standard dosimetry method (SDM) and the voxel dosimetry method (VDM). The biologically effective dose (BED) distribution was also evaluated. Dose/volume and BED volume histograms were generated to derive equivalent uniform BED (EUBED) and equivalent uniform dose (EUD) values.

Results

No “cold spots” were highlighted by voxel dosimetry. The median absorbed-dose in the target region was 20?Gy (range 15–27?Gy) by SDM, and the median EUD was 20.4?Gy (range 16.5–29.4?Gy) by the VDM; SDM and VDM estimates differed by about 6?%. The EUD/mean voxel absorbed dose ratio was >0.9 in all patients, indicative of acceptable uniformity in the target. The median BED and EUBED values were 21.8?Gy (range 15.9–29.3?Gy) and 22.8?Gy (range 17.3–31.8?Gy), respectively.

Conclusion

VDM highlighted the absence of significant heterogeneity in absorbed dose in the target. The EUD/mean absorbed dose ratio indicated a biological efficacy comparable to that of uniform distribution of absorbed dose. The VDM is recommended for improving accuracy, taking into account actual activity distribution in the target region. The radiobiological model applied allowed us to compare the effects of IART? with those of EBRT and to match the two irradiation modalities.     

Magnetresonanztomographie bei Patienten mit magnetversorgten Hörimplantaten

Background

Every year in Germany approximately 3,500 patients receive a cochlear implant or other hearing implants with an implantable magnet. At the same time more and more patients are examined by magnetic resonance imaging (MRI). For the indications and execution of this imaging modality a number of restrictions and safety measures have to be considered.

Methods

This article is based on the restrictions of the manufacturers and a selective literature search in PubMed using the following keywords: MRI compatibility/MRI safety + cochlea implant/auditory brainstem implant/Bonebridge/Carina/Esteem/Otomag/Sophono alpha/Vibrand Soundbridge. We included all 20 publications of this search concerning the MRI compatibility of the hearing implants complemented by papers cited in the primary articles.

Results

High electromagnetic field intensities as used in MRI can cause malfunction and dislocation of the implant or the magnet in the device. Older cochlear implants (CI) and the current CIs produced by Advanced bionics without explantation of the magnet, some CI models produced by the company Cochlear and the middle ear implants Carina®/Esteem® (older models) and Vibrant-Soundbridge® are not approved for MRI examinations. Other hearing prostheses are approved for 0.2 T, 1.0 T or 1.5 T MRI and in exceptional circumstances 3 T MRI. Recommendations of the manufacturers have to be followed, notably wearing a head bandage during the imaging procedure. The longitudinal axis of the patient’s head has to be to positioned parallel to the main magnetic field of the scanner. The patient may not move the head laterally during the examination. Possible artefacts and the reduced validity of the results of skull MRI have to be considered when evaluating the indications for the examination.

Conclusion

For patients wearing hearing implants with an implantable magnet the indications for MRI in devices with MRI certification should be rigorously restricted. Possible defects/dislocation of the implants may occur and the quality of the skull MRI images is reduced. A close contact between the radiologist and the implanting team is required. Other diagnostic procedure options should be exhausted before employing MRI.     

Weiss lecture. The dose-rate factor in radiation biology.

The dose-rate effect has been the topic of extensive radiobiological studies and has important implications in radiation therapy and in the field of radiation protection. Three examples will be discussed: two in radiation therapy and one in protection. First, continuous low dose-rate interstitial brachytherapy may be replaced by pulsed brachytherapy, using a single source moving through the catheters of the implant. This strategy, using a modern computer-controlled afterloading device, would allow better dose optimization and result in a considerable cost saving. Radiobiological data have proved useful in defining the pulse length and pulse frequency that is equivalent to continuous low dose-rate. Second, in the intracavitary treatment of carcinoma of the cervix, a few high dose-rate (HDR) fractions on an outpatients basis can replace the low dose-rate (LDR) treatment that requires the patient to be hospitalized for several days. Radiobiological data can be used to estimate the dose levels at HDR that are equivalent to conventional LDR protocols. Third, it is usually assumed in radiation protection that doses accumulated over a period of time at low dose-rate are less effective biologically than the same dose delivered in a single acute exposure. While this may be true for X- or gamma-rays, radiobiological data indicate that neutrons delivered at low dose or in a series of fractions spread out over a period of time may produce more oncogenic transformation than a single acute exposure. This has important implications in radiation protection.     

The potential of ytterbium 169 in brachytherapy: a brief physical and radiobiological assessment.

Ytterbium 169 (half-life 32 days; mean gamma emission 93 keV, after excluding photons of energy less than 10 keV) is a radionuclide with interesting potential for brachytherapy applications. Although not yet commercially available, its possible application as a clinical radionuclide is currently being considered by Amersham International. This article presents an assessment of some properties of the nuclide that may be clinically relevant. Use is made of some new ideas that allow quantification of the likely dose homogeneity that can be obtained in a brachytherapy distribution, and in this context ytterbium 169 is shown to be superior to some currently available brachytherapy nuclides. The assessment also uses recent extensions to the linear-quadratic model to consider the likely radiobiological implications associated with the use of the nuclide. From this it is suggested that the main potential for ytterbium 169 would be as a source that may be re-used for a number of short-term applications, rather than as a permanently implantable nuclide.     

Radiobiological compensation for unintended treatment interruptions during palliative radiotherapy

Unlike radical treatment protocols, in which radiobiological methods have been used in an attempt to overcome the risk of reduced tumour control, the problem of compensation for unintended treatment interruptions during palliative radiotherapy has received little attention. For palliative radiotherapy, unnecessarily extended treatment times could theoretically reduce the duration of tumour regression and symptomatic relief. It can be shown, using a simple argument, that the overall extension of the treatment time is likely to be at least equal to the reduced duration of benefit. In most practical instances, this duration would amount to relatively few days, but it can sometimes be as long as 1-2 weeks. The mechanisms for gap compensations are the same as for radical radiotherapy, although there is greater scope for hypo-fractionated compensation provided that tissue tolerances are respected. It is debatable whether compensation should be applied in all patients, but there might be clinical situations where this would be indicated. Such decisions might influence waiting times for other patients requiring radical radiotherapy, and therefore must be balanced against the available resources.     

Combined external and interstitial radiotherapy of vocal cord carcinoma

Since 1984 an Ir-192 source with a high dose rate has been used for interstitial implants, and since 1986 in the treatment of the tumor-bearing vocal cord in the organ-preserving management of larynx carcinoma. The combined percutaneous and interstitial treatment has been administered either as the primary treatment or after incomplete removal of the tumour. So fat 16 patients have been treated, two of them presented with tumours on both cords. All patients refused radical surgical interventions, four of them refused cord stripping too. The treatment method included external radiotherapy with a dose of 4600 to 5000 cGy to the larynx. One to two weeks after external XRT an interstitial implant into the vocal cord followed. Using two needles per cord, a boost dose of 1000 cGy was given to the tumour area. The median follow-up time is 21.3 months (range five to 49, calculated October 1990). So far no local or regional failures occurred. None of the patients had intra- or postoperative complications. All patients have preserved their voice. No severe late effects could be observed. The number of patients is very low, but the preservation of voice is high psychosocial value.     

Magnification rate of digital panoramic radiographs and its effectiveness for pre-operative assessment of dental implants

Objectives

The purpose of this study was to determine the accuracy and effectiveness of digital panoramic radiographs for pre-operative assessment of dental implants.

Methods

We selected 86 patients (221 implants) and calculated the length of the planned implant based on the distance between a selection of critical anatomical structures and the alveolar crest using the scaling tools provided in the digital panoramic system. We analysed the magnification rate and the difference between the actual inserted implant length and planned implant length according to the location of the implant placement and the clarity of anatomical structures seen in the panoramic radiographs.

Results

There was no significant difference between the planned implant length and actual inserted implant length (P > 0.05). The magnification rate of the width and length of the inserted implants, seen in the digital panoramic radiographs, was 127.28 ± 13.47% and 128.22 ± 4.17%, respectively. The magnification rate of the implant width was largest in the mandibular anterior part and there was a significant difference in the magnification rate of the length of implants between the maxilla and the mandible (P < 0.05). When the clarity of anatomical structures seen in the panoramic radiographs is low, the magnification rate of the width of the inserted implants is significantly higher (P < 0.05), but there is no significant difference between the planned implant length and actual inserted implant length according to the clarity of anatomical structures (P < 0.05).

Conclusions

Digital panoramic radiography can be considered a simple, readily available and considerably accurate pre-operative assessment tool in the vertical dimension for dental implant therapy.     

Dual-energy CT for the evaluation of silicone breast implants

Objective

The evaluation of breast implants for rupture is currently the domain of ultrasound and MRI, while mammography is of very limited diagnostic value. Recently, specific visualisation of silicone has become feasible using dual-energy CT. Our objective was to evaluate whether it is feasible to identify silicone in breast implants by dual-energy CT and to reliably diagnose or rule out ruptures.

Methods

Seven silicone breast implant specimens were examined on dual-source CT at 100- and 140-kV tube potential with a 0.8-mm tin filter (collimation 128 × 0.6 mm, current–time products 165 and 140 mAsref with modulation, rotation time 0.28 s, pitch 0.55). Two patients scheduled for implant removal or replacement were examined with identical parameters.

Results

The silicone of the implant specimens showed a strong dual-energy signal. In one patient, both implants were intact, while a rupture was identified in the other patient. Ultrasound, MRI, surgical findings and histology confirmed the dual-energy CT diagnosis.

Conclusion

Dual-energy CT may serve as an alternative technique for speedy evaluation of silicone breast implants. Specific clinical studies are required to determine the diagnostic accuracy and define indications for this technique.

Key Points

? Dual-energy CT makes it possible to visualise silicone in breast implants. ? Silicone provides a strong photoelectric effect that can be detected. ? Initial experience suggests that implant ruptures can be identified or ruled out.     

Technical and dosimetric aspects of iodine-125 seed reimplantation in suboptimal prostate implants

Objective:

Brachytherapy employing iodine-125 seeds is an established treatment for low-risk prostate cancers. Post-implant dosimetry (PID) is an important tool for identifying suboptimal implants. The aim of this work was to improve suboptimal implants by a subsequent iodine-125 seed top-up (reimplantation), based on the PID results.

Methods:

Of 255 patients treated between 2009 and 2012, 6 were identified as having received suboptimal implants and were scheduled for seed top-up. Needle configurations and the number of top-up seeds were determined based on post-implant CT images as well as a reimplantation treatment plan. An average of 14 seeds per patient were implanted during each top-up. Dosimetric outcome was assessed via target parameters and doses received by organs at risk.

Results:

All six patients had a successful top-up, with a 67% increase in the mean dose delivered to 90% of the prostate volume and a 40% increase in the volume that receives 100% of the prescribed dose. However, the final dosimetric assessment was based on the same seed activity, as the planning system does not account for the decay of the initially implanted seeds. Although physical dosimetry is not influenced by different seed activities (doses are calculated to infinity), the radiobiological implications might be slightly different from the situation when optimal implantation is achieved with one treatment only.

Conclusion:

Seed reimplantation in suboptimal prostate implants is feasible and leads to successful clinical outcomes.

Advances in knowledge:

Suboptimal prostate implants can occur for various reasons. This work shows that seed reimplantation as salvage therapy can lead to an optimal dosimetric outcome with manageable normal tissue effects.Low dose rate (LDR) brachytherapy employing radioactive seeds is a well-established treatment for low-risk prostate cancers. In our centre, implants are conducted with iodine-125 seeds (Oncura RAPID Strand, model 6711; Oncura Inc., Arlington Heights, IL) with an average seed activity of 0.395 mCi to deliver a prescribed dose of 145 Gy (to >98% of the prostate). Treatment planning and post-implant dosimetry (PID) are completed using SPOT-PRO™ v. 3.1 (Nucletron, Utrecht, Netherlands) software based on transrectal ultrasound images (for treatment planning) and CT images (for PID).The most commonly reported parameters that are indicative of the dosimetric quality of the implant are D₉₀ (the dose delivered to 90% of the prostate volume) and V₁₀₀ (the volume that receives 100% of the prescribed dose). Several studies showed a link between the quality of implants and the biochemical outcome [1–3]. Therefore, to minimise the risk of recurrence, it is recommended to achieve a post-implant D₉₀>140 Gy and V₁₀₀>90% [1].PID is an important quantitative tool for the assessment of LDR implants; therefore, it is recommended as a routine procedure by several professional organisations [2–5]. Besides evaluating the overall quality of the implant, PID can assist in the dosimetric assessment of the organs at risk (OARs). Although the dosimetry of OARs cannot be adjusted if overdosed, the radiation oncologist can have a closer follow-up of those patients at risk of developing normal tissue sequelae.Another role of PID is to identify suboptimal implants that can arise owing to organ movement during the procedure, geographical misses of seeds or technical equipment errors. Despite all the efforts and experience of the brachytherapy team, suboptimal implants do occur and they have to be dealt with. Although several centres encounter such events, there is a lack of guidelines or even indications as to how to proceed to improve the final outcome. The major challenge is perhaps the planning, which cannot be done in a conventional way, i.e. based on the ultrasound study of the transrectal volume, owing to lack of previously implanted seed visibility [6]. Therefore, post-implant CT images are the most convenient to use for this task, as the original seeds can be seen and extra seeds can be added to cover the underdosed areas of the prostate.The aim of this work was to present our experience with iodine-125 seed reimplantation (top-up) in a cohort of six patients whose initial implant was suboptimal as identified by PID. The technical and dosimetric challenges of seed top-up implants are investigated.     

Imaging breasts with silicone implants

Over the last two decades, the use of breast implants both for breast augmentation and for breast reconstruction following mastectomy has increased substantially. It is estimated that around two million women have undergone breast augmentation, while hundreds of thousands have had breast reconstruction surgery. Different types of material have been used for breast implants, but silicone gel implants have been the dominating implant type. Many implants can lead to complications, such as hardening and rupture, and may therefore need in vivo evaluation by imaging, particularly if they lead to clinical symptoms. They can also pose problems in the assessment of surrounding breast tissue by conventional mammography. In this respect, imaging modalities such as ultrasound, computed tomography and magnetic resonance imaging offer greater possibilities to assess a failing implant, as well as surrounding breast tissue. Several factors, mainly of a psychological nature, lead to requests for breast implants. In this review article, only the imaging aspects of breasts with silicone gel implants will be dealt with. Each modality is concisely presented with its possibilities and limitations. Received: 5 February 1998; Revision received: 18 May 1998; Accepted: 25 May 1998     

Weiss Lecture

Summary

The dose-rate effect has been the topic of extensive radiobiological studies and has important implications in radiation therapy and in the field of radiation protection. Three examples will be discussed: two in radiation therapy and one in protection. First, continuous low dose-rate interstitial brachytherapy may be replaced by pulsed brachytherapy, using a single source moving through the catheters of the implant. This strategy, using a modern computer-controlled afterloading device, would allow better dose optimization and result in a considerable cost saving. Radiobiological data have proved useful in defining the pulse length and pulse frequency that is equivalent to continuous low dose-rate. Second, in the intracavitary treatment of carcinoma of the cervix, a few high dose-rate (HDR) fractions on an outpatients basis can replace the low dose-rate (LDR) treatment that requires the patient to be hospitalized for several days. Radiobiological data can be used to estimate the dose levels at HDR that are equivalent to conventional LDR protocols. Third, it is usually assumed in radiation protection that doses accumulated over a period of time at low dose-rate are less effective biologically than the same dose delivered in a single acute exposure. While this may be true for X- or γ-rays, radiobiological data indicate that neutrons delivered at low dose or in a series of fractions spread out over a period of time may produce more oncogenic transformation than a single acute exposure. This has important implications in radiation protection.