Dosimetry Planning Research Articles

Preventing radiation retinopathy with hyperfractionation

The purpose of this study was to determine factors associated with the development of radiation retinopathy in a large series of patients with head and neck cancer. In particular, we addressed whether the use of hyperfractionated radiation therapy was effective in reducing the risk of retinopathy. One hundred eighty-six patients received a significant dose to the retina as part of curative radiotherapy. Primary sites were: nasopharyngeal, 46; paranasal sinus tumors, 64; nasal cavity, 69; and palate, 7. Prescription doses varied depending on primary site and histology. Hyperfractionated (twice-daily) radiation was delivered to 42% of the patients in this study, typically at 110 to 120 cGy per fraction. The remainder were treated once-daily. Retinal doses were determined from computerized dosimetry plans when available. For all other patients, retinal doses were retrospectively calculated using reconstructed off-axis dosimetry taken from contours through the center of the globes. Retinal dose was defined as the minimum dose received by at least 25% of the globe. The median retinal dose was 5685 cGy (range). Patients were followed for a median of 7.6 years. Thirty-one eyes in 30 patients developed radiation retinopathy, resulting in monocular blindness in 25, bilateral blindness in one, and decreased visual acuity in four. The median time to the diagnosis of retinopathy was 2.6 years (range, 10 months to 5.3 years). The actuarial incidence of developing radiation retinopathy was 20% at both 5 and 10 years. The incidence of developing ipsilateral blindness due to retinopathy was 16% at 5 years and 17% at 10 years. Site-specific incidences varied significantly with ethmoid sinus (9 of 25, 36%), nasal cavity (13 of 69, 19%), and maxillary sinus (6 of 35, 17%) being the most common sites associated with radiation retinopathy. Three of seventy-two patients (4%) receiving retinal doses less than 5000 cGy developed retinopathy. Higher retinal doses resulted in a steady increase in the incidence of retinopathy, with twenty-five of the thirty cases occurring after 6000 cGy or more. Of the patients receiving more than 5000 cGy to the retina, hyperfractionation was associated with a significantly lower incidence of radiation retinopathy (55% vs. 15%; p = 0.0020). On multivariate analysis, retinal dose (p < 0.0001), fractionation schedule (p = 0.0005), age (p = 0.027), and overall treatment time (p = 0.028) were significant predictors of radiation retinopathy. The incidence of radiation retinopathy following treatment of nasal cavity paranasal tumors is 20% at five and ten years. Retinal dose and fractionation schedule are the strongest predictors of retinopathy. Hyperfractionated radiotherapy is associated with a significant reduction in the incidence of radiation retinopathy, especially when the retina receives more than 5000 cGy

Preventing radiation retinopathy with hyperfractionation

Preventing radiation retinopathy with hyperfractionation

Sequential evaluation of prostate edema following permanent seed prostate brachytherapy using CT-MRI fusion

Post implant prostate edema can have a profound effect on post plan dosimetry. We analyzed the extent and time course of prostate edema and its effect on dosimetry for 20 patients following permanent seed prostate brachytherapy.

Correlating the degree of needle trauma during prostate brachytherapy and the development of acute urinary toxicity

Purpose To determine if there is an association between the degree of prostate trauma during prostate brachytherapy and development of acute urinary toxicity. Methods and materials In a consecutive prospective cohort of permanent 125I prostate brachytherapy patients, the number of times each needle was repositioned was tracked, and the dosimetry plans were used to determine the number of times needles within 1 cm of the urethra were manipulated. Additionally, prostate volume, total number of needles, number of needles/prostate volume, and the number of periurethral needle manipulations/prostate volume were determined. The need for catheterization beyond 24 hours and the Radiation Therapy Oncology Group (RTOG) urinary toxicity score at 4 weeks were recorded. The independent samples t test was used to search for a correlation between these parameters and the recorded toxicity scores. Results Twenty-eight consecutive implant patients were evaluated in the study. Median (range) values were as follows: prostate volume 35 cc ( range, 15–51 cc), number of needles per patient 32 (range, 21–41), number of needle manipulations per patient 94.5 ( range, 55–147), and number of periurethral needle manipulations 42 (range, 17–65). The only significant association between urinary toxicity and these variables was for the number of periurethral needle manipulations ( p = 0.025). Conclusions These data provide evidence that needle prostate trauma during brachytherapy contributes to acute urinary toxicity.

The effect of patient inhomogeneities in oesophageal 192Ir HDR brachytherapy: a Monte Carlo and analytical dosimetry study

The effect of patient inhomogeneities surrounding the oesophagus on the dosimetry planning of an upper thoracic oesophageal 192Ir HDR brachytherapy treatment is studied. The MCNPX Monte Carlo code is used for dosimetry in a patient-equivalent phantom geometry and results are compared in terms of isodose contours as well as dose volume histograms with corresponding calculations by a contemporary treatment planning system software featuring a full TG-43 dose calculation algorithm (PLATO BPS version 14.2.4). It is found that the presence of patient inhomogeneities does not alter the delivery of the planned dose distribution to the planning treatment volume. Regarding the organs at risk, the common practice of current treatment planning systems (TPSs) to consider the patient geometry as a homogeneous water medium leads to a dose overestimation of up to 13% to the spinal cord and an underestimation of up to 15% to the sternum bone. These findings which correspond to the dose region of about 5–10% of the prescribed dose could only be of significance when brachytherapy is used as a boost to external beam therapy. Additionally, an analytical dosimetry model, which is efficient in calculating dose in mathematical phantoms containing inhomogeneity shells of materials of radiobiological interest, is utilized for dosimetry in the patient-equivalent inhomogeneous phantom geometry. Analytical calculations in this work are in good agreement with corresponding Monte Carlo results within the bone inhomogeneities of spinal cord and sternum bone but, like treatment planning system calculations, the model fails to predict the dose distribution in the proximal lung surface as well as within the lungs just as the TPS does, due to its inherent limitation in treating lateral scatter and backscatter radiation.

264 poster Quality of implant dosimetry for intraoperative versus preoperative planning for permanent prostate seed implants: are there differences in coverage of the base, mid-zone and apex of the prostate?

264 poster Quality of implant dosimetry for intraoperative versus preoperative planning for permanent prostate seed implants: are there differences in coverage of the base, mid-zone and apex of the prostate?

207 poster The benefits provided by using intra-operative dosimetry and the fusion of MRI to CT for post plan dosimetry in the learning curve of transperineal interstitial permanent prostate brachytherapy

207 poster The benefits provided by using intra-operative dosimetry and the fusion of MRI to CT for post plan dosimetry in the learning curve of transperineal interstitial permanent prostate brachytherapy

72 poster Gamma Knife radiosurgery for the treatment of low grade gliomas : integration of the metabolic data of positron emission tomography (PET) in the dosimetry planning

72 poster Gamma Knife radiosurgery for the treatment of low grade gliomas : integration of the metabolic data of positron emission tomography (PET) in the dosimetry planning

Linear accelerator–based radiosurgery for vestibular schwannoma

Despite major advances in skull base surgery and microsurgical techniques, surgery for vestibular schwannoma (VS) carries a risk of complications. Some are inherent to general anesthesia and surgery of any type and include myocardial infarction, pneumonia, pulmonary embolism, and infection. Some are specific to neurosurgery in this area of the brain, and include hydrocephalus, cerebrospinal fluid leak, facial nerve paralysis, facial numbness, hearing loss, ataxia, dysphagia, and major stroke. Even in the hands of very experienced acoustic surgeons, these risks cannot be eliminated. Radiosurgery provides an outpatient, noninvasive alternative for the treatment of small acoustic schwannomas. Initially radiosurgery was undertaken in "high-risk" patients, including the elderly, those with severe medical comorbidities, and those in whom tumors recurred after surgery. Additionally, a high rate of cranial nerve morbidity was reported. With improvements in dosimetry planning and dose selection, however, authors practicing at radiosurgical centers now report very low complication rates, as well as high tumor control rates. In this report the authors specifically review the results of linear accelerator-based radiosurgery for VS and compare these outcomes with the best surgical alternatives.

Intraoperative dosimetry planning does not improve the postoperative dose coverage after LDR - brachytherapy - a prospective clinical study

Intraoperative dosimetry planning does not improve the postoperative dose coverage after LDR - brachytherapy - a prospective clinical study

Shielding effects of metallic encapsulations and radiographic contrast agents for catheter-based intravascular brachytherapy

Purpose/Objective: Both photon- and beta-emitting radionuclides for intravascular brachytherapy (IVB) are under active investigation for prevention of restenosis following conventional angioplasty with or without stents. High atomic number materials are usually present in the coronary vessels undergoing treatment in the form of metallic encapsulations, stents, calcified plaque, or radiographic contrast agent. The high atomic number materials are likely to interfere with the photons and betas and, thus, change the dosimetry in the treatment volume. The purpose of this study is to investigate the shielding effects caused by the presence of high atomic number materials in IVB. Materials and Methods: Dose rates at various distances in water, with and without the presence of various high atomic number materials, were calculated using Monte Carlo simulation techniques for photon and electron transport in extended media. The high atomic number materials investigated included titanium, stainless steel, calcified plaque, Hypaque, and Omnipaque. A wide range of monoenergetic photon and electron sources and several photon- and beta-emitting radionuclides, which have been under consideration for IVB, were used. The energy of the monoenergetic photon sources was in the range from 10 keV to 1 MeV, and that of the monoenergetic electron sources in the range from 0.5 to 2 MeV. Photon-emitting radionuclides 192Ir, 125I, and 103Pd and beta-emitting radionuclides 90Y, 32P, and 188Re were also considered. Results: It was found that the high atomic number materials interfere considerably with the transport of photons of relatively low energies (below 40 keV) and all electron sources. When the energy of photon exceeds 100 keV, the interference becomes minimum for the high atomic number materials that are likely to be present in clinical situations. The shielding correction factors (SCFs) for dose rate at 2 mm from center were essentially 1.00 for photon energies above 100 keV; as the energy decreased below 100 keV, the SCF became smaller reaching a value of almost 0 for the lowest energy studied, 10 keV. For the photon source of 192Ir, the SCF was essentially 1.00; while for the photon sources of 125I and 103Pd, shielding corrections were considerably lower than 1.00 depending on the type and thickness of the high atomic number material. For the beta emitting sources, the shielding effect can be expressed as a loss in effective penetration depth. This loss depends both on the material and its thickness. For titanium and stainless steel, the loss of range was about two and four times the thickness of the metal. Conclusions: The effects of high atomic number materials, such as metallic stents, calcified plaque, and contrast agents are minimal for high energy photon emitters, such as 192Ir. The effects are pronounced for beta emitters and low energy photon emitters, and must be included in dosimetry planning.

Integration of the metabolic data of positron emission tomography in the dosimetry planning of radiosurgery with the Leksell gamma knife: early experience with brain tumors

✓ The purpose of this study was to assess the use of positron emission tomography (PET) as a stereotactic planning modality for gamma knife radiosurgery (GKS). The authors developed and validated a technique for fiducial marker imaging, importation, and handling of PET data for integration into GammaPlan planning software. The clinical feasibility in applying this approach to a selected group of patients presenting with recurrent glial tumors or metastases was evaluated. Positron emission tomography data can be integrated into GammaPlan, allowing a high spatial accuracy, as validated using a phantom. Positron emission tomography data were successfully combined with magnetic resonance (MR) images to define the target volume for the radiosurgical treatment of patients with recurrent glioma or metastasis. This approach may contribute to optimizing target selection for infiltrating or ill-defined brain lesions. Because PET is also useful for the pretreatment and follow-up evaluation, the use of stereotactic PET in these patients can enable an accurate comparison of PET-based metabolic data with MR-based anatomical data. This could give a better understanding of the metabolic changes following radiosurgery. The ability to use PET data in GKS represents a crucial step toward further developments in radiosurgery, as this approach provides additional information that may open new perspectives for the optimization of the treatment of brain tumors.

Identifying the predictors of acute urinary retention following magnetic-resonance-guided prostate brachytherapy

Purpose: Larger prostate gland volumes have been associated with long-term urinary morbidity in prostate interstitial radiation therapy utilizing ultrasound image guidance technique. This study was performed to identify the clinical and technical predictors of acute urinary retention following magnetic-resonance (MR)-guided prostate interstitial brachytherapy. Methods and Materials: Fifty patients underwent MR-guided prostate brachytherapy between December 1997 and March 1999. Patient selection was limited to men with stage T1cNXM0 disease, PSA of less than10 ng/mL, biopsy Gleason score not more than 3 + 4, and endorectal coil MR stage T2 disease. Dosimetry plans were developed in the operating room and 125Iodine sources were implanted using MR real-time guidance. The peripheral zone (PZ) of the prostate gland was defined as the clinical target volume (CTV) and the minimum prescribed dose to the CTV was 137 Gy. The volumes of the PZ, transition zone (TZ), and total prostate gland volume were also determined by MR. Individual source strength ranged from 0.35 to 0.54 μGym 2/h (NIST 99, median 0.46 μGym 2/h) and the total implanted activity ranged from 17.0 to 43.1 mCi (median, 28.1 mCi) using 43–120 seeds (median, 79). The seeds were placed using MR-compatible biopsy needles (14–28, median, 19). Results: The ability of clinical (MR defined prostate, PZ, and TZ volumes) and technical (number of catheters, number of seeds implanted, and total activity) factors to predict AUR for 50 men undergoing MR-guided prostate interstitial brachytherapy were evaluated using univariable and logistic regression multivariable analyses. Six men (12%) experienced AUR within 24 h after removal of the Foley catheter subsequent to prostate brachytherapy. The total number of seeds ( p = 0.05), MR determined prostate volume ( p < 0.01), and the MR-determined TZ volume ( p < 0.01) were significant predictors of AUR on univariable analysis. Utilizing a multivariable logistic regression analysis, the TZ volume was the only significant predictor of AUR ( p < 0.01). The prostate volume is highly correlated to the TZ volume (Spearman correlation coefficient of 0.91) and was thus significant in the univariable analysis; however, the prostate volume did not add prognostic value in multivariable analysis. Conclusion: Benign prostatic hyperplasia (BPH) resulting in an enlarged TZ volume, is the most important predictor of AUR following MR-guided prostate interstitial radiation therapy. Although AUR was significant (60%) in men with moderate BPH (TZ volume ≥ 50 cc), it was also self-limiting.

Inhibition of peridural fibrosis after laminectomy using low-dose external beam radiation in a dog model.

Clinical studies have demonstrated a significant association between the presence of extensive postlumbar discectomy peridural scar formation and the recurrence of low back and radicular pain. Low-dose perioperative radiation therapy has previously been demonstrated to inhibit peridural fibrosis after laminectomy in a rat model. The current study was designed to measure the effect of low-dose radiation on postlaminectomy peridural fibrosis development in a larger animal model. Three dogs underwent a total of 12 lumbar hemilaminectomies. For each animal, two levels received 1) external beam radiation 24 hours before surgery, 2) surgery alone, or 3) radiation alone. Radiation was administered in a single fraction of 700 cGy using computed tomographic guidance for dosimetry planning. The isodose distribution was such that the dose conformed to the posterior epidural space with minimal exit dose to normal tissue. Port films were used to confirm the correct levels. Gadolinium-enhanced magnetic resonance imaging (MRI) of the lumbar spines was obtained before the animals were killed 12 weeks after surgery. The spines were harvested, and axial sections through the laminectomy defect were stained with hematoxylin and eosin and Masson's trichrome. All specimens were evaluated for extent of fibrosis along the dura, density of fibrosis, nerve root entrapment, and sublaminar fibrosis. There were no complications from the surgery, and no new neurological deficits were noted. There was a statistically significant difference between the irradiated and nonirradiated groups regarding the extent of fibrosis (P = 0.001) and the density of fibroblasts (P = 0.001). There was also a marked difference in nerve root entrapment (P = 0.182) and the presence of sublaminar fibrosis (P = 0.061) between the treatment and control groups. MRI revealed less gadolinium enhancement at the irradiated levels compared with the nonirradiated levels, confirming the usefulness of MRI in predicting the degree of epidural fibrosis. Low-dose external beam radiation therapy administered 24 hours before laminectomy in a dog model significantly decreased the extent and density of peridural fibrosis as well as nerve root entrapment and sublaminar fibrosis. This treatment strategy may be efficacious in patients with recurrent radicular pain after lumbar discectomy that is thought to be secondary to peridural fibrosis on the basis of gadolinium-enhanced MRI studies, and who might benefit from reoperation for nerve root decompression.

Clinical utility of three-dimensional US.

Three-dimensional (3D) ultrasonography (US) is rapidly gaining popularity as it moves out of the research environment and into the clinical setting. This modality offers several distinct advantages over conventional US, including 3D image reconstruction with a single pass of the US beam, virtually unlimited viewing perspectives; accurate assessment of long-term effects of treatment; and more accurate, repeatable evaluation of anatomic structures and disease entities. In obstetric imaging, 3D US provides a novel perspective on the fetal anatomy, makes anomalies easier to recognize, facilitates maternal-fetal bonding, and helps families better understand fetal abnormalities. Three-dimensional pelvic US allows volume data sets to be acquired with both transvaginal and transabdominal probes. Viewing multiple 3D power Doppler US images in a fast cine loop has proved useful in angiographic applications. Three-dimensional prostate US can help make accurate volume assessments for dosimetry planning or for estimating prostate-specific antigen levels. In breast imaging, 3D US has the capacity to demonstrate lesion margins and topography, thereby helping differentiate benign from malignant masses. Three-dimensional US can also help determine the need for biopsy and help facilitate needle localization and guidance during biopsy. With recent advances in computer technology and display techniques, 3D US will likely play an increasingly important role in medicine.

70 A device to facilitate the planning and in-vivo rectal tld dosimetry of gynaecologycal HDR brachytherapy applications

70 A device to facilitate the planning and in-vivo rectal tld dosimetry of gynaecologycal HDR brachytherapy applications

A CT-based Monte Carlo simulation tool for dosimetry planning and analysis.

The Los Alamos code MCNP4A (Monte Carlo N-Particle version 4A) is currently used to simulate a variety of problems ranging from nuclear reactor analysis to boron neutron capture therapy. A graphical user interface has been developed that automatically sets up the MCNP4A geometry and radiation source requirements for a three-dimensional Monte Carlo simulation using computed tomography data. The major drawback for this dosimetry system is the amount of time to obtain a statistically significant answer. A specialized patch file has been developed that optimizes photon particle transport and dose scoring within the standard MCNP4A lattice geometry. The transport modifications produce a performance increase (number of histories per minute) of approximately 4.7 based upon a 6 MV point source centered within a 30 x 30 x 30 cm3 lattice water phantom and 1 x 1 x 1 mm3 voxels. The dose scoring modifications produce a performance increase of approximately 470 based upon a tally section of greater than 1 x 10(4) lattice elements and a voxel size of 5 mm3. Homogeneous and heterogeneous benchmark calculations produce good agreement with measurements using a standard water phantom and a high- and low-density heterogeneity phantom. The dose distribution from a typical mediastinum treatment planning setup is presented for qualitative analysis and comparison versus a conventional treatment planning system.

Ultrasound anatomy for prostate brachytherapy.

Even for surgeons experienced in prostate anatomy, the manipulation of perineally-placed needles into the prostate under the guidance of an ultrasound probe in the rectum can be disorienting. For those embarking on ultrasound-guided brachytherapy for prostate cancer, there can be no substitute for experience gained performing transrectal ultrasound examinations. A review of the anatomy of the region is presented, together with images showing how that anatomy appears sonographically. Some of the concepts involved in the role of prostate planimetry for dosimetry planning will be touched upon.

Abdominal SPECT/MRI fusion applied to the study of splenic and hepatic uptake of radiolabeled thrombocytes and colloids.

The importance of applying MRI (CT)/SPECT fusion in the abdominal and thoracic areas has been recognized in recent studies aiming at radionuclide therapy of cancer. According to our earlier results spleen and liver volume determination with different segmentation methods is inaccurate with SPECT alone. We therefore applied a SPECT/MRI registration procedure to the estimation of spleen and liver volumes and spleen/liver activity ratios in three male volunteers administered 111In-labeled thrombocytes and 99mTc-labeled colloids. The objectives of the study were to investigate if the uptake of thrombocytes in the spleen and liver can be measured more accurately when the anatomical borders of these organs are transferred from MRI to SPECT, and to test a SPECT/MRI registration method for improving three-dimensional dosimetry for radiotherapy treatment planning. A good correlation was found between spleen/liver activity ratios calculated from volumetric average activity per pixel values and from total volumetric counts derived from registered data but not from projection data. The average registration residual with this SPECT/MRI fusion method is approximately 1-2 cm in the abdominal area. Combining anatomical images with SPECT is therefore important for improving quantitative SPECT also in the abdomen.

99 Radionuclide therapy dosimetry and treatment planning

99 Radionuclide therapy dosimetry and treatment planning