Isodose Curves Research Articles

Monte Carlo and experimental investigations of multileaf collimated electron beams for modulated electron radiation therapy.

Modulated electron radiation therapy (MERT) has been proposed as a means of delivering conformal dose to shallow tumors while sparing distal structures and surrounding tissues. Conventional systems for electron beam collimation are labor and time intensive in their construction and are therefore inadequate for use in the sequential delivery of multiple complex fields required by MERT. This study investigates two proposed methods of electron beam collimation: the use of existing photon multileaf collimators (MLC) in a helium atmosphere to reduce in-air electron scatter, and a MLC specifically designed for electron beam collimation. Monte Carlo simulations of a Varian Clinac 2100C were performed using the EGS4/BEAM system and dose calculations performed with the MCDOSE code. Dose penumbras from fields collimated by photon MLCs both with air and with helium at 6, 12, and 20 MeV at a range of SSDs from 70 to 90 cm were examined. Significant improvements were observed for the helium based system. Simulations were also performed on an electron specific MLC located at the level of the last scraper of a 25x25 cm2 applicator. A number of leaf materials, thicknesses, end shapes, and widths were simulated to determine optimal construction parameters. The results demonstrated that tungsten leaves 15 mm thick and 5 mm wide with unfocused ends would provide sufficient collimation for MERT fields. A prototype electron MLC was constructed and comparisons between film measurements and simulation demonstrate the validity of the Monte Carlo model. Further simulations of dose penumbras demonstrate that such an electron MLC would provide improvements over the helium filled photon MLC at all energies, and improvements in the 90-10 penumbra of 12% to 45% at 20 MeV and 6 MeV, respectively. These improvements were also seen in isodose curves when a complex field shape was simulated. It is thus concluded that an MLC specific for electron beam collimation is required for MERT.

The Flow-Cytometric Analysis of Nasopharyngeal Carcinoma Treated with Stereotactic Radiosurgery

Stereotactic radiosurgery has been used in the treatment of nasopharyngeal carcinoma. To analyze the effect of radiosurgery on the flow cytometry DNA index and progression of cells through the cycles, 4 recurrent nasopharyngeal carcinoma cases were treated with stereotactic radiosurgery. The distribution of cells through the cycles was measured by flow cytometry at various times thereafter. The 50% isodose curve was placed at the tumor margin and the margin dose was 20 Gy. Biopsy of the nasopharynx was performed before and 1, 3, and 6 months after treatment. The specimens were then subjected to flow-cytometric analysis. The percentage of cells in the S phase was 27.6 ± 1.0, 14.6 ± 0.8, 12.1 ± 1.6 and 11.3 ± 1.3%; the proliferating index was 39.1 ± 1.4, 17.0 ± 0.9, and 14.3 ± 1.2 and 14.1 ± 1.5%, and the DNA index was 1.5 ± 0.1, 1.6 ± 0.2, 1.7 ± 0.1 and 1.9 ± 0.1 before and 1, 3, and 6 months after treatment, respectively. It is suggested that the percentage of cells in the S phase and the proliferating index decreased to normal after treatment with stereotactic radiosurgery. This study explores the preliminary radiobiological effect of stereotactic radiosurgery on the DNA content and the distribution of cells through the cycles in nasopharyngeal carcinoma.

Effect of stent on radiation dosimetry in an in-stent restenosis model

Purpose: Intravascular brachytherapy is the leading modality being evaluated for treatment of in-stent restenosis. Stent struts may have an effect on the dose distributions of various radiation sources. We evaluated dosimetry in a stented coronary artery model using a variety of beta and gamma sources and stent materials. Methods: We determined the dose distributions with and without stent in the in-stent restenosis model. Three beta-particle emitting radionuclides, 90Y (2.3 MeV), 144Pr (3.0 MeV), and 106Rh (3.5 MeV), and two gamma-ray emitters, 192Ir (380 keV) and 125I (30 keV), were studied. Stent materials included stainless steel, nitinol, and tantalum. Monte Carlo dose calculations were performed in a stent model of multiple stent struts placed at 1.5 mm from the source. Isodose curves were generated and the ratios of dose rates with and without stent, the stent factors, were evaluated. A stent factor of greater or less than unity represents dose enhancement or reduction in the presence of a stent. Results: For the three beta radionuclides, dose reduction was found on the adventitial side of the stent strut and dose enhancement was noted on the luminal side. On the luminal side, the maximum dose enhancement ranges from 7% to 29%, and the dose reduction on the adventitial side ranges from 13% to 43%. Both the reduction and enhancement effects were most pronounced for the high atomic number material, tantalum. For a given stent material, the dose reduction and enhancement are similar for the three beta radionuclides. For the gamma sources, the stent had no effect for the high-energy 192Ir, but for the low-energy 125I, drastic dose reduction on the adventitial side was observed (up to 86% for tantalum stent), and about 10% dose enhancement on the luminal side was also noted. The dose reduction with 125I was more pronounced than that seen with the beta sources. Conclusions: The presence of stent struts significantly affects dose distributions of 90Y, 106Rh, 144Pr, and 125I. The maximum dose reduction can be as much as 86%. 192Ir was unaffected. These factors need to be considered in choosing radionuclides and dose prescriptions in treating in-stent restenosis.

The effect of whole pleural cavity irradiation with combined 60Co and electron beam plus local biological agents on malignant pleural effusion

To study the effect and complications of combined 60Co γ ray and electron beam irradiation to malignant pleural effusion. From January,1996 to December,1998,55 patients with malignant pleural effusion (unilateral; 49 cases of primary lung cancer,4 breast cancer and 2 thymic tumor) received whole pleural cavity irradiation of 60Co γ ray with centrally placed lead blocks to protect the vital organs and normal lung tissue,and the electron beam irradiation at the area which was covered by lead blocks in 60Co γ ray irradiation.The pleural effusion was completely drained before radiotherapy,and dose distribution of middle level was calculated by TPS using EXT 2.4 version software (Multidata Co.USA).The 100% isodose curve was 2Gy,15 fractions were given and another 20Gy/10Fx irradiation was added to the visible tumor.All patients received sequential chemotherapy for 3-6 cycles (cisplatin-based regimens for lung cancer and thymic tumor,cyclophosphamide and adriamycin and fluoracil for breast cancer).Kaplan-Meier curve was used to evaluate the pleural effusion control rate and survival rate of patients. Complete remission of the malignant pleural effusion was seen in 9 patients and partial in 46 when treatment completed.The 6-,12-,18-month pleural effusion control rate and survival rate of the patients were 76%,53%,44% and 64%,34%,26%,respectively.The median control time of effusion and survival time were 14 months (2-32 months) and 9 months (4-32 months) respectively.We did not find any abnormality in liver function as well as in kidney function before and after treatment.Myelosuppression was the main side effect after combined chemotherapy and radiotherapy.Nineteen patients suffered from acute oesophagitis in grade 1-2,and 3 serious pleural fibrosis.There was no acute irradiation pneumonitis. The treatment for malignant pleural effusion with combined 60Co γ ray and electron beam irradiation is tolerable and effective in clinical use.

Dosimetric verification of a commercial 3D treatment planning systemfor conformal radiotherapy with a dynamic multileaf collimator

The dosimetric accuracy of a 3D treatment planning system (TPS)for conformal radiotherapy with a computer-assisted dynamic multileafcollimator (DMLC) was evaluated.The DMLC and the TPS have been developed for clinical applications wheredynamic fields not greater than 10×10 cm2 and multiplenon-coplanar arcs are required. Dosimetric verifications were performed bysimulating conformal treatments of irregularly shaped targets using severalarcs of irradiation with 6 MV x-rays and a spherical-shaped, tissue-simulatingphantom. The accuracy of the delivered dose at the isocentre was verifiedusing an ionization chamber placed in the centre of the phantom. Isodosedistributions in the axial and sagittal planes passing through the centre ofthe phantom were measured using double-layer radiochromic films. Measured doseat the isocentre as well as isodose distributions were compared to thosecalculated by the TPS. The maximum percentage difference between measured andprescribed dose was less than 2.5% for all the simulated treatment plans. Themean (±SD) displacement between measured and calculated isodoseswas, in the axial planes, 1.0±0.6 mm, 1.2±0.7 mm and1.5±1.1 mm for 80%, 50% and 20% isodose curves, respectively,whereas in the sagittal planes it was 2.0±1.2 mm and 2.2±2 mm for 80% and 50% isodose curves, respectively.The results indicate that the accuracy of the 3D treatment planning systemused with the DMLC is reasonably acceptable in clinical applications whichrequire treatments with several non-coplanar arcs and small dynamic fields.

Stereotactic dose planning system used in Leksell Gamma Knife model-B: EGS4 Monte Carlo versus GafChromic films MD-55

The accuracy of the dose planning system (Leksell GammaPlan) used in Gamma Knife (model-B) radiosurgery was verified using both the GafChromic films MD-55 (improved) and the EGS4 Monte Carlo technique. The Monte Carlo technique was employed to calculate the dose distribution along the x-, y- and z-axes when a single shot with opening of all 201 sources was delivered at the centre of a simulated water phantom with a diameter of 160 mm. Collimator helmets with different size of the Gamma Knife unit were verified. Good consistency (typical discrepancy less than 2%) was obtained between the results of Monte Carlo and GammaPlan. Small discrepancies, however, were obtained by GafChromic films. Discrepancies, as great as 10% when using the 4 mm collimator helmet, at the low percentage isodose curve along the z-axis of the measurement results were probably due to the small energy dependency of the GafChromic films. Significant discrepancies were not observed along x- and y-axes because such small discrepancies were easily over-washed by other gamma beams coming from the x- and y-directions. Similar results showing the discrepancies between the GafChromic films and GammaPlan were obtained when using the 8, 14 and 18 mm collimator helmets. However, the discrepancies along the z-axis became smaller as scattering effect increased when using larger collimator sizes. We suggest that the Monte Carlo technique should also be applied in stereotactic dose planning system verification as it is an ideal and reliable computational technique.

Dose-Volume Effect in Gamma Knife Radiosurgery of Meningiomas

Objective: To study the relationship between dose-volume effect and complication in gamma knife radiosurgery of intracranial meningiomas. Method: By using an integrated logistic formula to establish mathematical models, the authors analyzed the relationship between treatment volume and logistic probability for 15 Gy, and the relationship between prescribed dose and treatment volume in 3% logistic probability. Furthermore, 37 meningiomas treated by gamma knife radiosurgery were analyzed retrospectively to investigate how the 3% isoeffect curve impacted on complication. Result: There was a linear relation between treatment volume and logistic probability for 15 Gy. The regression formula was f(x) = 0.0042x + 0.0007. The 3% isoeffect curve indicated that the prescribed dose was inversely proportional to the treatment volume. During 16.3 months follow-up on average, 18 cases below the 3% isodose curve were without complication, but in 19 cases above the 3% isodose curve, 5 cases suffered complications, the difference being statistically significant. Conclusion: Dose selection in gamma knife radiosurgery is volume dependent. The 3% isodose curve may possibly be the guideline in gamma knife radiosurgery of meningiomas.

Optimizing computerized treatment planning for the gamma knife by source culling

Purpose: A good plan is crucial to the success of gamma knife treatment, which depends not only on parameters such as the number of shots, shot position, collimator sizes, and shot weight, but also on the number of blocked cobalt sources. However, during treatment, a plug is generally used to block those cobalt sources, so the beam cannot reach critical tissues. We present here an automated method to optimize all of those parameters, and to choose a source set, although the beams of some blocked sources do not hit any critical tissue. This strategy is used to achieve a high dose that better conforms to the tumor shape, and at the same time, avoids healthy tissue. Methods and Materials: Using a workstation that integrates the gamma knife treatment planning system, we developed a two-step optimization algorithm. First, we used a modified Powell’s method to optimize the location of the shot, collimator size, and shot weight; we used simulated annealing to determine if the number of shots was adequate using this parameter. Then, simulated annealing was used to determine which cobalt sources we needed to block. Results: Application of this optimization method in two cases showed that the treatment plan can be much improved when the set of blocked cobalt sources has been taken into consideration. Conclusion: Determining the set of blocked sources is necessary in certain cases. This technique better conforms the desired isodose curves to the outline of the target volume and minimizes damage to the surrounding normal tissues.

Measurement and comparison of prostate treatment dose to the femoral heads with 6-MV versus 10-MV photon energies

The purpose of this paper is to compare the femoral head dose distribution of 6-MV x-rays vs. 10-MV x-rays when treating prostate patients using a 4-field initial box technique with a Michigan technique boost. First, tissue maximum ratio (TMR) calculations were utilized to project the expected dose contributions to the femoral heads from each energy based upon the average male pelvis. Then, plans for both the 6- and 10-MV energies were developed for 5 prostate patients using the ADAC Pinnacle 3 (Milpitas, CA) treatment planning computer. Average doses and dose gradients were determined by examination of isodose curves. The dose contributions to the femoral heads were analyzed and compared with the tolerance dose (TD) 5/5 for this region. No significant dose differences existed between the 6- and 10-MV photon energies. This conclusion was supported by examination of point doses at depths of 3, 6, 12, and 15 cm for each energy.

A simple technique for treating age-related macular degeneration with external beam radiotherapy

Purpose: To develop a simple external beam photon radiotherapy technique to treat age-related macular degeneration without the need for simulation, planning computed tomography (CT) or computer dosimetry.Methods and Materials: The goal was to enable the treatment to be set up reliably on the treatment machine on Day 1 with the patient supine in a head cast without any prior planning. Using measurements of ocular globe topography from Karlsson et al. (Int J Radiat Oncol Biol Phys 1996; 33: 705–712), we chose a point 1.5 cm behind the anterior surface of the upper eyelid (ASUE) as the isocentre of a half-beam, blocked, 5.0 × 3.0–cm, angled lateral field to treat the involved eye. This would position the isocentre about 0.5 cm behind the posterior surface of the lens, and a little over 1 cm in front of the macula, according to Karlsson et al. The setup requires initial adjustment of the gantry from horizontal (to account for any asymmetry of position of the eyes), then angling 15° posteriorly to avoid the contralateral eye. Finally, the couch is raised to position the isocentre 1.5 cm behind the ASUE.Results: To verify the applicability of the technique, we performed CT and computer dosimetry on the first 11 eyes so treated. Our CT measurements were in good agreement with Karlsson et al. The lens dose was < 5% and the macula was within the 95% isodose curve in each case (6-MV linac). Treatment setup time is approximately 10 min each day. The 11 patients were treated with 5 × 2.00 Gy (2 patients) or 5 × 3.00 Gy (9 patients), and subjective response on follow-up over 1 to 12 months (median 4 months) was comparable to previously reported results, with no significant acute side effects.Conclusion: Our technique is easy to set up and reliably treats the macula, with sparing of the lens and contralateral eye. It enables treatment to commence rapidly and cost-effectively without the need for simulation or CT computer planning.

Commissioning of a micro multi-leaf collimator and planning system for stereotactic radiosurgery

Purpose: A computer controlled micro multi-leaf collimator, m3 mMLC, has been commissioned for conformal, fixed-field radiosurgery applications. Measurements were made to characterise the basic dosimetric properties of the m3, such as leaf transmission, leakage and beam penumbra. In addition, the geometric and dosimetric accuracy of the m3 was verified when used in conjunction with a BrainSCAN v3.5 stereotactic planning system. Materials and methods: The m3 was detachably mounted to a Varian Clinac 2100C accelerator delivering 6 MV X-rays. Leaf transmission, leakage, penumbra and multiple, conformal fixed field dose distributions were measured using calibrated film in solid water. Beam data were collected using a diamond detector in a scanning water tank and planned dose distributions were verified using LiF TLDs and film. A small, shaped phantom was also constructed to confirm field shaping accuracy using portal images. Results: Mean transmission through the closed multi-leaves was 1.9±0.1% and leakage between leaves was 2.8±0.15%. Between opposing leaves abutting along the central beam-axis transmission was ∼15±3%, but was reduced to a mean of 4.5±0.6% by moving the abutmen position 4.5 cm off-axis. Beam penumbrae were effectively constant as a function of increasing square field size and asymmetric fields and was seen to vary non-linearly when shaped to diagonal, straight edges. TMR, OAR and relative output beam data measurements of circular m3 fields were comparable to conventional, circular stereotactic collimators. Multiple, conformal field dose distributions were calculated with good spatial and dosimetric accuracy, with the planned 90% isodose curves agreeing with measurements to within 1–2 mm and to ±3% at isocentre. Portal films agreed with planned beams eye-view field shaping to within 1 mm. Conclusions: The m3 micro multi-leaf collimator is a stable, high precision field-shaping device suitable for small-field, radiosurgery applications. Dose distributions can be accurately calculated by a planning system using only a few beam data parameters.

60 Gy isodose volumes in carcinoma of the cervix and their relation to the geometry of uterine tube and ovoids.

The International Commission on Radiation Units and Measurements recommends the use of 60 Gy isodose volumes for reporting doses in the intracavity treatment of carcinoma of the uterine cervix. This study was aimed at determining the variation in isodose volumes while using different sizes of intrauterine tubes and ovoids, with different applicator geometries. It was based on the treatment plans of 175 patients with cervical cancer, treated with low dose rate intracavitary brachytherapy with or without additional external beam radiotherapy. The volumes encompassed by the 60 Gy isodose curves were calculated using the Nucletron planning system. Applicator positions in 15 patients who were treated to the same point A dose, using 6 cm intrauterine tube and medium ovoids, were recorded. This was to discover how variations in applicator geometry influences isodose volumes. The 60 Gy isodose volumes increased with increasing point A dose. For a constant point A dose prescription, reference isodose volume increased with ovoid applicator size used, but showed no consistent variation with the length of intrauterine tube. There were individual variations in the isodose volumes within a standard set-up (same sized intrauterine tube and ovoids and same point A dose), due to variations in applicator geometry. Displacement of the ovoids changed the volumes encompassed by the reference isodose. There are significant variations in the volumes encompassed by the 60 Gy isodose during intracavitary treatment using a standard set-up, while treatment using applicators of different sizes can give equivalent values of 60 Gy isodose volume. 60 Gy isodose volumes may hence be useful in dosimetric comparisons but have a limited role in predicting clinical response.

Three-dimensional optimization of treatment planning for gamma unit treatment system.

During a treatment using the Leksell gamma unit, the physician and physicist need to determine a treatment plan by changing the parameters such as collimator sizes, the position of isocenters and isocenters' weights. This is a complex problem because the set of parameters is large, especially when targets are geometrically close to a critical structure. For this reason, we present here an optimization algorithm, namely the multiplier penalty method, to mathematically determine those parameters. Two cases are presented in this article: the first one is really planned by a physicist in a clinical treatment, and is redone in our optimization algorithm to show the effectiveness of this method; the second one is theoretical where a critical structure is placed close to the target volume. The results show that this method achieves an excellent conformation to the specified isodose curve with the contour of the target volume, allowing minimal damage to surrounding healthy tissue.

Computed tomography analysis of causes of local failure in radiotherapy for cervical carcinoma.

The authors analyzed the radiation dose to the periphery of the cervix and area of the cervix in relation to local failure of radiotherapy for carcinoma of the cervix using computed tomography (CT) images. Between 1981-1990, 127 consecutive patients were treated with definitive radiotherapy. Ninety-nine of these patients had CT images taken at the time of intracavitary therapy. Of these 99 patients, 80 were eligible for this analysis. After CT scanning, isodose curves relative to the point A dose were superimposed on the CT images. The minimum percent dose and minimum dose at the periphery of the cervix were estimated. The area of the cervix also was measured. These factors were examined in relation to the local tumor control rate. Histograms of both the minimum percent dose and the cervical area showed significant differences between the local control and local failure groups (P <0.001). The local control rates were related to both the minimum percent dose and the cervical area, and differed significantly over and below the values of 60% and 18 cm2 (P <0.001 each), respectively. The local control patients, over and below the line: Y = -0.220X + 21.2, in which X (gray [Gy]) and Y (Gy) are the whole pelvis dose and the minimum dose, respectively, could be well differentiated with significance (91.7% vs. 25.0%; P <0.001). Computed tomography analysis indicated that the local tumor control rate was related strongly to the minimum percent dose, the cervical area, and the pair of whole pelvis and minimum dose values. These factors were found to be more useful than the point A dose in predicting local tumor control.

A software tool for the quantitative evaluation of 3D dose calculation algorithms.

Current methods for evaluating modern radiation therapy treatment planning (RTP) systems include the manual superposition of calculated and measured isodose curves and the comparison of a limited number of calculated and measured point doses. Both techniques have significant limitations in providing quantitative evaluations of the large number of dose data generated by modern RTP systems. More sophisticated comparison techniques have been presented in the literature, including dose-difference and distance-to-agreement (DTA) analyses. A software tool has been developed that uses superimposed isodose plots, dose-difference, and DTA distributions to quantify errors in computed dose distributions. Dose-difference and DTA analyses are overly sensitive in regions of high- and low-dose gradient, respectively. The logical union of locations that fail both dose-difference and DTA acceptance criteria, termed the composite evaluation, is calculated and displayed. The composite evaluation provides a method for the physicist to efficiently identify regions that fail both the dose-difference and DTA acceptance criteria. The tool provides a computer platform for the quantitative comparison of calculated and measured dose distributions.

Effect of Gamma Knife Radiosurgery on Rat Brain Sodium Channel Subunit mRNA Expression

Several lines of evidence underscore a possible and delayed antiepileptic effect of Gamma Knife irradiation. This effect could be related to structural and molecular changes. Since voltage-gated Na⁺ channels (NaChs) play a crucial role in neuron excitability, we studied the effect of Gamma Knife (GK) irradiation on the distribution of Na⁺ channel (NaCh) subunit mRNAs in rat brains. A left side irradiation was performed in rats using a stereotactic device adapted for GK radiosurgery. A dose of 100 Gy was administered with the 4 mm collimator. The left dentate gyrus and thalamus coordinates were based on De Groot’s rat stereotactic atlas. The isodose curve distribution was calculated with the dose planning software used in Gamma Knife and superimposed on the target. Na⁺ channels α unit and mRNAs (subtype II and subtype III) expression was studied 1 hour, 30 days and 60 days later. We used non-radioactive in situ hybridization with subtype-specific digoxigenin-labeled cRNA probes. Labeling intensity was evaluated with a densitometric analysis of digitized images from the control side (right) and lesioned side (left) in each rat. No morphological changes were observed one hour after GK irradiation. 30 days later, the upper thalamic nuclei exhibited a few necrotic regions associated with gliosis. In contrast, no lesions were observed in the hippocampus. 60 days later, the necrotic region involving thalamic nuclei was enlarged. NaCh II and III mRNAs expression did not appear to be modified after GK at the three times studied here. In particular, neurons surrounding the GK necrosis continued to express high levels of NaCh mRNAs. Thus, regulation of NaCh II and III subtypes do not appear to explain the functional antiepileptic effect of GK.

Protection of Visual Pathway in Gamma Knife Radiosurgery for Craniopharyngiomas

Craniopharyngiomas present a major challenge to Gamma Knife radiosurgery (GKRS) due to their proximity to the optic apparatus. Based on observations of the evolving tumoral change on MRI and clinical results, an optimization of the treatment strategy and dose selection is possible. From March 1993 to September 1996, 21 patients with craniopharyngiomas were treated by GKRS. Every patient received stereotactic MRI exclusively for targeting and dose planning. The tumor and adjacent structures, including optic nerves, chiasm, and tracts were carefully identified and delineated on sagittal, coronal and axial films. The tumor volume ranged from 0.3 to 28 ml (average 9 ml). We purposefully apply multiple isocenters (average 9.1 shots) to create an isodose curve that covered the tumor optimally while sparing the optic pathway. The marginal dose prescribed was 9.5 to 16 Gy (50%). The maximal dose was 19 to 32 Gy. The maximal dose to the optic apparatus was 3.2 to 12.5 Gy. After GKRS, all patients were followed up clinically every month. MR studies were conducted every six months with the same techniques on the same scanner to investigate evolution of tumor volume and any adverse radiation effect. The follow-up period ranged from 6 to 40 months (mean: 18.4, median: 19). All patients were followed more than 6 months. Nineteen out of 21 cases (90.5%) achieved tumor control; that is, 18 tumor shrinkage (volume reduction: 15–95%) and 1 stabilized tumor growth. Among these 21 patients, 7 had improved visual acuity or visual field after GKRS, and the rest remained stable. Two patients developed mild T₂ change on MRI without any endocrinological disturbance or visual impairment. Protection of the visual pathway can be secured by a sophisticated delineation on 3-dimensional stereotactic images with multiple-shot dose planning. Craniopharyngiomas with tumor volume up to 25 ml were treated safely and effectively, because the dose to the optic apparatus was kept as low as possiby this strategy. Further follow-up is needed to determine the highest tolerable dose to surrounding critical structures and the long-term outcome of tumor control.

Radiographic findings and morbidity in patients treated with stereotactic radiosurgery

Purpose: To determine the prognostic significance of pretreatment edema, lesion size and location on morbidity following stereotactic radiosurgery (SRS).Methods and Materials: Forty-seven evaluable patients with 63 lesions were treated on a 6-MV linear accelerator radiosurgery system at Memorial Sloan Kettering Cancer Center. All patients received a 10-mg intravenous bolus of dexamethasone sodium phosphate (Decadron) prior to SRS. Thirteen patients were treated for asymptomatic lesions while 34 were treated because of neurologic symptoms. The median dose delivered was 1800 cGy and the median prescription isodose curve was 85%. Pretreatment edema was measured on a transaxial T2-weighted MR image acquired within 1 month of the SRS.Results: Ten patients experienced morbidity as a result of their treatment. The complication rate was measured by neurologic events following SRS and was not significantly influenced by the extent of peritumoral edema. Lesion size was also unrelated to the development of post-treatment symptoms as assessed by the ease of tapering steroids. The only parameter found to influence post-SRS complications was lesion location. Four of six (66%) patients treated to lesions in the motor cortex suffered post-SRS seizure activity, whereas only 6 of 37 (16%) patients treated to lesions elsewhere in the brain parenchyma experienced seizure activity.Conclusion: The presence of pretreatment edema and lesion size are not predictors of post-SRS complication rates or the ability to taper Decadron. Lesion location is predictive of post-SRS seizure activity.

Dose mapping of dried figs treated by gamma radiation

The irradiation parameters of two varieties of dried figs were determined in a fixed position. The Dose Uniformity (UDmax/Dmin) obtained for Fricke Dosimeter was 1.4 and for YR Gammachrome 1.3. The isodose curves were built using a geostatistical gridding method, the Kriging method.

An efficient method for small field treatment dose calculation for stereotactic radiosurgery using a LINAC.

The normal procedure for a physician-physicist team designing a treatment plan for multiarc stereotactic radiosurgery is the trial-and-error approach of changing the collimator size and the location of the isocenter of radiation and viewing the isodose curves on two-dimensional computed tomography (CT) or magnetic resonance imaging (MRI) image planes. Automatic optimization procedures have also been used to optimize beam weight or beam size. However, either process is very time consuming. To improve the speed of the dose calculation, a random sampling method has been proposed. Unfortunately, the sampled values of an objective function are different from one sample to another. Such a sampling method cannot be used in automatic optimization because the next move in an optimization process is based on the current and past objective function values. To this end, an adaptive method based on the size of the collimators is proposed and used to determine a small volume in the shape of a hollow sphere for which the dose is calculated. With an appropriate choice of an adaptive hollow sphere, the objective function calculated based on such a hollow sphere is the same as that calculated with the traditional three-dimensional (3-D) cube matrix. However, with the new adaptive method, the speed of calculating a dose can be improved by a factor of 4 to a factor of 100. Because of the improvement in the speed of calculating a treatment dose, the new adaptive hollow sphere method for calculating a treatment dose can be used routinely in designing a treatment plan.