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Shepard DM, Earl MA, Li XA, Naqvi S, Yu C. Direct aperture optimization: a turnkey solution for step-and-shoot IMRT. Med Phys 2002; 29:1007-18. [PMID: 12094970 DOI: 10.1118/1.1477415] [Citation(s) in RCA: 236] [Impact Index Per Article: 10.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022] Open
Abstract
IMRT treatment plans for step-and-shoot delivery have traditionally been produced through the optimization of intensity distributions (or maps) for each beam angle. The optimization step is followed by the application of a leaf-sequencing algorithm that translates each intensity map into a set of deliverable aperture shapes. In this article, we introduce an automated planning system in which we bypass the traditional intensity optimization, and instead directly optimize the shapes and the weights of the apertures. We call this approach "direct aperture optimization." This technique allows the user to specify the maximum number of apertures per beam direction, and hence provides significant control over the complexity of the treatment delivery. This is possible because the machine dependent delivery constraints imposed by the MLC are enforced within the aperture optimization algorithm rather than in a separate leaf-sequencing step. The leaf settings and the aperture intensities are optimized simultaneously using a simulated annealing algorithm. We have tested direct aperture optimization on a variety of patient cases using the EGS4/BEAM Monte Carlo package for our dose calculation engine. The results demonstrate that direct aperture optimization can produce highly conformal step-and-shoot treatment plans using only three to five apertures per beam direction. As compared with traditional optimization strategies, our studies demonstrate that direct aperture optimization can result in a significant reduction in both the number of beam segments and the number of monitor units. Direct aperture optimization therefore produces highly efficient treatment deliveries that maintain the full dosimetric benefits of IMRT.
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Earl MA, Shepard DM, Naqvi S, Li XA, Yu CX. Inverse planning for intensity-modulated arc therapy using direct aperture optimization. Phys Med Biol 2003; 48:1075-89. [PMID: 12741503 DOI: 10.1088/0031-9155/48/8/309] [Citation(s) in RCA: 131] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
Abstract
Intensity-modulated arc therapy (IMAT) is a radiation therapy delivery technique that combines gantry rotation with dynamic multi-leaf collimation (MLC). With IMAT, the benefits of rotational IMRT can be realized using a conventional linear accelerator and a conventional MLC. Thus far, the advantages of IMAT have gone largely unrealized due to the lack of robust automated planning tools capable of producing efficient IMAT treatment plans. This work describes an inverse treatment planning algorithm, called 'direct aperture optimization' (DAO) that can be used to generate inverse treatment plans for IMAT. In contrast to traditional inverse planning techniques where the relative weights of a series of pencil beams are optimized, DAO optimizes the leaf positions and weights of the apertures in the plan. This technique allows any delivery constraints to be enforced during the optimization, eliminating the need for a leaf-sequencing step. It is this feature that enables DAO to easily create inverse plans for IMAT. To illustrate the feasibility of DAO applied to IMAT, several cases are presented, including a cylindrical phantom, a head and neck patient and a prostate patient.
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Comparative Study |
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McPartlin AJ, Li XA, Kershaw LE, Heide U, Kerkmeijer L, Lawton C, Mahmood U, Pos F, van As N, van Herk M, Vesprini D, van der Voort van Zyp J, Tree A, Choudhury A. MRI-guided prostate adaptive radiotherapy - A systematic review. Radiother Oncol 2016; 119:371-80. [PMID: 27162159 DOI: 10.1016/j.radonc.2016.04.014] [Citation(s) in RCA: 118] [Impact Index Per Article: 13.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/24/2016] [Revised: 04/08/2016] [Accepted: 04/09/2016] [Indexed: 11/29/2022]
Abstract
Dose escalated radiotherapy improves outcomes for men with prostate cancer. A plateau for benefit from dose escalation using EBRT may not have been reached for some patients with higher risk disease. The use of increasingly conformal techniques, such as step and shoot IMRT or more recently VMAT, has allowed treatment intensification to be achieved whilst minimising associated increases in toxicity to surrounding normal structures. To support further safe dose escalation, the uncertainties in the treatment target position will need be minimised using optimal planning and image-guided radiotherapy (IGRT). In particular the increasing usage of profoundly hypo-fractionated stereotactic therapy is predicated on the ability to confidently direct treatment precisely to the intended target for the duration of each treatment. This article reviews published studies on the influences of varies types of motion on daily prostate position and how these may be mitigated to improve IGRT in future. In particular the role that MRI has played in the generation of data is discussed and the potential role of the MR-Linac in next-generation IGRT is discussed.
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Systematic Review |
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118 |
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Semenenko VA, Li XA. Lyman–Kutcher–Burman NTCP model parameters for radiation pneumonitis and xerostomia based on combined analysis of published clinical data. Phys Med Biol 2008; 53:737-55. [DOI: 10.1088/0031-9155/53/3/014] [Citation(s) in RCA: 97] [Impact Index Per Article: 5.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
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5
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Cai M, Song XL, Li XA, Chen M, Guo J, Yang DH, Chen Z, Zhao SC. Current therapy and drug resistance in metastatic castration-resistant prostate cancer. Drug Resist Updat 2023; 68:100962. [PMID: 37068396 DOI: 10.1016/j.drup.2023.100962] [Citation(s) in RCA: 85] [Impact Index Per Article: 42.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/21/2022] [Revised: 03/06/2023] [Accepted: 04/10/2023] [Indexed: 04/19/2023]
Abstract
Castration-resistant prostate cancer (CRPC), especially metastatic castration-resistant prostate cancer (mCRPC) is one of the most prevalent malignancies and main cause of cancer-related death among men in the world. In addition, it is very difficult for clinical treatment because of the natural or acquired drug resistance of CRPC. Mechanisms of drug resistance are extremely complicated and how to overcome it remains an urgent clinical problem to be solved. Thus, a comprehensive and thorough understanding for mechanisms of drug resistance in mCRPC is indispensable to develop novel and better therapeutic strategies. In this review, we aim to review new insight of the treatment of mCRPC and elucidate mechanisms governing resistance to new drugs: taxanes, androgen receptor signaling inhibitors (ARSIs) and poly (ADP-ribose) polymerase (PARP) inhibitors (PARPi). Most importantly, in order to improve efficacy of these drugs, strategies of overcoming drug resistance are also discussed based on their mechanisms respectively.
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Review |
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85 |
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Li XA, Soubra M, Szanto J, Gerig LH. Lateral electron equilibrium and electron contamination in measurements of head-scatter factors using miniphantoms and brass caps. Med Phys 1995; 22:1167-70. [PMID: 7565391 DOI: 10.1118/1.597508] [Citation(s) in RCA: 77] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023] Open
Abstract
The head-scatter factor (Sh) can be measured with a narrow miniphantom or a metal cap provided it is completely covered by the photon beam and its lateral size is thick enough to prevent electron contamination contributions. The effects of lateral electron equilibrium (LEE) and electron contamination on the Sh values were studied. The EGS4 Monte Carlo technique was used to calculate the minimum beam radii (rLEE) required to achieve complete LEE for photon beams ranging from 60Co to 24 MV. The measurement shows that the error introduced to the Sh value due to lateral electron disequilibrium is negligible. The radii of the miniphantoms or the sidewall thicknesses of the caps can be reduced below rLEE provided they are thick enough to prevent the effect of electron contamination.
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Li XA, Sutcliffe MJ, Schwartz TW, Dobson CM. Sequence-specific 1H NMR assignments and solution structure of bovine pancreatic polypeptide. Biochemistry 1992; 31:1245-53. [PMID: 1734969 DOI: 10.1021/bi00119a038] [Citation(s) in RCA: 59] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/28/2022]
Abstract
Sequence-specific 1H NMR assignments for the 36 residue bovine pancreatic polypeptide (bPP) have been completed. The secondary and tertiary structure of bPP in solution has been determined from experimental NMR data. It is shown that bPP has a very well-defined C-terminal alpha-helix involving residues 15-32. Although regular secondary structure cannot be clearly defined in the N-terminal region, residues 4-8 maintain a rather ordered conformation in solution. This is attributed primarily to the hydrophobic interactions between this region and the C-terminal helix. The two segments of the structure are joined by a turn which is poorly defined. The four end residues both at the N-terminus and the C-terminus are highly disordered in solution. The overall fold of the bPP molecule is very closely similar to that found in the crystal structure of avian pancreatic polypeptide (aPP). The RMS deviation for backbone atoms of residues 4-8 and 15-32 between the bPP mean structure and the aPP crystal structure is 0.65 A, although there is only 39% identity of the residues. Furthermore, the average conformations of some (mostly from the alpha-helix) side chains of bPP in solution are closely similar to those of aPP in the crystal structure. A large number of side chains of bPP, however, show significant conformational averaging in solution.
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Wang R, Li XA. A Monte Carlo calculation of dosimetric parameters of 90Sr/90Y and 192Ir SS sources for intravascular brachytherapy. Med Phys 2000; 27:2528-35. [PMID: 11128305 DOI: 10.1118/1.1319374] [Citation(s) in RCA: 52] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022] Open
Abstract
The AAPM TG-60 report has proposed various dose calculation parameters for intravascular brachytherapy (IVBT). These parameters include the dose rate constant (or the dose rate at a reference position for a beta-particle emitting source), the radial dose function, and the anisotropy function. In this work, we have used a modified EGS4 Monte Carlo system to calculate these parameters for the two most commonly used IVBT sources (the beta-particle emitting 90Sr/90Y source and the photon emitting 192Ir SS source). To ensure the calculation accuracy, the present calculation was compared with several measurements and calculations reported by other authors. Excellent agreement was found for the results with the photon source. For beta-particle source calculation, the present results for a variety of point sources agree very well with a previous work. The presently calculated radial dose functions for the 90Sr/90Y source are consistent with those of a published work for intermediate radial distances. The dose uniformity in the axial direction was also studied. The contributions of bremsstrahlung photons to total doses for the 90Sr/90Y beta source, and the influence of ignoring electron transport on calculated doses for the 192Ir SS photon source are discussed.
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Hatanaka K, Li XA, Masuda K, Yutani C, Yamamoto A. Immunohistochemical localization of C-reactive protein-binding sites in human atherosclerotic aortic lesions by a modified streptavidin-biotin-staining method. Pathol Int 1995; 45:635-41. [PMID: 8548035 DOI: 10.1111/j.1440-1827.1995.tb03515.x] [Citation(s) in RCA: 52] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/31/2023]
Abstract
One-step fluorescein-conjugated polyclonal antibody technique has shown that C-reactive protein (CRP) was located only extracellularly in human atherosclerotic lesions. In this report a more sensitive streptavidin-biotin technique was applied to detect the localization of CRP in human atherosclerotic lesions. Immunohistochemical staining with polyclonal and monoclonal anti-human CRP antibodies both produced a brown color extracellularly in the necrotic lesions, and intracellularly in CD68+ foam cells. The latter suggests an uptake of CRP-lipid complexes by macrophages. The staining is human CRP-specific because it was eliminated by preabsorption of the monoclonal antibody with pure human CRP, or by substitution of the primary antibody with non-immune rabbit serum. By overlaid CRP-binding study, a positive stain was observed on intimal smooth muscle cells and foam cells, suggesting that they have CRP-binding sites unless the CRP-binding activity was generated de novo through the fixation procedure. Accordingly, it is hypothesized that CRP may facilitate the uptake of lipids by macrophages accumulating in atherosclerotic lesions. Further, CRP might participate in cytolysis, which enlarges the necrotic area, and/or in phagocytosis that scavenges the necrotic tissue.
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Comparative Study |
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10
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Wang R, Li XA. Monte Carlo dose calculations of beta-emitting sources for intravascular brachytherapy: a comparison between EGS4, EGSnrc, and MCNP. Med Phys 2001; 28:134-41. [PMID: 11243335 DOI: 10.1118/1.1339880] [Citation(s) in RCA: 48] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022] Open
Abstract
The dose parameters for the beta-particle emitting 90Sr/90Y source for intravascular brachytherapy (IVBT) have been calculated by different investigators. At a distant distance from the source, noticeable differences are seen in these parameters calculated using different Monte Carlo codes. The purpose of this work is to quantify as well as to understand these differences. We have compared a series of calculations using an EGS4, an EGSnrc, and the MCNP Monte Carlo codes. Data calculated and compared include the depth dose curve for a broad parallel beam of electrons, and radial dose distributions for point electron sources (monoenergetic or polyenergetic) and for a real 90Sr/90Y source. For the 90Sr/90Y source, the doses at the reference position (2 mm radial distance) calculated by the three code agree within 2%. However, the differences between the dose calculated by the three codes can be over 20% in the radial distance range interested in IVBT. The difference increases with radial distance from source, and reaches 30% at the tail of dose curve. These differences may be partially attributed to the different multiple scattering theories and Monte Carlo models for electron transport adopted in these three codes. Doses calculated by the EGSnrc code are more accurate than those by the EGS4. The two calculations agree within 5% for radial distance <6 mm.
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Comparative Study |
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48 |
11
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Li XA, Yu C, Holmes T. A systematic evaluation of air cavity dose perturbation in megavoltage x-ray beams. Med Phys 2000; 27:1011-7. [PMID: 10841404 DOI: 10.1118/1.598966] [Citation(s) in RCA: 43] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022] Open
Abstract
The EGS4 Monte Carlo radiation transport code was used to systematically study the dose perturbation near planar and cylindrical air cavities in a water medium irradiated by megavoltage x-ray beams. The variables of the problem included x-ray energy, cavity shape and dimension, and depth of the cavity in water. The Monte Carlo code was initially validated against published measurements and its results were found to agree within 2% with the published measurements. The study results indicate that the dose perturbation is strongly dependent on x-ray energy, field size, depth, and size of cavity in water. For example, the Monte Carlo calculations show dose reductions of 42% and 18% at 0.05 and 2 mm, respectively, beyond the air-water interface distal to the radiation source for a 3 cm thick air slab irradiated by a single 5x5 cm2 15 MV beam. The dose reductions are smaller for a parallel-opposed pair of 5x5 cm2 15 MV x-ray beams, being 21% and 11% for the same depths. The combined set of Monte Carlo calculations showed that the dose reduction near an air cavity is greater for: (a) Smaller x-ray field size, (b) higher x-ray energy, (c) larger air-cavity size, and (d) smaller depth in water where the air cavity is situated. A potential clinical application of these results to the treatment of prostate cancer is discussed.
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Comparative Study |
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43 |
12
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Ma L, Yu CX, Earl M, Holmes T, Sarfaraz M, Li XA, Shepard D, Amin P, DiBiase S, Suntharalingam M, Mansfield C. Optimized intensity-modulated arc therapy for prostate cancer treatment. Int J Cancer 2001; 96:379-84. [PMID: 11745509 DOI: 10.1002/ijc.1039] [Citation(s) in RCA: 39] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022]
Abstract
We recently implemented intensity-modulated arc therapy (IMAT) at our institution. In this study, we evaluate the dosimetric merits of the application of this technique to the treatment of prostate cancer. Each IMAT treatment plan incorporated bilateral overlapping arcs. The dose from each beam segment was computed using the three-dimensional dose model of a clinical treatment planning system (Render Plan 3.5, Precision Therapy). The weights assigned to the individual arc segments were optimized using a gradient search method. For 12 patients, comparisons were made between the IMAT treatment plans and corresponding plans using fixed cone-beam intensity-modulated radiotherapy (IMRT) from a commercial inverse planning system (CORVUS, NOMOS Corp.). We found that the optimized IMAT treatments produced similar dose distributions to the IMRT deliveries. Compared with the IMRT treatments, the IMAT treatments produced slightly less target dose homogeneity with consistently greater sparing of the rectum in regions of lower dose. The trade-off between target dose conformity and rectum sparing can be adjusted in both optimization procedures. Because the total beam-on time for IMAT delivery is 1 to 2 minutes with approximately 5-6 minutes of patient setup time, the delivery efficiency of the IMAT treatment was significantly better than the multiple-beam IMRT treatment.
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Li XA, Ma CM, Salhani D. Measurement of percentage depth dose and lateral beam profile for kilovoltage x-ray therapy beams. Phys Med Biol 1997; 42:2561-8. [PMID: 9434308 DOI: 10.1088/0031-9155/42/12/019] [Citation(s) in RCA: 37] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023]
Abstract
In this work, nine commonly used dosimetry detectors have been investigated to determine suitable relative dosimeters for kilovoltage x-ray beams. By comparison with the Monte Carlo calculated data, it was determined that for the detectors studied the PTW N23342, Markus and NACP parallel-plate chambers are more suitable for the measurement of percentage depth dose (PDD) data for this beam quality range with an uncertainty of about 3%. A diode detector may be used to measure the PDD for the 100 kVp beam, but it is not suitable for higher energies (300 kVp). The Capintec parallel-plate chamber may be adequate for medium-energy photons, but it has a slightly higher uncertainty for low-energy x-rays (100 kVp). For the measurement of beam profiles, diode and film yield incorrect profile tails, which can be corrected using the RK ionization chamber.
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Comparative Study |
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Li XA, Wang R, Yu C, Suntharalingam M. Beta versus gamma for catheter-based intravascular brachytherapy: dosimetric perspectives in the presence of metallic stents and calcified plaques. Int J Radiat Oncol Biol Phys 2000; 46:1043-9. [PMID: 10705028 DOI: 10.1016/s0360-3016(99)00457-5] [Citation(s) in RCA: 34] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/25/2022]
Abstract
PURPOSE Both beta and gamma emitters are currently used in the catheter-based intravascular brachytherapy. The dosimetric effects due to the presence of metallic stents and calcified plaques have not been fully addressed. This work compares these effects for two most commonly used beta and gamma sources ( (90)Sr and (192)Ir). MATERIALS AND METHODS An EGS4 Monte Carlo package was used to calculate dose in water for a (90)Sr (supplied by NOVOSTE) and an (192)Ir (Supplied by BEST) source, with or without the presence of a calcified plaque or a metallic stent. Plaques of different shape (shell and disk), size and density, and two types of stainless-steel stents (ring or mesh stent) were studied. The ring stent consists of identical rings stacked along the long axis of the sources. The gap between two rings is 0.3 mm. The mesh stents are made of identical square (0.1 x 0.1 or 0.2 x 0.2 mm(2)) holes separated from each other by stainless-steel wire. The cross section of wire for both ring and mesh stents is 0.1 x 0.1 mm(2). A dose perturbation factor (DPF), defined as the ratio of the doses with and without the presence of a plaque or a stent, was introduced to quantify the effects. A carefully chosen set of EGS4 transport parameters for the small geometry in question was used in the calculation. RESULTS The radial and axial dose distributions calculated in water were found to agree with the published measurements to within 3%. The dose perturbations due to the presence of calcified plaques or metallic stents were found far more significant for the (90)Sr source than those for the (192)Ir source. Up to 30% dose reduction behind a plaque were observed for the (90)Sr source, while the dose reduction for the (192)Ir source was found to be negligible. The dose enhancement inside a plaque was as high as 10% for the beta source or 6% for the gamma source. In the presence of a stent, the DPF was in the range of 1.15-0.75 for the beta source, while it was almost equal to 1.0 for the gamma source. CONCLUSION The dose perturbation due to the presence of a calcified plaque or a metallic stent is significant for the beta source. The dose reduction in the region beyond a plaque or a stent could be more than 20%. For the gamma source, the dose effect behind a plaque or a stent is practically negligible. These dosimetric differences between the beta and gamma sources in the presence of a calcified plaque or metallic stent should be considered in the dose prescription of intravascular brachytherapy.
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Li XA, Hatanaka K, Ishibashi-Ueda H, Yutani C, Yamamoto A. Characterization of serum amyloid P component from human aortic atherosclerotic lesions. Arterioscler Thromb Vasc Biol 1995; 15:252-7. [PMID: 7749834 DOI: 10.1161/01.atv.15.2.252] [Citation(s) in RCA: 34] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023]
Abstract
Serum amyloid P component (SAP) is a glycoprotein in human plasma. We recently showed the localization of SAP in human atherosclerotic lesions by immunohistochemical staining. In this study, the presence of SAP in atherosclerotic lesions was confirmed, and the biochemical character of SAP in atherosclerotic intima was investigated and compared with that of native SAP. Atherosclerotic intima was sequentially extracted with 2 mmol/L CaCl2-Tris-buffered saline (TBS), 10 mmol/L EDTA-TBS, 3 mol/L guanidine-TBS, and collagenase digestion. The character of SAP in each extract was studied with double immunodiffusion, electroimmunoassay, crossed immunoelectrophoresis, and Western immunoblotting. The total amount of SAP in atherosclerotic intima was 190 +/- 64 micrograms/g wet tissue with an SAP-albumin ratio of 1:22.7, which is 44 times higher than the relative plasma ratio of 1:1000. This suggests that SAP is specifically localized in atherosclerotic lesions. SAP from the intima was indistinguishable from plasma or purified SAP with respect to immunological character and molecular weight. However, electrophoretic mobility and the binding of SAP to atherosclerotic intima appeared heterogeneous. Of total extractable SAP, about 43% appeared in the CaCl2-TBS fraction, 25% in the EDTA-TBS fraction, and 32% in the collagenase digestion fraction. SAP is one of the two pentraxins in human plasma; the other is C-reactive protein, which has also been reported to locate in atherosclerotic lesions. Our findings suggest a role for SAP in atherogenesis and encourage efforts to determine more precisely the physiological contributions of the pentraxin family to the development of atherosclerosis.
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Wang JZ, Li XA, Mayr NA. Dose escalation to combat hypoxia in prostate cancer: a radiobiological study on clinical data. Br J Radiol 2006; 79:905-11. [PMID: 16885177 DOI: 10.1259/bjr/18700614] [Citation(s) in RCA: 33] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/05/2022] Open
Abstract
Earlier studies have demonstrated that hypoxic regions exist in human prostate cancer and the degree of hypoxia correlates with the treatment outcome of radiotherapy. Using the concept of the clinical oxygen enhancement ratio (COER), the linear-quadratic (LQ) model was extended to account for the effect of tumour hypoxia. The clinical data collected at the Fox Chase Cancer Center for prostate cancer were analysed based on the LQ model as well as the tumour control probability (TCP) model. The LQ and TCP parameters (alpha = 0.15 Gy (-1), alpha/beta = 3.1 Gy and the number of clonogens K = 10(6) approximately 10(7) cells) determined in earlier studies were used to derive the COER for prostate cancer: COER = 1.4 with a standard confidence interval (CI) of (1.2, 1.8). The result is consistent with the in vitro OER measurements of human tumour cell lines under chronic hypoxia conditions. This implies that a higher dose is needed to overcome tumour hypoxia. For prostate tumours, the prescription dose required to overcome tumour hypoxia is 165 Gy (CI: 153 approximately 186 Gy) for permanent 125I implants and 88 Gy (CI: 74 approximately 118 Gy) in 2 Gy fractions for external-beam radiotherapy. The impact of LQ parameters on the calculations of COER and dose escalation was discussed. This study provides a preliminary estimate of the dose escalation needed to overcome tumour hypoxia based on clinical data. More clinical data with better statistics and longer follow-up time are required to further tune the radiobiological modelling of hypoxia for prostate cancer.
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Journal Article |
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Wilkins D, Li XA, Cygler J, Gerig L. The effect of dose rate dependence of p-type silicon detectors on linac relative dosimetry. Med Phys 1997; 24:879-81. [PMID: 9198022 DOI: 10.1118/1.597985] [Citation(s) in RCA: 33] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/04/2023] Open
Abstract
Cumulative radiation damage to silicon semiconductor diode detectors can induce dose rate dependent sensitivity, a concern in the pulsed beam of a linac. Two p-Si diode photon detectors were used in this study, diodes A and B. Both were preirradiated by the supplier to 5 kGy, with diode A receiving an estimated 8 kGy from measurements, and diode B, 25 kGy. At 6 MV, the PDD measured with diode B was lower (by 4.4% at a depth of 25 cm) than diode A. Using SSD to vary the dose per pulse from 0.02 to 0.64 mGy/pulse, diode A was dose rate independent (within 2%), while the sensitivity of diode B changed by 13%. Silicon diode detectors should be checked regularly against ionization chambers in the pulsed beam of a linac, especially older high-resistivity diodes that have accumulated dose from high-energy photon beams.
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Comparative Study |
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Ye SJ, Li XA, Zimmer JR, Chu JC, Choi CK. Dosimetric perturbations of linear array of beta-emitter seeds and metallic stent in intravascular brachytherapy. Med Phys 2000; 27:374-80. [PMID: 10718142 DOI: 10.1118/1.598841] [Citation(s) in RCA: 32] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022] Open
Abstract
The radiation treatment with catheter-based beta-emitter sources is under clinical trials to prevent restenosis following interventional coronary procedures. There are still large uncertainties in the dose calculation due to the complicated treatment geometry. We present the Monte Carlo simulations to account for the dosimetric perturbations due to neighboring trained seeds, proximal/distal gold markers, and a stainless steel stent. A catheter-based beta-emitter system is modeled using the Monte Carlo code, MCNP4B. Dose distributions and dose rates are calculated in voxels (0.64x0.64x0.5 mm3) around the long cylindrical trains of 90Sr/Y source with and without the stent (at 1.92 mm from the source axis). For the total activity of 70 mCi (2.59x10(9) Bq), the dose around most of the source length (except for edge seeds and gold markers) varies from 40 to 0.23 cGy/s as the radial distance from the source axis (r) increases from 0.64 to 6.4 mm. At the prescription range of r = 1.5-4.0 mm, the dose gradient is very steep and the contribution of neighboring seeds to the dose is significant. The dose enhancement due to neighboring seeds (the so-called "train effect") varies from 9% to 64% as r increases from 0.64 to 5.2 mm. The doses at r = 2 mm from the last edge seed and the gold marker are about 80% and 40% of that of the nonedge seed (8.7 cGy/s), respectively. The dose enhancement due to the secondary electrons and the primary electrons scattered with the stent is shown to be about 9.3% in the voxel including the stent. However, as r increases beyond the stent (r = 2.0-6.4 mm), the dose is slightly reduced by 4%-12%, compared to that without the stent.
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Li XA, Ma L, Naqvi S, Shih R, Yu C. Monte Carlo dose verification for intensity-modulated arc therapy. Phys Med Biol 2001; 46:2269-82. [PMID: 11580168 DOI: 10.1088/0031-9155/46/9/301] [Citation(s) in RCA: 31] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Abstract
Intensity-modulated arc therapy (IMAT), a technique which combines beam rotation and dynamic multileaf collimation, has been implemented in our clinic. Dosimetric errors can be created by the inability of the planning system to accurately account for the effects of tissue inhomogeneities and physical characteristics of the multileaf collimator (MLC). The objective of this study is to explore the use of Monte Carlo (MC) simulation for IMAT dose verification. The BEAM/DOSXYZ Monte Carlo system was implemented to perform dose verification for the IMAT treatment. The implementation includes the simulation of the linac head/MLC (Elekta SL20), the conversion of patient CT images and beam arrangement for 3D dose calculation, the calculation of gantry rotation and leaf motion by a series of static beams and the development of software to automate the entire MC process. The MC calculations were verified by measurements for conventional beam settings. The agreement was within 2%. The IMAT dose distributions generated by a commercial forward planning system (RenderPlan. Elekta) were compared with those calculated by the MC package. For the cases studied, discrepancies of over 10% were found between the MC and the RenderPlan dose calculations. These discrepancies were due in part to the inaccurate dose calculation of the RenderPlan system. The computation time for the IMAT MC calculation was in the range of 20-80 min on 15 Pentium-Ill computers. The MC method was also useful in verifying the beam apertures used in the IMAT treatments.
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Abstract
In this paper, the consistency of kilovoltage (tube potentials between 40 and 300 kV) x-ray beam dosimetry using the "in-air" method and the in-phantom measurement has been studied. The procedures for the measurement of the central-axis depth-dose curve, which serve as a link between the dose at the reference depth to the dose elsewhere in a phantom, were examined. The uncertainties on the measured dose distributions were analyzed with the emphasis on the surface dose measurement. The Monte Carlo method was used to calculate the perturbation correction factors for a photon diode and a NACP plane-parallel ionization chamber at different depths in a water phantom irradiated by 100-300 kV (2.43 mm Al-3.67 mm Cu half-value layer) x-ray beams. The depth-dose curves measured with these two detectors, after correcting for the perturbation effect (up to 15% corrections), agreed with each other to within 1.5%. Comparisons of the doses at the phantom surface and at 2 cm depth in water for photon beams of 100-300 kV tube potential obtained using the "backscatter" method and those using the "in-phantom" measurement have shown that the "in-air" method can be equally applied to this energy range if the depth-dose curve can be measured accurately. To this end, measured depth ionization curves require depth-dependent correction factors.
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Li XA, Chu JC, Chen W, Zusag T. Dose enhancement by a thin foil of high-Z material: a Monte Carlo study. Med Phys 1999; 26:1245-51. [PMID: 10435524 DOI: 10.1118/1.598619] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/18/2023] Open
Abstract
The purpose of this work is to study the dose enhancement by a thin foil (thickness of 0.2-4 mm) of high-Z material in a water phantom, irradiated by high-energy photon beams. EGS4 Monte Carlo technique was used. Perturbations on the beam spectra due to the presence of the foils, and dose enhancement dependence of photon-beam quality, beam incident angle, atomic number (Z), the thickness and size of the foil, and the depth of the foil situated in the phantom were studied. Analysis of photon and secondary-electron spectra indicates that the dose enhancement near an inhomogeneity interface is primarily due to secondary electrons. A calculation for 1-mm-thick planar lead foil in a water phantom shows that the dose enhancements at 0.25, 1, 2 and 3 mm away from the foil in the backward region were 58%, 37%, 24% and 17%, respectively, for a 15 MV beam. Calculations for a variety of planar foils and photon beams show that dose enhancement: (a) increases with Z; (b) decreases with decreasing foil thickness when the foils are thinner than a certain value (1 mm for lead foil for 15 MV); (c) decreases with decreasing incident photon-beam energies; (d) changes slightly for beam incident angles less than 45 degrees and more prominently for larger angles; (e) increases with size of foil; and (f) is almost independent of the depth at which the foil is situated when the foil is placed beyond the range of secondary electrons. The dose enhancement calculation is also performed for a cylindrically shaped lead foil irradiated by a four-field-box. The dose enhancement of 34%/13% was obtained at 0.25/2 mm away from the cylindrical outer interface for a 15 MV four-field-box.
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Li XA, Warren DW, Gregoire J, Pedersen RC, Lee AS. The rat 78,000 dalton glucose-regulated protein (GRP78) as a precursor for the rat steroidogenesis-activator polypeptide (SAP): the SAP coding sequence is homologous with the terminal end of GRP78. Mol Endocrinol 1989; 3:1944-52. [PMID: 2628731 DOI: 10.1210/mend-3-12-1944] [Citation(s) in RCA: 26] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/01/2023] Open
Abstract
Based on the striking sequence identity between the amino acid sequence of rat steroidogenesis-activator polypeptide (SAP) and the carboxyl terminus of the 78,000 dalton glucose-regulated protein (GRP78), the precursor-product relationship between GRP78 and SAP was investigated in Leydig cells. Immunoblot analysis with peptide antibodies specific for GRP78 and SAP showed that the putative SAP precursor is also immunoreactive with the anti-GRP78 antibody. Genomic blot hybridizations further revealed that GRP78 is neither rearranged nor amplified in the H-540 Leydig cell tumor, the original source for SAP. Further, there appears to be a single copy of the SAP coding sequence within the rat genome. This sequence resides within the last exon of GRP78. Our observations support the hypothesis that, in steroidogenic cells, SAP is likely to be derived from posttranslational processing of a very minor fraction of GRP78.
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Cygler J, Li XA, Ding GX, Lawrence E. Practical approach to electron beam dosimetry at extended SSD. Phys Med Biol 1997; 42:1505-14. [PMID: 9279902 DOI: 10.1088/0031-9155/42/8/003] [Citation(s) in RCA: 26] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023]
Abstract
This paper presents a practical approach to clinical dosimetry for electron beams at extended source-to-surface distance (SSD). Characteristics of electron beams from a Siemens MD-2 accelerator are presented at a nominal SSD of 100 cm and an extended SSD of 115 cm. Relative output factors are measured at both 100 and 115 cm SSDs for a range of square field sizes for each beam energy. The change of output with SSD does not follow the inverse square law, if the nominal SSD is used. This deviation is larger for lower beam energy and smaller field sizes. An effective SSD, SSDeff, has been determined at dmax for each beam energy as a function of field size. A comprehensive approach to treatment time calculations is proposed.
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Li XA, Rogers DW. Electron mass scattering powers: Monte Carlo and analytical calculations. Med Phys 1995; 22:531-41. [PMID: 7643788 DOI: 10.1118/1.597582] [Citation(s) in RCA: 24] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023] Open
Abstract
Values of electron mass scattering power, T/p, for various materials have been calculated by using the EGS4 Monte Carlo system and by integration of the Molière multiple-scattering distribution. The energy range covered is 0.5-100 MeV. Monte Carlo calculations test the concept of T/p "experimentally" and assess the contribution to electron mass scattering power from effects such as Moller scatter and energy-loss straggling. The Monte Carlo results agree within 2% with the analytical results calculated from Molière multiple-scattering theory at energies less than 20 MeV for high-Z materials and for energies less than 50 MeV for low-Z materials. At higher energies the Monte Carlo calculations include the effects of bremsstrahlung production which can significantly increase values of T/p. For low-Z materials and electron energies less than 60 MeV, the Monte Carlo calculated T/p values are generally 22% higher than those given by ICRU Report 35, while those for high-Z materials and energies less than 25 MeV are found to be consistent (within 1%) with ICRU Report 35. The effects of Moller scatter, which significantly affect T/p for low-Z materials, as well as bremsstrahlung effects, are included in the present Monte Carlo calculations. If the tabulated T/p data of ICRU Report 35 are modified to include the Moller scatter effect, then for energies less than 60 MeV they are generally 6% less than the present Monte Carlo data for low-Z materials as well as for copper. It is shown that T/p is a well-defined constant over an appropriate range of slab thickness except when bremsstrahlung effects are significant. It is found that T/p is proportional to E-n, where n is in the range of 1.5-2.0 for the energies considered here. The Monte Carlo calculations are shown to agree well with various relevant experimental measurements. Accurate T/p data, which should include the effect of Moller scatter, are necessary in electron-beam treatment planning, especially for a small field size. The choice of the depth step in the implementation of pencil-beam codes should not violate the slab-thickness limits for T/p data.
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Ma L, Li XA, Yu CX. An efficient method of measuring the 4 mm helmet output factor for the Gamma knife. Phys Med Biol 2000; 45:729-33. [PMID: 10730967 DOI: 10.1088/0031-9155/45/3/311] [Citation(s) in RCA: 21] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Abstract
It is essential to have accurate measurements of the 4 mm helmet output factor in the treatment of trigeminal neuralgia patients using the Gamma Knife. Because of the small collimator size and the sharp dose gradient at the beam focus, this measurement is generally tedious and difficult. We have developed an efficient method of measuring the 4 mm helmet output factor using regular radiographic films. The helmet output factor was measured by exposing a single Kodak XV film in the standard Leksell spherical phantom using the 18 mm helmet with 30-40 of its plug collimators replaced by the 4 mm plug collimators. The 4 mm helmet output factor was measured to be 0.876 +/- 0.009. This is in excellent agreement with our EGS4 Monte Carlo simulated value of 0.876 +/- 0.005. This helmet output factor value also agrees with more tedious TLD, diode and radiochromic film measurements that were each obtained using two separate measurements with the 18 mm helmet and the 4 mm helmet respectively. The 4 mm helmet output factor measured by the diode was 0.884 +/- 0.016, and the TLD measurement was 0.890 +/- 0.020. The radiochromic film measured value was 0.870 +/- 0.018. Because a single-exposure measurement was performed instead of a double exposure measurement, most of the systematic errors that appeared in the double-exposure measurements due to experimental setup variations were cancelled out. Consequently, the 4 mm helmet output factor is more precisely determined by the single-exposure approach. Therefore, routine measurement and quality assurance of the 4 mm helmet output factor of the Gamma Knife could be efficiently carried out using the proposed single-exposure technique.
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