Simon Lo - Academia.edu (original) (raw)
Papers by Simon Lo
International Journal of Radiation Oncology Biology Physics, 2009
The tumor oxygenation status is likely influenced by two major factors: local tumor blood supply ... more The tumor oxygenation status is likely influenced by two major factors: local tumor blood supply (tumor perfusion) and its systemic oxygen carrier, hemoglobin (Hgb). Each has been independently shown to affect the radiotherapy (RT) outcome in cervical cancer. This study assessed the effect of local tumor perfusion, systemic Hgb levels, and their combination on the treatment outcome in cervical cancer. A total of 88 patients with cervical cancer, Stage IB2-IVA, who were treated with RT/chemotherapy, underwent serial dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) before RT, at 20-22 Gy, and at 45-50 Gy. The DCE-MRI perfusion parameters, mean and lowest 10th percentile of the signal intensity distribution in the tumor pixels, and the Hgb levels, including pre-RT, nadir, and mean Hgb (average of weekly Hgb during RT), were correlated with local control and disease-specific survival. The median follow-up was 4.6 years. Local recurrence predominated in the group with both a low mean Hgb (<11.2 g/dL) and low perfusion (lowest 10th percentile of signal intensity <2.0 at 20-22 Gy), with a 5-year local control rate of 60% vs. 90% for all other groups (p = .001) and a disease-specific survival rate of 41% vs. 72% (p = .008), respectively. In the group with both high mean Hgb and high perfusion, the 5-year local control rate and disease-specific survival rate was 100% and 78%, respectively. These results suggest that the compounded effects of Hgb level and tumor perfusion during RT influence the radioresponsiveness and survival in cervical cancer patients. The outcome was worst when both were impaired. The management of Hgb may be particularly important in patients with low tumor perfusion.
International Journal of Radiation Oncology Biology Physics, 2004
Supratentorial primitive neuroectodermal tumor (PNET) accounts for 2–3% of all pediatric brain tu... more Supratentorial primitive neuroectodermal tumor (PNET) accounts for 2–3% of all pediatric brain tumors. We retrospectively reviewed all supratentorial PNET cases treated with radiotherapy (RT) at our institutions.A total of 25 patients (17 males and 8 females, median age 9 years) were treated with RT between 1980 and 2001. The primary site location was the pineal region in 7 (28%), temporal lobe in 5 (20%), thalamus in 5 (20%), frontal lobe in 4 (16%), parietal lobe in 2 (8%), and suprasellar region in 2 (8%). Five patients (20%) had neuraxis dissemination (M+ disease) at initial diagnosis. The RT treatment volumes were craniospinal (CS) in 17 (68%), whole brain (WB) followed by a boost in 2 (8%), and primary site (PS) alone in 6 (24%). The median dose to the primary site was 54 Gy (range, 31–55.8 Gy). The median dose to patients receiving WB and spinal fields was 36 Gy (range, 23.4–39.6 Gy). Sixteen patients (64%) received chemotherapy; the most common type was the “8 in 1” chemotherapy regimen in 9 children. The median follow-up of surviving patients was 70 months (range, 34–251 months).The 5-year and 10-year progression-free survival rate was 36% and 27%, respectively, and the median time to progression was 22 months. The 5-year and 10-year progression-free survival rate was 47.1% and 47.1% for those receiving CSRT and 12.5% and 0% for those receiving WBRT or PSRT, respectively. The 5-year and 10-year progression-free survival rate for those with M0 disease was 40.0% and 30.0%, respectively; for those with M+ disease, the corresponding figures were 20.0% and 0%. On multivariate analysis, only M status (p = 0.01) and RT volume (p = 0.02) were statistically significant according to the Cox proportional hazards model. The primary site control rate at 5 and 10 years was 62%. Failure at nontreated neuraxis sites was a common cause of progression in patients receiving WBRT or PSRT, as seen in 6 (75%) of 8 cases. Of the 17 patients undergoing CSRT, 8 had no recurrence. Eight of the nine CSRT relapses had a leptomeningeal component. Four (80%) of 5 M+ children and 4 (33%) of 12 M0 children who underwent CSRT developed recurrence in the neuraxis (p = 0.1, Fisher's exact test).The craniospinal axis is the standard volume that needs to be treated in supratentorial PNET. Leptomeningeal dissemination was the main obstacle for cure even in patients receiving CSRT, regardless of M status.
Surgical Endoscopy and Other Interventional Techniques, 2008
Background The Plicator™ (NDO Surgical, Inc., Mansfield, MA) endoscopically places a full-thickne... more Background The Plicator™ (NDO Surgical, Inc., Mansfield, MA) endoscopically places a full-thickness permanent suture to augment the antireflux barrier. At 3-years post-treatment, published results demonstrated a reduction in subjects’ gastroesophageal reflux disease (GERD) symptoms and related medication use. Aim To evaluate the Plicator’s safety and durability of effect at improving GERD symptoms at 5-years post-treatment. Methods A total of 33 chronic GERD sufferers across seven sites were followed for approximately 5 years (median follow-up: 59 months, range 50–65 months) after receiving a single full-thickness plication approximately 1 cm below the gastroesophageal (GE) junction in the anterior gastric cardia. At baseline, 30 out of 33 subjects required daily proton-pump inhibitor (PPI) therapy. Results Of the subjects who were PPI dependent prior to treatment 67% (20/30) remained off daily PPI therapy at 60 months and 5-year median GERD health-related quality-of-life (HRQL) scores show significant improvement from baseline off-meds scores (10 versus 19, p GERD-HRQL. No new adverse events were identified and all device-related events occurred acutely. These results were comparable to the results seen at 36 months follow-up. Conclusions Endoscopic full-thickness plication can reduce GERD symptoms and medication use for at least 5-years post procedure with no long-term adverse events post treatment.
Gastrointestinal Endoscopy, 2004
Background: The Plicator procedure has been designed to inhibit gastroesophageal (GE) reflux by f... more Background: The Plicator procedure has been designed to inhibit gastroesophageal (GE) reflux by fixating the full-thickness of the cardia wall under direct endoscopic visualization. The resulting serosa-to-serosa tissue union may help to accentuate and restore the valvular mechanism of the GE junction. Aim: To assess safety, efficacy, and 12-month durability of endoscopic full-thickness plication in patients with symptomatic GERD. Methods: Patients with chronic heartburn requiring maintenance anti-secretory therapy were recruited. Exclusions were hiatal hernia (>2cm), Grades III and IV esophagitis, and Barrett's. The following were assessed at baseline (on and off-meds) and 12 month post-plication: GERD-HRQL, GSRS, SF-36 questionnaires, and medication use. Additionally, 24-hr pH-metry and manometry were measured at baseline, three-months (pH/ manometry) and six-months (pH only) post-plication. All patients received a single, full-thickness plication in the gastric cardia 1-2cm below the GE junction. Results: Sixty-four patients (mean age 46.3, range 23-71) underwent endoscopic full-thickness plication. No re-treatments were performed. 12 months postprocedure, proton pump inhibitor (PPI) therapy remained completely discontinued in 68% (36/53) of PPI dependent patients and median GERD-HRQL scores (n=57) were improved when compared to baseline off-med (19.0 vs. 5.0) and on-med (13.0 vs. 5.0). In 24-hour pH-metry performed at six-months postprocedure (n = 46), 80% of patients demonstrated an improvement in distal esophageal acid exposure. Median % time pH < 4 decreased 39% with 30% of patients experiencing a normalization of pH at 6-months. No significant change in esophageal manometry was noted. All procedure related adverse events occurred acutely, including one gastric perforation which was managed conservatively without sequelae. Common adverse events included sore throat (41%) and abdominal pain (20%), resolving spontaneously within several days postprocedure. Conclusions: In this study, a single full-thickness plication placed at the GE junction reduced symptoms, medication use and esophageal acid exposure associated with GERD. Sustained reduction in PPI use at 1-year follow-up suggests durability of full-thickness tissue apposition.
Gastrointestinal Endoscopy, 2007
Gastrointestinal Endoscopy, 2004
A novel endoscopic full-thickness plication device has been designed to inhibit gastroesophageal ... more A novel endoscopic full-thickness plication device has been designed to inhibit gastroesophageal reflux by placing a transmural plication near the gastroesophageal junction under direct endoscopic visualization. This study assessed the safety and efficacy of endoscopic full-thickness plication in the treatment of patients with symptoms caused by GERD.Patients with chronic heartburn requiring maintenance therapy with antisecretory medication were recruited. Exclusion criteria were the following: hiatal hernia (>2 cm), grade III and IV esophagitis, and Barrett's esophagus. The following were assessed over a follow-up period of 6 months: GERD–Health-Related Quality of Life, Gastrointestinal Symptom Rating Scale, and SF-36 Health Survey, medication use, 24-hour esophageal pH monitoring and esophageal manometry. Patients underwent a single, full-thickness plication in the gastric cardia just distal to the gastroesophageal junction. Re-treatment was not permitted.A total of 64 patients (mean age 46.3 years, range 23-71 years) underwent endoscopic full-thickness plication (mean procedure time 17.2 minutes). At 6 months after plication, proton pump inhibitor therapy had been eliminated in 74% of previously medication-dependent patients. Median GERD–Health-Related Quality of Life scores improved 67% (19.0 vs. 5.0; p<0.001). Improvements also were observed in median Gastrointestinal Symptom Rating Scale and SF-36 Health Survey mental and physical composite scores. Median esophageal acid exposure improved significantly (10 vs. 8; p<0.008) with normalization of pH noted in 30% of patients. No significant change in esophageal manometry was noted. One gastric perforation occurred and was managed conservatively without sequelae.In this study, a single full-thickness plication placed at the gastroesophageal junction reduced symptoms, medication use, and esophageal acid exposure associated with GERD.
Gastrointestinal Endoscopy Clinics of North America, 2009
Small bowel endoscopy has made tremendous advances over the last 8 years. The introduction of cap... more Small bowel endoscopy has made tremendous advances over the last 8 years. The introduction of capsule endoscopy, double-balloon enteroscopy, single-balloon enteroscopy and spiral overtube-assisted enteroscopy have completely removed the mystery in investigating the small intestine. These new procedures are challenging and timeconsuming to perform. A brief overview on the technical issues and complications related to these small bowel endoscopy procedures is presented.
SSRN Electronic Journal, 2000
ABSTRACT It is well known that the competing risks model is identified if the dependence structur... more ABSTRACT It is well known that the competing risks model is identified if the dependence structure between risks (the copula function) is known or assumed. Special cases include independence of risks or independent censoring. If the copula function is not specified, parameters of interest are only set identified. As these sets are often wide in applications, it is difficult to obtain informative results. In this paper we strike a balance between imposing too much and too little structure. By establishing a general link between observable changes in subdistributions (cumulative incidence curves) and the sign of changes in marginal distributions (the causal treatment effect) we are able to show the identifiability of the latter if the copula function is independent of the varying covariate. This has two important implications: First, it is possible to obtain informative results even if the copula function is mainly unspecified or unknown. Second, the sign of the covariate effect tends to be invariant with respect to the chosen dependence structure. Our method is computationally very simple and our simulations suggest that it identifies and consistently estimates the sign of the treatment effect for large sets of duration times. An application to unemployment duration data illustrates the usefulness of our method for empirical research.
Techniques in Gastrointestinal Endoscopy, 2006
Capsule endoscopy has been around for 5 years, but formal training is still poorly incorporated i... more Capsule endoscopy has been around for 5 years, but formal training is still poorly incorporated into most gastroenterology fellowships. Practicing gastroenterologists may be considered adequately trained after a short 1-day training program. However, there is no accepted standard on the volume or competency requirement, and the current modes of training are probably insufficient. Capsule endoscopy is not a mere extension of regular endoscopy and should require specific reading and interpretive skills. We attempt to address these issues and make some suggestions on how we can become good at reading capsule endoscopy in this article. Tech Gastrointest Endosc 8:146-148
Medical Radiology, 2011
ABSTRACT
Comprehensive Management of Skull Base Tumors, 2008
Medical Radiology, 2010
Early diagnosis has been demonstrated to correlate with favorable treatment outcomes in patients ... more Early diagnosis has been demonstrated to correlate with favorable treatment outcomes in patients with nasopharyngeal carcinoma (NPC). Unfortunately, the clinical manifestation can often be very deceptive and confusing until the disease progresses into a relatively advanced stage. Coupled with the diffi culty associated with thorough examination of the nasopharynx, this disease presents a diagnostic challenge to physicians. Only about 10% of all new patients present with early disease despite recent advances in diagnostic techniques such as fi beroptic examination, diagnostic imaging, and tumor serology. The presenting symptoms are closely related to the location of the tumor in nasopharynx, the extent of local tumor invasion, and the degree of regional nodal metastasis. Because early symptoms are frequently minimal in nature, they are easily ignored by the physician and the patient. A high index of suspicion is crucial in a timely diagnosis of nasopharyngeal carcinoma, especially in endemic areas like Southeast Asia and areas with a large number of Chinese immigrants. As mentioned earlier, the treatment outcomes of NPC depend on the stage of the disease, and delaying the diagnosis of NPC can potentially be detrimental.
SSRN Electronic Journal, 2000
Many popular estimators for duration models require independent competing risks or independent ce... more Many popular estimators for duration models require independent competing risks or independent censoring. In contrast, copula based estimators are also consistent in presence of dependent competing risks. In this paper we suggest a computationally convenient extension of the Copula Graphic Estimator (Zheng and Klein, 1995) to a model with more than two dependent competing risks. We analyse the applicability of this estimator by means of simulations and real world unemployment duration data from Germany. We obtain evidence that our estimator yields nice results if the dependence structure is known and that it is a powerful tool for the assessment of the relevance of (in-)dependence assumptions in applied duration research.
International Journal of Radiation Oncology Biology Physics, 2009
The tumor oxygenation status is likely influenced by two major factors: local tumor blood supply ... more The tumor oxygenation status is likely influenced by two major factors: local tumor blood supply (tumor perfusion) and its systemic oxygen carrier, hemoglobin (Hgb). Each has been independently shown to affect the radiotherapy (RT) outcome in cervical cancer. This study assessed the effect of local tumor perfusion, systemic Hgb levels, and their combination on the treatment outcome in cervical cancer. A total of 88 patients with cervical cancer, Stage IB2-IVA, who were treated with RT/chemotherapy, underwent serial dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) before RT, at 20-22 Gy, and at 45-50 Gy. The DCE-MRI perfusion parameters, mean and lowest 10th percentile of the signal intensity distribution in the tumor pixels, and the Hgb levels, including pre-RT, nadir, and mean Hgb (average of weekly Hgb during RT), were correlated with local control and disease-specific survival. The median follow-up was 4.6 years. Local recurrence predominated in the group with both a low mean Hgb (&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt;11.2 g/dL) and low perfusion (lowest 10th percentile of signal intensity &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt;2.0 at 20-22 Gy), with a 5-year local control rate of 60% vs. 90% for all other groups (p = .001) and a disease-specific survival rate of 41% vs. 72% (p = .008), respectively. In the group with both high mean Hgb and high perfusion, the 5-year local control rate and disease-specific survival rate was 100% and 78%, respectively. These results suggest that the compounded effects of Hgb level and tumor perfusion during RT influence the radioresponsiveness and survival in cervical cancer patients. The outcome was worst when both were impaired. The management of Hgb may be particularly important in patients with low tumor perfusion.
International Journal of Radiation Oncology Biology Physics, 2004
Supratentorial primitive neuroectodermal tumor (PNET) accounts for 2–3% of all pediatric brain tu... more Supratentorial primitive neuroectodermal tumor (PNET) accounts for 2–3% of all pediatric brain tumors. We retrospectively reviewed all supratentorial PNET cases treated with radiotherapy (RT) at our institutions.A total of 25 patients (17 males and 8 females, median age 9 years) were treated with RT between 1980 and 2001. The primary site location was the pineal region in 7 (28%), temporal lobe in 5 (20%), thalamus in 5 (20%), frontal lobe in 4 (16%), parietal lobe in 2 (8%), and suprasellar region in 2 (8%). Five patients (20%) had neuraxis dissemination (M+ disease) at initial diagnosis. The RT treatment volumes were craniospinal (CS) in 17 (68%), whole brain (WB) followed by a boost in 2 (8%), and primary site (PS) alone in 6 (24%). The median dose to the primary site was 54 Gy (range, 31–55.8 Gy). The median dose to patients receiving WB and spinal fields was 36 Gy (range, 23.4–39.6 Gy). Sixteen patients (64%) received chemotherapy; the most common type was the “8 in 1” chemotherapy regimen in 9 children. The median follow-up of surviving patients was 70 months (range, 34–251 months).The 5-year and 10-year progression-free survival rate was 36% and 27%, respectively, and the median time to progression was 22 months. The 5-year and 10-year progression-free survival rate was 47.1% and 47.1% for those receiving CSRT and 12.5% and 0% for those receiving WBRT or PSRT, respectively. The 5-year and 10-year progression-free survival rate for those with M0 disease was 40.0% and 30.0%, respectively; for those with M+ disease, the corresponding figures were 20.0% and 0%. On multivariate analysis, only M status (p = 0.01) and RT volume (p = 0.02) were statistically significant according to the Cox proportional hazards model. The primary site control rate at 5 and 10 years was 62%. Failure at nontreated neuraxis sites was a common cause of progression in patients receiving WBRT or PSRT, as seen in 6 (75%) of 8 cases. Of the 17 patients undergoing CSRT, 8 had no recurrence. Eight of the nine CSRT relapses had a leptomeningeal component. Four (80%) of 5 M+ children and 4 (33%) of 12 M0 children who underwent CSRT developed recurrence in the neuraxis (p = 0.1, Fisher's exact test).The craniospinal axis is the standard volume that needs to be treated in supratentorial PNET. Leptomeningeal dissemination was the main obstacle for cure even in patients receiving CSRT, regardless of M status.
Surgical Endoscopy and Other Interventional Techniques, 2008
Background The Plicator™ (NDO Surgical, Inc., Mansfield, MA) endoscopically places a full-thickne... more Background The Plicator™ (NDO Surgical, Inc., Mansfield, MA) endoscopically places a full-thickness permanent suture to augment the antireflux barrier. At 3-years post-treatment, published results demonstrated a reduction in subjects’ gastroesophageal reflux disease (GERD) symptoms and related medication use. Aim To evaluate the Plicator’s safety and durability of effect at improving GERD symptoms at 5-years post-treatment. Methods A total of 33 chronic GERD sufferers across seven sites were followed for approximately 5 years (median follow-up: 59 months, range 50–65 months) after receiving a single full-thickness plication approximately 1 cm below the gastroesophageal (GE) junction in the anterior gastric cardia. At baseline, 30 out of 33 subjects required daily proton-pump inhibitor (PPI) therapy. Results Of the subjects who were PPI dependent prior to treatment 67% (20/30) remained off daily PPI therapy at 60 months and 5-year median GERD health-related quality-of-life (HRQL) scores show significant improvement from baseline off-meds scores (10 versus 19, p GERD-HRQL. No new adverse events were identified and all device-related events occurred acutely. These results were comparable to the results seen at 36 months follow-up. Conclusions Endoscopic full-thickness plication can reduce GERD symptoms and medication use for at least 5-years post procedure with no long-term adverse events post treatment.
Gastrointestinal Endoscopy, 2004
Background: The Plicator procedure has been designed to inhibit gastroesophageal (GE) reflux by f... more Background: The Plicator procedure has been designed to inhibit gastroesophageal (GE) reflux by fixating the full-thickness of the cardia wall under direct endoscopic visualization. The resulting serosa-to-serosa tissue union may help to accentuate and restore the valvular mechanism of the GE junction. Aim: To assess safety, efficacy, and 12-month durability of endoscopic full-thickness plication in patients with symptomatic GERD. Methods: Patients with chronic heartburn requiring maintenance anti-secretory therapy were recruited. Exclusions were hiatal hernia (>2cm), Grades III and IV esophagitis, and Barrett's. The following were assessed at baseline (on and off-meds) and 12 month post-plication: GERD-HRQL, GSRS, SF-36 questionnaires, and medication use. Additionally, 24-hr pH-metry and manometry were measured at baseline, three-months (pH/ manometry) and six-months (pH only) post-plication. All patients received a single, full-thickness plication in the gastric cardia 1-2cm below the GE junction. Results: Sixty-four patients (mean age 46.3, range 23-71) underwent endoscopic full-thickness plication. No re-treatments were performed. 12 months postprocedure, proton pump inhibitor (PPI) therapy remained completely discontinued in 68% (36/53) of PPI dependent patients and median GERD-HRQL scores (n=57) were improved when compared to baseline off-med (19.0 vs. 5.0) and on-med (13.0 vs. 5.0). In 24-hour pH-metry performed at six-months postprocedure (n = 46), 80% of patients demonstrated an improvement in distal esophageal acid exposure. Median % time pH < 4 decreased 39% with 30% of patients experiencing a normalization of pH at 6-months. No significant change in esophageal manometry was noted. All procedure related adverse events occurred acutely, including one gastric perforation which was managed conservatively without sequelae. Common adverse events included sore throat (41%) and abdominal pain (20%), resolving spontaneously within several days postprocedure. Conclusions: In this study, a single full-thickness plication placed at the GE junction reduced symptoms, medication use and esophageal acid exposure associated with GERD. Sustained reduction in PPI use at 1-year follow-up suggests durability of full-thickness tissue apposition.
Gastrointestinal Endoscopy, 2007
Gastrointestinal Endoscopy, 2004
A novel endoscopic full-thickness plication device has been designed to inhibit gastroesophageal ... more A novel endoscopic full-thickness plication device has been designed to inhibit gastroesophageal reflux by placing a transmural plication near the gastroesophageal junction under direct endoscopic visualization. This study assessed the safety and efficacy of endoscopic full-thickness plication in the treatment of patients with symptoms caused by GERD.Patients with chronic heartburn requiring maintenance therapy with antisecretory medication were recruited. Exclusion criteria were the following: hiatal hernia (>2 cm), grade III and IV esophagitis, and Barrett's esophagus. The following were assessed over a follow-up period of 6 months: GERD–Health-Related Quality of Life, Gastrointestinal Symptom Rating Scale, and SF-36 Health Survey, medication use, 24-hour esophageal pH monitoring and esophageal manometry. Patients underwent a single, full-thickness plication in the gastric cardia just distal to the gastroesophageal junction. Re-treatment was not permitted.A total of 64 patients (mean age 46.3 years, range 23-71 years) underwent endoscopic full-thickness plication (mean procedure time 17.2 minutes). At 6 months after plication, proton pump inhibitor therapy had been eliminated in 74% of previously medication-dependent patients. Median GERD–Health-Related Quality of Life scores improved 67% (19.0 vs. 5.0; p<0.001). Improvements also were observed in median Gastrointestinal Symptom Rating Scale and SF-36 Health Survey mental and physical composite scores. Median esophageal acid exposure improved significantly (10 vs. 8; p<0.008) with normalization of pH noted in 30% of patients. No significant change in esophageal manometry was noted. One gastric perforation occurred and was managed conservatively without sequelae.In this study, a single full-thickness plication placed at the gastroesophageal junction reduced symptoms, medication use, and esophageal acid exposure associated with GERD.
Gastrointestinal Endoscopy Clinics of North America, 2009
Small bowel endoscopy has made tremendous advances over the last 8 years. The introduction of cap... more Small bowel endoscopy has made tremendous advances over the last 8 years. The introduction of capsule endoscopy, double-balloon enteroscopy, single-balloon enteroscopy and spiral overtube-assisted enteroscopy have completely removed the mystery in investigating the small intestine. These new procedures are challenging and timeconsuming to perform. A brief overview on the technical issues and complications related to these small bowel endoscopy procedures is presented.
SSRN Electronic Journal, 2000
ABSTRACT It is well known that the competing risks model is identified if the dependence structur... more ABSTRACT It is well known that the competing risks model is identified if the dependence structure between risks (the copula function) is known or assumed. Special cases include independence of risks or independent censoring. If the copula function is not specified, parameters of interest are only set identified. As these sets are often wide in applications, it is difficult to obtain informative results. In this paper we strike a balance between imposing too much and too little structure. By establishing a general link between observable changes in subdistributions (cumulative incidence curves) and the sign of changes in marginal distributions (the causal treatment effect) we are able to show the identifiability of the latter if the copula function is independent of the varying covariate. This has two important implications: First, it is possible to obtain informative results even if the copula function is mainly unspecified or unknown. Second, the sign of the covariate effect tends to be invariant with respect to the chosen dependence structure. Our method is computationally very simple and our simulations suggest that it identifies and consistently estimates the sign of the treatment effect for large sets of duration times. An application to unemployment duration data illustrates the usefulness of our method for empirical research.
Techniques in Gastrointestinal Endoscopy, 2006
Capsule endoscopy has been around for 5 years, but formal training is still poorly incorporated i... more Capsule endoscopy has been around for 5 years, but formal training is still poorly incorporated into most gastroenterology fellowships. Practicing gastroenterologists may be considered adequately trained after a short 1-day training program. However, there is no accepted standard on the volume or competency requirement, and the current modes of training are probably insufficient. Capsule endoscopy is not a mere extension of regular endoscopy and should require specific reading and interpretive skills. We attempt to address these issues and make some suggestions on how we can become good at reading capsule endoscopy in this article. Tech Gastrointest Endosc 8:146-148
Medical Radiology, 2011
ABSTRACT
Comprehensive Management of Skull Base Tumors, 2008
Medical Radiology, 2010
Early diagnosis has been demonstrated to correlate with favorable treatment outcomes in patients ... more Early diagnosis has been demonstrated to correlate with favorable treatment outcomes in patients with nasopharyngeal carcinoma (NPC). Unfortunately, the clinical manifestation can often be very deceptive and confusing until the disease progresses into a relatively advanced stage. Coupled with the diffi culty associated with thorough examination of the nasopharynx, this disease presents a diagnostic challenge to physicians. Only about 10% of all new patients present with early disease despite recent advances in diagnostic techniques such as fi beroptic examination, diagnostic imaging, and tumor serology. The presenting symptoms are closely related to the location of the tumor in nasopharynx, the extent of local tumor invasion, and the degree of regional nodal metastasis. Because early symptoms are frequently minimal in nature, they are easily ignored by the physician and the patient. A high index of suspicion is crucial in a timely diagnosis of nasopharyngeal carcinoma, especially in endemic areas like Southeast Asia and areas with a large number of Chinese immigrants. As mentioned earlier, the treatment outcomes of NPC depend on the stage of the disease, and delaying the diagnosis of NPC can potentially be detrimental.
SSRN Electronic Journal, 2000
Many popular estimators for duration models require independent competing risks or independent ce... more Many popular estimators for duration models require independent competing risks or independent censoring. In contrast, copula based estimators are also consistent in presence of dependent competing risks. In this paper we suggest a computationally convenient extension of the Copula Graphic Estimator (Zheng and Klein, 1995) to a model with more than two dependent competing risks. We analyse the applicability of this estimator by means of simulations and real world unemployment duration data from Germany. We obtain evidence that our estimator yields nice results if the dependence structure is known and that it is a powerful tool for the assessment of the relevance of (in-)dependence assumptions in applied duration research.