Biomedical subjects

Ivan Yeung

Publications and source records attributed to Ivan Yeung.

13 recordsLinked to original sources

PubMed2006

Radiation dose to the internal pudendal arteries from permanent-seed prostate brachytherapy as determined by time-of-flight MR angiography.

PURPOSE: To determine the feasibility of time-of-flight magnetic resonance (MR) angiography to visualize the internal pudendal arteries (IPAs) in potent men undergoing permanent-seed prostate brachytherapy and to calculate the radiation dose received by these arteries. METHODS AND MATERIALS: Prostate brachytherapy is performed at the University Health Network/Princess Margaret Hospital by use of transrectal ultrasound (TRUS) preplanning and preloaded needles. All patients received (125)I, with a mean seed activity of 0.32 mCi/seed (0.41 U). Postplan evaluation is performed at 1 month by magnetic resonance-computed tomography fusion. Twenty consecutive potent men had time-of-flight MR angiography as part of their postplan evaluation. RESULTS: The mean V100 was 96.5%, and the mean D90 was171.5 Gy. The IPAs were easily visualized for 18 of the 20 men. The mean peak dose received by the IPA was 17 Gy. The highest peak dose received by any patient was 38.2 Gy, with only 1 other patient receiving a peak dose greater than 30 Gy. Eleven of 18 had a measurable portion of at least 1 IPA that received 10% of the prescribed dose (V10 = 14.5 Gy). Only 2 patients had nonzero values for V25. The distal third of the IPA received the highest dose for 16 of the 18 patients. CONCLUSIONS: The IPAs can be well visualized in the majority of potent men by use of time-of-flight MR angiography 1 month after brachytherapy. The IPAs receive a low but calculable dose from permanent-seed (125)I brachytherapy. Further research is needed to determine if this outcome has any correlation with subsequent potency.

Aged↗

PubMed2006

Rectal-wall dose dependence on postplan timing after permanent-seed prostate brachytherapy.

PURPOSE: Dose to rectal wall after permanent-seed prostate brachytherapy is dependent on distance between posterior prostatic seeds and anterior rectal wall and is influenced by postimplant periprostatic edema. We analyzed the effect of postplan timing on anterior rectal-wall dose. METHODS AND MATERIALS: Twenty patients received permanent seed 125I brachytherapy as monotherapy (145 Gy). Implants were preplanned by use of transrectal ultrasound (TRUS) and carried out by use of preloaded needles. Postimplant dosimetry was calculated by use of magnetic resonance imaging-computed tomography fusion on Days 1, 8, and 30. The anterior rectal-wall dose is reported as the isodose enclosing 1.0 or 2.0 cc of rectal wall and as the RV100 in cc. RESULTS: The dose to rectal wall increased progressively over time. The median increase in dose to 1.0 cc of rectal wall (RD [1 cc]) from Day 1 to 30 was 39.2 Gy (p < 0.001). RV100 increased from a median of 0.07 cc on Day 1 to 0.67 cc on Day 30. The most significant predictor of rectal-wall dose (RD [1 cc], RD [2 cc], or RV100) was the time of evaluation (p < 0.001). CONCLUSION: Although periprostatic edema cannot be quantified by postimplant imaging, the dose to the anterior rectal wall increases significantly over time as prostatic and periprostatic edema resolve. Critical-organ dose reporting and guidelines for minimizing toxicity must take into account the time of the assessment.

Aged↗

PubMed2005

Comparison of late toxicity between continuous low-dose-rate and pulsed-dose-rate brachytherapy in cervical cancer patients.

PURPOSE: To compare survival and severe late radiation toxicity between patients who received continuous low-dose-rate (LDR) brachytherapy (BT) and pulsed-dose-rate (PDR) BT for cervical cancer. METHODS AND MATERIALS: A retrospective review of cervical cancer patients who underwent primary radiotherapy with or without concurrent cisplatin was performed. Late Grade 3 or worse toxicities were assessed using the National Cancer Institute Common Toxicity Criteria. The study endpoints were overall and disease-free survival and the probability of severe late toxicity. RESULTS: A total of 109 patients (65.7%) received LDR BT and 57 (34.3%) received PDR BT. Seventy patients received concurrent chemotherapy with cisplatin. The 3-year overall survival and disease-free survival rate was 70% and 57% for the LDR group and 82% and 70% for the PDR group, respectively (p = 0.25 and p = 0.19). The 3-year probability rate for late Grade 3 or worse toxicity was 7.4% for LDR BT patients and 7.6% for PDR BT patients, respectively (p = 0.69) and 6.9% and 7.6%, respectively, for concurrent chemotherapy vs. none (p = 0.69). CONCLUSION: No difference was found in severe late toxicity, overall survival, or disease-free survival between the LDR and PDR groups.

Adult↗

PubMed2005

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

PURPOSE: To analyze the extent and time course of prostate edema and its effect on dosimetry after permanent seed prostate brachytherapy. METHODS AND MATERIALS: Twenty patients scheduled for permanent seed (125)I prostate brachytherapy agreed to a prospective study on postimplant edema. Implants were preplanned using transrectal ultrasonography. Postimplant dosimetry was calculated using computed tomography-magnetic resonance imaging (CT-MRI) fusion on the day of the implant (Day 1) and Days 8 and 30. The prostate was contoured on MRI, and the seeds were located on CT. Factors investigated for an influence on edema were the number of seeds and needles, preimplant prostate volume, transitional zone index (transition zone volume divided by prostate volume), age, and prostate-specific antigen level. Prostate dosimetry was evaluated by the percentage of the prostate volume receiving 100% of the prescribed dose (V(100)) and percentage of prescribed dose received by 90% of the prostate volume (D(90)). RESULTS: Prostate edema was maximal on Day 1, with the median prostate volume 31% greater than preimplant transrectal ultrasound volume (range, 0.93-1.72; p < 0.001) and decreased with time. It was 21% greater than baseline at Day 8 (p = 0.013) and 5% greater on Day 30 (p < 0.001). Three patients still had a prostate volume greater than baseline by Day 30. The extent of edema depended on the transition zone volume (p = 0.016) and the preplan prostate volume (p = 0.003). The median V(100) on Day 1 was 93.6% (range, 86.0-98.2%) and was 96.3% (range, 85.7-99.5%) on Day 30 (p = 0.079). Patients with a Day 1 V(100) >93% were less affected by edema resolution, showing a median increase in V(100) of 0.67% on Day 30 compared with 2.77% for patients with a V(100) <93 % on Day 1. CONCLUSION: Despite the extreme range of postimplant edema, the effect on dosimetry was less than expected. Dose coverage of the prostate was good for all patients during Days 1-30. Our data indicate that postimplant dosimetry on the day of implant is sufficient for patients with good dose coverage (Day 1 V(100) >93%).

Aged↗

PubMed2005

Assessment of the tumor microenvironment in cervix cancer using dynamic contrast enhanced CT, interstitial fluid pressure and oxygen measurements.

PURPOSE: Interstitial fluid pressure (IFP) and oxygen (pO(2)) measurements are prognostic factors in cervical cancer. The purpose of this study was to determine the relationship between IFP and oxygenation and parameters derived from dynamic contrast-enhanced computed tomography (DCE-CT). METHODS AND MATERIALS: Dynamic contrast-enhanced computed tomography was performed in 32 patients with cervical cancer before radiation therapy. Images were acquired during intravenous contrast injection at 1 per s for 120 s and 1 per 15 s for 60 s. DCE-CT was analyzed using CT Perfusion 3 software (GE Medical Systems) to derive tumor blood flow (BF), permeability surface area product, blood volume, and mean transit time. Further analysis was performed to obtain relative peak enhancement, residual enhancement at 3 min after contrast injection (RE), time to peak and initial slope. Nodal status and tumor size were assessed with MRI. From in vivo IFP (n = 31) and pO(2) (n = 31) tumor measurements median pO(2) (mO(2)), percentage measurements less than 5 mm Hg (HP5) and mean IFP values were calculated. RESULTS: There was a positive correlation between BF and mO(2) (r = 0.47, p = 0.007) and between RE and HP5 (r = 0.39, p = 0.03). There was no correlation between IFP and DCE-CT parameters. CONCLUSION: There is a moderately positive, correlation between tumor oxygenation and BF as well as RE and HP5. Further study is required to determine if DCE-CT parameters are useful predictors of tumor behavior in cervical cancer.

Extracellular Fluid↗

PubMed2005

Assessing perfusion changes during whole brain irradiation for patients with cerebral metastases.

PURPOSE: To assess perfusion changes within brain and correlate these with clinical symptoms during whole brain radiotherapy (WBRT) for cerebral metastases. MATERIALS AND METHODS: Fourteen patients with cerebral metastases underwent dynamic CT perfusion scans during palliative whole brain irradiation. Perfusion scans were performed on Day 1 prior to initial radiotherapy treatment, then on Day 2, and on Day 5 immediately after completion of radiotherapy. Measurements of cerebral blood flow (CBF), cerebral blood volume (CBV), mean transit time (MTT) and capillary surface area permeability product (PS) were performed for each perfusion scan, and daily symptom assessment was taken prior to initial perfusion scan and thereafter prior to each daily radiation treatment. RESULTS: Utilizing Day 1 as baseline, a 19% increase (P = 0.033) was noted in PS at Day 2, (median 1.47 ml/100 g/min), which returned to Day 1 range at Day 5 (median 1.31 ml/100 g/min). When symptoms were correlated with perfusion parameters, a statistically significant association between change in MTT with change in headache scores was observed, baseline to Day 2 (P = 0.019), and a trend between change in nausea scores with change in CBV (P = 0.059) as well as change in MTT (P = 0.098), baseline to Day 5. CONCLUSION: This study has demonstrated the feasibility of a non-invasive technique to assess changes occurring within the human brain during a course of radiation treatment. Dynamic perfusion tomography provides insight into the pathophysiological processes taking place and allows correlation with patient symptomatology.

Adult↗

PubMed2005

Radiation-induced craniofacial bone growth inhibition: efficacy of cytoprotection following a fractionated dose regimen.

BACKGROUND: Severe craniofacial growth disturbances are noted in 66 to 100 percent of children with head and neck cancers who received radiotherapy during their growing years. The authors have previously demonstrated the prevention of radiation-induced craniofacial bone growth inhibition following single-dose orthovoltage radiation to the orbitozygomatic complex in an infant rabbit model through the administration of the cytoprotective agent amifostine (WR-2721) before radiation treatment. The purpose of this study was to investigate the efficacy of cytoprotection using a fractionated dose regimen that better approximates the clinical application of radiation therapy. METHODS: Thirty 7-week-old male New Zealand rabbits were randomized into three groups (n = 10), each receiving six fractions of orthovoltage radiation to the right orbitozygomatic complex: group C, sham irradiation control; group F35, total dose of 35 Gy; and group F35A, total dose of 35 Gy with administration of amifostine 200 mg/kg intravenously 20 minutes before each fraction. Bone growth was evaluated up to skeletal maturity (age 21 weeks) with serial radiographs and computed tomography scans for cephalometric analysis, bone volume, and bone density measurements. RESULTS: Fractionated radiation resulted in significant (p < 0.05) bone growth inhibition compared with sham radiation in 16 of 21 cephalometric parameters measured and significantly (p < 0.05) reduced bone volume of the rabbit orbitozygomatic complex. Pretreatment with amifostine before each radiation fraction prevented growth deformities in four cephalometric parameters and significantly (p < 0.05) attenuated these effects in another seven parameters compared with radiated animals. Bone volumes were also significantly (p < 0.05) improved in F35A animals compared with F35 animals. CONCLUSIONS: This study establishes that fractionation of orthovoltage radiation does not prevent the development of growth disturbances of the rabbit craniofacial skeleton and also demonstrates that preirradiation administration of amifostine is highly effective in the prevention and attenuation of radiation-induced craniofacial bone growth inhibition.

Amifostine↗

PubMed2004

MRI-CT fusion to assess postbrachytherapy prostate volume and the effects of prolonged edema on dosimetry following transperineal interstitial permanent prostate brachytherapy.

PURPOSE: Quality assurance through postplan assessment is an integral part of permanent seed prostate implants. The use of MRI-CT fusion for 1-month postimplant dosimetry permits accurate assessment of prostate volume without seed induced artifact and uncertainties of prostate contour inherent to CT assessments. Routine use of MRI-CT fusion reveals significant prostate edema may persist several weeks. This study evaluates the effect of edema, and its subsequent resolution, on dosimetry. METHODS AND MATERIALS: From May 2001 to June 2003, 241 men were treated with (125)I seed implants based on a transrectal ultrasound (TRUS) preplan. Quality assessment was performed at 1 month by CT-MRI fusion using VariSeed software. Over this 24-month period, 29 patients (12%) with residual edema at 1 month (12-60% >TRUS plan volume), had repeat CT-MRI fusion at 2-4 months to reassess volume and dosimetry. Eleven of the 29 had received prior androgen ablation to shrink the prostate preimplant. RESULTS: For the entire group (n = 241), mean preimplant prostate volume was 33.7 cc and median postplan dosimetric parameters were: V100, 92.2%; D90, 153 Gy; and V150, 53%. For the 29 patients with prolonged edema, mean preimplant volume was 34.8 cc and 1-month volume was 46.1 cc (p <0.001). Mean volume reduction between 1 and 2 months was 13%. The decrease in prostate volume had a significant effect on dosimetry with median increase between 1 month and 2 months in calculated V100 of 9.5%, V150 22.6%, V200 30.1%, and D90 11.5%. CONCLUSIONS: Significant residual edema is seen 1-month postimplant in 12% of prostates and may have a profound effect on dosimetry. Further study is underway to characterize the time course of resolution of the edema, and to perform integral dosimetry based on the changing volume.

Brachytherapy↗

PubMed2004

Factors predicting an increased dose to the penile bulb in permanent seed prostate brachytherapy.

PURPOSE: Erectile dysfunction following permanent seed brachytherapy for prostate cancer may be related to the dose to the penile bulb. We investigated anatomic and dosimetric factors that might contribute to an increased dose to the penile bulb. METHODS AND MATERIALS: One-month CT and MR images were examined for 50 consecutive patients treated with exclusive (125)I permanent seed prostate brachytherapy to a prescribed dose of 145 Gy. Implants were preplanned by transrectal ultrasound (TRUS). Postimplant dosimetry was performed at 1 month using an MRI-CT fusion. Spearman's correlation was used to establish a correlation between dosimetric parameters, anatomical factors, and the dose to the penile bulb. RESULTS: Penile bulb volumes ranged from 1.2-8.5 cc (median, 3.9 cc). The distance from the penile bulb to the prostate apex ranged from 5-33 mm (median, 15.5 mm). D50 of the penile bulb ranged from 13-121 Gy. The range for the V45 (65 Gy) was 0-87%; only 3% of patients had >2 cc covered by this isodose and in 16% of patients the V45 covered more than 50% of the penile bulb. About one-third of the patients received a dose to the bulb that would put them at a high risk of erectile dysfunction after external beam radiation, if the dose were radiobiologically equivalent. There was a significant inverse correlation between the distance between the prostate apex and the penile bulb, and the dosimetric parameters of the bulb: r = -0.548, -0.656, p = < 0.01. The further caudal the apex was from the symphysis, the closer it was to the penile bulb (r = -0.564, p = <0.01). We could not find a correlation between the dose to the prostate or its apex and the dose to the penile bulb. CONCLUSION: When the prostate apex is close to the penile bulb, care should be taken to limit the dose to the penile bulb, if possible. This may reduce the incidence of erectile dysfunction and urinary toxicity after permanent seed prostate brachytherapy.

Brachytherapy↗

PubMed2002

Efficacy of radioprotection in the prevention of radiation-induced craniofacial bone growth inhibition.

It has been reported that radiotherapy-induced craniofacial deformities can occur in 66 to 100 percent of survivors of childhood head and neck cancers. Recent interest in the effectiveness of radioprotectors in the protection of normal tissue against radiation injury led us to investigate a possible role of radioprotection in the prevention of radiation-induced craniofacial bone growth inhibition. Therefore, the objective of this study was to use the radioprotective agent amifostine (Ethyol, WR-2721) as a probe to determine the effectiveness of radioprotection in the prevention of radiation-induced craniofacial bone growth inhibition after single-dose orthovoltage radiation to the infant rabbit orbital-zygomatic complex. Seven-week-old male New Zealand white rabbits were randomized into three groups (n = 10 each): group 1, 0 Gy (sham radiation); group 2, 35-Gy single-dose orthovoltage radiation; and group 3, 35-Gy single-dose orthovoltage radiation and amifostine (300 mg/kg intravenously, given 20 minutes before radiation). Serial radiographs and computed tomographic scans were obtained for cephalometric analysis, bone volume, and bone density measurements until skeletal maturity at 21 weeks. Significant (p < 0.05) reductions in orbital-zygomatic complex linear bone growth, bone volume, and bone density were observed after 35-Gy radiation compared with nonirradiated controls. No significant differences were noted between groups in cephalometric analysis of the nontreated (nonirradiated) left orbital-zygomatic complex, indicating no crossover effect from the radiation beam. However, pretreatment with amifostine, 20 minutes before 35-Gy radiation, resulted in significant (p < 0.05) preservation of linear bone growth, bone volume, and bone mineral density in the rabbit orbital-zygomatic complex compared with controls. This study demonstrated for the first time the effectiveness of a radioprotector in the prevention of radiation-induced craniofacial bone growth inhibition, and it paves the way for investigation into the pathogenic mechanism and prevention of radiotherapy-induced craniofacial deformities.

Amifostine↗

PubMed2002

Factors influencing risk of acute urinary retention after TRUS-guided permanent prostate seed implantation.

PURPOSE: To look for factors predictive of acute urinary retention (AUR) after permanent seed prostate brachytherapy. METHODS AND MATERIALS: From March 1999 to February 2001, 150 permanent seed prostate implants were performed at Princess Margaret Hospital (Stage T1c, n = 113; T2a, n = 37; mean prostate-specific antigen level 5.9 ng/mL, prescription dose 145 Gy per Task Group No. 43). alpha-Blockers were used routinely after implantation. Dosimetry was based on the 1-month postimplant CT scan. The International Prostate Symptom Score (IPSS) and catheterization were recorded at 1 month and 3 months and then every 3 months. The following variables were examined: age, baseline IPSS, prior androgen ablation, prostate transrectal ultrasound volume, number of seeds, D(90), V(100), V(200), and urethral dose. RESULTS: Twenty patients (13%) experienced AUR. No difference was seen in the mean D(90) (149 Gy vs. 152 Gy, p = 0.6), V(100) (90% vs. 91%, p = 0.6), V(200) (23% vs. 25% p = 0.4), IPSS (6.4 vs. 5.9, p = 0.8), or maximal urethral dose (204 Gy vs. 210 Gy, p = 0.5). The prostate volume was significantly larger in men with AUR (39.8 cm(3) vs. 34.3 cm(3), p = 0.003), and the mean number of seeds was higher (112 vs. 103, p = 0.006). Of the 20 patients experiencing AUR, 11 (55%) had received prior antiandrogen therapy to downsize their prostates vs. 35 (27%) of the 130 who did not have AUR (p = 0.02). Multivariate analysis showed prostate volume and prior hormone use to be independent predictors of AUR. CONCLUSIONS: Implant quality as determined by D(90), V(100), V(200), and urethral dose did not predict AUR. Prostate size was the major determinant of AUR. For any given prostate size, prior androgen ablation increased the risk of AUR. Men with larger prostates should be aware of the increased risk when contemplating brachytherapy.

Aged↗

PubMed2002

Interobserver variation in postimplant computed tomography contouring affects quality assessment of prostate brachytherapy.

PURPOSE: Permanent seed implants are accepted treatment of early stage prostate cancer. Implant quality is assessed by post implant CT-based dosimetry but prostate contours on CT images are obscured by metallic seed artefact and edema. Outcome depends on implant quality, but perceived implant quality depends on accurate prostate contouring. This study documents inter observer variation in prostate contouring on post implant CT scans. METHODS AND MATERIALS: Ten patients had implant dosimetry calculated on 4 copies of the post implant CT scan. Prostate contours from MRI-CT fusion were the gold standard for prostate edge identification. CTs were contoured by an experienced prostate brachytherapist matching CT images to the pre implant TRUS, and by 2 GU radiation oncologists experienced in conformal radiotherapy planning. Dosimetry was compared to that obtained using MRI-CT fusion in terms of D90 and V100. RESULTS: Contours and dosimetry were not reproducible among the 3 observers. The V100's of the experienced brachytherapist differed from that of MRI-CT fusion by a mean of 2.4% compared to 9.1% and 4.4% for observers 1 and 2, and the D90 by a mean of 9.3 Gy compared to 30.3 and 14.4 Gy for observers 1 and 2. CONCLUSIONS: Quality assessment of prostate brachytherapy based on 1 month post implant CT is difficult. This may obscure the dose-response relationship in brachytherapy as well as create problems for quality assurance in multicentre trials evaluating brachytherapy against standard modalities. Whenever possible, MRI-CT fusion should be employed to verify prostate contours post implant.

Brachytherapy↗

PubMed2002

The effect of voxel size on the accuracy of dose-volume histograms of prostate 125I seed implants.

Cumulative dose-volume histograms (DVH) are crucial in evaluating the quality of radioactive seed prostate implants. When calculating DVHs, the choice of voxel size is a compromise between computational speed (larger voxels) and accuracy (smaller voxels). We quantified the effect of voxel size on the accuracy of DVHs using an in-house computer program. The program was validated by comparison with a hand-calculated DVH for a single 0.4-U iodine-125 model 6711 seed. We used the program to find the voxel size required to obtain accurate DVHs of five iodine-125 prostate implant patients at our institution. One-millimeter cubes were sufficient to obtain DVHs that are accurate within 5% up to 200% of the prescription dose. For the five patient plans, we obtained good agreement with the VariSeed (version 6.7, Varian, USA) treatment planning software's DVH algorithm by using voxels with a sup-inf dimension equal to the spacing between successive transverse seed implant planes (5 mm). The volume that receives at least 200% of the target dose, V200, calculated by VariSeed was 30% to 43% larger than that calculated by our program with small voxels. The single-seed DVH calculated by VariSeed fell below the hand calculation by up to 50% at low doses (30 Gy), and above it by over 50% at high doses (>250 Gy).

Algorithms↗