Biomedical subjects

Jean Pouliot

Publications and source records attributed to Jean Pouliot.

35 records · Page 2Linked to original sources

PubMed2004

Dosimetric impact of prostate volume change between CT-based HDR brachytherapy fractions.

PURPOSE: The objective is to evaluate the prostate volume change and its dosimetric consequences after the insertion of catheters for high-dose-rate brachytherapy. METHODS AND MATERIALS: For 13 consecutive patients, a spiral CT scan was acquired before each of the 2 fractions, separated on average by 20 hours. The coordinates of the catheters were obtained on 3 axial CT slices corresponding to apex, mid portion, and base portion of the prostate. A mathematical expansion model was used to evaluate the change of prostate volumes between the 2 fractions. It is based on the difference in the cube of the average distance between the centroid and catheter positions. The variation of implant dose-volume histograms between fractions was computed for plans produced by either inverse planning based on simulated annealing or geometric optimization. RESULTS: The average magnitude of either increase or reduction in prostate volume was 7.8% (range, 2-17%). This volume change corresponds to an average prostate radius change of only 2.5% (range, 0.7-5.4%). For 5 patients, the prostate volume increased on average by 9% (range, 2-17%), whereas a reduction was observed for 8 patients by an average of 7% (range, 2-13%). More variation was observed at the prostate base than at mid or apex gland. The comparison of implant dose-volume histograms showed a small reduction of V100 receiving the prescription dose, with an average of 3.5% (range, 0.5-12%) and 2.2% (range, 1-6%) for inverse planning based on our simulated annealing and geometric optimization plans, respectively. CONCLUSION: Small volume change was observed between treatment fractions. This translates into small changes in dose delivered to the prostate volume.

Adult↗

PubMed2004

Inverse planning for HDR prostate brachytherapy used to boost dominant intraprostatic lesions defined by magnetic resonance spectroscopy imaging.

PURPOSE: To dose escalate selected regions inside the prostate without compromising the dose coverage of the prostate and the protection to the urethra, rectum, and bladder for prostate cancer patients treated with high-dose-rate brachytherapy. METHODS AND MATERIALS: Magnetic resonance imaging combined with magnetic resonance spectroscopy imaging was used to differentiate between normal and malignant prostate and define cancer-validated dominant intraprostatic lesions (DIL) on 10 patients. The DILs were then contoured on the planning scans (CT or MRI based, 5 patients each), and our inverse planning dose optimization algorithm (called IPSA) was used to generate dose distributions for 3 different boost levels. Dose-volume histograms of the target and each organ at risk were compared with optimized plans without DIL boost. RESULTS: Combined MRI/magnetic resonance spectroscopic imaging identified 2 DILs in 8/10 of the 10 patients studied and a single DIL in the remaining 2 patients. The average prostate dose coverage V100 was 97% (sigma = 1.0%). When the minimum DIL dose requested was 120% of the prescribed dose, the average DIL V120 was 97.1% (sigma = 1.8%). For a boost value of 150%, the average V150 ranged from 77.8% to 86.1%, depending on the upper limit of the dose constraints. The bladder V50 increased by 1%, independently of the boost levels. The absolute increases in V50 for the rectum varied from 1% to 3%, depending on the boost level. The urethra V120 were increased by 13.4% and 32.5% for the lowest and highest boost levels, respectively. CONCLUSION: The DIL dose can be escalated to a minimum of 120% while the entire prostate is treated simultaneously, without increasing the dose to surrounding normal tissues. Higher boost levels between 150% and 170% are feasible, but with slightly larger doses delivered to the rectum and urethra.

Algorithms↗

PubMed2004

Daily electronic portal imaging for morbidly obese men undergoing radiotherapy for localized prostate cancer.

PURPOSE: We summarize our experience with a series of morbidly obese men treated using daily online portal imaging and implanted gold markers to guide external beam radiation therapy (EBRT). METHODS AND MATERIALS: Three consecutive morbidly obese men were treated with EBRT for localized prostate cancer. Daily electronic portal imaging was used to verify patient position. The magnitude and direction of patient positioning error were documented for each fraction. RESULTS: The absolute magnitude of positioning error was greatest in the left-right direction with a mean of 11.4 mm/fraction (median, 8 mm; range, 0-42 mm). Mean error in the superior-inferior direction was also substantial at 7.2 mm/fraction (median, 5 mm; range, 0-47 mm). Anteroposterior error was the least problematic with a mean value of 2.6 mm/fraction (median, 2.5 mm; range, 0-8 mm). CONCLUSIONS: Daily electronic portal imaging combined with gold fiducial markers dramatically improves the precision of EBRT in the treatment of morbidly obese men with prostate cancer. Setup error rather than organ motion appears to be the dominant force in positioning error in obese men.

Gold↗

PubMed2004

The robustness of dose distributions to displacement and migration of 125I permanent seed implants over a wide range of seed number, activity, and designs.

PURPOSE: To investigate the robustness of permanent prostate implant dosimetry for various (125)I seed activities and various seed models. The dosimetric impact of seed misplacement and seed migration (seed loss) is also taken into account using various standard dose indices. METHODS AND MATERIALS: A dose-based inverse planning algorithm is used for automated dosimetric plan creation (45-60 s per plan) and provides an unbiased way to compare the robustness of various optimal dosimetric plans. Seed misplacement and seed migration are simulated by way of Monte Carlo, based on the measured displacement distributions from clinical postimplant cases. Plans were generated for seed activities between 0.2 and 1.4 mCi (0.25 to 1.78 U) and for 11 different seed models. RESULTS: The numbers of seeds and needles are shown to decrease rapidly for a seed activity between 0.3 mCi and 0.6 mCi (0.38 and 0.76 U). The loss in V100, from 100%, because of seed misplacement is below 10% for an apparent activity ranging from 0.2 to 0.9 mCi (0.25 to 1.14 U). A minimum degradation in V100 is observed around 0.6-0.7 mCi (0.76-0.89 U). D90 increases from 150 to 170 Gy between 0.3 and 0.7 mCi (0.38 and 0.89 U) and decreases afterward to fall below 140 Gy at higher activity. V200 and D10 to the target volume both show an increase in hot spots up to 0.7 mCi, and then decrease linearly at higher activities for all seed models. V200 and D10 to the urethra remain about constant for all seed activities up to 0.8 mCi (1.02 U), at which point they start to decrease. All seed models follow this general trend. CONCLUSIONS: Plans were shown to be robust to misplacement and migration of seeds over a wide range of seed activity and for various seed models. With a properly tuned inverse planning algorithm able to ensure the dose coverage and protection for the organs at risk in the presence of placement errors (displacement and migration), the choice of a preferred seed activity, in a range up to about 0.7 mCi (0.89 U), is open. The upper part of this range offers the opportunity to significantly reduce the number of seeds and needles, thus reducing surgical trauma to the patient, saving time in an operating room planning setting, and reducing the cost of a permanent prostate implant procedure.

Algorithms↗

PubMed2004

Comparison of inverse planning simulated annealing and geometrical optimization for prostate high-dose-rate brachytherapy.

PURPOSE: An inverse planning simulated annealing (IPSA) algorithm for optimization of high-dose-rate (HDR) brachytherapy has been previously described. In this study, IPSA is compared with geometrical optimization (GO) for prostate brachytherapy. METHODS AND MATERIALS: Using CT data collected from 10 patients, treatment plans were prepared using GO and IPSA. The clinical target volume (CTV) and critical organs (CO) including bladder, rectum, and urethra were contoured using Plato Version 14.2.1 (Nucletron Corp., Veenendaal, The Netherlands). Implant catheters were digitized using the CT planning system. All dwell positions outside of the CTV were turned off. Two optimized plans were generated for each implant using GO and IPSA. The same set of dose constraints were used for all inverse planning calculations and no manual adjustment of the dwell weight was used. Two prescription methods were used. Using the first method, coverage was prioritized: the prescription dose was normalized to the isodose volume that covers 98% of the CTV (V100 = 98% of CTV). The dose volume histograms (DVH) of CO were generated for comparison. Using the second method, sparing was prioritized: the prescription dose was normalized such that no urethra volume received 150% of the prescription dose (V150-urethra = 0 cc). The DVH of CTV and CO were generated, and the homogeneity index (HI) and conformal index (COIN) were calculated for comparison and compared using the Wilcoxon matched-pairs test. RESULTS: Using the coverage-prioritized method, the difference in V80-bladder dose was not statistically significant (p = 0.09; median: IPSA = 0.62 cc, GO = 1.05 cc). The V80-rectum ranged from 0.20-4.8 cc, and 0.05-1.4 cc using GO and IPSA, respectively. IPSA's V80-rectum was significantly lower (p = 0.005; median: IPSA=0.38 cc, GO = 1.31 cc). V150-urethra ranged from 0.02-0.75 cc and 0.0-0.01 cc using GO and IPSA, respectively. The V150-urethra was significantly lower using IPSA (p = 0.005; median: IPSA = 0.00 cc, GO = 0.33 cc). Using the sparing prioritized method, the V100-prostate ranged from 30-97% and 95-100% using GO and IPSA, respectively. This difference was statistically significant (p = 0.008). The HI and COIN were statistically higher using IPSA (p = 0.005). CONCLUSION: Anatomy-based inverse optimization using IPSA is superior to dwell-position-based optimization using GO as it: (1) Improves target coverage and conformality while sparing normal structures, (2) Improves dose homogeneity within the target, and (3) Minimizes volume of non-contoured normal tissue irradiated. Routine application of three-dimensional brachytherapy planning and anatomy-based inverse dwell time optimization is recommended.

Brachytherapy↗

PubMed2004

The effect of the radial function on I-125 seeds used for permanent prostate implantation.

The purpose of this study was to evaluate the integrity of eight commercially-available low-activity Iodine-125 (125I) seeds for their radial function g(r) and its effect on the dose delivered to the adjacent critical structures when used in permanent prostate implants (PPI). Ten previously treated patients were retrospectively used in this comparison. The Amersham Health Oncura seed was used to peripherally design an isodose distribution with urethral and anterior rectal wall sparing. Plan criteria included minimum coverage of 144 Gy to the planning target volume (PTV), < or = 70% dose to 150% of the PTV volume (V150-PTV), and the quantity of needles < or = 70% of the size of the PTV, in cc. Upon completion of the Oncura plan, the seed type was changed and the activity was adjusted until the V100-PTV for each of the other 7 seed types matched the V100-PTV defined by the Oncura seed. Computed tomography (CT)-based postimplant dosimetry was used to determine the dose to 40% (D40) of the bulb of the penis (in Gy). Dose-volume histograms (DVH) were used to evaluate the differences to V100 (in %) and D40 (in Gy) of the anterior rectal wall and bulb of the penis, and V100 (in %) of the urethra. The data was tabulated. Radioactive 125I sources included in this study were 125I Source 2301 (Best); I-Plant (MedTech), IoGold (Mentor), Oncura (Amersham Health), ProstaSeed (UroCor), SelectSeed (Nucletron), SourceTech (Bard), and Symmetra (UroMed). The sizes of the PTV for the 10 patients ranged from 18.82 cc to 48.99 cc. The Oncura seed was used as the reference seed and all other seed types were normalized to it for data comparison. It was determined that the dose rate constant (Delta) and anisotropy factor (phi) contribute to the activity needed to achieve comparable V100-PTV doses, but a strong dependence on the radial function g(r) was found to effect the doses to the critical structures studied. Values of g(r) at 4 cm were calculated and the IoGold and SourceTech seeds were determined to have the highest g(r) values, with ProstaSeed and SelectSeed having the lowest values. 125I Source 2301 and IoGold required less activity per seed to achieve the same dose to the V100-PTV due to the higher dose rate and anisotrophy constants (Delta.phi). The seed types with silver were less penetrating and resulted in the production of characteristic x-rays that modified the energy spectrum and influenced the radial function. The seeds requiring the lowest activity showed the highest dose to the anterior rectal wall, a posterior adjacent structure; the urethra, an interior structure; and the bulb, an inferior structure. This study was designed to investigate the integrity of eight different commercially-available seed types, and their dependence on the g(r) in seed choice. It was determined that the dose rate constant and anisotropy factor determine the activity needed for implantation but a strong dependence on the radial function was found to effect the doses to the adjacent structures.

Anisotropy↗

PubMed2004

Dose uncertainty due to computed tomography (CT) slice thickness in CT-based high dose rate brachytherapy of the prostate cancer.

In computed tomography (CT)-based high dose rate (HDR) brachytherapy, the uncertainty in the localization of the longitudinal catheter-tip positions due to the discrete CT slice thickness, results in a delivered dose uncertainty. Catheter coordinates were extracted from five patients treated for prostate cancer, and three simulation scenarios were followed to mimic the longitudinal imprecision of the catheter tips, hence the dwell positions. All catheters were displaced (1) forward, (2) backward, or (3) randomly distributed within the space defined by one CT slice thickness, for thicknesses ranging from 2 to 5 mm. Average and standard deviation values of the relative dose variations are reported for the various catheter displacement scenarios. Also, the dose points were grouped according to their relative position in the prostate, inner, peripheral and outer area of prostate and base, median and apex zones, in order to estimate the spatial sensitivity of the dose errors. For scenarios (1) and (2), the dose uncertainties due to the finite slice thickness increase linearly with the slice spacing, from 3% to 8% for the slice thickness values ranging from 2 to 5 mm, respectively. The more realistic scenario (3) yields average errors ranging from 0.7% to 1.7%. The apex and the base show larger dose errors and variability of dose errors than the median of the prostate. No statistical difference was observed among different transversal sections of the prostate. A CT slice thickness of 3 mm appears to be a good compromise showing an acceptable average dose uncertainty of 1%, without unduly increasing the number of slices.

Algorithms↗

PubMed2004

Radiation dosimetry of a conformal heat-brachytherapy applicator.

The purpose of this paper is to report the radiation dosimetric characteristics of a new combination applicator for delivering heat and radiation simultaneously to large area superficial disease <1.5 cm deep. The applicator combines an array of brachytherapy catheters (for radiation delivery) with a conformal printed circuit board microwave antenna array (for heat generation), and a body-conforming 5-10 mm thick temperature-controlled water bolus. The rationale for applying both modalities simultaneously includes the potential for significantly higher response rate due to enhanced synergism of modalities, and lower peak toxicity due to temporal extension of heat and radiation induced toxicities. Treatment plans and radiation dosimetry are calculated with IPSA (an optimization tool developed at UCSF) for 15 x 15 cm(2) and 35 x 24 cm(2) applicators, lesion thicknesses of 5 to 15 mm, flat and curved surfaces, and catheter separation of 5 and 10 mm. The effect on skin dose of bolus thickness and presence of thin copper antenna structures between radiation source and tissue are also evaluated. Results demonstrate the ability of the applicator to provide conformal radiation dose coverage for up to 15 mm deep target volumes under the applicator. For clinically acceptable plans, tumor coverage is > 98%, homogeneity index > 0.95 and the percentage of normal tissue irradiated is < 20%. The dose gradient at the skin surface varies from 3 to 5 cGy/mm depending on bolus thickness and lesion depth. Attenuation of the photon beam by the printed circuit antenna array is of the order 0.25% and secondary electron emissions are absorbed completely within 5 mm of water bolus and plastic layers. Both phenomena can then be neglected in dose calculations allowing commercial software to be used for treatment planning. This novel applicator should prove useful for the treatment of diffuse chestwall disease located over contoured anatomy that may be difficult to treat with single field external beam therapy. By delivering heat and radiation simultaneously, increased synergism is expected with a TER in the range of 2-5. Lowering radiation dose by an equivalent factor may produce lower radiation toxicity with similar efficacy, while preserving the option of subsequent retreatment(s) with thermoradiotherapy in order to further extend patient survival.

Algorithms↗

PubMed2003

(Non)-migration of radiopaque markers used for on-line localization of the prostate with an electronic portal imaging device.

PURPOSE: Radiopaque gold markers can be implanted in the prostate to visualize its position on portal images during radiation therapy. This procedure assumes that the markers do not move within the prostate. In this work we test this assumptiom. METHODS AND MATERIALS: Three markers were implanted transrectally in the prostate of patients undergoing external radiation therapy. An orthogonal pair of portal images was acquired periodically throughout the course of radiation therapy with an a-Si electronic portal imaging device (EPID). The marker coordinates were determined, and the distances between the implanted markers were recorded. The distance time trend is used to evaluate the magnitude of marker migration. RESULTS: The average standard deviation (SD) of the distances between markers was 1.3 mm (range 0.44 to 3.04 mm). Three of the 11 patients show a SD larger than 2 mm. For these patients, all three distances show a simultaneous reduction with time, compatible with a shrinking of the prostate. All had been treated with neoadjuvant hormone therapy. For 1 of the 3 patients, this reduction in volume was confirmed with a repeat computed tomographic scan. CONCLUSION: None of the 33 markers studied migrated significantly. The implantation of three radiopaque gold markers enables accurate and precise on-line verification of the prostate position during external beam radiation therapy. The use of three markers provides a tool to monitor prostate position and volume changes that can occur over time due to hormone or radiation therapy.

Gold↗

PubMed2003

Analysis of interaction between number of implant catheters and dose-volume histograms in prostate high- dose-rate brachytherapy using a computer model.

PURPOSE: In prostate high-dose-rate brachytherapy, to determine before implant, using the standard geometric optimization algorithm, whether there is an optimal number of catheters. MATERIALS AND METHODS: Transrectal ultrasound images of the prostate from 24 patients were transferred into the brachytherapy planning system. Urethra and prostate contours were digitized onto each axial slice of a CT scan, as well as hypothetical locations of the catheters (2/3 of the catheters along the prostate contour, 1/3 around the urethra). Each prostate was implanted with 9, 12, 15, 18, and 21 catheters. Dosimetry was optimized using a geometric optimization algorithm; prescription isodose was chosen so that 95% of planning target volume was covered by the 100% isodose. RESULTS: A significant increase in mean volume of prostate receiving 150% of the dose (V150) when the number of catheters decreased (p < 0.0001). The 9-catheter group significantly differed from each of the other groups; no difference was seen in V150 among the 21-, 18-, and 15-catheter groups. Parallel results were observed for urethra V150 and homogeneity index; there was no difference in conformity index by catheter group. CONCLUSION: V150 increased when fewer catheters were used. There was no significant difference among the 21-, 18-, and 15-catheter groups: the geometric optimization routine probably compensated for the larger distance between dwell positions. Based on the technique described in our study, we conclude that 15 to 21 catheters seem to cover the prostate adequately without creating excess hot spots.

Algorithms↗

PubMed2003

A comparison of methods to calculate biological effectiveness (RBE) from Monte Carlo simulations.

The relative biological effectiveness (RBE) of radiation is assessed and easily calculated by Monte Carlo simulations of the passage of radiation through matter. The expression to calculate the RBE provided by microdosimetry requires the use of the energy spectrum of charged particles. This paper compares the RBE values obtained for Palladium-103 (103Pd) and iodine-125 (125I) when calculated with 2 different spectra: the electron slowing-down spectrum and the ejection spectrum. The former yields a value of 10.6%, twice the value obtained with the latter (4.5%). Which spectrum to use is an open question. A theoretical argument is presented in favor of the ejection spectrum.

Electrons↗

PubMed2003

Simulating needle insertion and radioactive seed implantation for prostate brachytherapy.

We are developing a simulation of needle insertion and radioactive seed implantation to facilitate surgeon training and planning for brachytherapy for treating prostate cancer. Inserting a needle into soft tissues causes the tissues to displace and deform: ignoring these effects during seed implantation leads to imprecise seed placements. Surgeons should learn to compensate for these effects so seeds are implanted close to their pre-planned locations. We describe a new 2-D dynamic FEM model based on a 7-phase insertion sequence where the mesh is updated to maintain element boundaries along the needle shaft. The locations of seed implants are predicted as the tissue deforms. The simulation, which achieves 24 frames per second using a 1250 triangular element mesh on a 750Mhz Pentium III PC, is available for surgeon testing by contacting ron@ieor.berkeley.edu.

Brachytherapy↗

PubMed2002

Inverse planning for interstitial gynecologic template brachytherapy: truly anatomy-based planning.

PURPOSE: Commercially available optimization schemes generally result in an undesirable dose distribution, because of the particular shapes of tumors extending laterally from the tandem. Dose distribution is therefore manually obtained by adjusting relative dwell time values until an acceptable solution is found. The objective of this work is to present the clinical application of an inverse planning dose optimization tool for the automatic determination of source dwell time values in the treatment of interstitial gynecologic templates. METHODS AND MATERIALS: In cases where the tumor extends beyond the range of the tandem-ovoid applicator, catheters as well as the tandem are inserted into the paravaginal and parametrial region in an attempt to cover the tumor volume. CT scans of these patients are then used for CT-based dose planning. Dose distribution is obtained manually by varying the relative dwell times until adequate dose coverage is achieved. This manual planning is performed by an experienced physician. In parallel, our in-house inverse planning based on simulated annealing is used to automatically determine which of all possible dwell positions will become active and to calculate the dwell time values needed to fulfill dose constraints applied to the tumor volume and to each organ at risk. To compare the results of these planning methods, dose-volume histograms and isodose distributions were generated for the target and each organ at risk. RESULTS: This procedure has been applied for the dose planning of 12 consecutive interstitial gynecologic templates cases. For all cases, once the anatomy was contoured, the routine of inverse planning based on simulated annealing found the solution to the dose constraints within 1 min of CPU time. In comparison, manual planning took more than 45 min. The inverse planning-generated plans showed improved protection to organs at risk for the same coverage compared to manual planning. CONCLUSION: This inverse planning tool reduced the planning time significantly and produced improved plans with reduced dose to the organs at risk. Furthermore, the inverse planning approach improves the physician's control over treatment. The focus becomes the physician's prescription to the target and his or her compromise due to dose to normal structures.

Algorithms↗

PubMed2002

Early clinical experience with anatomy-based inverse planning dose optimization for high-dose-rate boost of the prostate.

PURPOSE: To present an exhaustive dosimetric comparison between three geometric optimization methods and our inverse-planning simulated annealing (IPSA) algorithm, with two different prescriptions for high-dose-rate (HDR) boost of the prostate. The objective of this analysis was to quantify the dosimetric advantages of the IPSA algorithm compared with more standard geometric optimizations. METHODS AND MATERIALS: Between September 1999 and June 2001, 34 patients were treated to a dose of 40-44 Gy by external pelvic fields, followed by an HDR boost of 18 Gy in 3 fractions. The first 4 patients were treated with HDR using geometric optimization, and anatomy-based inverse-planning dose optimization was used for the remaining 30 patients. We retrospectively used the data from these 30 patients to create HDR dose distributions according to five different dose optimization protocols, including our IPSA algorithm. The various geometric optimization procedures differed in the way the dwell positions were activated and plan normalization was performed. Dose-volume histograms from all these plans were analyzed and multiple implant quality indexes extracted. RESULTS: The IPSA algorithm provided better clinical tumor volume prescription dose coverage than did the geometric optimizations. The average prostate volume receiving 100% of the prescribed dose (V100) was 96.3% and 94.5% for IPSA with two different prescriptions compared with 92.1%, 92.6%, and 88.8% for the three geometric optimization schemes. The average urethra V150 value was 0.0% and 0.7% for IPSA with two different prescriptions, and the three geometric optimization protocols generated average values of 22.9%, 33.9%, and 38.8%. The bladder and rectal dose-volume histograms were similar, although the latest version of the IPSA algorithm slightly decreases the dose to these organs at risk because of organ-specific dose constraints included in the objective function. CONCLUSION: We found that planning an HDR prostate boost could be performed in a fast, secure, and effective manner with the IPSA algorithm. We demonstrated that our inverse-planning algorithm produces superior HDR plans than more conventional geometric optimizations for adenocarcinoma of the prostate. The organs at risk protection included in the objective function is a major feature of the algorithm and should allow us to escalate the HDR dose to the prostate without increasing undesirable side effects.

Adenocarcinoma↗

PubMed2002

Dosimetric impact of the variation of the prostate volume and shape between pretreatment planning and treatment procedure.

PURPOSE: The goal of this study is to evaluate the dosimetric impact on a pretreatment planning of prostatic volume and shape variations occurring between the moment of the volume study (preplanning) and just before a transperineal permanent seed implant procedure. Such variations could be an obvious source of misplacement of the seeds relative to the prostate gland and organs at risk. Other sources of dosimetric uncertainties, such as misplacement due to the procedure itself or edema, are eliminated by looking at these variations before the implant procedure. METHODS AND MATERIALS: For 35 clinical cases, prostate contours were taken at preplanning time as well as in the operating room (OR) minutes before the procedure. Comparison of shape and volume between the two sets was made. The impact on V100 was evaluated by placing the seeds in their planned positions in the new volume (clinical situation) and also by performing a new plan with the second set of contours to simulate an intraoperative approach. RESULTS: The volume taken in the OR remained unchanged compared to the pretreatment planning volume in only 37% of the cases. While on average the dose coverage loss from pretreatment planning due to a combination of variations of volume and shape was small at 5.7%, a V100 degradation of up to 20.9% was observed in extreme cases. Even in cases in which no changes in volume were observed, changes in shape occurred and strongly affected implant dosimetry. CONCLUSIONS: Variations of volume and shape between pretreatment planning and the implant procedure can have a strong impact on the dosimetry if the planning and the implant procedure are not performed on the same day. This is an argument in favor of performing implant dosimetry in the OR.

Brachytherapy↗

PubMed2002

A comparison between tandem and ovoids and interstitial gynecologic template brachytherapy dosimetry using a hypothetical computer model.

PURPOSE: To evaluate the dose distribution within the clinical target volume between two gynecologic brachytherapy systems---the tandem and ovoids and the Syed-Neblett gynecologic template---using a hypothetical computer model. METHODS AND MATERIALS: Source positions of an intracavitary system (tandem and ovoids) and an interstitial system (GYN template) were digitized into the Nucletron Brachytherapy Planning System. The GYN template is composed of a 13-catheter implant (12 catheters plus a tandem) based on the Syed-Neblett gynecologic template. For the tandem and ovoids, the dwell times of all sources were evenly weighted to produce a pear-shaped isodose distribution. For the GYN template, the dwell times were determined using volume optimization. The prescribed dose was then normalized to point A in the intracavitary system and to a selected isodose line in the interstitial system. The treated volume in the two systems was kept approximately the same, and a cumulative dose-volume histogram of the treated volume was then generated with the Nucletron Brachytherapy Planning System to use for comparison. To evaluate the dose to a hypothetical target, in this case the cervix, a 2-cm-long, 3-cm-diameter cylinder centered along the tandem was digitized as the clinical target volume. The location of this hypothetical cervix was based on the optimal application of the brachytherapy system. A visual comparison of clinical target coverage by the treated volume on three different orthogonal planes through the treated volume was performed. The percentage dose-volume histograms of the target were generated for comparison. Multiple midline points were also placed at 5-mm intervals away from the tandem in the plane of the cervix to simulate the location of potential bladder and rectal dose points. Doses to these normal structures were calculated for comparison. RESULTS: Although both systems covered the hypothetical cervix adequately, the interstitial system had a better coverage of the region lateral to the cervix. Smaller volumes of the vagina and uterine fundus received the full dose from the interstitial implant. The cumulative dose-volume histograms revealed larger high-dose regions within the treatment volume for the intracavitary system. The volumes receiving > or = 180% of the prescription dose were 31 cc and 17 cc for the intracavitary system and interstitial system, respectively. The isodose lines showed that most of this difference results from the high-dose region around the tandem. The percentage dose-volume histograms showed that a larger percentage of cervix received a higher dose in the intracavitary system. Fifty-two percent of the target volume received 200% or higher of the prescription dose with tandem and ovoids, compared with only 20% with the template system. Analysis of dose points outside of the 100% isodose lines showed a slightly more rapid dose drop-off with the interstitial system compared to the intracavitary system. Point doses at 20, 25, and 30 mm from the tandem in the interstitial system were 100%, 69%, and 51% of prescribed dose, and from the intracavitary system were 101%, 76%, and 58%, respectively. CONCLUSIONS: Our dosimetric analysis revealed a better coverage in the parametrial regions, but underdosage of the central cervical region, for the interstitial system. On the other hand, because of the increased distance of source to dose point, there is a more rapid dose drop-off outside the treated volume with the interstitial system, which has the potential to improve tissue sparing. Based on this analysis, we caution against using a radiotherapy system with a homogeneous central dose distribution when treating cervical cancer with an intact uterus. We recommend differential loading of the implant catheters with the majority of dose delivered from the tandem when using an interstitial GYN template with remote afterloader.

Brachytherapy↗

PubMed2002

Relative biological effectiveness enhancement of a 125I brachytherapy seed with characteristic x rays from its constitutive materials.

The isotopes used for permanent prostate implants, 125I and 103Pd, provide about equivalent tumor control. The purpose of this study is to investigate how characteristic x rays may be used to raise the relative biological effectiveness (RBE) of an iodine seed at short distances to increase the differential effect between tumor and healthy tissue. Within the theoretical framework of microdosimetry, the GEANT4 Monte Carlo simulation toolkit has been used to calculate the RBE of experimental seed designs in which shell and core dimensions and composition were varied independently. A new seed model was also simulated based on the best results obtained. The RBE could be enhanced by increasing the shell thickness and for the range considered, optimum results were obtained by using gradually lower atomic number elements. For a practical 50-60 microm shell, molybdenum is the material of choice. The core diameter has little influence on RBE, but maximum effectiveness is obtained with yttrium or zirconium. These results were put together to design a Mo-shell and Y-core seed for which the RBE enhancement was at least 5-7% (close to the source), which is higher than palladium. This enhanced RBE combined with the longer half-life of iodine could mean comparable tumor control and better protection to organs at risk than with current seeds. The RBE dependence on distance is an interesting feature that could benefit other applications such as ocular melanoma or coronary brachytherapy where a highly localized dose distribution is desired.

Brachytherapy↗