Brachytherapy Dose Delivery Research Articles

AAPM Task Group Report 267: A joint AAPM GEC-ESTRO report on biophysical models and tools for the planning and evaluation of brachytherapy.

Brachytherapy utilizes a multitude of radioactive sources and treatment techniques that often exhibit widely different spatial and temporal dose delivery patterns. Biophysical models, capable of modeling the key interacting effects of dose delivery patterns with the underlying cellular processes of the irradiated tissues, can be a potentially useful tool for elucidating the radiobiological effects of complex brachytherapy dose delivery patterns and for comparing their relative clinical effectiveness. While the biophysical models have been used largely in research settings by experts, it has also been used increasingly by clinical medical physicists over the last two decades. A good understanding of the potentials and limitations of the biophysical models and their intended use is critically important in the widespread use of these models. To facilitate meaningful and consistent use of biophysical models in brachytherapy, Task Group 267 (TG-267) was formed jointly with the American Association of Physics in Medicine (AAPM) and The Groupe Européen de Curiethérapie and the European Society for Radiotherapy & Oncology (GEC-ESTRO) to review the existing biophysical models, model parameters, and their use in selected brachytherapy modalities and to develop practice guidelines for clinical medical physicists regarding the selection, use, and interpretation of biophysical models. The report provides an overview of the clinical background and the rationale for the development of biophysical models in radiation oncology and, particularly, in brachytherapy; a summary of the results of literature review of the existing biophysical models that have been used in brachytherapy; a focused discussion of the applications of relevant biophysical models for five selected brachytherapy modalities; and the task group recommendations on the use, reporting, and implementation of biophysical models for brachytherapy treatment planning and evaluation. The report concludes with discussions on the challenges and opportunities in using biophysical models for brachytherapy and with an outlook for future developments.

Method of Dose Delivery in Brachytherapy for Cervical Cancer

A method is proposed for delivering an absorbed dose to a target during brachytherapy for cervical cancer by using a new applicator, which consists of an annular part and two uterine canals. The design of the applicator makes it possible to implement it to the required depth, taking into account the anatomical features, to widely cover the cervix and corporis of a womb with a 100 % isodose of irradiation, which makes it possible to deliver the planned dose. The patient admitted to the brachytherapy department received the first treatment fraction using the Interstitial Ring applicator, the second and subsequent fractions were performed using the proposed applicator (6 fractions in total). In comparison with the standard applicator Interstitial Ring, the use of the proposed applicator made it possible to increase the coverage of the target with the prescribed dose from 69.2 to 95.5 %. This led to an increase in the absorbed dose per target for a course of radiation therapy from the planned 70.4 to 85.4 Gy. Twelve treatment plans for 3 patients were analyzed using different dose fractionation schemes. In all three cases, it was possible to increase the coverage of the target with the prescribed dose, which led to an increase in the total absorbed dose per target, which was more than 85 Gy. The proposed method has been successfully applied in the N. N. Alexandrov National Cancer Centre during brachytherapy for cancer of the cervix and corporis of a womb.

Development and validation of a scatter-corrected CBCT image-guided method for cervical cancer brachytherapy.

Multiple patient transfers have a nonnegligible impact on the accuracy of dose delivery for cervical cancer brachytherapy. We consider using on-site cone-beam CT (CBCT) to resolve this problem. However, CBCT clinical applications are limited due to inadequate image quality. This paper implements a scatter correction method using planning CT (pCT) prior to obtaining high-quality CBCT images and evaluates the dose calculation accuracy of CBCT-guided brachytherapy for cervical cancer. The CBCT of a self-developed female pelvis phantom and five patients was first corrected using empirical uniform scatter correction in the projection domain and further corrected in the image domain. In both phantom and patient studies, the CBCT image quality before and after scatter correction was evaluated with registered pCT (rCT). Model-based dose calculation was performed using the commercial package Acuros®BV. The dose distributions of rCT-based plans and corrected CBCT-based plans in the phantom and patients were compared using 3D local gamma analysis. A statistical analysis of the differences in dosimetric parameters of five patients was also performed. In both phantom and patient studies, the HU error of selected ROIs was reduced to less than 15 HU. Using the dose distribution of the rCT-based plan as the baseline, the γ pass rate (2%, 2mm) of the corrected CBCT-based plan in phantom and patients all exceeded 98% and 93%, respectively, with the threshold dose set to 3, 6, 9, and 12 Gy. The average percentage deviation (APD) of D90 of HRCTV and D2cc of OARs was less than 1% between rCT-based and corrected CBCT-based plans. Scatter correction using a pCT prior can effectively improve the CBCT image quality and CBCT-based cervical brachytherapy dose calculation accuracy, indicating promising prospects in both simplified brachytherapy processes and accurate brachytherapy dose delivery.

Yttrium-90 Hepatic Therapy and the Increasing Role of Volumetric Voxel-based Post Therapy Dosimetry: A Case Report

Yttrium-90 (Y-90) therapy has become an important component to the care of patients with primary hepatic malignancies and lesions that have metastasized to the liver. Therapy is administered through an intra-arterial procedure after an interventional procedure is performed using an albumin labeled product to ensure therapy will be delivered to the target volume of interest with minimal migration from the target of choice. In the past, dose to target has been measured by activity delivered and qualitative deposition of dose on metabolic imaging post application. Imaging tools such as single positron emission computer tomography (SPECT) and digital positron emission tomography have given us insight into quantitative dose to volumetric tumor target and dose to normal tissue. Recent validation of computational software has provided voxel-based dosimetry similar to applied processes established in radiation oncology planning systems. This development presents an opportunity to create dose volume analysis similar to teletherapy and brachytherapy dose delivery for Y-90 therapy. In this case report, we review Y-90 dosimetry on a patient with dual diagnosis of a locally advanced high-risk adenocarcinoma of the prostate which required treatment to the para-aortic lymph nodes located in the same axial plane with renal parenchyma. Although not clinically anticipated, hepatocellular carcinoma was serendipitously discovered at the time of staging for prostate cancer. Treatment dosimetry of the hepatocellular carcinoma is reviewed in retrospect with voxel-based commercial software. Same day SPECT study suggested dose localized to the liver, however voxel planning software confirmed unintentional dose to additional structures including the right kidney and uninvolved liver which influenced radiation therapy treatment planning for prostate carcinoma. With modern available tools, post therapy dosimetry for Y-90 can be performed in a manner similar to volumetric dosimetry used in radiation oncology and provide valuable dose volume analysis of dose delivered to target and additional tissue.

Brachytherapy with surgical resection as salvage treatment for recurrent high-grade meningiomas: a matched cohort study.

To evaluate surgical resection with brachytherapy placement as a salvage treatment in patients with recurrent high-grade meningioma who exhausted prior external beam treatment options. Single-center retrospective review of our institutional experience of brachytherapy implantation from 2012 to 2018. The primary outcome of the study was progression free survival (PFS). Secondary outcomes included overall survival (OS) and complications. A matched cohort of patients not treated with brachytherapy over the same time period was evaluated as a control group. All patients had received prior radiation treatment and underwent planned gross total resection (GTR) surgery. A total of 27 cases were evaluated. Compared with prior treatment, brachytherapy implantation demonstrated a statistically significant improvement in tumor control [HR 0.316 (0.101 - 0.991), p = 0.034]. PFS-6 and PFS-12 were 92.3% and 84.6%, respectively. Compared with the matched control cohort, brachytherapy treatment demonstrated improved PFS [HR 0.310 (0.103 - 0.933), p = 0.030]. Overall survival was not statistically significantly different between groups [HR 0.381 (0.073 - 1.982), p = 0.227]. Overall postoperative complications were comparable between groups, although there was a higher incidence of radiation necrosis in the brachytherapy cohort. Brachytherapy with planned GTR improved PFS in recurrent high-grade meningioma patients who exhausted prior external beam radiation treatment options. Future improvement of brachytherapy dose delivery methods and techniques may continue to prolong control rates and improve outcomes for this challenging group of patients.

Analysis of local control rate and toxicity of radiotherapy dose of cervical cancer.

136 Background: The standard of care for treatment of advanced cervical cancer is the combination of external beam radiation therapy (EBRT) and intracavitary brachytherapy with cisplatin based concurrent chemotherapy. Radiotherapy plays a crucial role in treatment of cervical cancer. Recent GEC-ESTRO guidelines recommend that the dose to 90% (D90) of the high-risk clinical target volume (HRCTV) in cervical cancer be at least 85Gy with higher doses for poor response to radiotherapy. This study was aim to analyze our institution’s patients with locally advanced cervical cancer in regards to whether higher brachytherapy dose delivery lead to a better outcome, and to investigate the proper dose to balance the local control rate and toxicity. Methods: A total of 262 patients with local advanced cervical cancer treated at a single institution were retrospectively analyzed for the association between dosimetry and outcomes. Youden index was used to identify the optimal cut-off point of continuous tumor parameters and divide patients into subgroups. Significance of radiotherapy dose parameters on OS, PFS, LRFS and DMFS and toxicity was evaluated. Results: In the univariate analysis, for both HR-CTV and LR-CTV, the high dose group (EQD2 D90 >75Gy, LR-CTV>68Gy, respectively) have a better LRFS than low-dose groups (P<0.05). LR-CTV remains significance after adjusted for age and FIGO stage. Moreover, in the high-dose LR-CTV group, there is no association between dose of LR-CTV and LRFS was found, however, higher dose of HR-CTV significant associated with higher ratio of side effect was found. In addition, no association of dose of HR-CTV or LR-CTV and OS were found for all patients. Conclusions: Our results showed that dose of LR-CTV may be a useful prognostic factor of LRFS of patients with cervical cancer. Moreover, after D90 of LR-CTV reaches 68Gy, increasing dose did not show a better LRFS but lead to higher ratio of toxicity, supporting that LR-CTV at 68Gy might be a safety and efficacy dose of radiotherapy to the patients with cervical cancer. However, further improved in dose had no significant benefit on local control rate, and it might increase the risks of toxicity.

Single-Institution Experience in 3D MRI-Based Brachytherapy for Cervical Cancer for 230 Women: Can Dose Overcome Poor Response?

Brachytherapy is an essential component of the treatment of locally advanced cervical cancer; recent GEC-ESTRO guidelines recommend that the dose to 90% (D90) of the high-risk clinical target volume (HRCTV) be at least 85Gy with even higher doses for poor response to external beam radiation. A retrospective review of brachytherapy plans delivered at a single institution were evaluated for dose parameters and local control following treatment. The HRCTV D90 was retrieved from all plans. Time to local failure was defined as time to event from completion of the last brachytherapy fraction. Significance of tumor parameters on local control was evaluated with univariate and multivariable Cox regression analysis. Correlations were determined with a linear regression model. A total of 230 women underwent high dose-rate brachytherapy for cervical cancer between 2007 and 2016 with evaluable dosimetry. Median follow up was 35.1 months. The median prescribed dose was 27.5Gy/5fx with a median HRCTV D90 of 83.7Gy (range: 74.8-93.5Gy), HRCTV volume of 31cc (range: 14.91-83.76cc), and treatment time of 7.1 weeks. Local control for the entire cohort at 3 years was 90.1%. Factors significantly correlated with higher rates of local failure were: HRCTV volume per cc increase (HR 1.047 [95%CI 1.017-1.078, p=0.002] and adenocarcinoma histology vs squamous (HR 2.773 [95%CI 1.148-6.698], p=0.023). Three-year local control for tumors < 31cc and ≥ 31cc, 3-year local control rates were not statistically different (93.5% vs 86.7%, p=0.055) but when 40cc was used as a cutoff, 3-year local control was 92.6% vs 82.2%, p=0.010. Adenocarcinomas had worse 3-year local control vs squamous cell carcinomas: 83.5% vs 91.7%, p=0.018. On multivariable analysis, HRCTV volume per cc increase (HR 1.044 [95%CI 1.014-1.075], p=0.004) and adenocarcinoma histology (HR 2.597 [95%CI 1.071-6.297], p=0.035) were both predictive of local failure. HRCTV dose at multiple thresholds (85, 90, 95Gy) did not predict for local control. There was a positive correlation between higher HRCTV volume and higher prescribed dose (R2 = 0.131, p<0.001). Adenocarcinomas were also significantly associated with higher prescribed brachytherapy dose (p=0.009). Women with HRCTV < 40cc had 3-year progression-free survival (including distant metastases) of 80.9% vs. 63.7% for ≥ 40cc (p=0.007). This translated to three-year overall survival for women with HRCTV < 40cc of 80.8% vs 61.0%% for ≥ 40cc (p=0.002). Local control is excellent with MRI based planning in the entire cohort of patients. Our data suggest that a poor response to external beam radiation (larger HRCTV volume) predicted for worse local control despite a higher brachytherapy dose delivery. These poor responders are also at higher risk of overall progression and death and may require additional intervention to mitigate this risk.

Commissioning of a well type chamber for HDR and LDR brachytherapy applications: a review of methodology and outcomes.

For safe and accurate dose delivery in brachytherapy, associated equipment is subject to commissioning and ongoing quality assurance (QA). Many centres depend on the use of a well-type chamber ('well chamber') for performing brachytherapy dosimetry. Documentation of well chamber commissioning is scarce despite the important role the chamber plays in the whole brachytherapy QA process. An extensive and structured commissioning of the HDR 1000 plus well chamber (Standard Imaging Inc, Middleton WI) for HDR and LDR dosimetry was undertaken at Sir Charles Gairdner Hospital. The methodology and outcomes of this commissioning is documented and presented as a guideline to others involved in brachytherapy. The commissioning tests described include mechanical integrity, leakage current, directional dependence, response, length of uniform response, the influence of insert holders, ion collection efficiency, polarity effect, accuracy of measured air kerma strength (S(K)) or reference air kerma rate (K(R)) and baseline setting (for ongoing constancy checks). For the HDR 1000 plus well chamber, some of the insert holders modify the response curve. The measured sweet length was 2.5 cm which is within 0.5% of that specified by the manufacturer. Correction for polarity was negligible (0.9999) and ion recombination was small (0.9994). Directional dependence was small (less than 0.2%) and leakage current was negligible. The measured K(R) for (192)Ir agreed within 0.11% compared with a second well chamber of similar model and was within 0.5% of that determined via a free-in-air measurement method. Routine constancy checks over a year agreed with the baseline within 0.4%.

Salvage brachytherapy in combination with interstitial hyperthermia for locally recurrent prostate carcinoma following external beam radiation therapy: a prospective phase II study.

Optimal treatment for patients with only local prostate cancer recurrence after external beam radiation therapy (EBRT) failure remains unclear. Possible curative treatments are radical prostatectomy, cryosurgery, and brachytherapy. Several single institution series proved that high-dose-rate brachytherapy (HDRBT) and pulsed-dose-rate brachytherapy (PDRBT) are reasonable options for this group of patients with acceptable levels of genitourinary and gastrointestinal toxicity. A standard dose prescription and scheme have not been established yet, and the literature presents a wide range of fractionation protocols. Furthermore, hyperthermia has shown the potential to enhance the efficacy of re-irradiation.Consequently, a prospective trial is urgently needed to attain clear structured prospective data regarding the efficacy of salvage brachytherapy with adjuvant hyperthermia for locally recurrent prostate cancer. The purpose of this report is to introduce a new prospective phase II trial that would meet this need. The primary aim of this prospective phase II study combining Iridium-192 brachytherapy with interstitial hyperthermia (IHT) is to analyze toxicity of the combined treatment; a secondary aim is to define the efficacy (bNED, DFS, OS) of salvage brachytherapy. The dose prescribed to PTV will be 30 Gy in 3 fractions for HDRBT, and 60 Gy in 2 fractions for PDRBT. During IHT, the prostate will be heated to the range of 40–47°C for 60 minutes prior to brachytherapy dose delivery. The protocol plans for treatment of 77 patients.

SU-E-J-172: Dose Estimation in Lung Brachytherapy Using Voxelized Phantom 4DCT and MCNP

Purpose: The purpose of this study is to accurately estimate the difference between 4DCT and 3DCT dose estimation due to respiratory motion and free breathing. Methods: A 10‐phase 4DCT DICOM images of a patient, provided by the department of ?radiation oncology, University of Arizona, with a tumor in left lung is used to produce voxelized phantoms as input to MCNP. The voxelized phantoms are prepared with an in‐house developed MATLAB program. The image resolution is 512×512 with voxel dimension of 0.9765625 × 0.9765625 × 3 mm3. The resolution of the voxel phantoms are reduced to half so that MCNP could handle the dosimetry calculation. 1 billion histories are used to achieve a relative error of less than 1 percent. Treatment planning for brachytherapy dose delivery is done based on the free‐breathing data with 36 iodine‐125 seeds implanted in the tumor, 2×2×1.8 cm3, to deliver 140 Gy to PTV. MCNP execution are performed on a 10 node cascade computer which compromises 10 processors of Intel Core i7, 3.4 GHz. MCNP execution for 10 phases takes a month to complete on the aforementioned platform. Results: Comparing the dose estimation of 4DCT and 3DCT shows that 3DCT underestimates dose in lung and low density tissue regions. Meanwhile, 4DCT underestimates dose in the organ at risks, which is due to taking into consideration their movements by 4DCT dosimetry. The dose to segments of heart and spinal cord in the level of tumor are underestimated by 15% ?and 20% respectively. The dose to lung tissue is overestimated by 32% with 4DCT. Conclusion: For an accurate treatment planning in lung brachytherapy, a 4DCT dose estimation is essential for optimum dose delivery.

Sci-Thur PM: YIS - 10: A new optically encoded single-fiber plastic scintillation detector for multi-point radiation dosimetry

To develop a new multi-point plastic scintillation detector (mPSD) that allows for simultaneous dose measurements at multiple points and uses a single optical guide. Two different prototypes were built. A two-point mPSD was built and light discrimination was based on the use of multiple color filters at the outputs of a network of optical fiber splitters. Light intensity was measured by an EMCCD camera. For the three-point mPSD, the light discrimination setup was replaced by a low-noise spectrometer. Depth-dose and profiles measurements were obtained on a 6 MV photon beam with the mPSDs inside a water phantom. An ion chamber was also used for comparison purpose. Finally, the three-point mPSD was tested under an Ir-192 high-dose-rate (HDR) brachytherapy dose delivery and compared to the treatment planning system. A good agreement was found between the measured and expected dose for both mPSDs. The average relative differences to the ion chamber measurement for the two-point mPSD were of (2.4 ± 1.6)% and (1.3 ± 0.8)%. For the three-point mPSD, these differences were of (2.3±1.1)%, (1.6±0.4)% and (0.32±0.19)%. The latter mPSD was shown very versatile, being able to measure dose from HDR brachytherapy with an average accuracy of (2.3±1.0)% per catheter. The practical feasibility of mPSDs using a single optical guide has been demonstrated under irradiation from a 6 MV photon beam and an Ir-192 HDR brachytherapy source. Their application for pre-treatment quality assurance and in vivo dosimetry will be various.

A simple method for dose fusion from multimodality treatment of prostate cancer: Brachytherapy to external beam therapy

Purpose Suboptimal dosage evaluated from postimplant dosimetry of prostate brachytherapy creates conundrum that needs resolution. This pilot study was undertaken to explore the feasibility of summing and visualizing radiation dosage from multimodality treatment. Methods and Materials Four weeks after 125I permanent prostate seed implant, CT scans were performed on the whole pelvis of patients using our standard protocol for prostate planning. The acquired CT data sets were reconstructed using different sizes of field of view (FOV). The images with limited FOV focusing on prostate were imported into Variseed (Varian Medical Systems, Inc., Palo Alto, CA) for postimplant evaluation, whereas images with full FOV were imported to Eclipse (Varian Medical Systems, Inc., Palo Alto, CA) treatment planning system (TPS) for future managements, that is, for external beam salvage. Results The dose matrix resulted from the postimplant dosimetry was exported from Variseed in standard DICOM format and imported into Eclipse TPS. The brachytherapy dose matrix was registered with the patient images with full FOV in Eclipse TPS. Targets for dose boost were defined based on the isodose curves generated from brachytherapy. An external photon beam plan was successfully generated to deliver dose for selected underdose regions. Conclusion Accurate external beam radiation treatment planning can be accomplished using our planning protocols when inadequate brachytherapy dose delivery occurs. The proposed technique can be used to safely deliver additional external radiation dose using intensity-modulated radiation therapy technique after suboptimal brachytherapy procedure.

Time‐resolved in vivo luminescence dosimetry for online error detection in pulsed dose‐rate brachytherapy

The purpose of this study is to present and evaluate a dose-verification protocol for pulsed dose-rate (PDR) brachytherapy based on in vivo time-resolved (1 s time resolution) fiber-coupled luminescence dosimetry. Five cervix cancer patients undergoing PDR brachytherapy (Varian GammaMed Plus with 192Ir) were monitored. The treatments comprised from 10 to 50 pulses (1 pulse/h) delivered by intracavitary/interstitial applicators (tandem-ring systems and/or needles). For each patient, one or two dosimetry probes were placed directly in or close to the tumor region using stainless steel or titanium needles. Each dosimeter probe consisted of a small aluminum oxide crystal attached to an optical fiber cable (1 mm outer diameter) that could guide radioluminescence (RL) and optically stimulated luminescence (OSL) from the crystal to special readout instrumentation. Positioning uncertainty and hypothetical dose-delivery errors (interchanged guide tubes or applicator movements from +/-5 to +/-15 mm) were simulated in software in order to assess the ability of the system to detect errors. For three of the patients, the authors found no significant differences (P>0.01) for comparisons between in vivo measurements and calculated reference values at the level of dose per dwell position, dose per applicator, or total dose per pulse. The standard deviations of the dose per pulse were less than 3%, indicating a stable dose delivery and a highly stable geometry of applicators and dosimeter probes during the treatments. For the two other patients, the authors noted significant deviations for three individual pulses and for one dosimeter probe. These deviations could have been due to applicator movement during the treatment and one incorrectly positioned dosimeter probe, respectively. Computer simulations showed that the likelihood of detecting a pair of interchanged guide tubes increased by a factor of 10 or more for the considered patients when going from integrating to time-resolved dose verification. The likelihood of detecting a +/-15 mm displacement error increased by a factor of 1.5 or more. In vivo fiber-coupled RL/OSL dosimetry based on detectors placed in standard brachytherapy needles was demonstrated. The time-resolved dose-rate measurements were found to provide a good way to visualize the progression and stability of PDR brachytherapy dose delivery, and time-resolved dose-rate measurements provided an increased sensitivity for detection of dose-delivery errors compared with time-integrated dosimetry.

Pulsed dose rate brachytherapy

Pulsed dose rate (PDR) is a new modality for dose delivery in brachytherapy. It uses modern afterloading technology (miniaturized source, cable driven, software controlled), with source activities in the range of 1 Ci, which is actually one tenth of the normal activity used for high dose rate (HDR) brachytherapy. Modern technology allows dose optimization, and source strength in the above-mentioned range creates a new dose rate condition. For small fractions (pulses) with short interpulse intervals, PDR mimics the radiobiology of high dose rate brachytherapy, whereas for bigger doses per fraction, dose adjustments are needed to compensate for the loss of therapeutic ratio. Clinical series showed good figures for local control and toxicity. Almost every clinical site has been reported to have been treated with PDR, with some thousand of patients having been reported. Technical difficulties in some body sites can be overcome by slightly modifying the implant technique. PDR brachytherapy is an ideal environment for the development of new dose fractionation schedules. It creates unique conditions in which to operate. Knowledge of tissue repair kinetics is extremely important for adequate selection of dose per pulse and interpulse interval. Therapeutic ratio can be improved by adjusting interpulse intervals to the repair half-times for normal tissues. On the other hand, superfractionated schedules with low dose per pulse can be explored in conditions of tumor hypoxia, thanks to the predicted hypersensitivity at low dose per fraction. The use of chemical agents (nicotinamide and others) in concomitance with this superfractionated schedules is foreseen in controlled clinical trials. In conclusion, PDR brachytherapy can be considered a new paradigm for dose delivery. It is safe and reliable, can be used in the setting of image-guided radiation therapy, and exploit the differential effect of ionizing radiations by a thorough knowledge of tissue kinetics for an improved therapeutic ratio.

TH‐C‐AUD A‐01: A Systematic Analysis of Errors and Uncertainties in Partial Breast Irradiation Using the MammoSite Balloon Catheter

Purpose: The objective of this study was to evaluate systematic and random errors associated with HDR brachytherapy treatment delivery of partial breast irradiation with a MammoSite Balloon. The impact of these errors on the actual delivered dose was studied. Method and Materials: The effect of respiratory motion, balloon contrast concentration and intra‐observer variability on accuracy of applicator delineation were investigated. Additionally, the consequence of variability of target delineation and dose matrix voxel size was considered in relationship to target coverage. Lastly, an analysis of 15 patients treated with the MammoSite was performed to simulate the effect of allowed source position uncertainty for each fraction. Results: Respiratory motion resulted in an under‐estimation of the balloon volume of 8% and 18% for computed tomography and cone‐beam computed tomography, respectively. The coverage was assessed based on the volume of the target receiving 95% of the prescribed dose (V95). On average, balloon volume underestimation combined with the daily ±1.0mm source position uncertainty degraded the coverage from an initial 96.3% to 91.7% over the course of 10 fractions. Additionally, a 3mm difference between the true tip of the applicator and its delineated position resulted in a decrease in coverage from 96.3% to 92.6% and from 98.7% to 79.3%, for a small and a large balloon, respectively. Conclusion: Current treatment protocols require that 90% of the target volume receives 90% of the prescription dose. Our findings suggest that significantly higher dosimetric coverage goals are necessary to compensate for the identified errors inherent in balloon based brachytherapy dose delivery. This will insure that the expected degradation of the actual dose delivered from the planned dose, as a consequence of random and systematic uncertainties, does not fall below an acceptable threshold.

Comparison of125I Stereotactic Brachytherapy and LINAC Radiosurgery Modalities based on Physical Dose Distribution and Radiobiological Efficacy

The goal of this study was to make a comparison between stereotactic brachytherapy implants and linear accelerator-based radiosurgery of brain tumors with respect to physical dose distributions and radiobiological efficacy. Twenty-four treatment plans made for irradiation of brain tumors with low-dose-rate (125)I brachytherapy and multiple-arc LINAC-based radiosurgery were analyzed. Using the dose-volume histograms and the linear-quadratic model, the brachytherapy doses were compared to the brachytherapy-equivalent LINAC radiosurgery doses with respect to the predicted late effects of radiation on normal brain tissue. To characterize the conformity and homogeneity of dose distributions, the conformal index, external volume index, and relative homogeneity index were calculated for each dose plan and the mean values were compared. The average tumor volume was 5.6 cm(3) (range: 0.1-19.3 cm(3)). At low doses, the calculated radiobiological late effect on normal tissue was equivalent for external-beam and brachytherapy dose delivery. For brachytherapy at doses greater than 30 Gy, the calculated equivalent dose to normal tissues was less than for external-beam radiosurgery. However, the dose-calculated homogeneity was better for the LINAC radiosurgery, with a mean relative homogeneity index of 0.62 compared to the calculated value of 0.19 for the brachytherapy (P=0.0002). These results are only predictions based on calculations concerning normal tissue tolerance. More data and research are needed to understand the clinical relevance of these findings.

Application of imaging-derived parameters to dosimetry of intravascular brachytherapy sources: perturbation effects of residual plaque burden.

The dosimetric effect of geometric and material heterogeneities on intravascular brachytherapy dose delivery has been studied recently. Residual plaque within the coronary vessel appears to have an impact on the uniform delivery of radiation dose to the arterial tissue. In this study, we have examined the effect of residual plaque burden and post-PCI (percutaneous coronary intervention) plaque configuration on the dose to the arterial wall from clinical intravascular brachytherapy beta-emitting sources containing 32P and 90Sr/90Y. Monte Carlo simulations using the MCNP4B code were performed for these catheter-based sources with residual plaque burden ranging between 25% and 50%. The residual plaque burden values were derived from post-PCI data provided in several recent clinical studies. Dose calculations were performed for three different values of plaque density (1.45 g cm(-3), 2.20 g cm(-3), and 3.1 g cm(-3)) and three different plaque morphologies for the same residual plaque burden. The dose perturbation factor (DPF), defined as the ratio of dose at 2 mm radial distance for a given case to the dose at the same radial distance in homogeneous water medium, was determined for each of the three different plaque densities. The range of DPF values was 0.81-1.01, 0.62-0.99, and 0.41-0.97 for different plaque densities for the 32P source. Corresponding DPF values for the 90Sr/90Y source were 0.90-1.01, 0.84-1.01, and 0.62-1.01. The results indicate the need for accurate assessment of post-PCI clinical measurements such as minimal lumen diameter and residual plaque burden and incorporation of these values into dose calculations.

Determination of the accuracy of implant reconstruction and dose delivery in brachytherapy in The Netherlands and Belgium

Purpose: To gain insight into the accuracy of brachytherapy treatments, the accuracy of implant reconstruction and dose delivery was investigated in 33 radiotherapy institutions in The Netherlands and Belgium.Materials and methods: The accuracy of the implant reconstruction method was determined using a cubic phantom containing 25 spheres at well-known positions. Reconstruction measurements were obtained on 41 brachytherapy localizers, 33 of which were simulators. The reconstructed distances between the spheres were compared with the true distances. The accuracy of the dose delivery was determined for high dose rate (HDR), pulsed dose rate (PDR) and low dose rate (LDR) afterloading systems using a polymethyl methacrylate cylindrical phantom containing a NE 2571 ionization chamber in its centre. The institutions were asked to deliver a prescribed dose at the centre of the phantom. The measured dose was compared with the prescribed dose.Results: The average reconstruction accuracy was −0.07 mm (±0.4 mm, 1 SD) for 41 localizers. The average deviation of the measured dose from the prescribed dose was +0.9% (±1.3%, 1 SD) for 21 HDR afterloading systems, +1.0% (±2.3%, 1 SD) for 12 PDR afterloaders, and +1.8% (±2.5%, 1 SD) for 15 LDR afterloaders.Conclusions: This comparison showed a good accuracy of brachytherapy implant reconstruction and dose delivery in The Netherlands and Belgium.

Automated data collection and analysis system for MOSFET radiation detectors

Metal oxide semiconductor field effect transistors (MOSFET) have been used as radiation dosimeters. Because of their small detector size, minimal power requirements, and signal integration characteristics, they offer unique possibilities as real-time dose monitors in radiotherapy. An automated data collection and analysis system for use with MOSFET radiation dosimeters has been designed and built. The objective was to design a system which can acquire and process the MOSFET signals in real time, in any radiation field encountered in radiotherapy. In particular, major problems have been solved arising from the intrinsic drifts of the MOSFET signal during low dose rate measurements. These signal drifts are significant when the MOSFET detector is used in applications such as on-line monitoring of radiation dose delivery in brachytherapy or radioimmunotherapy. The data collection and analysis system includes a portable IBM-compatible personal computer fitted with digital-to-analog and analog-to-digital converter boards. A single-chip programmable current supply is used to power the MOSFET dosimeters. Intrinsic and extrinsic drifts in signal due to ion diffusion and electron tunneling are corrected by deconvolution of the collected data in real time or after data collection. The data acquisition system and signal-processing methodologies are described.