Evaluation of rotational errors in treatment setup of stereotactic body radiation therapy of liver cancer.

PURPOSE: To evaluate the dosimetric impact of rotational setup errors in stereotactic body radiotherapy (SBRT) treatment of liver tumors and to investigate whether translational shifts can compensate for rotation. METHODS AND MATERIALS: The positioning accuracy in 20 patients with liver malignancies treated with SBRT was reevaluated offline by matching the patients' cone-beam computed tomography (CT) scans (n=75) to the planning CT scans and adjusting the 3 rotational angles (pitch, roll, and yaw). Systematic and random setup errors were calculated. The dosimetric changes caused by rotational setup errors were quantified for both simulated and observed patient rotations. Dose distributions recalculated on the rotated CT scans were compared with the original planned doses. Translational corrections were simulated based on manual translational registration of the rotated images to the original CT scans. The correction efficacy was evaluated by comparing the recalculated plans with the original plans. RESULTS: The systematic rotational setup errors were -0.06° ± 0.68°, -0.29° ± 0.62°, and -0.24° ± 0.61°; the random setup errors were 0.80°, 1.05°, and 0.61° for pitch, roll, and yaw, respectively. Analysis of CBCT images showed that 56.0%, 14.7%, and 1.3% of treated fractions had rotational errors of >1°, >2°, and >3°, respectively, in any one of the rotational axes. Rotational simulations demonstrated that the reduction of gross tumor volume (GTV) coverage was <2% when rotation was <3°. Recalculated plans using actual patient roll motions showed similar reduction (<2%) in GTV coverage. Translational corrections improved the GTV coverage to within 3% of the original values. For organs at risk (OAR), the dosimetric impact varied case by case. CONCLUSION: Actual rotational setup errors in SBRT for liver tumors are relatively small in magnitude and are unlikely to affect GTV coverage significantly. Translational corrections can be optimized to compensate for rotational setup errors. However, caution regarding possible dose increases to OAR needs to be exercised.

Dose correction in lung for HDR breast brachytherapy.

PURPOSE: To evaluate the dosimetric impact of lung tissue in Ir-192 APBI. MATERIAL AND METHODS: In a 40 × 40 × 40 cm(3) water tank, an Accelerated Partial Breast Irradiation (APBI) brachytherapy balloon inflated to 4 cm diameter was situated directly below the center of a 30 × 30 × 1 cm(3) solid water slab. Nine cm of solid water was stacked above the 1 cm base. A parallel plate ion chamber was centered above the base and ionization current measurements were taken from the central HDR source dwell position for channels 1, 2, 3 and 5 of the balloon. Additional ionization data was acquired in the 9 cm stack at 1 cm increments. A comparable data set was also measured after replacing the 9 cm solid water stack with cork slabs. The ratios of measurements in the two phantoms were calculated and compared to predicted results of a commercial treatment planning system. RESULTS: Lower dose was measured in the cork within 1 cm of the cork/solid water interface possibly due to backscatter effects. Higher dose was measured beyond 1 cm from the cork/solid water interface, increasing with path length up to 15% at 9 cm depth in cork. The treatment planning system did not predict either dose effect. CONCLUSIONS: This study investigates the dosimetry of low density material when the breast is treated with Ir-192 brachytherapy. HDR dose from Ir-192 in a cork media is shown to be significantly different than in unit density media. These dose differences are not predicted in most commercial brachytherapy planning systems. Empirical models based on measurements could be used to estimate lung dose associated with HDR breast brachytherapy.

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 (125)I 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.

Dose perturbation study in a multichannel breast brachytherapy device.

PURPOSE: A study was conducted to determine the dosimetric effects resulting from air pockets and high atomic number (Z) contrast medium within a multichannel breast brachytherapy device. MATERIAL AND METHODS: A 5-6 cm diameter Contura (SenoRx) brachytherapy device was inflated using 37 cm(3) of saline. Baseline dose falloff from an HDR Iridium-192 source was measured with the Iridium source centered in the central channel and an anterior off-center channel. Data were collected at distances from 1 to 50 mm. Comparison studies were conducted with identically inflated volume containing varied air pocket volumes (1-4 cm(3)) and concentrations of contrast solution (3%, 6%, and 9% by volume). Dose perturbation factors (DPF) were computed and evaluated. RESULTS: Dose perturbations due to air pockets and contrast solutions were observed. As the volume of air increased, the DPF increased by approximately 2.25%/cm(3). The effect was consistent for both channels. The contrast effects were more complex. The 3% contrast media had minimal dose perturbation. The 6% contrast solution caused dose reduction of 1.0% from the central channel but 1.5% dose increase from the anterior channel. The 9% contrast solution caused dose reductions by 4.0% (from central channel) and 3.0% (from anterior channel). The DPF from all contrast solutions moderated with increasing distance. CONCLUSIONS: Dose perturbations due to air pockets and high-Z contrast solution can be significant. It is important to control these effects to avoid dose errors.

Practical considerations for prostate HDR brachytherapy.

PURPOSE: A process for prostate high-dose-rate (HDR) brachytherapy was developed and implemented successfully in the community hospital setting. The practical aspects of the program are reviewed and may serve as a foundation for clinics interested in offering this clinical service. METHODS AND MATERIALS: A generic needle distribution geometry was established to accommodate target volumes of variable size. A system to identify and assign treatment channels to each implant needle was devised. The computerized tomography (CT)-based treatment planning was used with dose constraints defined for sensitive structures and target uniformity. Implant needle stability was promoted by supporting the patient on a CT compatible padded sliding board. A process that aligns dwell position to CT imaging without the use of radiographic markers was followed. Graphical optimization of dwell times was used to generate the treatment dose distributions. RESULTS: Prostate HDR brachytherapy as a boost or as monotherapy has been offered in a program that has evolved over the past 8 years. Practical aspects of the program promote its feasibility and precision. Collaboration with commercial entities has also led to the development of products that support the technique. CONCLUSIONS: Prostate HDR brachytherapy offers a relatively high degree of dose distribution control in comparison with other prostate radiotherapy modalities. The practical aspects described offer assurance to achieve that goal.

Treatment delays using an automated afterloading low-dose-rate brachytherapy system.

PURPOSE: Low-dose-rate (LDR) brachytherapy is an integral treatment modality in radiation oncology. Clinical efficacy is based on experience with manual source loading and continuous dose delivery. With remote afterloading technology, sources may be loaded and unloaded during the treatment course to prevent radiation exposure to nursing staff members and visitors. The aim of this study was to investigate treatment interruptions in terms of frequency and duration as well as extension of the overall treatment time period. The potential clinical impact of treatment interruptions was also considered. MATERIALS AND METHODS: The treatment records of 20 patients who underwent brachytherapy in the Indiana University Department of Radiation Oncology administered with a Selectron LDR remote afterloader were reviewed. Results were tabulated and analysis performed with respect to 1) the number of interruptions, 2) delay time, 3) delay time (T(d)) as a function of total implant time (T), 4) the time of day that each interruption occurred, and 5) the time in minutes of each individual interruption. RESULTS: The mean number of interruptions was 44.9 per patient, (range, 24-76), with a mean prescription implantation duration of 45.7 hours and a mean actual treatment time of 51.2 hours resulting in a mean interruption time of 6.4 minutes per treatment hour. The number of interruptions was standardized and divided by the number of prescribed dose in grays, translating to 1.2 to 3.7 interruptions per gray delivered, with a mean of 1.6, resulting in an average T(d) of 11.21% (range, 7.35%-17.12%). CONCLUSION: Significant interruptions are frequent using remote afterloading LDR techniques, reducing the effective dose rate. Careful monitoring of such interruptions is warranted.