Dose measurements nearby low energy electronic brachytherapy sources using radiochromic film

Comprehensive commissioning and clinical implementation of GammaTiles STaRT for intracranial brain tumors

Purpose

To validate the dose calculation accuracy and dose distribution of GammaTiles for brain tumors, and to suggest a surgically targeted radiation therapy (STaRT) workflow for planning, delivery, radiation safety documentation, and posttreatment validation.

Methods and Materials

Novel surgically targeted radiation therapy, GammaTiles, uses Cs-131 radiation isotopes embedded in collagen-based tiles that can be resorbed after surgery. GammaTile target delineation and dose calculation were performed on MIM Symphony software. Point-based and complex seed distribution calculations in MIM Symphony were verified with hand calculations and BrachyVision calculations. Vendor-provided 2-dimensional dose distribution calculation accuracy was validated using gafchromic EBT3 film measurements at various depths. A workflow was established for safe and effective GammaTile implants.

Results

Good agreement was observed between different calculations. Calculation accuracy of less than 0.5% was achieved for all points except one, which had rounding issues for very low doses and resulted in just below 5% difference. Differences in anisotropy and geometry positioning were noticed in the delineation of Cs-131 IsoRay seeds in the compared systems, resulting in minor discrepancies in the calculated dosimetry distributions. Film measurements showed profiles with relatively good agreement of 0% to 5% in nongradient regions with higher differences between 5% to 10% in the sharp dose fall-off regions.

Conclusions

A comprehensive evaluation of GammaTile geometry, dose distribution, and clinical workflow was conducted. Safe intro-operative implantation of GammaTiles requires extensive preplanning and interdisciplinary collaboration. A STaRT workflow was outlined to provide a guideline for an accurate treatment planning and safe implant process at other institutions.

Alkali Metal Salts of 10,12-Pentacosadiynoic Acid and Their Dosimetry Applications

Wide-dose-range 2D radiochromic films for radiotherapy, such as GAFchromic EBT, are based on the lithium salt of 10,12-pentacosadiynoic acid (Li-PCDA) as the photosensitive component. We show that there are two solid forms of Li-PCDA-a monohydrated form A and an anhydrous form B. The form used in commercial GAFchromic films is form A due to its short needle-shaped crystals, which provide favorable coating properties. Form B provides an enhanced photoresponse compared to that of form A, but adopts a long needle crystal morphology, which is difficult to process. The two forms were characterized by powder X-ray diffraction, Fourier transform infrared spectroscopy, CP-MAS ¹³C solid-state NMR spectroscopy, and thermogravimetric analysis. In sum, these data suggest a chelating bridging bidentate coordination mode for the lithium ions. The sodium salt of PCDA (Na-PCDA) is also reported, which is an ionic cocrystal with a formula of Na⁺PCDA^-·3PCDA. The PCDA and PCDA^- ligands display monodentate and bridging bidentate coordination to the sodium ion in contrast to the coordination sphere of the Li-PCDA forms. In contrast to its lithium analogues, Na-PCDA is photostable.

Alkali Metal Salts of 10,12-Pentacosadiynoic Acid and Their Dosimetry Applications

Wide-dose-range 2D radiochromic films for radiotherapy, such as GAFchromic EBT, are based on the lithium salt of 10,12-pentacosadiynoic acid (Li-PCDA) as the photosensitive component. We show that there are two solid forms of Li-PCDA-a monohydrated form A and an anhydrous form B. The form used in commercial GAFchromic films is form A due to its short needle-shaped crystals, which provide favorable coating properties. Form B provides an enhanced photoresponse compared to that of form A, but adopts a long needle crystal morphology, which is difficult to process. The two forms were characterized by powder X-ray diffraction, Fourier transform infrared spectroscopy, CP-MAS 13C solid-state NMR spectroscopy, and thermogravimetric analysis. In sum, these data suggest a chelating bridging bidentate coordination mode for the lithium ions. The sodium salt of PCDA (Na-PCDA) is also reported, which is an ionic cocrystal with a formula of Na+PCDA-·3PCDA. The PCDA and PCDA- ligands display monodentate and bridging bidentate coordination to the sodium ion in contrast to the coordination sphere of the Li-PCDA forms. In contrast to its lithium analogues, Na-PCDA is photostable.

In vivo dosimetry in low-voltage IORT breast treatments with XR-RV3 radiochromic film

PURPOSE

The objectives of the study were to establish a procedure for in vivo film-based dosimetry for intraoperative radiotherapy (IORT), evaluate the typical doses delivered to organs at risk, and verify the dose prescription.

MATERIALS AND METHODS

In vivo dose measurements were studied using XR-RV3 radiochromic films in 30 patients with breast cancer undergoing IORT using the Axxent® device (Xoft Inc.). The stability of the radiochromic films in the energy ranges used was verified by taking measurements at different depths. The stability of the scanner response was tested, and 5 different calibration curves were constructed for different beam qualities. Six pieces of film were placed in each of the 30 patients. All the pieces were correctly sterilized and checked to ensure that the process did not affect the outcome. All calibration and dose measurements were analyzed using the Radiochromic.com software application.

RESULTS

The doses were measured for 30 patients. The doses in contact with the applicator (prescription zone) were 19.8 ± 0.9 Gy. In the skin areas, the doses were as follows: 1-2 cm from the applicator, 1.86 ± 0.77 Gy; 2-5 cm, 0.73 ± 0.14 Gy; and greater than 5 cm, 0.28 ± 0.17 Gy. The dose delivered to the pectoral muscle (tungsten shielding disc) was 0.51 ± 0.27 Gy.

CONCLUSIONS

The study demonstrated the viability of XR-RV3 films for in vivo dose measurement in the dose and energy ranges applied in a complex procedure, such as breast IORT. The doses in organs at risk were far below the tolerances for cases such as those studied.

Clinical Implication of Dosimetry Formalisms for Electronic Low-Energy Photon Intraoperative Radiation Therapy

PURPOSE

Intraoperative radiation therapy (IORT) using the INTRABEAM, a miniature x-ray source, has shown to be effective in treating breast cancer. However, recent investigations have suggested a significant deviation between the reported and delivered doses. In this work, the dose delivered by INTRABEAM in the TARGIT breast protocol was investigated, along with the dose from the Xoft Axxent, another source used in breast IORT.

METHODS AND MATERIALS

The absorbed dose from the INTRABEAM was determined from ionization chamber measurements using: (a) the manufacturer-recommended formula (Zeiss V4.0 method), (b) a Monte Carlo calculated chamber conversion factor (C_Q method), and (c) the formula consistent with the TARGIT breast protocol (TARGIT method). The dose from the Xoft Axxent was determined from ionization chamber measurements using the Zeiss V4.0 method and calculated using the American Association of Physicists in Medicine TG-43 formalism.

RESULTS

For a nominal TARGIT prescription of 20 Gy, the dose at the INTRABEAM applicator surface ranged from 25.2 to 31.7 Gy according to the C_Q method for the largest (5 cm) and smallest (1.5 cm) diameter applicator, respectively. The Zeiss V4.0 method results were 7% to 10% lower (23.2 to 28.6 Gy). At 1 cm depth, the C_Q and Zeiss V4.0 absorbed doses were also larger than those predicted by the TARGIT method. The dose at 1 cm depth from the Xoft Axxent for a surface dose of 20 Gy was slightly less than INTRABEAM (3%-7% compared with C_Q method). An exception was for the 3 cm applicator, where the Xoft dose was appreciably lower (31%).

CONCLUSIONS

The doses delivered in the TARGIT breast protocol with INTRABEAM were significantly greater than the prescribed 20 Gy and depended on the size of spherical applicator used. Breast IORT treatments with the Xoft Axxent received less dose compared with TARGIT INTRABEAM, which could have implications for studies comparing clinical outcomes between the 2 devices.

Comparison of the 2-D Dose Distribution Calculated by Planning System and Measured by Gafchromic Film Physical Dosimetry for 60Co and 192Ir Brachytherapy Sources

Background:

High Dose Rate (HDR) brachytherapy sources with high photon energy have been widely used in treating tumors. Dosimetric parameter of these brachytherapy sources should be determined according to the AAPM TG-43 recommendation. Gafchoromic films are reliable tools for this evaluation.

Objective:

The aim of this study is to evaluate and compare dose accuracy of the two-brachytherapy sources in a dedicated phantom.

Material and Methods:

In this analytical study, two common sources, including Cobalt and Iridium, were loaded into the dedicated phantom. The two-dimensional dose distribution around the source was calculated by TPS system for certain activities and geometries around the sources. Then, the experimental dose measured by Gafchromic film dosimetry was reported for different angles ranging from 0 to 180 degrees.

Results:

The difference between calculated and measured doses was less than 6 percent (-5 to +6 percent) for all of the channels and angles. These errors are smaller and mainly more than zero (D_film>D_TPS) for angles less than 20 and larger than 110 degrees. There is no statistically significant discrepancy in dose calculation by treatment planning system.

Conclusion:

Although the estimated error in dose calculation is not significant, there is still an opportunity to increase the treatment precision. The correlation between the error and the angle should be considered in further plans of brachytherapy. The present study showed comparable errors compared to results of other research studies.

On the use of EBT3 film for relative dosimetry of kilovoltage X ray beams

EBT3 films were evaluated for relative dosimetry in water, in the energy range of therapeutic kV X ray beams. A film batch was calibrated in air for all nine beam qualities of a clinical unit (XStrahl 200). Monte Carlo (MC) simulations using MCNP v.6 facilitated the calculation of the film absorbed dose (f), and beam quality (k_bq) energy dependences in air. Results were found in agreement with corresponding data in the literature. Film samples from the same batch were irradiated in water along the central beam axis for each beam quality. Experimental percentage depth dose (PDD) results obtained using calibration data in air showed quality and depth dependent differences from corresponding MC simulations. These differences increased beyond film dosimetry uncertainty (<3.3%), reaching up to 8% at increased depth. The observed differences reduced only slightly when spectral variation as a function of measurement point was accounted for, using photon effective energy. PDD measurements and corresponding MC results facilitated the determination of f and k_bq in water. Results showed that the origin of the observed differences between experimental and MC PDD results is the difference between film response in air and water, as a result of radiation field perturbation from the film oriented along the central beam axis. This implies a directional dependence of film response which necessitates that the angular distribution of photons impinging on the film is the same in the calibration and measurement geometries.

Energy and dose dependence of GafChromic EBT3-V3 film across a wide energy range

AIM

To determine the energy and dose dependence of GafChromic EBT3-V3 film over an energy range 0.2 mm Al HVL to 6 MV.

BACKGROUND

The decay scheme of a brachytherapy source may be complex and the spectrum of energy can be wide. LiF TLDs are the golden standard recommended for dosimetric measures in brachytherapy, for their energy independence, but TLDs could be not available in some centres. An alternative way to perform dose measurements is to use GafChromic films, but they show energy dependence.

METHODS AND MATERIALS

Films have been irradiated at increasing dose with three different beams: 6 MV beam, TPR20, 10 = (0.684 ± 0.01), HVL = (2.00 ± 0.01)mmAl and HVL = (0.20 ± 0.01)mmAl. Calibration curves were generated using the same dose range (0cGy to 850cGy) for the three energies. Using the 6 MV calibration curve as reference, the film response in terms of net optical density (OD) was evaluated.

RESULTS

The difference in the calibration curve obtained by irradiating the film with 6 MV and 2 mm Al HVL energy beams is less than 3 %, within the calibration uncertainty, in the dose range 500-850cGy. The OD of EBT3-V3 film is significantly lower at 0.2 mmAl HVL compared to 6 MV, showing differences up to 25 %.

CONCLUSION

Within the range 6 MV-2 mm Al HVL and dose higher than 500cGy, GafChromic EBT3-V3 films are energy independent. In this dose range, films can be calibrated in a simple geometry, using a 6 MV Linac beam, and can be used for brachytherapy sources dose measures. The use of EBT3 films can be extended to reference dosimetry in Ir-192 clinical brachytherapy.