Alexandra Rink

Characterization and real-time optical measurements of the ionizing radiation dose response for a new radiochromic medium

A new radiochromic film, GafChromic EBT, was investigated for use in a real-time radiation dosimetry system. It was found to be approximately eight times more sensitive to ionizing radiation dose, exhibited less postexposure development and achieved stable readout faster than one of its predecessors, GafChromic MD-55. A clear distinction in change in optical density between exposure and postexposure was observed, but the measurements obtained during exposure were not linear with time or dose. This could not be explained by a shift in wavelength of maximum change in absorbance, as it was stable at approximately 636 nm during the entire exposure range (up to 9.52 Gy). Increasing the spectral window of interest over which calculations were performed did little to correct the nonlinearity. The radiochromic film exhibited small dose rate dependence in real-time measurements, with an increase in standard deviation of change in optical density measurements from 0.9% to 1.8% over a sixfold variation in dose rate. Overall, GafChromic EBT has increased sensitivity and decreased postexposure darkening, and this bodes well for its potential role as a radiation dosimeter, including real-time applications.

Investigation of energy dependence of EBT and EBT-2 Gafchromic film

PURPOSE The purpose of this study was to quantify the extent of energy dependence of Gafchromic film to x-ray energies ranging in quality from 105 kVp to 6 MV, and relate this dependency to the films chemical composition and date of production. METHODS Lots of Gafchromic EBT film manufactured in 2004 and 2005 together with more recent batches produced in 2007 were evaluated for energy dependence. Multiple batches of EBT-2 film were also evaluated. Energy dependence was quantified as Rx-the ratio of net optical density (netOD) measured at a given energy x relative to the netOD measured at 6 MV, as measured on a linear accelerator. Rx was evaluated for beam qualities of 105 and 220 kVp on a clinical orthovoltage unit using two separate techniques-a flatbed scanner (Epson) and a real-time fiber-optic readout system. Neutron activation analysis for chlorine and bromine content was performed on all the films to determine whether the composition of the film had changed between batches of film exhibiting different energy dependence responses. RESULTS For batches of EBT manufactured in 2007, R105 kVp was 0.75 and R220 kVp was 0.85, indicating an under-response at orthovoltage energies. These results were confirmed using both the Epson flatbed scanner as well as the real-time readout system. For batches of EBT film manufactured before 2006, Rl05 kVp ranged from 0.9 to 1.0. The results from the neutron activation analysis confirmed a direct relationship between the concentration of chlorine and the magnitude of under-response at orthovoltage energies. EBT-2 film exhibited R105 kVp values ranging from 0.79 (under-response) to 1.20 (over-response) among batches containing varying concentrations of bromine, chlorine, and potassium. CONCLUSIONS The results of this study indicated that differences in energy response of EBT and EBT-2 films were due to differences in the chemical composition and therefore the effective atomic number of the film, which have changed over time. To achieve an energy independent dosimeter over a range of kilovoltage energies, the effective atomic number of the dosimeter must be closely matched to that of water. Small deviations in chemical composition can lead to large deviations in response as a function of energy.

Energy dependence "75 kVp to 18 MV… of radiochromic films assessed using a real-time optical dosimeter

The response of radiochromic film, GafChromic EBT, was investigated for dependence on x-ray beam energy using a previously reported real-time optical readout approach. X-ray beams of energy from 75 kVp to 18 MV were employed. The dose-induced change in optical density for the EBT film was compared to values obtained for GafChromic HS and MD-55 films, exposed under the same conditions. All responses were normalized to that obtained for 60Co irradiation. While change in optical density for 1 Gy of applied dose as measured with HS and MD-55 films decreased by approximately 40% at low energies, the mean change in optical density of EBT film remained within 3% of that in the 60Co beam over the entire energy range.

Temperature and hydration effects on absorbance spectra and radiation sensitivity of a radiochromic medium

The effects of temperature on real time changes in optical density (DeltaOD) of GAFCHROMIC EBT film were investigated. The spectral peak of maximum change in absorbance (lambdamax) was shown to downshift linearly when the temperature of the film was increased from 22 to 38 degrees C. The DeltaOD values were also shown to decrease linearly with temperature, and this decrease could not be attributed to the shift in lambdamax. A compensation scheme using lambdamax and a temperature-dependent correction factor was investigated, but provided limited improvement. Part of the reason may be the fluctuations in hydration of the active component, which were found to affect both position of absorbance peaks and the sensitivity of the film. To test the effect of hydration, laminated and unlaminated films were desiccated. This shifted both the major and minor absorbance peaks in the opposite direction to the change observed with temperature. The desiccated film also exhibited reduced sensitivity to ionizing radiation. Rehydration of the desiccated films did not reverse the effects, but rather gave rise to another form of the polymer with absorbance maxima upshifted further 20 nm. Hence, the spectral characteristics and sensitivity of the film can be dependent on its history, potentially complicating both real-time and conventional radiation dosimetry.

A Facility for Magnetic Resonance-Guided Radiation Therapy

Magnetic resonance (MR) imaging is routinely employed in the design of radiotherapy (RT) treatment plans for many disease sites. It is evident that tighter integration of MR imaging into the RT process would increase confidence in dose placement and facilitate the integration of new MR imaging information (including anatomical and functional imaging) into the therapy process. To this end, a dedicated MR-guided RT (MRgRT) facility has been created that integrates a state-of-the-art linear accelerator delivery system, high-dose rate brachytherapy afterloader, and superconducting MR scanner to allow MR-based online treatment guidance, adaptive replanning, and response monitoring while maintaining the clinical functionality of the existing delivery systems. This system is housed within a dedicated MRgRT suite and operates in a coordinated fashion to assure safe and efficient MRgRT treatments.

Suitability of radiochromic medium for real-time optical measurements of ionizing radiation dose

A system, consisting of a novel optical fiber-based readout configuration and model-based method, has been developed to test suitability of a certain radiochromic medium for real-time measurements of ionizing radiation dose. Using this system with the radiochromic film allowed dose measurements to be performed during, and immediately after, exposure. The rates of change in OD before, during, and after exposure differ, and the change in OD during exposure was found to be proportional to applied dose in the tested range of 0-4 Gy. Estimating applied dose within an average error of less than 5% did not require a waiting time of 24-48 h as generally recommended with this radiochromic film. The errors can be further reduced by performing a calibration for each individual dosimeter setup instead of relying on batch calibration. Measurements of change in OD were found to be independent of dose-rate in the 95-570 cGy/min range for applied dose of 1 Gy or less. Some error was introduced due to dose-rate variation for doses of 2 Gy and above. The major limiting factor in utilizing this radiation sensitive medium for real-time in vivo dosimetry is the strong dependence on temperature in the clinically relevant range of 20-38 degrees C.

In vivo dosimetry with radiochromic films in low-voltage intraoperative radiotherapy of the breast.

PURPOSE EBT2 radiochromic films were studied and used for in vivo dosimetry in targeted intraoperative radiotherapy (TARGIT), a technique in which the Intrabeam system (Carl Zeiss, Oberkochen, Germany) is used to perform intraoperative partial breast irradiation with x-rays of 50 kV(p). METHODS The energy of the radiation emitted by the Intrabeam with the different spherical applicators, under 1 and 2 cm of solid water, and under the tungsten impregnated rubber used for shielding of the heart in TARGIT of the breast, was characterized with measurements of half-value layer (HVL). The stability of response of EBT2 was verified inside this range of energies. EBT2 films were calibrated using the red and green channels of the absorption spectrum in the 0-20 Gy dose range delivered by the Intrabeam x-rays. The dependence of film response on temperature during irradiation was measured. For in vivo dosimetry, pieces of radiochromic films wrapped in sterile envelopes were inserted after breast conserving surgery and before TARGIT into the excision cavity, on the skin and on the shielded pectoralis fascia for treatments of the left breast. RESULTS HVLs of the Intrabeam in TARGIT of the breast correspond to effective energies of 20.7-36.3 keV. The response of EBT2 was constant inside this range of energies. We measured the dose to the target tissue and to organs at risk in 23 patients and obtained an average dose of 13.52 ± 1.21 Gy to the target tissue. Dose to the skin in close proximity to the applicator was 2.22 ± 0.97 Gy, 0.29 ± 0.17 Gy at 5-10 cm from the applicator, and 0.08 ± 0.07 Gy at more than 10 cm from the applicator. Dose to the pectoral muscle for left breast treatment was 0.57 ± 0.23 Gy. CONCLUSIONS Our results show that EBT2 films are accurate at the beam energies, dose range, and irradiation temperature found in TARGIT and that in vivo dosimetry in TARGIT with EBT2 films wrapped in sterile envelopes is a feasible procedure. Measured dose to the organs at risk indicates that the technique is safe from side effects to the skin and the heart.

Lessons learned using an MRI-only workflow during high-dose-rate brachytherapy for prostate cancer

PURPOSE We report clinical observations of a technique using an MRI-only workflow for catheter insertion and treatment planning in patients receiving standard-care high-dose-rate brachytherapy before external beam radiotherapy for prostate cancer. METHODS AND MATERIALS Forty patients with intermediate or high-risk prostate cancer were enrolled on a prospective clinical trial approved by our institutions research ethics board. Multiparametric MRI with stereotactic navigation was used to guide insertion of brachytherapy catheters, followed by MRI-based treatment planning. RESULTS Sixty-two implants were performed. Median catheter insertion + imaging time was 100 minutes, and overall anesthesia time was 4.0 hours (range, 2.1-6.9 hours). MRI at the time of brachytherapy restaged 14 patients (35%) who were found to have a higher stage of disease. In 6 patients, this translated in directed insertion of brachytherapy catheters outside the prostate boundary (extracapsular disease [n = 2] or seminal vesicle invasion [n = 4]). Most patients (80%) had gross tumor visible on MRI, which influenced catheter insertion and treatment planning. MRI depicted postimplant anatomic boundaries clearly, with the exception of the apical prostate which was blurred by trauma after catheter insertion. Conventional dose-planning objectives for the rectum (V75 < 1.0 cc) were difficult to achieve, but toxicities were low (acute grade ≥ 2 genitourinary = 20%, late grade ≥ 2 genitourinary = 15%, and late grade ≥ 2 gastrointestinal = 7%). Urethral trauma visualized on MRI led to two transient Grade 3 events. CONCLUSIONS Despite a standard-care approach, MRI acquired throughout the procedure altered catheter insertion and dose-planning strategies. An MRI-only workflow is feasible but must be streamlined for broader acceptance.

Intra-irradiation changes in the signal of polymer-based dosimeter (GAFCHROMIC EBT) due to dose rate variations

The effect of dose rate on the real-time change in optical density (DeltaOD) of a GAFCHROMIC EBT film is quantified using a previously reported optical readout approach. A range of doses (5-1000 cGy) and dose rates (16-520 cGy min(-1)) are used, and a statistically significant difference between DeltaOD of films exposed at different dose rates occurs within approximately one order of magnitude change in the dose rate. A small increase in per cent standard deviation of measured DeltaOD values is also observed when the entire dose rate range was used, but in all cases combining all DeltaOD values produces per cent standard deviation of <4.5%. Thus, whether the dose rate effect is clinically significant depends on the specific application of EBT and the desired accuracy.

Dosimetric feasibility of ablative dose escalated focal monotherapy with MRI-guided high-dose-rate (HDR) brachytherapy for prostate cancer

PURPOSE To determine the dosimetric feasibility of dose-escalated MRI-guided high-dose-rate brachytherapy (HDR-BT) focal monotherapy for prostate cancer (PCa). METHODS In all patients, GTV was defined with mpMRI, and deformably registered onto post-catheter insertion planning MRI. PTV included the GTV plus 9mm craniocaudal and 5mm in every other direction. In discovery-cohort, plans were obtained for each PTV independently aiming to deliver ⩾16.5Gy/fraction (two fraction schedule) while respecting predefined organs-at-risk (OAR) constraints or halted when achieved equivalent single-dose plan (24Gy). Dosimetric results of original and focal HDR-BT plans were evaluated to develop a planning protocol for the validation-cohort. RESULTS In discovery-cohort (20-patients, 32-GTVs): PTV D95% ⩾16.5Gy could not be reached in a single plan (3%) and was accomplished (range 16.5-23.8Gy) in 15 GTVs (47%). Single-dose schedule was feasible in 16 (50%) plans. In the validation-cohort (10-patients, 10-GTVs, two separate implants each): plans met acceptable and ideal criteria in 100% and 43-100% respectively. Migration to single-dose treatment schedule was feasible in 7 implants (35%), without relaxing OARs constraints or increasing the dose (D100% and D35%) to mpMRI-normal prostate (p>0.05). CONCLUSION Focal ablative dose-escalated radiation is feasible with the proposed protocol. Prospective studies are warranted to determine the clinical outcomes.