Dual-Energy CT: Balance Between Iodine Attenuation and Artifact Reduction for the Evaluation of Head and Neck Cancer.

OBJECTIVE: Dual-energy computed tomography high energy virtual monochromatic images (VMIs) can reduce artifact but suppress iodine attenuation in enhancing tumor. We investigated this trade-off to identify VMI(s) that strike the best balance between iodine detection and artifact reduction. METHODS: The study was performed using an Alderson radiation therapy phantom. Different iodine solutions (based on estimated tumor iodine content in situ using dual-energy computed tomography material decomposition) and different dental fillings were investigated. Spectral attenuation curves and quality index (QI: 1/SD) were evaluated. RESULTS: The relationship between iodine attenuation and QI depends on artifact severity and iodine concentration. For low to average concentration solutions degraded by mild to moderate artifact, the iodine attenuation and QI curves crossed at 95 keV. CONCLUSIONS: High energy VMIs less than 100 keV can achieve modest artifact reduction while preserving sufficient iodine attenuation and could represent a useful additional reconstruction for evaluation of head and neck cancer.

Radiochromic film based dosimetry of image-guidance procedures on different radiotherapy modalities.

In this work we compare doses from imaging procedures performed on today's state-of-the-art integrated imaging systems using a reference radiochromic film dosimetry system. Skin dose and dose profile measurements from different imaging systems were performed using radiochromic films at different anatomical sites on a humanoid RANDO phantom. EBT3 film was used to measure imaging doses from a TomoTherapy MVCT system, while XRQA2 film was used for dose measurements from kilovoltage imaging systems (CBCT on 21eX and TrueBeam Varian linear accelerators and CyberKnife stereoscopic orthogonal imagers). Maximum measured imaging doses in cGy at head, thorax, and pelvis regions were respectively 0.50, 1.01, and 4.91 for CBCT on 21eX, 0.38, 0.84, and 3.15 for CBCT on TrueBeam, 4.33, 3.86, and 6.50 for CyberKnife imagers, and 3.84, 1.90, and 2.09 for TomoTherapy MVCT. In addition, we have shown how an improved calibration system of XRQA2 film can achieve dose uncertainty level of better than 2% for doses above 0.25 cGy. In addition to simulation-based studies in literature, this study provides the radiation oncology team with data necessary to aid in their decision about imaging frequency for image-guided radiation therapy protocols.

Investigation of dosimetric characteristics of radiochromic film in response to alpha particles emitted from Americium-241.

BACKGROUND: In radiotherapy, it is essential to deliver prescribed doses to tumors while minimizing damage to surrounding healthy tissue. Accurate measurements of absorbed dose are required for this purpose. Gafchromic® external beam therapy (EBT) radiochromic films have been widely used in radiotherapy. While the dosimetric characteristics of the EBT3 model film have been extensively studied for photon and charged particle beams (protons, electrons, and carbon ions), little research has been done on α $\alpha$ -particle dosimetry. α $\alpha$ -emitting radionuclides have gained popularity in cancer treatment due to their high linear energy transfer, short range in tissue, and ability to spare surrounding organs at risk, thereby delivering a more localized dose distribution to the tumor. Therefore, a dose-calibration film protocol for α $\alpha$ -particles is required. PURPOSE: This study aimed to develop a dose-calibration protocol for the α $\alpha$ -particle emitting radionuclide 241Am, using Monte Carlo (MC) simulations and measurements with unlaminated EBT3 films. METHODS: In this study, a MC-based user code was developed using the Geant4 simulation toolkit to model and simulate an 241Am source and an unlaminated EBT3 film. Two simulations were performed: one with voxelized geometries of the EBT3 active volume composition and the other using water. The dose rate was calculated within a region of interest in the voxelized geometries. Unlaminated EBT3 film pieces were irradiated with the 241Am source at various exposure times inside a black box. Film irradiations were compared to a 6-MV photon beam from a Varian TrueBeam machine. The simulated dose rate was used to convert the exposure times into absorbed doses to water, describing a radiochromic-film-based reference dosimetry protocol for α $\alpha$ -particles. The irradiated films were scanned and through an in-house Python script, the normalized pixel values from the green-color channel of scanned film images were analyzed. RESULTS: The 241Am energy spectra obtained from the simulations were in good agreement with IAEA and NIST databases, having differences < $<$ 0.516% for the emitted γ $\gamma$ -rays and produced characteristic x-rays and < $<$ 0.006% for the α $\alpha$ -particles. Due to the short range of α $\alpha$ -particles, there was no energy deposition in the voxels outside the active 241Am source region projected onto the film surface. Thus, the total dose rate within the voxels covering the source was 0.847 ± $\pm$ 0.003 Gy/min within the sensitive layer of the film (LiPCDA) and 0.847 ± $\pm$ 0.004 Gy/min in water, indicating that the active volume can be considered water equivalent for the 241Am beam quality. A novel approach was employed in α $\alpha$ -film dosimetry using an exponential fit for the green channel, which showed promising results by reducing the uncertainty in dose estimation within 5%. Although the statistical analysis did not reveal significant differences between the 6-MV photon beam and the α $\alpha$ calibration curves, the dose-response curves exhibited the expected behavior. CONCLUSIONS: The developed MC user code simulated the experimental setup for α $\alpha$ -dosimetry using radiochromic film with acceptable uncertainty. Unlaminated EBT3 film is suitable for the dosimetry of α $\alpha$ -radiation at low doses and can be used in conjunction with other unlaminated GafChromic® films for quality assurance and research purposes.

Physical wedge as a tool for radiochromic film calibration.

Reliable calibration is one of the major challenges in using radiochromic films (RCF) for radiation dosimetry. In this study the feasibility of using dose gradients produced by a physical wedge (PW) for RCF calibration was investigated. The aim was to establish an efficient and reproducible method for calibrating RCF using a PW. Film strips were used to capture the wedge dose profile for five different exposures and the acquired scans were processed to generate corresponding net optical density wedge profiles. The proposed method was compared to the benchmark calibration, following the guidelines for precise calibration using uniform dose fields. The results of the benchmark comparison presented in this paper showed that using a single film strip for measuring wedge dose profile is sufficient for estimating a reliable calibration curve within the recorded dose range. Furthermore, the PW calibration can be extrapolated or extended by using multiple gradients for the optimal coverage of the desired calibration dose range. The method outlined in this paper can be readily replicated using the equipment and expertise commonly found in a radiotherapy center. Once the dose profile and central axis attenuation coefficient of the PW are determined, they can serve as a reference for a variety of calibrations using different types and batches of film. This investigation demonstrated that the calibration curves obtained with the presented PW calibration method are within the bounds of the measurement uncertainty evaluated for the conventional uniform dose field calibration method.

Sorafenib in combination with ionizing radiation has a greater anti-tumour activity in a breast cancer model.

High expression of vascular endothelial growth factor (VEGF) in patients with breast cancer has been associated with a poor prognosis, indicating that VEGF could be linked to the efficacy of chemotherapy and radiotherapy. It has also been suggested that radiation resistance is partly due to tumour cell production of angiogenic cytokines, particularly VEGF receptor (VEGFR). This evidence indicates that inhibition of VEGFR might enhance the radiation response. Sorafenib tosylate (Bay 54-9085) is an oral, small-molecule multikinase inhibitor of several targets including RAF/MEK/ERK MAP kinase signalling, VEGFR-2, VEGFR-3 and platelet-derived growth factor receptor-beta. Sorafenib has shown clinical efficacy in treating solid tumours such as renal cell and hepatocellular carcinomas. However, strategies are yet to be identified to prolong and maximize the anticancer effect of this multikinase inhibitor. The objective of this study was to determine whether a combination of Sorafenib and radiation will enhance the treatment response in vitro and in vivo. Radio-modulating effect of Sorafenib was assessed by performing clonogenic assays. In addition, cell cycle analyses as well as annexin-V apoptosis assays were performed 24 and 48 h after treatment, respectively. To confirm our in-vitro results, tumour growth delay assays were performed. Our results showed a strong and supra-additive antitumour effect of radiation combined with Sorafenib in vitro (dose enhancement factor of 1.76). The combined therapy demonstrated a strong and significant G2/M cell cycle arrest (combined treatment vs. irradiated alone: P<0.0008). Moreover, annexin-V staining showed a significant increase in the level of apoptosis (combined treatment vs. irradiated alone: P<0.0004). Study of the syngeneic model demonstrated the superior potency of the Sorafenib combined with radiotherapy. Our results demonstrate that higher antitumour activity can be achieved when radiation and Sorafenib are combined.

MRI simulation for radiotherapy treatment planning.

Over the last two decades, the computed tomography simulator became the standard of the contemporary radiotherapy treatment planning (RTP) process. Along the same time, the superb soft tissue contrast of magnetic resonance imaging (MRI) was widely incorporated into RTP through the process of image coregistration. This review summarizes the efforts of incorporation of MRI data into target definition process for RTP based on gained clinical evidence so far and opens a question whether the time is up for bringing a MRI-simulator as an additional standard imaging tool into radiation oncology departments.

EBT2 film as a depth-dose measurement tool for radiotherapy beams over a wide range of energies and modalities.

PURPOSE: One of the fundamental parameters used for dose calculation is percentage depth-dose, generally measured employing ionization chambers. There are situations where use of ion chambers for measuring depth-doses is difficult or problematic. In such cases, radiochromic film might be an alternative. The EBT-2 model GAFCHROMIC™ film was investigated as a potential tool for depth-dose measurement in radiotherapy beams over a broad range of energies and modalities. METHODS: Pieces of the EBT-2 model GAFCHROMIC™ EBT2 film were exposed to x-ray, electron, and proton beams used in radiotherapy. The beams employed for this study included kilovoltage x-rays (75 kVp), (60)Co gamma-rays, megavoltage x-rays (18 MV), electrons (7 and 20 MeV), and pristine Bragg-peak proton beams (126 and 152 MeV). At each beam quality, film response was measured over the dose range of 0.4-8.0 Gy, which corresponds to optical densities ranging from 0.05 to 0.4 measured with a flat-bed document scanner. To assess precision in depth-dose measurements with the EBT-2 model GAFCHROMIC™ film, uncertainty in measured optical density was investigated with respect to variation in film-to-film and scanner-bed uniformity. RESULTS: For most beams, percentage depth-doses measured with the EBT-2 model GAFCHROMIC™ film show an excellent agreement with those measured with ion chambers. Some discrepancies are observed in case of (i) kilovoltage x-rays at larger depths due to beam-hardening, and (ii) proton beams around Bragg-peak due to quenching effects. For these beams, an empirical polynomial correction produces better agreement with ion-chamber data. CONCLUSIONS: The EBT-2 model GAFCHROMIC™ film is an excellent secondary dosimeter for measurement of percentage depth-doses for a broad range of beam qualities and modalities used in radiotherapy. It offers an easy and efficient way to measure beam depth-dose data with a high spatial resolution.

Linearization of dose-response curve of the radiochromic film dosimetry system.

PURPOSE: Despite numerous advantages of radiochromic film dosimeter (high spatial resolution, near tissue equivalence, low energy dependence) to measure a relative dose distribution with film, one needs to first measure an absolute dose (following previously established reference dosimetry protocol) and then convert measured absolute dose values into relative doses. In this work, we present result of our efforts to obtain a functional form that would linearize the inherently nonlinear dose-response curve of the radiochromic film dosimetry system. METHODS: Functional form [ζ = (-1)[middle dot]netOD((2∕3))∕ln(netOD)] was derived from calibration curves of various previously established radiochromic film dosimetry systems. In order to test the invariance of the proposed functional form with respect to the film model used we tested it with three different GAFCHROMIC™ film models (EBT, EBT2, and EBT3) irradiated to various doses and scanned on a same scanner. For one of the film models (EBT2), we tested the invariance of the functional form to the scanner model used by scanning irradiated film pieces with three different flatbed scanner models (Epson V700, 1680, and 10000XL). To test our hypothesis that the proposed functional argument linearizes the response of the radiochromic film dosimetry system, verification tests have been performed in clinical applications: percent depth dose measurements, IMRT quality assurance (QA), and brachytherapy QA. RESULTS: Obtained R(2) values indicate that the choice of the functional form of the new argument appropriately linearizes the dose response of the radiochromic film dosimetry system we used. The linear behavior was insensitive to both film model and flatbed scanner model used. Measured PDD values using the green channel response of the GAFCHROMIC™ EBT3 film model are well within ±2% window of the local relative dose value when compared to the tabulated Cobalt-60 data. It was also found that criteria of 3%∕3 mm for an IMRT QA plan and 3%∕2 mm for a brachytherapy QA plan are passing 95% gamma function points. CONCLUSIONS: In this paper, we demonstrate the use of functional argument to linearize the inherently nonlinear response of a radiochromic film based reference dosimetry system. In this way, relative dosimetry can be conveniently performed using radiochromic film dosimetry system without the need of establishing calibration curve.

Image-Guided Brachytherapy for Rectal Cancer: Reviewing the Past Two Decades of Clinical Investigation.

(1) Background: The introduction of total mesorectal excision (TME) for rectal cancer has led to improvement in local recurrence (LR) outcomes. Furthermore, the addition of preoperative external beam radiotherapy to TME reduces LR to less than 6%. As a trade-off to these gradual improvements in local therapies, the oncology community's work is now focusing on mitigating treatment-related toxicities. In other words, if a small proportion of 4-6% of rectal cancer patients benefit from additional local therapy beyond TME, the burden of acute and long-term side effects must be considered with care. (2) Methods: With the introduction of better-quality imaging for tumor visualization and treatment planning, a new conformed radiation treatment was introduced with high-dose-rate endorectal brachytherapy. The treatment concept was tested in phase I and II studies: first in the pre-operative setting, and then as a boost after external beam radiation therapy, as a dose-escalation study, to achieve higher local tumor control. (3) Results: HDREBT is safe and effective in achieving a high tumor regression rate and was well tolerated in a phase II multicenter and two matched-pair studies. (4) Conclusions: HDREBT is a conformed radiation therapy that is safe and effective, and is presently explored in a phase III dose-escalation study in the NOM of patients with operable rectal cancer.

MORPHEUS Phase II-III Study: A Pre-Planned Interim Safety Analysis and Preliminary Results.

BACKGROUND: We explored image-guided adaptive endorectal brachytherapy patients electing non-operative management for rectal cancer. We present the first pre-planned interim analysis. METHODS: In this open-label phase II-III randomized study, patients with operable cT2-3ab N0 M0 rectal cancer received 45 Gy in 25 fractions of pelvic external beam radiotherapy (EBRT) with 5-FU/Capecitabine. They were randomized 1:1 to receive either an EBRT boost of 9 Gy in 5 fractions (Arm A) or three weekly adaptive brachytherapy (IGAEBT) boosts totaling 30 Gy (Arm B). Patient characteristics and toxicity are presented using descriptive analyses; TME-free survival between arms with the intention to treat the population is explored using the Kaplan-Meier method. RESULTS: A total of 40 patients were in this analysis. Baseline characteristics were balanced; acute toxicities were similar. Complete clinical response (cCR) was 50% (n = 10/20) in Arm A and 90% in Arm B (n = 18/20). Median follow-up was 1.3 years; 2-year TME-free survival was 38.6% (95% CI: 16.5-60.6%) in the EBRT arm and 76.6% (95% CI: 56.1-97.1%) in the IGAEBT arm. CONCLUSIONS: Radiation intensification with IGAEBT is feasible. This interim analysis suggests an improvement in TME-free survival when comparing IGAEBT with EBRT, pending confirmation upon completion of this trial.

Reference dosimetry during diagnostic CT examination using XR-QA radiochromic film model.

PURPOSE: The authors applied 2D reference dosimetry protocol for dose measurements using XR-QA radiochromic film model during diagnostic computed tomography (CT) examinations carried out on patients and humanoid Rando phantom. METHODS: Response of XR-QA model GAFCHROMIC™ film reference dosimetry system was calibrated in terms of Air-Kerma in air. Four most commonly used CT protocols were selected on their CT scanner (GE Lightspeed VCT 64), covering three anatomical sites (head, chest, and abdomen). For each protocol, 25 patients ongoing planned diagnostic CT examination were recruited. Surface dose was measured using four or eight film strips taped on patients' skin and on Rando phantom. Film pieces were scanned prior to and after irradiation using Epson Expression™ 10000XL document scanner. Optical reflectance of the unexposed film piece was subtracted from exposed one to obtain final net reflectance change, which is subsequently converted to dose using previously established calibration curves. RESULTS: The authors' measurements show that body skin dose variation has a sinusoidal pattern along the scanning axis due to the helical movement of the x-ray tube, and a comb pattern for head dose measurements due to its axial movement. Results show that the mean skin dose at anterior position for patients is (51 ± 6) mGy, (29 ± 11) mGy, (45 ± 13) mGy and (38 ± 20) mGy for head, abdomen, angio Abdomen, and chest and abdomen protocol (UP position), respectively. The obtained experimental dose length products (DLP) show higher values than CT based DLP taken from the scanner console for body protocols, but lower values for the head protocol. Internal dose measurements inside the phantom's head indicate nonuniformity of dose distribution within scanned volume. CONCLUSIONS: In this work, the authors applied an Air-Kerma in air based radiochromic film reference dosimetry protocol for in vivo skin dose measurements. In this work, they employed green channel extracted from the scanned RGB image for dose measurements in the range from 0 to 200 mGy. Measured skin doses and corresponding DLPs were higher than DLPs provided by the CT scanner manufacturer as they were measured on patients' skin.

Radiochromic film dosimetry of HDR (192)Ir source radiation fields.

PURPOSE: A radiochromic film based dosimetry system for high dose rate (HDR) Iridium-192 brachytherapy source was described. A comparison between calibration curves established in water and Solid Water™ was provided. METHODS: Pieces of EBT-2 model GAFCHROMIC™ film were irradiated in both water and Solid Water™ with HDR (192)Ir brachytherapy source in a dose range from 0 to 50 Gy. Responses of EBT-2 GAFCHROMIC™ film were compared for irradiations in water and Solid Water™ by scaling the dose between media through Monte Carlo calculated conversion factor for both setups. To decrease uncertainty in dose delivery due to positioning of the film piece with respect to the radiation source, traceable calibration irradiations were performed in a parallel-opposed beam setup. RESULTS: The EBT-2 GAFCHROMIC™ film based dosimetry system described in this work can provide an overall one-sigma dose uncertainty of 4.12% for doses above 1 Gy. The ratio of dose delivered to the sensitive layer of the film in water to the dose delivered to the sensitive layer of the film in Solid Water™ was calculated using Monte Carlo simulations to be 0.9941 ± 0.0007. CONCLUSIONS: A radiochromic film based dosimetry system using only the green color channel of a flatbed document scanner showed superior precision if used alone in a dose range that extends up to 50 Gy, which greatly decreases the complexity of work. In addition, Solid Water™ material was shown to be a viable alternative to water in performing radiochromic film based dosimetry with HDR (192)Ir brachytherapy sources.

Influence of electron density spatial distribution and X-ray beam quality during CT simulation on dose calculation accuracy.

Impact of the various kVp settings used during computed tomography (CT) simulation that provides data for heterogeneity corrected dose distribution calculations in patients undergoing external beam radiotherapy with either high-energy photon or electron beams have been investigated. The change of the Hounsfield Unit (HU) values due to the influence of kVp settings and geometrical distribution of various tissue substitute materials has also been studied. The impact of various kVp settings and electron density (ED) distribution on the accuracy of dose calculation in high-energy photon beams was found to be well within 2%. In the case of dose distributions obtained with a commercially available Monte Carlo dose calculation algorithm for electron beams, differences of more than 10% were observed for different geometrical setups and kVp settings. Dose differences for the electron beams are relatively small at shallow depths but increase with depth around lower isodose values.

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 (CQ 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 CQ 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 CQ 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 CQ 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.

Reference radiochromic film dosimetry in kilovoltage photon beams during CBCT image acquisition.

PURPOSE: A common approach for dose assessment during cone beam computed tomography (CBCT) acquisition is to use thermoluminescent detectors for skin dose measurements (on patients or phantoms) or ionization chamber (in phantoms) for body dose measurements. However, the benefits of a daily CBCT image acquisition such as margin reduction in planning target volume and the image quality must be weighted against the extra dose received during CBCT acquisitions. METHODS: The authors describe a two-dimensional reference dosimetry technique for measuring dose from CBCT scans using the on-board imaging system on a Varian Clinac-iX linear accelerator that employs the XR-QA radiochromic film model, specifically designed for dose measurements at low energy photons. The CBCT dose measurements were performed for three different body regions (head and neck, pelvis, and thorax) using humanoid Rando phantom. RESULTS: The authors report on both surface dose and dose profiles measurements during clinical CBCT procedures carried out on a humanoid Rando phantom. Our measurements show that the surface doses per CBCT scan can range anywhere between 0.1 and 4.7 cGy, with the lowest surface dose observed in the head and neck region, while the highest surface dose was observed for the Pelvis spot light CBCT protocol in the pelvic region, on the posterior side of the Rando phantom. The authors also present results of the uncertainty analysis of our XR-QA radiochromic film dosimetry system. CONCLUSIONS: Radiochromic film dosimetry protocol described in this work was used to perform dose measurements during CBCT acquisitions with the one-sigma dose measurement uncertainty of up to 3% for doses above 1 cGy. Our protocol is based on film exposure calibration in terms of "air kerma in air," which simplifies both the calibration procedure and reference dosimetry measurements. The results from a full Monte Carlo investigation of the dose conversion of measured XR-QA film dose at the surface into dose to water (or water kerma) at the surface of the phantom indicate that, for typical beam qualities used in CBCT, this conversion can be approximated by simple mass-energy absorption coefficient ratios water-to-air.

Evaluation of EBT-2 model GAFCHROMIC film performance in water.

PURPOSE: The authors present results of the measurements on the impact of radiochromic film immersion in water. The impact of film piece size, initial optical density, postimmersion waiting time prior to scanning, and the time film was kept in water has been investigated. The authors also investigated the pathways of water penetration into the film during the film immersion in water. METHODS: To study the impact of water immersion on change in optical density, the authors used various sizes of the latest EBT-2 model GAFCHROMICTM film: 2 x 2, 4 x 4, and 8 x 8 in.2. In addition, to test any existing dependence of the film's optical density on water diffusion, the authors used two sets of films: Unexposed (0 Gy) and film pieces exposed to a dose of 3 Gy. Times that film pieces were left in water ranged from 30 min to 24 h, and once the film was permanently removed from water, the authors also studied the impact of the scanning time (deltat) that ranged from 0 (films scanned right after removal from water) to 72 h postimmersion. RESULTS: While the penetration depth can reach as much as 9 mm around the edges of the EBT-2 GAFCHROMIC film, the anticipated dose error due to the change in optical density due to the water immersion appears to be negligible for the short immersions of the order of 30 min. However, as the immersion time increases, the anticipated dose error may reach 22 cGy on a 2 x 2 in.2 piece of film, which corresponds to 7% dose error at 3 Gy of measured dose. CONCLUSIONS: In this work, the authors report on an undoubted impact of radiochromic film immersion in water on the measured change in optical density, which may lead to systematic errors in dose measurements if the film is kept in water for longer periods of time. The magnitude of the impact depends on many parameters: Size of the film piece, initial optical density, postimmersion waiting time prior to scanning (defined by the current radiochromic film dosimetry protocol in. place), and the time film was kept in water. The authors also suggested various approaches in correcting for the change in netOD due to water penetration into the film, but the authors believe that the use of the control film piece would be the most appropriate.

Absorption spectra time evolution of EBT-2 model GAFCHROMIC film.

PURPOSE: One of the major drawbacks of the current radiochromic film dosimetry protocols is the postirradiation waiting time. In this article, the authors study the postirradiation time evolution of the absorption spectrum of radiochromic EBT-2 GAFCHROMIC film model. METHODS: Postirradiation scanning times range from 3 min to 5 days and a dose range extends from 0 to 6 Gy. The authors compare the results of absorption spectra measurements for the latest GAFCHROMIC EBT-2 film model to the absorption spectra of the previous EBT GAFCHROMIC film model. The authors also describe a method that can establish the time error constraints on the postirradiation scanning time that will still provide an acceptable dose error for clinical applications if the protocol employing the shorter postirradiation scanning time is implemented in the clinic. RESULTS: The two film models experience the very same dose change in net absorbance with sensitivity of the latest EBT-2 model GAFCHROMIC film being slightly lower than its predecessor. The authors show that for two postirradiation scanning times of 30 min and 24 h, the 1% dose error can be achieved if the scanning time window is less than +/- 5 min and +/- 2 h, respectively. CONCLUSIONS: By comparing the resultant change in net absorbance between the latest EBT-2 and previous EBT GAFCHROMIC film models, the authors conclude that the addition of the yellow marker dye to the sensitive layer does not affect dosimetric properties of the latest film model. The authors also describe a procedure by which one can establish an acceptable time window around chosen postirradiation scanning time protocol that would provide an acceptable dose error for practical purposes.

Positional and angular tracking of HDR 192 Ir source for brachytherapy quality assurance using radiochromic film dosimetry.

PURPOSE: To quantify and verify the dosimetric impact of high-dose rate (HDR) source positional uncertainty in brachytherapy, and to introduce a model for three-dimensional (3D) position tracking of the HDR source based on a two-dimensional (2D) measurement. This model has been utilized for the development of a comprehensive source quality assurance (QA) method using radiochromic film (RCF) dosimetry including assessment of different digitization uncertainties. METHODS: An algorithm was developed and verified to generate 2D dose maps of the mHDR-V2 192 Ir source (Elekta, Veenendaal, Netherlands) based on the AAPM TG-43 formalism. The limits of the dosimetric error associated with source (0.9 mm diameter) positional uncertainty were evaluated and experimentally verified with EBT3 film measurements for 6F (2.0 mm diameter) and 4F (1.3 mm diameter) size catheters at the surface (4F, 6F) and 10 mm further (4F only). To quantify this uncertainty, a source tracking model was developed to incorporate the unique geometric features of all isodose lines (IDLs) within any given 2D dose map away from the source. The tracking model normalized the dose map to its maximum, then quantified the IDLs using blob analysis based on features such as area, perimeter, weighted centroid, elliptic orientation, and circularity. The Pearson correlation coefficients (PCCs) between these features and source coordinates (x, y, z, θy , θz ) were calculated. To experimentally verify the accuracy of the tracking model, EBT3 film pieces were positioned within a Solid Water® (SW) phantom above and below the source and they were exposed simultaneously. RESULTS: The maximum measured dosimetric variations on the 6F and 4F catheter surfaces were 39.8% and 36.1%, respectively. At 10 mm further, the variation reduced to 2.6% for the 4F catheter which is in agreement with the calculations. The source center (x, y) was strongly correlated with the low IDL-weighted centroid (PCC = 0.99), while the distance to source (z) was correlated with the IDL areas (PCC = 0.96) and perimeters (PCC = 0.99). The source orientation θy was correlated with the difference between high and low IDL-weighted centroids (PCC = 0.98), while θz was correlated with the elliptic orientation of the 60-90% IDLs (PCC = 0.97) for a maximum distance of z = 5 mm. Beyond 5 mm, IDL circularity was significant, therefore limiting the determination of θz (PCC ≤ 0.48). The measured positional errors from the film sets above and below the source indicated a source position at the bottom of the catheter (-0.24 ± 0.07 mm). CONCLUSIONS: Isodose line features of a 2D dose map away from the HDR source can reveal its spatial coordinates. RCF was shown to be a suitable dosimeter for source tracking and dosimetry. This technique offers a novel source QA method and has the potential to be used for QA of commercial and customized applicators.

Optimizing the dynamic range extension of a radiochromic film dosimetry system.

The authors present a radiochromic film dosimetry protocol for a multicolor channel radiochromic film dosimetry system consisting of the external beam therapy (EBT) model GAFCHROMIC film and the Epson Expression 1680 flat-bed document scanner. Instead of extracting only the red color channel, the authors are using all three color channels in the absorption spectrum of the EBT film to extend the dynamic dose range of the radiochromic film dosimetry system. By optimizing the dose range for each color channel, they obtained a system that has both precision and accuracy below 1.5%, and the optimized ranges are 0-4 Gy for the red channel, 4-50 Gy for the green channel, and above 50 Gy for the blue channel.

Optimization of HDRBT boost dose delivery for patients with rectal cancer.

PURPOSE: We describe methods to improve dose delivery for patients with rectal cancer receiving boost brachytherapy after external beam radiotherapy. METHODS AND MATERIALS: Patients with rectal cancer who were ineligible or refusing surgery are treated with external beam radiotherapy and subsequently with three weekly image-guided volume-adapted high-dose-rate brachytherapy boosts of 10 Gy to the residual clinical target volume, for a total of 30 Gy in three fractions. Tungsten shielding placed at the center of intracavitary mold applicator and double-balloon technique was used to improve dose conformity to the target. RESULTS: Our results show that the use of tungsten shield and double balloon reduces the dose gradient within the target volume to receive the prescription boost dose of 10 Gy from maximum dose of 60 Gy down to 20 Gy. CONCLUSIONS: We outlined two methods for achieving higher high-dose-rate brachytherapy dose conformity using the tungsten shielding rods (to spare contralateral healthy tissues) and double-balloon technique (to decrease dose gradient within the target to minimize dose to the proximal mucosa).