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

Abstract

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 192Ir 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.

Evaluating dosimetric accuracy of the 6 MV calibration on EBT3 film in the use of Ir‐192 high dose rate brachytherapy

“…The dose range of this calibration was between 0 and 10 Gy, with the output of the TrueBeam was measured using a NIST-traceable ionization chamber (PTW TN30013) in a PTW MP1 Water Phantom Tank (PTW-Freiburg, Germany). The measured output, 1.004 cGy/MU combined with the TMR (10) and output factor of the 20 × 20 cm 2 field size, was then used to calculate the MU for delivering each calibration dose level. We chose to irradiate the film inside the same water tank instead of a more convenient solid phantom because a direct measurement in water would eliminate the need for additional dose conversion from solid phantom to water.…”

“…The radiochromic film has replaced its predecessor, the radiographic film, as the film of choice for clinical radiology/radiotherapy applications since it eliminates the need for post‐processing and exhibits a wider dynamic range (up to hundreds of Gy). Therapeutic applications of radiochromic films include the patient‐specific Quality Assurance (QA), 1–4 linac commissioning, 5 brachytherapy QA, 6–10 treatment planning system (TPS) dose validation, 11 and so forth. The major advantage of radiochromic film over other dosimeters in brachytherapy applications is its high spatial resolution, customizability into any shape, and 2D measurement capability.…”

Feasibility of 4D HDR brachytherapy source tracking using x‐ray tomosynthesis: Monte Carlo investigation

“…21 Ideally, such methods can localize and monitor an HDR source in vivo with spatiotemporal resolution on the order of typical source dimensions (0.5 × 0.5 × 5 mm 3 ) and minimum dwell times (∼1 s), while also imposing minimal burden in cost, space and workflow. Multiple source tracking methods have been proposed with variable success.Some investigators have localized Ir-192 sources by detecting their high-energy gamma emissions using film 22 , pinhole imaging systems 23,24 , point dosimeters 25 , electronic portal imaging devices (EPIDs) [26][27][28] or flat panel detectors (FPDs). While these efforts have demonstrated HDR source tracking at large, their value for dosimetry may be fundamentally limited, as they require additional means to reference the source position to patient anatomy during treatment.…”

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

“… 9 , 10 More recently, sensitive self-developing films such as GAFchromicExternal Beam Therapy (EBT) have been developed by Ashland that are based on the lithium salt of PCDA (Li-PCDA) 11 − 18 and span dose ranges of 0.01–40 Gy. 1 , 2 , 19 As the more recent models of GAFchromic EBT films (especially EBT3 and EBT-XD) exhibit high spatial resolutions, 20 , 21 near-tissue equivalence, 21 , 22 and dose rate and energy dependence 23 , 24 along with being insensitive to visible light, 25 they are routinely employed in low-dose gradient (e.g., a quality assurance tool in intensity-modulated radiation therapy) 25 − 28 and high-dose gradient settings (e.g., brachytherapy 29 , 30 ). 18 , 23 , 31 Therefore, continued research into photoactive ingredients with a tunable reactivity is very important to improve dosimetry technologies for different therapeutic uses.…”

“… ,, The X-ray structure and topochemical parameters of PCDA were recently reported . As PCDA exhibits some photoreactivity, it has been incorporated into radiochromic films in the past (e.g., GAFchromic MD-55); however, the PCDA films are relatively insensitive and can only be used to measure doses around 5 Gy. , More recently, sensitive self-developing films such as GAFchromicExternal Beam Therapy (EBT) have been developed by Ashland that are based on the lithium salt of PCDA (Li-PCDA) − and span dose ranges of 0.01–40 Gy. ,, As the more recent models of GAFchromic EBT films (especially EBT3 and EBT-XD) exhibit high spatial resolutions, , near-tissue equivalence, , and dose rate and energy dependence , along with being insensitive to visible light, they are routinely employed in low-dose gradient (e.g., a quality assurance tool in intensity-modulated radiation therapy) − and high-dose gradient settings (e.g., brachytherapy , ). ,, Therefore, continued research into photoactive ingredients with a tunable reactivity is very important to improve dosimetry technologies for different therapeutic uses. In this paper, we highlight the spectroscopic characterization of lithium PCDA salts and the crystallographic and spectroscopic characterization of a sodium PCDA salt and compare their photoreactivities, particularly in the context of the impact of the solid-state crystal form.…”