Commissioning a CT-compatible LDR tandem and ovoid applicator using Monte Carlo calculation and 3D dosimetry

Abstract

Purpose: To determine the geometric and dose attenuation characteristics of a new commercially available CT-compatible LDR tandem and ovoid (T&O) applicator using Monte Carlo calculation and 3D dosimetry. Methods: For geometric characterization, we quantified physical dimensions and investigated a systematic difference found to exist between nominal ovoid angle and the angle at which the afterloading buckets fall within the ovoid. For dosimetric characterization, we determined source attenuation through asymmetric gold shielding in the buckets using Monte Carlo simulations and 3D dosimetry. Monte Carlo code MCNP5 was used to simulate 1.5 × 10 9 photon histories from a 137 Cs source placed in the bucket to achieve statistical uncertainty of 1% at a 6 cm distance. For 3D dosimetry, the distribution about an unshielded source was first measured to evaluate the system for 137 Cs, after which the distribution was measured about sources placed in each bucket. Cylindrical PRESAGE R dosimeters (9.5 cm diameter, 9.2 cm height) with a central channel bored for source placement were supplied by Heuris Inc. The dosimeters were scanned with the Duke Large field of view Optical CT-Scanner before and after delivering a nominal dose at 1 cm of 5-8 Gy. During irradiation the dosimeter was placed in a water phantom to provide backscatter. Optical CT scan time lasted 15 min during which 720 projections were acquired at 0.5• increments, and a 3D distribution was reconstructed with a (0.05 cm) 3 isotropic voxel size. The distributions about the buckets were used to calculate a 3D distribution of transmission rate through the bucket, which was applied to a clinical CT-based T&O implant plan. Results: The systematic difference in bucket angle relative to the nominal ovoid angle (105 • ) was 3.1• -4.7• . A systematic difference in bucket angle of 1 • , 5• , and 10• caused a 1% ± 0.1%, 1.7% ± 0.4%, and 2.6% ± 0.7% increase in rectal dose, respectively, with smaller effect to dose to Point A, bladder, sigmoid, and bowel. For 3D dosimetry, 90.6% of voxels had a 3D γ -index (criteria = 0.1 cm, 3% local signal) below 1.0 when comparing measured and expected dose about the unshielded source. Dose transmission through the gold shielding at a radial distance of 1 cm was 85.9% ± 0.2%, 83.4% ± 0.7%, and 82.5% ± 2.2% for Monte Carlo, and measurement for left and right buckets, respectively. Dose transmission was lowest at oblique angles from the bucket with a minimum of 56.7% ± 0.8%, 65.6% ± 1.7%, and 57.5% ± 1.6%, respectively. For a clinical T&O plan, attenuation from the buckets leads to a decrease in average Point A dose of ∼3.2% and decrease in D 2cc to bladder, rectum, bowel, and sigmoid of 5%, 18%, 6%, and 12%, respectively. Conclusions: Differences between dummy and afterloading bucket position in the ovoids is minor compared to effects from asymmetric ovoid shielding, for which rectal dose is most affected. 3D dosimetry can fulfill a novel role in verifying Monte Carlo calculations of complex dose distributions as are common about brachy...

On the feasibility of optical‐CT imaging in media of different refractive index

“…In addition, for the brachytherapy evaluation, usable radius was measured outwards from the r = 0.5 cm point, due to the noise associated with highfrequency portions of the image near the Cs-137 seed. 11 An example measurement of usable radius r u is shown in Fig. 3 (bottom row).…”

“…These distributions span a range of clinical interest in 3D dosimetry as seen in recent publications. 11,12 Simulations are also conducted for four different media and three different scanning configurations. The RIs of these media were chosen as: a PRESAGE R -matched fluid (1.501); a slightly mismatched fluid, e.g., mineral oil in DLOS telecentric scanner 13 (1.49); water (1.33); and air (1.00).…”

Systematic Review of Intensity-Modulated Brachytherapy (IMBT): Static and Dynamic Techniques

“…"Static" IMBT approaches include those in which the shield does not move relative to the source or surrounding tissues during BT delivery, such as shielded applicators (eg, shielded cylinders, [3][4][5][6][7][8][9][10][11] shielded tandem and ovoids, [12][13][14][15][16][17][18][19][20] Strut-Adjusted Volume Implants [SAVI], 21 intracavitary mold applicators [ICMA], [22][23][24][25][26] direction-modulated brachytherapy [DirMBT], 21,25,[27][28][29][30][31][32][33][34] and cup or D-shaped applicators [35][36][37][38][39][40][41][42][43][44][45][46][47][48][49]…”

An investigation of a PRESAGE®in vivodosimeter for brachytherapy

“…It has the benefits of stability in the clinical setting, linear radiation-induced light absorption contrast rather than scattering contrast, and can be cast into different shapes and sizes without a requirement for an external container (Guo et al 2006b) It has been used across a wide range of clinical applications (Brady et al 2010, Clift et al 2010, Thomas et al 2010, Doran et al 2010, Rankine et al 2013, Zhao et al 2012, Adamson et al 2012, Oldham et al 2012, Wai et al 2009, Palmer et al 2013), but has never been used in-vivo. These qualities, and the need for in-vivo dose verification in brachytherapy, prompted the development of PRESAGE ® In-Vivo (PRESAGE ® -IV).…”