Table 1b

SAVI�

DV (CC) D%

Mean Max Median Std Mean Max Median Std

PTV90 -0.11 -0.27 -0.07 0.12 -0.13 -0.34 -0.06 0.15

PTV95 -0.23 -0.70 -0.21 0.35 -0.29 -0.90 -0.24 0.42

PTV100 -0.12 -0.82 -0.27 0.71 -0.21 -1.09 -0.38 0.80

PTV150 -0.03 -1.21 -0.09 0.83 -0.17 -2.92 -0.29 1.99

PTV200 -0.13 -0.68 -0.04 0.33 -0.91 -4.83 -0.36 2.23

DDose (cGy) D%

Rib_0.1cc -6.80 -37.66 -1.31 16.29 -0.72 -4.09 -0.43 2.33

Rib_0.01cc -7.50 -38.52 -1.53 16.32 -1.08 -4.16 -0.77 2.38

Skin_0.1cc -1.32 -7.44 -1.27 4.88 -0.37 -2.18 -0.35 1.39

Skin_0.01cc -2.05 -6.79 -1.79 3.78 -0.53 -1.86 -0.42 1.05

Note:

DV(CC)5 VTG186-VTG43, volume difference for PTV.

DDose(cGy)5 DTG186-DTG43, dose differences for Rib or Skin.

D% 5 (TG186-TG43)/TG43*100, percentage difference of TG186 and TG43 respect to TG43.

S51Abstracts / Brachytherapy 13 (2014) S15eS126

Oncentra� Brachy 4.4 brachytherapy TPS [Nucletron, an Elekta company(Elekta AB, Stockholm, Sweden)].All material external to the patient body contour was considered to be air(r50.001205 g/cm3). The lung was contoured and set to lung inflateddensity (r50.26 g/cm3). Ribs were assigned a density identical tocortical bone (r51.92 g/cm3). All clinical plans were then recomputedwith the CC algorithm with the standard accuracy setting, in which amultiple resolution calculation was used for shorter calculation time andsufficient precision. The final 3D dose grids were then set to 1.0 mm inall cases. Following the recommendation of the TG-186 for therecalibration of prescribed and reported doses, we compared the dose tomedium in medium (Dm,m) from the CC calculation algorithm with theclinical standard TG-43 dose to water in water (Dw,w-TG43). TheContura� and SAVI� were analyzed separately. The effects from theSAVI� heterogeneities were modeled by contouring the air between thestruts and assigning it air density. The percent of the planning targetvolume (PTV_EVAL) that received 90%, 95%, 100% (V90, V95,V100)of the prescription dose were compared with these two algorithms as wellas the volumes of V150 and V200 (cc). The maximum doses to 0.01 ccand 0.1 cc of skin and ribs were compared. These dosimetric parameterswere compared based on percent difference (mean, median, andmaximum) with TG-43 calculated values as the standard.Results: For the Contura� PTV_EVAL (Table 1a), the mean percentagedifferences in V90, V95, V100, V150, and V200 were 1.2%, 2.2%, 3.4%,6.1%, and 7.6%, respectively. The maximum percentage differences inV90, V95, V100, V150, and V200 were 2.3%, 3.2%, 4.0%, 9.1%, and10.2%, respectively, while the maximum volume differences in V150 andV200 were 2.8 cc and 0.6 cc. For the ribs, the mean difference ofmaximum doses to 0.01 cc and 0.1 cc were 2.5% and 2.6%, while themax difference were 3.6% and 3.7%. For the skin, the mean differencesof maximum doses to 0.01 cc and 0.1 cc were 4.7% and 4.8%, while themax differences were found to be 5.6% and 5.9%. For the PTV_EVALwith SAVI� (Table 1b), the mean percentage differences in V90, V95,V100, V150, and V200 were 0.1%, 0.3%, 0.2%, 0.2%, and 0.9%,respectively. The maximum percentage differences in V90, V95, V100,V150, and V200 were 0.3%, 0.9%, 1.1%, 2.9%, and 4.8%, respectively,while the maximum volume differences in V150, and V200 were 1.2 ccand 0.7 cc. For the ribs, the mean differences of maximum doses to 0.01cc and 0.1 cc were 0.7% and 1.1%, while the max differences were foundto be 4.1% and 4.2%. For the skin, the mean differences of maximumdoses to 0.01 cc and 0.1 cc were 0.4% and 0.5%, while the maxdifferences were 2.2% and 1.9%.In general, the TG-43 algorithm overestimated the dose as compared to theCC dose calculation. The dosimetric differences between the two algorithmsfor the SAVI� were in general less than the Contura�. This discrepancycould be attributed to the effect of the air cavity in the SAVI� device.Conclusions: The CC algorithm with heterogeneity modeling wascompared to TG-43 for 10 APBI brachytherapy cases, and the dosimetric

differences were generally within 5% for Contura� and 1% for SAVI�.However, maximum differences in dose were 10.2% and 4.8%, respectively.

PD18

EPID-based Quality Assurance Technique for HDR Ring and

Tandem Source Dwell Positions with the Nucletron Flexitron

Afterloader

Cindy Tam, MSc, Marcus Sonier, MSc, Matt Wronski, PhD, Philip Au, BSc,

Ananth Ravi, PhD. Medical Physics, Sunnybrook Odette Cancer Centre,

Toronto, ON, Canada.

Purpose: To develop a novel EPID based Quality Assurance (QA) methodfor evaluating dwell positions with HDR remote after loaded applicators.Materials and Methods: Using a retired linac EPID flat panel detectortogether with a mobile C-arm fluoroscopy unit, images of the HDRsource at defined dwell positions in 14 tandem and 6 ring applicatorswere obtained. For the tandem applicators the distance from theapplicator tip to the first dwell position was measured. For the ringapplicators the radial distance from the center of the ring and the angularsource position were determined, in addition to the offset within the ringof the most distal dwell position. These polar coordinates were transferredinto the CT space of the Oncentra Brachy Treatment Planning System viaa spherical transformation. The modified source trajectories werecompared against the clinically commissioned source trajectories basedon conventional film based methods. The impact of any discrepancies onthe dose distribution was examined.Results: The EPID flat panel detector produced high resolution (300 mmpixel pitch) images from which the source dwell positions could bedetermined quantitatively; see Figure 1. Analysis showed a markeddeviation from the nominal first dwell position quoted by Nucletron as 6mm from the tip of the applicators. Tandem applicator measurementsindicated the distal dwell was within 7.6þ/-2.2 mm from the tip of theapplicator. The most distal dwell position for the ring applicator wasfound to be 7.8þ/-0.9 mm from the tip of the ring. The radial distances ofthe source dwell positions from the ring’s geometric center were alsomeasured and found to vary for each applicator. The locations of thesources obtained with the EPID imaging system did not coincide with theclinically commissioned source trajectories. The treatment time adjustedfor source decay based on the EPID-verified source positions was reducedby as much as 7%. Using the last ring dwell position as a reference,treatment isodoses according to the actual source positions may in fact berotated by 14þ/-4 degrees from the standard plan. This result could havesignificant implications on dose toxicity to sensitive structures such as therectum and bladder.Conclusions: QA techniques for HDR source dwell positions must evolvebeyond the use of conventional film-based techniques into strategies

S52 Abstracts / Brachytherapy 13 (2014) S15eS126

utilizing higher resolution. With this EPID-based QA technique, objectiveand accurate source dwell positions can be determined, unaffected bymetallic streaking artifacts present in reconstructed CT images. Thetechniques described here can easily be incorporated into routineprocedures such as annual QA, or testing following source changes andcommissioning new applicators.

PD19

Investigation of the GliaSite� Balloon Brachytherapy System Using

I-125 and Cs-131 Solutions

Mark J. Rivard, PhD, Yun Yang, PhD. Radiation Oncology, Tufts

University School of Medicine, Boston, MA.

Purpose: The GliaSite� Radiation Therapy System (IsoRay Medical, Inc,Richland, WA) contains a single-use double-walled brachytherapy balloonapplicator having a central catheter for external filling and draining. Thesystem is approved for intracavitary I-125 treatment of patients withmalignant brain tumors following surgical resection of a tumor. Thecurrent study is a modern assessment of the I-125 dosimetry using MonteCarlo (MC) methods with a new evaluation that considered using a Cs-131 solution.Materials and Methods: The modern brachytherapy balloon design wasexamined through measurements of six samples as well as utilizing CADdata obtained from the manufacturer. This information was used todevelop a simulation model for the GliaSite balloon under differentinflation levels for both I-125 and Cs-131 solutions. The MCNP5 MCradiation transport code was used to estimate the dose to water and brainin a phantom external to the balloon. Dose was calculated per injectedmCi of radioactive solution in a polar coordinate system analogous to theTG-43 dose calculation formalism. The radial range spanned from theballoon surface to 15 cm from the balloon center in 0.1 cm increments,and the polar angles spanned 0� to 180� in 1� increments. The MCNP5default photoatomic cross-section library was changed for comparisonwith results from Dempsey et al. [IJROBP 1998] and Monroe et al.[MedPhys 2001]. Sensitivity of results was also determined for ballooncomposition, balloon design, inflation shape, and solution self-shielding.Results: For Cs-131 in comparison to I-125, the maximum dose on thesurface of a 3 cm diameter balloon decreased by 4% and 8% forprescription depths of 0.5 cm and 1.0 cm, respectively. With the slightlyhigher mean photon energy, Cs-131 provided a more uniform dosedistribution than I-125. Similarly, the dose rate at larger distances (suchas 5 cm) from the balloon center, where the dose rate is approximatelyten times less than the prescription dose, were about 16% lower for I-125than for Cs-131. In an absolute sense, this equates to less than 2%

difference in absorbed dose at larger distances, which may be considerednegligible given the standard clinical approximation of human tissue (i.e.,brain) as liquid water. However, for a 3 cm diameter balloon, the dose tobrain was about 4% higher and 15% lower than the dose to water at radialdistances for 1.5 cm and 10 cm, respectively. The figure depicts the doseanisotropy as a function of polar angle for the 3 cm diameter balloonfilled with Cs-131 solution. Perturbations from unity are evident only nearthe source long axis at positions mostly inside the balloon nipple tip (near0�) and catheter (near 180�).

For a balloon having a 3.0 cm diameter on the transverse plane, but being 2.7cm (i.e., 10% shorter) on the long axis, the dose rate on the balloon equatorsurface was 3% higher than if the balloon shape were approximated as asphere. However, this diminished to unity with increasing radial distance.For both radionuclide solutions, the effect of self-shielding on the overalldose distribution was ! 0.1% of the high-Z atoms (Ziodine553,Zcesium555) for solution concentrations up to 0.01% radionuclide by mass.Conclusions: Reevaluation of the GliaSite� brachytherapy balloon systemusing I-125 gave similar results as previously reported by Dempsey et al.and Monroe et al. However, differences of up to 10% were attributed to