Dose For Treatments Research Articles

SU‐G‐BRA‐16: Target Dose Comparison for Dynamic MLC Tracking and Mid‐ Ventilation Planning in Lung Radiotherapy Subject to Intrafractional Baseline Drifts

Purpose:Lung tumor motion during radiotherapy can be accounted for by expanded treatment margins, for example using a mid‐ventilation planning approach, or by localizing the tumor in real‐time and adapting the treatment beam with multileaf collimator (MLC) tracking. This study evaluates the effect of intrafractional changes in the average tumor position (baseline drifts) on these two treatment techniques.Methods:Lung stereotactic treatment plans (9‐beam IMRT, 54Gy/3 fractions, mean treatment time: 9.63min) were generated for three patients: either for delivery with MLC tracking (isotropic GTV‐to‐PTV margin: 2.6mm) or planned with a mid‐ventilation approach and delivered without online motion compensation (GTV‐to‐PTV margin: 4.4‐6.3mm). Delivery to a breathing patient was simulated using DynaTrack, our in‐house tracking and delivery software. Baseline drifts in cranial and posterior direction were simulated at a rate of 0.5, 1.0 or 1.5mm/min. For dose reconstruction, the corresponding 4DCT phase was selected for each time point of the delivery. Baseline drifts were accounted for by rigidly shifting the CT to ensure correct relative beam‐to‐target positioning. Afterwards, the doses delivered to each 4DCT phase were accumulated deformably on the mid‐ventilation phase using research RayStation v4.6 and dose coverage of the GTV was evaluated.Results:When using the mid‐ventilation planning approach, dose coverage of the tumor deteriorated substantially in the presence of baseline drifts. The reduction in D98% coverage of the GTV in a single fraction ranged from 0.4‐1.2, 0.6‐3.3 and 4.5‐6.2Gy, respectively, for the different drift rates. With MLC tracking the GTV D98% coverage remained unchanged (+/− 0.1Gy) regardless of drift.Conclusion:Intrafractional baseline drifts reduce the tumor dose in treatments based on mid‐ventilation planning. In rare, large target baseline drifts tumor dose coverage may drop below the prescription, potentially affecting clinical outcome in hypofractionated treatment protocols. Dynamic MLC tracking preserves tumor dose coverage even in the presence of extreme baseline drifts.We acknowledge financial and technical support of the MLC tracking research from Elekta AB. Research at ICR is supported by CRUK under Programme C33589/A19727 and NHS funding to the NIHR Biomedical Research Centre at RMH and ICR. MFF is supported by CRUK under Programme C33589/A19908.

SU‐F‐T‐151: Measurement Evaluation of Skin Dose in Scanning Proton Beam Therapy for Breast Cancer

Purpose:To measure the skin dose and compare it with the calculated dose from a treatment planning system (TPS) for breast cancer treatment using scanning proton beam therapy (SPBT).Methods:A single en‐face‐beam SPBT plan was generated by a commercial TPS for two breast cancer patients. The treatment volumes were the entire breasts (218 cc and 1500 cc) prescribed to 50.4 Gy (RBE) in 28 fractions. A range shifter of 5 cm water equivalent thickness was used. The organ at risk (skin) was defined to be 5 mm thick from the surface. The skin doses were measured in water with an ADCL calibrated parallel plate (PP) chamber. The measured data were compared with the values calculated in the TPS. Skin dose calculations can be subject to uncertainties created by the definition of the external contour and the limitations of the correction based algorithms, such as proton convolution superposition. Hence, the external contours were expanded by 0, 3 mm and 1 cm to include additional pixels for dose calculation. In addition, to examine the effects of the cloth gown on the skin dose, the skin dose measurements were conducted with and without gown.Results:On average the measured skin dose was 4% higher than the calculated values. At deeper depths, the measured and calculated doses were in better agreement (< 2%). Large discrepancy occur for the dose calculated without external expansion due to volume averaging. The addition of the gown only increased the measured skin dose by 0.4%.Conclusion:The implemented TPS underestimated the skin dose for breast treatments. Superficial dose calculation without external expansion would result in large errors for SPBT for breast cancer.

SU‐F‐T‐08: Brachytherapy Film Dosimetry in a Water Phantom for a Ring and Tandem HDR Applicator

Purpose:The feasibility of dose measurement using new generation EBT3 film was explored in a water phantom for a ring and tandem HDR applicator for measurements tracking mucosal dose during cervical brachytherapy.Methods:An experimental fixture was assembled to position the applicator in a water phantom. Prior to measurement, calibration curves for EBT3 film in water and in solidwater were verified. EBT3 film was placed at different known locations around the applicator in the water tank. A CT scan of the phantom with applicator was performed using clinical protocol. A typical cervical cancer treatment plan was then generated by Oncentra brachytherapy planning system. A dose of 500 cGy was prescribed to point A (2 cm, 2 cm). Locations measured by film included the outer surface of the ring, measurement point A_m (2.2 cm, 2.2 cm), and profiles extending from point A_m parallel to the tandem. Three independent measurements were conducted. The doses recorded by film were carefully analyzed and compared with values calculated by the treatment planning system.Results:Assessment of the EBT3 films indicate that the dose at point A matches the values predicted by the planning system. Dose to the point A_m was 411.5 cGy, and the outer circumferential surface dose of the ring was between 500 and 1150 cGy. It was found that from the point A_m, the dose drops 60% within 4.5 cm on the line parallel to the tandem. The measurement doses agree with the treatment planning system.Conclusion:Use of EBT3 film is feasible for in‐water measurements for brachytherapy. A carefully machined apparatus will likely improve measurement accuracy. In a typical plan, our study found that the ring surface dose can be 2.5 times larger than the point A prescription dose. EBT3 film can be used to monitor mucosal dose in brachytherapy treatments.

In situ solidification and in situ chemical oxidation combined in a single application to reduce contaminant mass and leachability in soil

A contaminated soil was treated in the laboratory with in situ chemical oxidation and in situ solidification/stabilization combined in a single application. Portland cement can activate the persulfate anion (S2O82−) in sodium persulfate (Na2S2O8), chemically oxidizing organic contaminants. Ten doses of Na2S2O8, ranging from 0.25% to 5% (by weight) with a cement:Na2S2O8 ratio of 2:1, completely activated S2O82− within 2.5h of mixing. Chemical oxidation removed a significant portion of the contaminants of concern (COCs), including BTEX, naphthalene, and 17 higher molecular weight PAHs. COC oxidation showed saturation-type dose-response curves, with pronounced contaminant removal at Na2S2O8 doses below 1.5%, and little removal at higher doses. Plots of reduced COC leachability vs. cement dose for combined oxidation and stabilization treatments were similar to those for COC oxidation, but for stabilization treatments the dose-response was roughly linear, and inversely related to the molecular weight of the COCs. Even with low Na2S2O8 doses, combined oxidation and stabilization treatments reduced COC leachability far more than stabilization treatments alone using the same PC dose, because of the COC removal achieved by Na2S2O8 oxidation. The increase in soil strength and decrease in hydraulic conductivity were unaffected by Na2S2O8.

Redesign of the Intracavitary Ring and Tandem Applicator for Improved Dosimetry in High Dose-Rate Brachytherapy

The commercial ring and tandem brachytherapy applicators (RTA) do not consistently match the specific anatomy of the endometrium and cervix of the patient. To conform variability in such situations we propose a redesign of the RTA used for cervical cancer treatments with the goal of improving patient-specific dosimetry. Unlike in the commercially available RTAs, the novel design of the RTA includes flexibility in positioning the ring with respect to the tandem. In particular, beside the possibility to place the tandem in the center of the ring, we propose an RTA capable of positioning the tandem distally and proximally with respect to the center of the ring (Fig.1). Therefore, the novel RTA has three possible variations in its final configuration. This RTA possesses additional flexibility to adapt different patient anatomies including daily variations in position and size of the organs at risk (OAR). For the evaluation of the benefit of the proposed methodology, 2 patients diagnosed with cervical cancer were treated with high dose-rate (HDR) brachytherapy using standard RTA and RTA in which the ring was placed distally towards the patient’s anterior surface during the same HDR brachytherapy course. A total of 10 brachytherapy treatment plans (TP) were analyzed; 5 per each patient. The HDR brachytherapy TPs were delivered as a boost after the patients had previously received 45 Gy in the initial external beam (EB) course. The prescription dose for the HDR treatments was 27.5 Gy in 5 fractions. The first patient was treated with 2 TPs generated using the standard RTA and 3 plans with the proposed RTA. The second patient was treated with 3 TPs generated using the standard RTA and 2 plans with the proposed RTA. The relevant dosimetric parameters such as D90 for the clinical target volumes (CTV) and D2cm3 for the OARs were compared to evaluate the patient dosimetry. An equivalent dose (EQD2) calculation was performed so that the total dose from 2 courses (EB and HDR) could be evaluated. In addition, we generated another 5 plans on the same computer tomography image sets to evaluate additional specific differences in dosimetry. When the modified RTA was used, the first patient received an average lower daily dose to the D2cm3 of the rectum, bladder and sigmoid of 54, 78 and 18 cGy, respectively, while the same CTV coverage (D90) was maintained. The average increase of the dose to the D2cm3 of the bowel was 29 cGy. This treatment resulted in a decrease of the total EQD2 of the rectum, bladder and sigmoid of 223, 488 and 105 cGy. The increase of the total EQD2 of the bowel was 182 cGy; however, the total dose to the bowel was significantly below the limits. The second patient received an average lower daily dose to the D2cm3 of the rectum and bladder of 52 and 44 cGy, which resulted in a decrease of the total EQD2 of 320 and 455 cGy, respectively. In this case, the increase of the total EQD2 for the CTV was 167 cGy. The total increase of the dose to the D2cm3 of the sigmoid was 156 cGy. The additional TPs generated for these patients revealed a possible decrease of the daily dose to the bladder, bowel, rectum and sigmoid of 33 (SD = 136) cGy, 1 (SD = 48) cGy, 34 (SD = 65) cGy and 67 (SD = 63) cGy, respectively. The novel design of the RTA has a strong potential for the complementary adjustment of the dose distributions. This RTA is capable of succeeding the daily variations of the patient’s anatomy so the daily dose to the critical OAR can be reduced. The increase of the total dose to the OARs (bowel in the first and sigmoid in the second case) can be regulated using the different position of the ring, so a greater decrease of the dose to the more affected OARs can be achieved. Therefore, the total equivalent dose can be modulated while the maximum dose constraints to the OARs are respected. Extended studies are required for additional conclusions.

A.88 - Impact of the MLC delivery errors on patient dose for IMRT treatments: A comparison between planned DVH and reconstructed DVH based on MLC log file (DynaLog)

A.88 - Impact of the MLC delivery errors on patient dose for IMRT treatments: A comparison between planned DVH and reconstructed DVH based on MLC log file (DynaLog)

On-line estimations of delivered radiation doses in three-dimensional conformal radiotherapy treatments of carcinoma uterine cervix patients in linear accelerator.

Transmission of radiation fluence through patient's body has a correlation to the planned target dose. A method to estimate the delivered dose to target volumes was standardized using a beam level 0.6 cc ionization chamber (IC) positioned at electronic portal imaging device (EPID) plane from the measured transit signal (St) in patients with cancer of uterine cervix treated with three-dimensional conformal radiotherapy (3DCRT). The IC with buildup cap was mounted on linear accelerator EPID frame with fixed source to chamber distance of 146.3 cm, using a locally fabricated mount. Sts were obtained for different water phantom thicknesses and radiation field sizes which were then used to generate a calibration table against calculated midplane doses at isocenter (Diso,TPS), derived from the treatment planning system. A code was developed using MATLAB software which was used to estimate the in vivo dose at isocenter (Diso,Transit) from the measured Sts. A locally fabricated pelvic phantom validated the estimations of Diso,Transit before implementing this method on actual patients. On-line dose estimations were made (3 times during treatment for each patient) in 24 patients. The Diso,Transit agreement with Diso,TPS in phantom was within 1.7% and the mean percentage deviation with standard deviation is −1.37% ±2.03% (n = 72) observed in patients. Estimated in vivo dose at isocenter with this method provides a good agreement with planned ones which can be implemented as part of quality assurance in pelvic sites treated with simple techniques, for example, 3DCRT where there is a need for documentation of planned dose delivery.

Cadaveric verification of the Eclipse AAA algorithm for spine SBRT treatments with titanium hardware

PurposeTo assess the accuracy of the Eclipse Analytical Anisotropic Algorithm when calculating dose for spine stereotactic body radiation therapy treatments involving surgically implanted titanium hardware. Methods and materialsA human spine was removed from a cadaver, cut sagittally along the midline, and then separated into thoracic and lumbar sections. The thoracic section was implanted with titanium stabilization hardware; the lumbar section was not implanted. Spine sections were secured in a water phantom and simulated for treatment planning using both standard and extended computed tomography (CT) scales. Target volumes were created on both spine sections. Dose calculations were performed using (1) the standard CT scale with relative electron density (RED) override of image artifacts and hardware, (2) the extended CT scale with RED override of image artifacts only, and (3) the standard CT scale with no RED overrides for hardware or artifacts. Plans were delivered with volumetric modulated arc therapy using a 6-MV beam with and without a flattening filter. A total of 3 measurements for each plan were made with Gafchromic film placed between the spine sections and compared with Eclipse dose calculations using gamma analysis with a 2%/2 mm passing criteria. A single measurement in a homogeneous phantom was made for each plan before actual delivery. ResultsGamma passing rates for measurements in the homogeneous phantom were 99.6% or greater. Passing rates for measurements made in the lumbar spine section without hardware were 99.3% or greater; measurements made in the thoracic spine containing titanium were 98.6 to 99.5%. ConclusionsEclipse Analytical Anisotropic Algorithm can adequately model the effects of titanium implants for spine stereotactic body radiation therapy treatments using volumetric modulated arc therapy. Calculations with standard or extended CT scales give similarly accurate results.

Cadaveric Verification of VMAT Dose Distributions for Spine SBRT Treatments With Titanium Implants

Cadaveric Verification of VMAT Dose Distributions for Spine SBRT Treatments With Titanium Implants

Dosimetric results in treatments of neuroblastoma and neuroendocrine tumors with (131)I-metaiodobenzylguanidine with implications for the activity to administer.

The aim was to investigate whole-body and red marrow absorbed doses in treatments of neuroblastoma (NB) and adult neuroendocrine tumors (NETs) with (131)I-metaiodobenzylguanidine and to propose a simple method for determining the activity to administer when dosimetric data for the individual patient are not available. Nine NB patients and six NET patients were included, giving in total 19 treatments as four patients were treated twice. Whole-body absorbed doses were determined from dose-rate measurements and planar gamma-camera imaging. For six NB and five NET treatments, red marrow absorbed doses were also determined using the blood-based method. Dosimetric data from repeated administrations in the same patient were consistent. In groups of NB and NET patients, similar whole-body residence times were obtained, implying that whole-body absorbed dose per unit of administered activity could be reasonably well described as a power function of the patient mass. For NB, this functional form was found to be consistent with dosimetric data from previously published studies. The whole-body to red marrow absorbed dose ratio was similar among patients, with values of 1.4 ± 0.6-1.7 ± 0.7 (1 standard deviation) in NB treatments and between 1.5 ± 0.6 and 1.7 ± 0.7 (1 standard deviation) in NET treatments. The consistency of dosimetric results between administrations for the same patient supports prescription of the activity based on dosimetry performed in pretreatment studies, or during the first administration in a fractionated schedule. The expressions obtained for whole-body absorbed doses per unit of administered activity as a function of patient mass for NB and NET treatments are believed to be a useful tool to estimate the activity to administer at the stage when the individual patient biokinetics has not yet been measured.

SU‐E‐T‐731: The Effect of Thermoplatic Mask to the Surface Dose in Sterotactic Brain Treatments with Flattening Filter Free Beams with EBT3 Gafchromic Film and OSL Dosimeter

Purpose:The surface dose of flattening filter free beams is higher than conventional flattening filter beams. In this study, The effect of thermoplatic mask to the surface dose in sterotactic brain treatments was compared using EBT3 Gafchromic film and OSL dosimeter.Methods:Treatment planning were done using 2 full arc VMAT technique with 6 FFF beam on the CT scan of Randophantom simulating the treatment of sterotactic treatments for one brain metastasis.1×1 cm2 Gafchromic EBT 3 film pieces were placed under the thermoplastic mask on the lens of the phantom. Then OSL dosimeter was also placed at the same way in the same place. Then phantom was irradiated without mask with film and OSL again. The surface doses with or without mask was compared for film and OSL dosimeter seperately.Results:The calibration of film and OSL dosimetry were done at first. Surface dosemeasurements made using the EBT3 film showed good agreement with results of the OSL dosimetry for standard fields of FFF beams within the measurement uncertainty of 3.1%. The surface dose determined using EBT3 film was found to change from 57 % to 93 % to with the mask. The surface dose determined using OSL dosimeter was found to change from 56 % to 93 % to with the mask. This study also demonstrates the suitability of Gafchromic EBT3 film and OSL dosimeter for surface dose measurements in FFF beams.Conclusion:The skin sparing effect FFF beams are lower than conventional beams. Significant bolus effect on skin dose in the presence of thermoplastic mask was seen. The skin reactions resulting from thermoplastic masks should be monitored especially for FFF beams and corrective measures should be taken during treatment to avoid skin toxicity.

SU‐E‐T‐352: Effects of Skull Attenuation and Missing Backscatter On Brain Dose in HDR Treatment of the Head with Surface Applicators

Purpose:To calculate the effect of lack of backscatter from air and attenuation of bone on dose distributions in brachytherapy surface treatment of head. Existing treatment planning systems based on TG43 do not account for heterogeneities, and thus may overestimate the dose to the brain. While brachytherapy generally has rapid dose falloff, the dose to the deeper tissues (in this case, the brain) can become significant when treating large curved surfaces.Methods:Applicator geometries representing a range of clinical cases were simulated in MCNP5. An Ir‐192 source was modeled using the energy spectrum presented by TG‐43. The head phantom was modeled as a 7.5‐cm radius water sphere, with a 7 ‐mm thick skull embedded 5‐mm beneath the surface. Dose values were calculated at 20 points inside the head, in which 10 of them were on the central axis and the other 10 on the axis connecting the central of the phantom with the second to last source from the applicator edge.Results:Central and peripheral dose distributions for a range of applicator and head sizes are presented. The distance along the central axis at which the dose falls to 80% of the prescribed dose (D80) was 7 mm for a representative small applicator and 9 mm for a large applicator. Corresponding D50 and D30 for the same small applicator were 17 mm and 32 mm respectively. D50 and D30 for the larger applicator were 32 mm and 60 mm respectively. These results reflect the slower falloff expected for larger applicators on a curved surface.Conclusion:Our results can provide guidance for clinicians to calculate the dose reduction effect due to bone attenuation and the lack of backscatter from air to estimate the brain dose for the HDR treatments of surface lesions.

Effect of organic manures and microbial inoculants superimposed over inorganic fertilizers on production and profitability of wheat (Triticum aestivum)

An experiment was conducted during two consecutive winter seasons (rabi) of 2011–12 and 2012–13 to assess the response of superimposed nutritional levels consisting vermicompost @ 5 t ha−1, FYM @ 10 t ha−1, seed inoculation with Azotobacter and PSB, individually and in combination over recommended dose of fertilizers in wheat crop under irrigated condition. The response of fertility levels was analysed on growth characters, yield attributes and yield as well as economic parameters of normal sown wheat variety ‘PBW-502’. The integration of vermicompost, FYM, Azotobacter and PSB along with inorganic fertilizers dose in different treatments exhibited significant response in terms of increasing grain yield of wheat from 12.66 to 18.36 per cent and 16.21 to 27.90 per cent compared to the control, which recorded lowest 4.63 and 4.13 t ha−1 grain yield during 2011–12 and 2012–13, respectively. Plots receiving recommended dose of fertilizer (RDF) + vermicompost 5 t ha−1 + Azotobacter and PSB as seed treatment and spraying at I and II irrigation recorded maximum grain yield (5.67 and 5.73 tha−1), straw yield (7.29 and 8.87t ha−1), gross income (87443 and97127 ha−1) and net income (37001 and 45462 ha−1) during 2011–12 and 2012–13, respectively

Comparison of RPL GD-301 and TLD-100 detectors responses by Monte Carlo simulation

(LiF:Mg,Ti) Thermo Luminescent Detectors are widely used for monitoring patient dose in radiotherapy treatments whereas Radio-Photoluminescent Dosimeters (RPL) are increasingly devoted to radiological protection purposes. A study, aiming at extending the use of RPL glasses to clinical applications, is conducted by comparing the dosimetric characteristics of a RPL glass dosimeter, commercially known as GD-301 to those of a TLD -100 detector. In this paper, preliminary Monte Carlo simulation results describing these dosimeters responses in terms of absorbed dose, source-detector distance and characteristics of the incident gamma field are presented.

A new online detector for estimation of peripheral neutron equivalent dose in organ.

Peripheral dose in radiotherapy treatments represents a potential source of secondary neoplasic processes. As in the last few years, there has been a fast-growing concern on neutron collateral effects, this work focuses on this component. A previous established methodology to estimate peripheral neutron equivalent doses relied on passive (TLD, CR39) neutron detectors exposed in-phantom, in parallel to an active [static random access memory (SRAMnd)] thermal neutron detector exposed ex-phantom. A newly miniaturized, quick, and reliable active thermal neutron detector (TNRD, Thermal Neutron Rate Detector) was validated for both procedures. This first miniaturized active system eliminates the long postprocessing, required for passive detectors, giving thermal neutron fluences in real time. To validate TNRD for the established methodology, intrinsic characteristics, characterization of 4 facilities [to correlate monitor value (MU) with risk], and a cohort of 200 real patients (for second cancer risk estimates) were evaluated and compared with the well-established SRAMnd device. Finally, TNRD was compared to TLD pairs for 3 generic radiotherapy treatments through 16 strategic points inside an anthropomorphic phantom. The performed tests indicate similar linear dependence with dose for both detectors, TNRD and SRAMnd, while a slightly better reproducibility has been obtained for TNRD (1.7% vs 2.2%). Risk estimates when delivering 1000 MU are in good agreement between both detectors (mean deviation of TNRD measurements with respect to the ones of SRAMnd is 0.07 cases per 1000, with differences always smaller than 0.08 cases per 1000). As far as the in-phantom measurements are concerned, a mean deviation smaller than 1.7% was obtained. The results obtained indicate that direct evaluation of equivalent dose estimation in organs, both in phantom and patients, is perfectly feasible with this new detector. This will open the door to an easy implementation of specific peripheral neutron dose models for any type of treatment and facility.

Effects of Gibberellic Acid on Hemolymph-Free Amino Acids of Galleria mellonella (Lepidoptera: Pyralidae) and Endoparasitoid Pimpla turionellae (Hymenoptera: Ichneumonidae)

Abstract The impacts of the plant growth regulator, gibberellic acid (GA3) were investigated on hemolymph free amino acids (FAAs) of Galleria mellonella L. (Lepidoptera: Pyralidae) larvae and its endoparasitoid Pimpla turionellae L. (Hymenoptera: Ichneumonidae) reared on host treated with different doses (50–5,000 ppm) of GA3 in diet. High-performance liquid chromatographic analysis of the hemolymph revealed that out of the 20 amino acids detected, 18 FAAs were found in the hemolymph of treated and untreated host larvae, whereas 19 were present in that of parasitoid. The total pool of FAAs in host larvae after treatment by GA3 fluctuated among doses and the ratio of essential amino acids in the pool increased to reach 9.54% at 1,000 ppm. Parasitoid larval total FAA concentration exhibited an increase at all treatments. However, the ratio of essential amino acids in the pool decreased >50% at all doses except for 50 ppm. Dose-dependent variations in individual FAAs in larval hemolymph of both species were also detected. Tyrosine, methionine, isoleucine, lysine, and tryptophan generally occurred in lowest amount both in control and treated G. mellonella larvae. The amount of serine, asparagine, glycine, isoleucine, leucine, and phenylalanine displayed differences only among GA3 doses in treatments. Great variations in the amount of all amino acids occurred at most of the treatment doses in wasp larvae hemolymph. Our study indicated that application of GA3 resulted in different effects on the quantity of FAAs associated with energy metabolisms of the pest species and its natural enemy.

SU‐E‐T‐318: The Effect of Patient Positioning Errors On Target Coverage and Cochlear Dose in Stereotactic Radiosurgery Treatment of Acoustic Neuromas

Purpose:Setup errors on the order of millimeters may cause under‐dosing of targets and significant changes in dose to critical structures especially when planning with tight margins in stereotactic radiosurgery. This study evaluates the effects of these types of patient positioning uncertainties on planning target volume (PTV) coverage and cochlear dose for stereotactic treatments of acoustic neuromas.Methods:Twelve acoustic neuroma patient treatment plans were retrospectively evaluated in Brainlab iPlan RT Dose 4.1.3. All treatment beams were shaped by HDMLC from a Varian TX machine. Seven patients had planning margins of 2mm, five had 1–1.5mm. Six treatment plans were created for each patient simulating a 1mm setup error in six possible directions: anterior‐posterior, lateral, and superiorinferior. The arcs and HDMLC shapes were kept the same for each plan. Change in PTV coverage and mean dose to the cochlea was evaluated for each plan.Results:The average change in PTV coverage for the 72 simulated plans was −1.7% (range: −5 to +1.1%). The largest average change in coverage was observed for shifts in the patient's superior direction (−2.9%). The change in mean cochlear dose was highly dependent upon the direction of the shift. Shifts in the anterior and superior direction resulted in an average increase in dose of 13.5 and 3.8%, respectively, while shifts in the posterior and inferior direction resulted in an average decrease in dose of 17.9 and 10.2%. The average change in dose to the cochlea was 13.9% (range: 1.4 to 48.6%). No difference was observed based on the size of the planning margin.Conclusion:This study indicates that if the positioning uncertainty is kept within 1mm the setup errors may not result in significant under‐dosing of the acoustic neuroma target volumes. However, the change in mean cochlear dose is highly dependent upon the direction of the shift.

SU‐E‐T‐43: Analytical Model for Photon Peripheral Dose in Radiotherapy Treatments

Purpose:The higher survival rate of radiotherapy patients entails a growing concern on second cancers associated to peripheral doses. Currently, dosimetry of out‐of field doses is still under development. Our group has developed a methodology to estimate neutron equivalent dose in organs (1,2). We aimed to propose a model to estimate out‐of‐field photon doses in isocentric treatments from basic clinical data.Methods:The proposed function models the dose as the sum of leakage and scatter terms. The latter is modeled as a virtual source at the collimator, which suffers from attenuation in air and tissue, corrected by the inverse‐square‐law. The model was parameterized using experimental measurements with TLD700 chips placed inside an anthropomorphic phantom (6–18MV) irradiated with conformal and modulated techniques in Elekta, Siemens and Varian linacs. This model provides photon dose at a point as a function of clinical parameters as prescription dose/UM, PTV volume, distance to the field edge, height of the MLC leaves and distance from the the MLC to the isocenter. Model was tested against independent measurements (TLD100) for a VMAT treatment on a Elekta. Dose to organs is modeled from dose to points along the head‐to‐feet axis of the organ of a “standard man” escalated by patient height.Results:Our semi‐empirical model depends on 3 given parameters (leakage parameter can be individualized). A novelty of our model, over other models (e.g., PERIDOSE), arises from its applicability to any technique (independently of the number of MU needed to deliver a dose). Differences between predictions and measurements were < 0.005mSv/UM.Conclusion:We have proposed a unique model which successfully account for photon peripheral organ dose. This model can be applied in the day‐to‐day clinic as it only needs a few basic parameters which are readily accessible.1. Radiother. Oncol. 107:234–243, 2013. 2. Phys. Med. Biol. 57:6167–6191, 2012.

SU‐E‐J‐160: Comparing the Setup Accuracy of Non‐Ionizing Patient Localization Systems with CBCT to Reduce Imaging Dose in Prone Breast Treatments

Purpose:CBCT is the current gold standard to verify prone breast patient setup. We investigated in a phantom if non‐ionizing localization systems can replace ionizing localization systems for prone breast treatments.Methods:An anthropomorphic phantom was positioned on a prone breast board. Electromagnetic transponders were attached on the left chest surface. The CT images of the phantom were imported to the treatment planning system. The isocenter was set to the center of the transponders. The positions of the isocenter and transponders transferred to the transponder tracking system. The posterior phantom surface was contoured and exported to the optical surface tracking system. A CBCT was taken for the initial setup alignment on the treatment machine. Using the electromagnetic and optical localization systems, the deviation of the phantom setup from the original CT images was measured. This was compared with the difference between the original CT and kV‐CBCT images.Results:For the electromagnetic localization system, the phantom position deviated from the original CT in 1.5 mm, 0.0 mm and 0.5 mm in the anterior‐posterior (AP), superior‐inferior (SI) and left‐right (LR) directions. For the optical localization system, the phantom position deviated from the original CT in 2.0 mm, −2.0 mm and 0.1 mm in the AP, SI and LR directions. For the CBCT, the phantom position deviated from the original CT in 4.0 mm, 1.0 mm and −1.0 mm in the AP, SI and LR directions. The measured values from the non‐ionizing localization systems differed from those with the CBCT less than 3.0 mm in all directions.Conclusions:This phantom study showed the feasibility of using a combination of non‐ionizing localization systems to achieve a similar setup accuracy as CBCT for prone breast patients. This could potentially eliminate imaging dose. As a next step, we are expanding this study to actual patients.This work has been in part supported by Departmental Research Award RODEPT1‐JS001, Department of Radiation Oncology, UC Davis Medical Center.

Four-Dimensional Patient Dose Reconstruction for Scanned Ion Beam Therapy of Moving Liver Tumors

Estimation of the actual delivered 4-dimensional (4D) dose in treatments of patients with mobile hepatocellular cancer with scanned carbon ion beam therapy. Six patients were treated with 4 fractions to a total relative biological effectiveness (RBE)-weighted dose of 40 Gy (RBE) using a single field. Respiratory motion was addressed by dedicated margins and abdominal compression (5 patients) or gating (1 patient). 4D treatment dose reconstructions based on the treatment records and the measured motion monitoring data were performed for the single-fraction dose and a total of 17 fractions. To assess the impact of uncertainties in the temporal correlation between motion trajectory and beam delivery sequence, 3 dose distributions for varying temporal correlation were calculated per fraction. For 3 patients, the total treatment dose was formed from the fractional distributions using all possible combinations. Clinical target volume (CTV) coverage was analyzed using the volumes receiving at least 95% (V95) and 107% (V107) of the planned doses. 4D dose reconstruction based on daily measured data is possible in a clinical setting. V95 and V107 values for the single fractions ranged between 72% and 100%, and 0% and 32%, respectively. The estimated total treatment dose to the CTV exhibited improved and more robust dose coverage (mean V95 > 87%, SD < 3%) and overdose (mean V107 < 4%, SD < 3%) with respect to the single-fraction dose for all analyzed patients. A considerable impact of interplay effects on the single-fraction CTV dose was found for most of the analyzed patients. However, due to the fractionated treatment, dose heterogeneities were substantially reduced for the total treatment dose. 4D treatment dose reconstruction for scanned ion beam therapy is technically feasible and may evolve into a valuable tool for dose assessment.