bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐F‐T‐523: Radiobiological Comparison of Helical Tomotherapy and VMAT in the Treatment of Head and Neck Tumors
Authors: Woods R , Mavroidis P , Lehman‐Davis M , Kostich M , Cook T , Chera B , Das S , Lian J
Published: July 2016

Purpose:

This study aims at comparing the efficacy of Helical Tomotherapy (TOMO) and Volumetric Modulated Arc Therapy (VMAT) in producing highly conformal dose distributions in challenging head & neck cancer patients. Furthermore, to interpret the dosimetric findings into expected tissue response rates in order to estimate their clinical impact.

Methods:

Seven patients were studied and for each patient two treatment plans (one TOMO and one VMAT) were created. Structures used for optimization were: high risk PTV, standard risk PTV, brainstem, spinal cord, parotid gland, larynx, mandible and surrounding tissue. The prescription varied between 60 – 74.4Gy to the HR. The same dosimetric constraints were used for both pairs of plans. Additionally, the TCP and NTCP values of the different targets and organs at risk (OAR) were calculated and compared together with the P+, which is the probability of achieving tumor control without normal tissue complications.

Results:

The proposed plan evaluation shows that the VMAT gives better results than TOMO in terms of expected clinical outcome. Specifically, the average difference in P+ = 3.3±1.8%, TCP = 3.3±1.8%, NTCP = 0.0±0.0%. The difference observed mainly stems from the lower TCP with TOMO (average difference of 2.9% for the high risk PTV and 3.1% for the standard risk PTV). The NTCP differences between the two modalities are very small even though their average differences in BEUD can be large (e.g. −21.9 Gy in brainstem, 27.2 Gy in larynx).

Conclusion:

The findings of the analysis indicate that VMAT achieves better coverage to the targets with lower doses to the OARs compared to TOMO for the patients tested. However, the dosimetric differences appear to have a measurable impact only in the expected TCP rates of the targets. Radiobiological evaluation of treatment plans should provide a closer association of the delivered treatment with the clinical outcome.
SU‐F‐T‐524: Investigation of the Dosimertric Benefits of Interchangeable Source Size of a Novel Rotating Gamma System
Authors: Eldib A , Chibani O , Chen L , Li J , Price R , Ma C
Published: July 2016

Purpose:

Tremendous technological developments were made for conformal therapy techniques with linear accelerators, while less attention was paid to cobalt‐60 units. The aim of the current study is to explore the dosimetric benefits of a novel rotating gamma ray system enhanced with interchangeable source sizes and multi‐leaf collimator (MLC).

Material and Methods:

CybeRT is a novel rotating gamma ray machine with a ring gantry that ensures an iso‐center accuracy of less than 0.3 mm. The new machine has a 70cm source axial distance allowing for improved penumbra compared to conventional machines. MCBEAM was used to simulate Cobalt‐60 beams from the CybeRT head, while the MCPLAN code was used for modeling the MLC and for phantom/patient dose calculation. The CybeRT collimation will incorporate a system allowing for interchanging source sizes. In this work we have created phase space files for 1cm and 2cm source sizes. Evaluation of the system was done by comparing CybeRT beams with the 6MV beams in a water phantom and in patient geometry. Treatment plans were compared based on isodose distributions and dose volume histograms.

Results:

Profiles for the 1cm source were comparable to that from 6MV in the order of 6mm for 10×10 cm2 field size at the depth of maximum dose. This could ascribe to Cobalt‐60 beams producing lowerenergy secondary electrons. Although, the 2cm source have a larger penumbra however it could be still used for large targets with proportionally increased dose rate. For large lung targets, the difference between cobalt and 6MV plans is clinically insignificant. Our preliminary results showed that interchanging source sizes will allow cobalt beams for volumetric arc therapy of both small lesions and large tumors.

Conclusion:

The CybeRT system will be a cost effective machine capable of performing advanced radiation therapy treatments of both small tumors and large target volumes.
SU‐F‐T‐525: Monitordeep‐Inspiratory Breathhold with a Laser Sensor for Radiation Therapy of Left Breast Cancer
Authors: Tai A , Li DJ , Currey A , Li X Allen
Published: July 2016

Purpose:

Radiation therapy (RT) of left sided breast cancers with deep‐inspiratory breathhold (DIBH) can reduce the dose to heart. The purpose of this study is to develop and test a new laser‐based tool to improve ease of RT delivery using DIBH.

Methods:

A laser sensor together with breathing monitor device (Anzai Inc., Japan) was used to record the surface breathing motion of phantom/volunteers. The device projects a laser beam to the chestwall and the reflected light creates a focal spot on a light detecting element. The position change of the focal spot correlates with the patient's breathing motion and is measured through the change of current in the light detecting element. The signal is amplified and displayed on a computer screen, which is used to trigger radiation gating. The laser sensor can be easily mounted to the simulation/treatment couch with a fixing plate and a magnet base, and has a sensitivity range of 10 to 40 cm from the patient. The correlation of breathing signals detected by laser sensor and visionRT is also investigated.

Results:

It is found that the measured breathing signal from the laser sensor is stable and reproducible and has no noticeable delay. It correlates well with the VisionRT surface imaging system. The DIBH reference level does not change with movement of the couch because the laser sensor and couch move together.

Conclusion:

The Anzai laser sensor provides a cost‐effective way to improve beam gating with DIBH for treating left breast cancer. It can be used alone or together with VisionRT to determine the correct DIBH level during the radiation treatment of left breast cancer with DIBH.
SU‐F‐T‐527: A Novel Dynamic Multileaf Collimator Leaf‐Sequencing Algorithm in Radiation Therapy
Authors: Jing J , Lin H , Chow J
Published: July 2016

Purpose:

A novel leaf‐sequencing algorithm is developed for generating arbitrary beam intensity profiles in discrete levels using dynamic multileaf collimator (MLC). The efficiency of this dynamic MLC leaf‐sequencing method was evaluated using external beam treatment plans delivered by intensity modulated radiation therapy technique.

Methods:

To qualify and validate this algorithm, integral test for the beam segment of MLC generated by the CORVUS treatment planning system was performed with clinical intensity map experiments. The treatment plans were optimized and the fluence maps for all photon beams were determined. This algorithm started with the algebraic expression for the area under the beam profile. The coefficients in the expression can be transformed into the specifications for the leaf‐setting sequence. The leaf optimization procedure was then applied and analyzed for clinical relevant intensity profiles in cancer treatment.

Results:

The macrophysical effect of this method can be described by volumetric plan evaluation tools such as dose‐volume histograms (DVHs). The DVH results are in good agreement compared to those from the CORVUS treatment planning system.

Conclusion:

We developed a dynamic MLC method to examine the stability of leaf speed including effects of acceleration and deceleration of leaf motion in order to make sure the stability of leaf speed did not affect the intensity profile generated. It was found that the mechanical requirements were better satisfied using this method.

The Project is sponsored by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry.
SU‐F‐T‐528: Relationship Between Tumor Size and Plan Quality Using FFF and Non‐FFF Modes in Rapidarc
Authors: Chen F
Published: July 2016

Purpose:

For a give PTV dose, beam‐on time is shorter in the FFF than the non‐FFF mode because of higher MU/min. Larger tumors usually require more complex intensity modulation, which might affect plan quality and total MU. We investigated the relationship between PTV size and plan quality using FFF and non‐FFF modes.

Methods:

Two different PTV volumes (PTV and PTV+1 cm margin) were drawn in brain, lung and liver. 3‐full to 7‐partial arc (Rapidarc) of 6 MV, 1400 MU/min were studied. Plan quality was evaluated by: (a) DVH for PTV and normal tissues, (b) total MU and beam‐on time, and (c) passing rate for IMRT plan QA.

Results:

For the same PTV coverage, DVH for normal tissue was the same or slightly lower in the FFF compared with non‐FFF. Total MU was 13% higher in FFF than non‐FFF in the 3‐arc, 7 Gy treatment, but the difference became smaller when arc number increased to 6–7 for 10–24 Gy. Larger PTV did not affect the difference in the total MU. FFF required a short beam‐on time and the ratio of FFF and non‐FFF was 0.34 to 0.88 for 7‐ and 3‐arc, respectively. For larger PTV, the ratio increased to 0.45–0.90. Ratio of total MU for large PTV was 3–8% lower in the non‐FFF plans. Although the small difference in MU, beam‐on time was 1.1 to‐1.6 times longer in the 3‐ and 7‐arc non‐FFF plans. Plan verification showed the similar gamma index passing rate.

Conclusion:

While total MU was similar with FFF and non‐FFF modes, the beam‐on time was shorter in the FFF treatment. The advantage of FFF was greater in treatments with high dose per fraction using more arc numbers. For dose less than 10 Gy, using FFF and non‐FFF modes, tumor size did not affect the relationship of total MU, beam‐on time.
SU‐F‐T‐529: Dosimetric Investigation of a Rotating Gamma Ray System for ImagedGuided Modulated Arc Radiotherapy
Authors: Ma C , Chibani O , Eldib A , Li J , Chen L
Published: July 2016

Purpose:

Because of their effectiveness and efficiency, rotational arc radiotherapy (MAT) techniques have been developed on both specialty machines such as Tomotherapy and conventional linear accelerators. This work investigates a new rotating Gamma therapy system for image guided MAT and SBRT of intra/extracranial tumors.

Methods:

The CyberMAT system (Cyber Medical Corp., China) consists of a ring gantry with a gamma source (effective source size 1cm and 1.7cm respectively), a 120‐leaf MLC, a kV CBCT and an EPID. The treatment couch provides 6‐degrees‐of‐freedom motion compensation and the kV CBCT system has a spatial resolution of 0.4mm for target localization. The maximum dose rate is >4.0 Gy/min and the maximum field size is 40cm × 40cm. Monte Carlo simulations were used to compute dose distributions and compare with measurements. A retrospective study of 125 previously treated SBRT patients was performed to evaluate the dosimetric characteristics of CyberMAT in comparison with existing VMAT systems.

Results:

Monte Carlo results confirmed the CyberMAT design parameters including output factors and 3D dose distributions. Its beam penumbra is 5mm to 10mm for field sizes 1cm to 10cm, respectively and its isocenter accuracy is <0.5mm. Compared to the 6 MV photons of Tomotherapy and conventional linacs, Cobalt beams produce lower‐energy secondary electrons that exhibit better dose properties in low‐density lung tissues. Cobalt beams are ideal for peripheral lung tumors with half‐arc arrangements to spare the opposite lung and other critical structures. Superior dose distributions have been obtained for brain, head and neck, breast, spine and lung tumors with half/full arc arrangements.

Conclusion:

Because of the unique dosimetric properties of Cobalt sources and its accurate stereotaxy/dose delivery CyberMAT is ideally suited for image guided MAT and SBRT. Full‐arc arrangements are superior for brain and H&N treatments while half‐arc arrangements produce best dose distributions for thoracic tumors.
SU‐F‐T‐531: Determination of Site‐Specific Dynamic‐Jaw Versus Static‐Jaw RapidArc Delivery
Authors: Tien C , Brewer M , Studenski M
Published: July 2016

Purpose:

Dynamic‐jaw tracking maximizes the area blocked by both jaw and MLC in RapidArc. We developed a method to quantify jaw tracking.

Methods:

An Eclipse Scripting API (ESAPI) was used to export beam parameters for each arc's control points. The specific beam parameters extracted were: gantry angle, control point number, meterset, x‐jaw positions, y‐jaw positions, MLC bank‐number, MLC leaf‐number, and MLC leaf‐position. Each arc contained 178 control points with 120 MLC positions. MATLAB routines were written to process these parameters in order to calculate both the beam aperture (unblocked) size for each control point. An average aperture size was weighted by meterset. Jaw factor was defined as the ratio between dynamic‐jaw to static‐jaw aperture size. Jaw factor was determined for forty retrospectively replanned patients treated with static‐jaw delivery sites including lung, brain, prostate, H&N, rectum, and bladder.

Results:

Most patients had multiple arcs and reduced‐field boosts, resulting in 151 fields. Of these, the lowest (0.4722) and highest (0.9622) jaw factor was observed in prostate and rectal cases, respectively. The median jaw factor was 0.7917 meaning there is the potential unincreased blocking by 20%. Clinically, the dynamic‐jaw tracking represents an area surrounding the target which would receive MLC‐only leakage transmission of 1.68% versus 0.1% with jaws. Jaw‐tracking was more pronounced at areas farther from the target. In prostate patients, the rectum and bladder had 5.5% and 6.3% lower mean dose, respectively; the structures closer to the prostate such as the rectum and bladder both had 1.4% lower mean dose.

Conclusion:

A custom ESAPI script was coupled with a MATLAB routine in order to extract beam parameters from static‐jaw plans and their replanned dynamic‐jaw deliveries. The effects were quantified using jaw factor which is the ratio between the meterset weighted aperture size for dynamic‐jaw fields versus static‐jaw fields.
SU‐F‐T‐532: Wobble: A New Metric to Quantify Dynamic‐Jaw VMAT Delivery
Authors: Tien C , Studenski M
Published: July 2016

Purpose:

Dynamic‐jaw tracking minimizes the unblocked area to account for changes in the target's projection as a function of gantry angle. To more fully quantify the benefit of dynamic‐jaw tracking, we have introduced a “wobble” metric that accounts for both target offset and eccentricity as opposed to a jaw factor which only accounts for the increase in blocked area

Methods:

The wobble is the unblocked area divided by target crosssectional area, and incorporates target offset and eccentricity. The jaw factor is simply the ratio of unblocked area with and without dynamic‐jaws. Forty patients treated to different sites were replanned with dynamic‐jaw tracking in Eclipse. The majority had multiple arcs and reduced‐field boosts resulting in 151 fields. We quantified the relationship between wobble and jaw factor.

Results:

The highest jaw factor measured was 0.9692, for a prostate case, and corresponded to a wobble of 0.355. The lowest jaw factor was 0.4722, for a rectal case, and corresponded to a wobble of 2.410. The median jaw factor was 0.7917. The wobble values ranged from 0.2145 to 2.575. The minimum value of wobble was found in a PTV containing prostate‐only site while the maximum value of wobble was found in a PTV containing prostate, seminal vesicle, and lymph nodes. The median wobble was 1.013. For all sites other than brain, there is approximately 20% reduction in jaw factor per 1 unit of wobble. For brain, there was 30% jaw factor reduction per unit of wobble.

Conclusion:

The wobble metric was introduced to fully quantify dynamic‐jaw tracking and potential dose reduction. There was good correlation observed between wobble and jaw factor. Increasing wobble indicates either large offset or more eccentricity and the potential for substantial dose reduction with dynamic tracking.
SU‐F‐T‐536: Contra‐Lateral Breast Study for Prone Versus Supine Patients
Authors: Marrero M , Joseph K , Klein E
Published: July 2016

Purpose:

There are several advantages to utilizing the prone technique for intact breast cancer patients. However, as the topography changes, accompanied by the influence of a supporting breast board and patient treatment couch, the question that arises is to whether there is a concern for contralateral breast dose for intact breast cancer patients being treated with this technique.

Methods:

An anthropomorphic phantom with breast mounds to duplicate intact breast cancer treatment was planned in prone and supine position. Two tangential beams were executed in the similar manner for as the radiotherapy planning system. For the prone setup, a breast dense foam board was used to support the phantom. A grid of 24 OSL nanodots was placed at 6cm, 4cm, and 2cm apart from the medial border for both prone and supine setups. The phantom was set up using megavoltage imaging and treated as per plan. Additional, a similar study was performed on a patient treated in prone position.

Results:

Overall, the contralateral breast dose was generally higher for prone setups at all locations especially when close to the medial border. The average mean dose was found to be 1.8%, 2.5% of the prescribed dose for supine respectively prone position. The average of the standard deviation is 1.04%, 1.38% for supine respectively prone position. As for patient treated in prone position average mean dose was found to be 1.165% of the prescribed dose and average of the standard deviation is 9.456%.

Conclusion:

There is minimal influence of scatter from the breast board. It appears that the volatility of the setup could lead to higher doses than expected from the planning system to the contralateral breast when the patient is in the prone position
SU‐F‐T‐547: Off‐Isocenter Winston‐Lutz Test for Stereotactic Radiosurgery/stereotactic Body Radiotherapy
Authors: Gao J , Liu X
Published: July 2016

Purpose:

To perform a quantitative study to verify that the mechanical field center coincides with the radiation field center when both are off from the isocenter during the single‐isocenter technique in linear accelerator‐based SRS/SBRT procedure to treat multiple lesions.

Methods:

We developed an innovative method to measure this accuracy, called the off‐isocenter Winston‐Lutz test, and here we provide a practical clinical guideline to implement this technique. We used ImagePro V.6 to analyze images of a Winston‐Lutz phantom obtained using a Varian 21EX linear accelerator with an electronic portal imaging device, set up as for single‐isocenter SRS/SBRT for multiple lesions. We investigated asymmetry field centers that were 3 cm and 5 cm away from the isocenter, as well as performing the standard Winston‐Lutz test. We used a special beam configuration to acquire images while avoiding collision, and we investigated both jaw and multileaf collimation.

Results:

For the jaw collimator setting, at 3 cm off‐isocenter, the mechanical field deviated from the radiation field by about 2.5 mm; at 5 cm, the deviation was above 3 mm, up to 4.27 mm. For the multileaf collimator setting, at 3 cm off‐isocenter, the deviation was below 1 mm; at 5 cm, the deviation was above 1 mm, up to 1.72 mm, which is 72% higher than the tolerance threshold.

Conclusion:

These results indicated that the further the asymmetry field center is from the machine isocenter, the larger the deviation of the mechanical field from the radiation field, and the distance between the center of the asymmetry field and the isocenter should not exceed 3 cm in of our clinic. We recommend that every clinic that uses linear accelerator, multileaf collimator‐based SRS/SBRT perform the off‐isocenter Winston‐Lutz test in addition to the standard Winston‐Lutz test and use their own deviation data to design the treatment plan.
SU‐F‐T‐549: Validation of a Method for in Vivo 3D Dose Reconstruction for SBRT Using a New Transmission Detector
Authors: Nakaguchi Y , Shimohigashi Y , Onizuka R , Ohno T
Published: July 2016

Purpose:

Recently, there has been increased clinical use of stereotactic body radiation therapy (SBRT). SBRT treatments will strongly benefit from in vivo patient dose verification, as any errors in delivery can be more detrimental to the radiobiology of the patient as compared to conventional therapy. In vivo dose measurements, a commercially available quality assurance platform which is able to correlate the delivered dose to the patient's anatomy and take into account tissue inhomogeneity, is the COMPASS system (IBA Dosimetry, Germany) using a new transmission detector (Dolphin, IBA Dosimetry). In this work, we evaluate a method for in vivo 3D dose reconstruction for SBRT using a new transmission detector, which was developed for in vivo dose verification for intensity‐modulated radiation therapy (IMRT).

Methods:

We evaluated the accuracy of measurement for SBRT using simple small fields (2×2−10×10 cm2), a multileaf collimator (MLC) test pattern, and clinical cases. The dose distributions from the COMPASS were compared with those of EDR2 films (Kodak, USA) and the Monte Carlo simulations (MC). For clinical cases, we compared MC using dose‐volume‐histograms (DVHs) and dose profiles.

Results:

The dose profiles from the COMPASS for small fields and the complicated MLC test pattern agreed with those of EDR2 films, and MC within 3%. This showed the COMPASS with Dolphin system showed good spatial resolution and can measure small fields which are required for SBRT. Those results also suggest that COMPASS with Dolphin is able to detect MLC leaf position errors for SBRT. In clinical cases, the COMPASS with Dolphin agreed well with MC. The Dolphin detector, which consists of ionization chambers, provided stable measurement.

Conclusion:

COMPASS with Dolphin detector showed a useful in vivo 3D dose reconstruction for SBRT. The accuracy of the results indicates that this approach is suitable for clinical implementation.
SU‐F‐T‐551: Beam Hardening and Attenuation of Photon Beams Using Integral Quality Monitor in Radiotherapy
Authors: Casar B , Carot I Mendez , Peterlin P
Published: July 2016

Purpose:

Aim of the multi‐centre study was to analyse beam hardening effect of the Integral Quality Monitor (IQM) for high energy photon beams used in radiotherapy with linear accelerators. Generic values for attenuation coefficient k(IQM) of IQM system were additionally investigated.

Methods:

Beam hardening effect of the IQM system was studied for a set of standard nominal photon energies (6 MV–18 MV) and two flattening filter free (FFF) energies (6 MV FFF and 10 MV FFF). PDD curves were measured and analysed for various square radiation fields, with and without IQM in place. Differences between PDD curves were statistically analysed through comparison of respective PDD_20,10 values. Attenuation coefficients k(IQM) were determined for the same range of photon energies.

Results:

Statistically significant differences in beam qualities for all evaluated high energy photon beams were found, comparing PDD_20,10 values derived from PDD curves with and without IQM in place. Significance of beam hardening effect was statistically proven with high confidence (p < 0,01) for all analysed photon beams except for 15 MV (p = 0,078), although relative differences in beam qualities were minimal, ranging from 0,1 % to 0,5 %. Attenuation of the IQM system showed negligible dependence on radiation field size. However, clinically important dependence of kIQM versus TPRs20,10 was found: 0,941 for 6 MV photon beams, to 0,959 for 18 MV photon beams, with highest uncertainty below 0,006. k(IQM) versus TPRs were tabulated and polynomial equation for the determination of k(IQM) is suggested for clinical use.

Conclusion:

There was no clinically relevant beam hardening, when IQM system was on linear accelerators. Consequently, no additional commissioning is needed for the IQM system regarding the determination of beam qualities. Generic values for k(IQM) are proposed and can be used as tray factors for complete range of examined photon beam energies.
SU‐F‐T‐559: High‐Resolution Scintillating Fiber Array for In‐Vivo Real‐Time SRS and SBRT Patient QA
Authors: Knewtson T , Pokhrel S , Loyalka S , Izaguirre E
Published: July 2016

Purpose:

A high‐resolution scintillating fiber detector was built for in‐vivo real‐time patient specific quality assurance (QA). The detector is designed for stereotactic body radiotherapy (SBRT) and stereotactic radiosurgery (SRS) to monitor treatment delivery and detect real‐time deviations from planned dose to increase patient safety and treatment accuracy.

Methods:

The detector consists of two high‐density scintillating fiber arrays layered to form an X‐Y grid which can be attached to the accessory tray of a medical linac for SBRT and cone SRS treatment QA. Fiber arrays consist of 128 scintillating fibers embedded within a precision‐machined, high‐transmission polymer substrate with 0.8mm pitch. The fibers are coupled on both ends to high‐sensitivity photodetectors and the output is recorded through a high‐speed analog‐to‐digital converter to capture the linac pulse sequence as treatment delivery progresses. The detector has a software controlled 360 degree rotational system to capture angular beam projections for high‐resolution beam profile reconstruction.

Results:

The detector was validated using SRS cone sizes from 6mm to 34mm and MLC defined field sizes from 5×5mm2 to 100×100mm2. The detector output response is linear with dose and is dose rate independent. Each field can be reconstructed accurately with a spatial resolution of 0.8mm and the current beam output is displayed every 50msec. Dosimetric errors of 1% with respect to the treatment plan can be identified and clinically significant deviations from the expected treatment can be displayed in real‐time to alert the therapists.

Conclusion:

The high resolution detector is capable of reconstructing beam profiles in real‐time with submillimeter resolution and 1% dose resolution. This system has the ability to project in‐vivo both spatial and dosimetric errors during SBRT and SRS treatments when only a non‐clinically significant fraction of the intended dose was delivered. The device has the potential to establish new standards for in‐vivo patient specific QA.
SU‐F‐T‐627: A Conformity Index Study of SBRT Plans for Lung Cancer Patient
Authors: Zhu Dengsong , Fan Guoqing , Peng Chong , Austerlitz Carlos
Published: July 2016

Purpose:

To evaluate the conformity index for SBRT lung plan in Eclipse (version 11) treatment planning system.

Methods:

In the Eclipse treatment planning system the conformity index was defined as VRI/TV, where VRI is prescription isodose surface, and TV is the target volume. In 1997, Van't Riet et al introduced TVRI/TV × TVRI/VRI as a conformal index, where TVRI is the target volume closed by the prescription isodose surface. We chose a lung cancer patient who was assigned to be treated with SBRT. SBRT immobilization device was used and non‐gated CT/4DCT was performed. The tumor movement was less than 1 cm based on 4DCT. A MIP image set was generated and used to delineate the internal target volume (ITV). A 5 mm margin was added on the ITV to generate the PTV. The prescription was 5000 cGy in 5 fractions and 100% prescription dose covered 95% of the PTV. A VMAT plan (plan I) was made to achieve the physician's goal. Then a high dose structure was generated by VRI subtracting the TV, a low dose structure was generated by TV subtracting VRI, and medium dose structure was named for the overlapping area between VRI and TV. The other VMAT plan (plan II) was made by adding constraints to high/low/medium dose structures, respectively. The conformity index was determined and compared between plan I and plan II.

Results:

For plan I, the Eclipse's conformal index was 1.06; the Van't Riet's conformal index was 0.82. For plan II, the Eclipse's conformal index was 1.02; the Van't Riet's conformal index was 0.85. The conformal index was improved in plan II.

Conclusion:

The high/low dose/medium dose structures may be useful to make more conformal plans to treat lung area using SBRT based on the conformal index values.
SU‐F‐T‐632: On the Use of Intensity Modulated Arc Therapy for Thoracic Vertebral Metastases SBRT Treatment
Authors: Pokhrel D , Mallory M , Badkul R , Jiang H , Saleh H , Wang F , Lominska C
Published: July 2016

Purpose:

To retrospectively evaluate quality, efficiency and delivery accuracy of intensity modulated arc therapy (IMAT) plans for thoracic‐vertebral metastases using stereotactic body radiotherapy (SBRT).

Methods:

After obtaining approval of RPC‐benchmark plan, seven previously treated thoracic‐vertebral metastases patients with non‐coplanar hybrid arcs(NC‐HA)using 1–2 3D‐dynamic conformal partial‐arcs plus 7–9 IMRT‐beams were re‐optimized with IMAT using 3 full co‐planar arcs. Tumors were located between T2–T7. T1/T2‐weighted MRI images were co‐registered with planning‐CT. PTVs were between 24.3–240.1cc(median=48.1cc). Prescription was 30Gy in 5 fractions with 6‐MV beams at Novalis‐TX consisting of HD‐MLC.Plans were compared for target coverage:conformality index(CI),homogeneity index(HI),PTVD90. Organs‐at‐risks(OARs)was evaluated for spinal cord(Dmax, D0.35cc, and D1.2cc), esophagus(Dmax and D5cc),heart(Dmax, D15cc)and lung(V5 and V10). Dose delivery efficiency and accuracy of each IMAT plan was assessed via quality assurance(QA) plan. Beam‐on time was recorded and a gamma index was used to compare agreement between planned and measured doses.

Results:

SBRT IMAT plans resulted in superior CI(1.02 vs. 1.36, p=0.05) and HI (0.14 vs. 0.27, p=0.01). PTVD90 was improved but statistically insignificant (31.0 vs. 30.4Gy, p=0.38). IMAT resulted in statistically significant improvements in OARs sparing: esophagus max(22.5 vs. 27.0Gy, p=0.03), esophagus 5cc (17.6 vs. 21.5Gy, p=0.02) and heart max(13.1 vs. 15.8Gy, p=0.03). Spinal cord,lung V5 and V10 were lower but statistically insignificant. Average total MU and beam‐on time were 2598±354 vs. 3542±495 and 4.7±0.6 min vs. 7.1±1.0min for IMAT vs. NC‐HA (without accounting for couch kicks time for NC‐HA). IMAT plans demonstrated an accurate dose delivery of 95.5±1.0% for clinical gamma passing‐rate of 2%/2mm criteria on MapCHECK, that was comparable to NC‐HA plans.

Conclusion:

IMAT plans provided highly conformal and homogeneous dose distributions to target and reduced OARs doses compared to NC‐HA. Total MU was reduced by a factor of 1.4 and subsequently decreased treatment times significantly ‐ potentially minimizing intra‐fraction motion error and owing to patient comfort. SBRT using IMAT planning for single fraction thoracic‐vertebrae metastases will be investigated.
SU‐F‐T‐637: Single‐Isocenter Versus Multiple‐Isocenter VMAT SRS for Unusual Multiple Metastasis Case with Two Widely Separated Lesions
Authors: Thomas EM , Popple RA , Fiveash JB
Published: July 2016

Purpose:

Single‐isocenter (SI) volumetric modulated arc therapy has been shown to be an effective and efficient approach to multiple metastasis radiosurgery. However, certain extreme cases raise the question of whether multiple‐isocenter (MI) approaches can still generate superior plans. In this study, we ask this question with respect to a clinical case with two very widely separated lesions.

Methods:

A patient with two widely separated (d = 12cm) tumors was treated with SI‐VMAT SRS using 10MV flattening filter free (FFF) beam with high‐definition multi‐leaf collimator (HD‐MLC, 2.5/5mm) in two non‐coplanar arcs using concentric rings to enforce steep gradient. Because of lesion positioning with respect to collimator angle selection, lesions were treated by 5mm leaves. We re‐planned the case with a congruent arc arrangement but separate isocenter for each lesion. In this manner, lesions were treated by 2.5mm leaves. Conformity index (CI), V50%, and mean brain dose were compared.

Results:

Neither conformity (CI_SI = 1.12, CI_MI = 1.08) nor V50% (V50%_SI =8.82cc, V50%_MI =8.81cc) were improved by utilizing a separate isocenter for each lesion. Mean brain dose was slightly reduced (dmean_SI = 118.4 cGy, dmean_MI = 88.7 cGy) by using multiple isocenters.

Conclusion:

For this case with a lesion at the apex of the brain and another distantly located at the base of skull, employing a separate isocenter for each target did not meaningfully improve plan quality. Single‐isocenter VMAT has been shown feasible and equivalent to multiple‐isocenter VMAT for multiple metastasis cases in general. In this extreme case, single‐ and multiple‐ isocenter VMAT were also equivalent. If rotational setup errors are appropriately corrected, the increased delivery efficiency of the single‐isocenter approach renders it preferable to the multiple isocenter approach.

Dr's Thomas, Popple, and Fiveash have all received honoraria from Varian Medical Systems for discussing their experiences with stereotactic radiosurgery.
SU‐F‐T‐638: Is There A Need For Immobilization in SRS?
Authors: Masterova K , Sethi A , Anderson D , Prabhu V , Rusu I , Gros S , Melian E
Published: July 2016

Purpose:

Frameless Stereotactic radiosurgery (SRS) is increasingly used in the clinic. Cone‐Beam CT (CBCT) to simulation‐CT match has replaced the 3‐dimensional coordinate based set up using a stereotactic localizing frame. The SRS frame however served as both a localizing and immobilizing device. We seek to measure the quality of frameless (mask based) and frame based immobilization and evaluate its impact on target dose.

Methods:

Each SRS patient was set up by kV on‐board imaging (OBI) and then fine‐tuned with CBCT. A second CBCT was done at treatment‐end to ascertain intrafraction motion. We compared pre‐ vs post‐treatment CBCT shifts for both frameless and frame based SRS patients. CBCT to sim‐CT fusion was repeated for each patient off‐line to assess systematic residual image registration error. Each patient was re‐planned with measured shifts to assess effects on target dose.

Results:

We analyzed 11 patients (12 lesions) treated with frameless SRS and 6 patients (11 lesions) with a fixed frame system. Average intra‐fraction iso‐center positioning errors for frameless and frame‐based treatments were 1.24 ± 0.57 mm and 0.28 ± 0.08 mm (mean ± s.d.) respectively. Residual error in CBCT registration was 0.24 mm. The frameless positioning uncertainties led to target dose errors in Dmin and D95 of 15.5 ± 18.4% and 6.6 ± 9.1% respectively. The corresponding errors in fixed frame SRS were much lower with Dmin and D95 reduced by 4.2 ± 6.5% and D95 2.5 ± 3.8% respectively.

Conclusion:

Frameless mask provides good immobilization with average patient motion of 1.2 mm during treatment. This exceeds MRI voxel dimensions (∼0.43mm) used for target delineation. Frame‐based SRS provides superior patient immobilization with measureable movement no greater than the background noise of the CBCT registration. Small lesions requiring submm precision are better served with a frame based SRS.
SU‐F‐T‐646: SBRT Lung: Moving Beyond the 3D Conformal Paradigm with An Elekta VersaHD Accelerator
Authors: Dalhart A , Hyer D , Allen B , Flynn R , Johnston H
Published: July 2016

Purpose:

To develop a volumetric modulated arc therapy (VMAT) planning and delivery methodology for lung stereotactic body radiation therapy (SBRT) that addresses the unique geometric challenges presented when using an Elekta VersaHD linear accelerator.

Methods:

The Elekta VersaHD imaging panels are fixed at 160cm SID when deployed, limiting the ability to perform non‐coplanar treatments due to collisions with the treatment table. The panels can be folded in a stored position for non‐coplanar treatments, but in this configuration, longitudinal table travel is restricted by the space required for the folded panels. To overcome these design features, a non‐coplanar VMAT technique was designed and evaluated for 6 patients that uses a superiorly placed isocenter near the apex of the lung. A coplanar VMAT technique with isocenter placement within the target was also assessed for each patient. Non‐coplanar plans included three arcs, totalling 340° of arc angle, while coplanar plans included two arcs, totalling 280° of arc angle. Each technique avoided the contralateral lung and major overlap on patient skin. Corresponding 3DCRT plans were used as a baseline for each patient in evaluating VMAT plans.

Results:

Coplanar and noncoplanar VMAT plans yielded a high dose conformity index (CI) improvement of 6% and low dose CI improvements of 9% and 11%, respectively, over corresponding 3DCRT plans. While both VMAT techniques provided greater skin sparing compared to 3DCRT, the non‐coplanar technique offered an improvement of 20% over the coplanar approach. Treatment time with for VMAT was ∼2.5 minutes compared to ∼10 minutes for 3DCRT.

Conclusion:

Two VMAT techniques were developed for lung SBRT that account for machine design limitations and provide greater dose conformity and normal tissue sparing than 3DCRT plans. The non‐coplanar VMAT plans showed a marginal improvement over coplanar VMAT plans and may not warrant the additional complexity.
SU‐F‐T‐652: Verification of Inter‐Fractional Tumor Motion Using Daily Fiducial Length Measurement for Pancreatic SBRT
Authors: Jang S , Hwang M , Michalski D , Lalonde R , Huq M
Published: July 2016

Purpose:

To evaluate possible inter‐fractional tumor motion variability for pancreatic SBRT with length measurement of the fiducial depiction on the scan and to develop an empirical correlation using a moving phantom and fiducials for adjusting gating window on a daily basis

Methods:

In this retrospective study, motion data for fifty pancreatic SBRT patients with fiducials were selected to find the tumor motion statistics. Three gold fiducials of 5mm were placed around a solid target inside the Quasar Phantom and the 4D CT images of the Phantom were acquired with tumor motion of 1cm in superior‐inferior direction. The breathing period was set to 4 seconds with a sinusoidal breathing form. After 4D‐CT acquisition, CT50% images were imported into the Eclipse and setup fields were created for acquiring CBCT images. CBCT images were acquired by using the TrueBeam‐STx and Pelvis technique. For sinusoidal tumor motions, CBCT images were acquired with tumor motion from zero to 2cm with 0.5cm increment along SI direction. The breathing period was varied from 2∼6 seconds with 2‐second increment. Also, real patients’ breathing files acquired from Varian‐RPM system were used to simulate real patients’ breathing patterns. Then length of fiducial depiction was measured with a HU threshold of zero and was used to find its correlation with real tumor motion.

Results:

Tumor motions in SI, AP, and LR directions were 0.7±0.5cm, 0.2±0.3cm, and 0.1±0.2cm, respectively. Breathing period was 4.3±0.8seconds. For a sinusoidal breathing form, there was a significant correlation between length of fiducial and real tumor motions with R2 of 0.98. However, the length of fiducial was independent of varying breathing periods. For a real patient's breathing file, the correlation between fiducial length(Y) and tumor motion(X) was Y=1.1268X+0.2696 with R2 of 0.99.

Conclusion:

With daily fiducial length measurement, we can evaluate possible changes in the inter‐fractional tumor motion in pancreatic SBRT.
SU‐F‐T‐653: Radiation Exposure from Cs‐131 Permanent Seed Implants
Authors: Giaddui T , Hardin M , To D , Kremmel E , Peng C , Hann P , Richardson S , Yu Y , Harrison A , Doyle L
Published: July 2016

Purpose:

Permanent seed implants have traditionally been used to treat prostate, lung and head or neck cancers using I‐125 and Pd‐103. Cs‐131, which has higher dose rate is being used to treat brain, head and/or neck cancers in our clinic, therefore, we chose to monitor the dose received by surgeons during the extensive procedure. The aims of this work are to assess the level of radiation exposure to surgeons and the instantaneous exposure at bedside and 1 m from patients.

Methods:

Ten patients received Cs‐131 implants for recurrent brain,head and/or neck cancer; the median implanted activity, number of implanted seeds and prescription dose at 0.5 cm from the perpendicular plane of the implant were: 54.3 mCi (14.52 – 77); 19 (4 – 24) and 60 Gy (range 42 – 60) respectively. Radiation exposure was recorded at bedside and 1 m from the patient using Victoreen ion chamber (Fluke Biomedical, Cleveland, OH). Exposure to surgeons was measured using TLD (Mirion Technologies (GDS), Inc., USA).

Results:

The median equivalent dose rate at 1 m and bedside immediately following implantation were 1.49×10‐2 mSv/h (8.77×10‐3–2.63×10‐2) and 7.76×10‐2 mSv/h (3.1×10‐2– 1.53×10‐1) respectively. Median equivalent dose to surgeons’ hands was 0.60 mSv (0.33 – 1.48) and no doses were detected for whole‐body. Surgical reconstruction for one patient was performed 71 days post‐implant and resulted in zero exposure to surgeons.

Conclusion:

The recorded exposure rates were low when compared with the literature. Post procedure surveys at bed site and 1 m indicated that all patients were within safe limits for discharge (< 0.05 mSv/h at 1 m). However, as a precautionary measure, patients were advised to avoid direct contact with children and pregnant women within four weeks of the implant and stay at least at 3 ft from other people. Surgeons doses were well within occupational dose limits.
SU‐F‐T‐658: Out‐Of‐Field Dose Comparison for TrueBeam Low Energy Beams for Extended Distances: Measurement Vs Monte Carlo Simulation
Authors: Wijesooriya K , Liyanage N , Kaluarachchi M , Sawkey D
Published: July 2016

Purpose:

Patient dose far from the treatment field is comprised of scatter from within the patient, and treatment head leakage. We quantify the treatment head leakage for TrueBeam linear accelerator for 6X and 6X‐FFF beams by comparing measurements to Monte Carlo simulations for a variety of jaw sizes and collimator rotations. This work is conceptually similar to that of Kry et al. (Medical Physics 2006; 33: 4405), who considered a Clinac linear accelerator.

Methods:

Measurements were made using an EXRADIN A101 ion chamber positioned in the patient plane, at distances up to 100 cm from isocenter. Simulations were done using VirtuaLinac, the GEANT4‐based Monte Carlo model of the TrueBeam treatment head, and an in‐house (U. Virginia) GEANT4‐based code. In‐house code modelled an ion chamber with build‐up, based on a CT scan of the chamber. VirtuaLinac included a detailed model of the treatment head shielding, and was run on the Amazon Web Services cloud to generate spherical phase space files surrounding the treatment head. These phase space files were imported into the in‐house code.

Results:

Initial comparisons between measurements and simulation showed an excess of dose in the in‐plane direction, away from the gantry, in the simulations. This was traced to an incomplete model of the shielding—specifically, the component holding the primary collimator was smaller in the model than in the TrueBeam. Modifications were made to VirtuaLinac to more closely match the engineering drawings. In the in‐plane direction, the lowest out of field dose was away from gantry (negative abscissa values) at around 60 cm from isocenter, for fields smaller than 10×10 cm2. Out of field dose decreased with decreasing jaw size. Flattening‐filter free beam produced out‐of‐field doses as low as 65% of those with flattened beam.

Conclusion:

Doses determined from simulation and measurement were in close agreement.

Funding support from the Jefferson Trust Foundation
SU‐F‐T‐666: Molecular‐Targeted Gold Nanorods Enhances the RBE of Proton Therapy
Authors: Khoo A , Sahoo N , Krishnan S , Diagaradjane P
Published: July 2016

Purpose:

In recent years, proton beam radiation therapy (PBRT) has gained significant attention in the treatment of tumors in anatomically complex locations. However, the therapeutic benefit of PBRT is limited by a relative biological effectiveness (RBE) of just 1.1. The purpose of this study is to evaluate whether this limitation can be overcome by artificially enhancing the RBE using molecular‐targeted gold nanorods (GNRs).

Methods:

Molecular‐targeting of GNRs was accomplished using Cetuximab (antibody specific to epidermal growth factor receptor that is over‐expressed in tumors) conjugated GNRs (cGNRs) and their binding affinity to Head and Neck cancer cells was confirmed using dark field microscopy and Transmission Electron Microscopy (TEM). The radiosensitization potential of cGNRs when irradiated with photon (6MV) and proton (100 and 160 MeV) beams was determined using clonogenic assays. The RBE at 10% surviving fraction (RBE10) for proton therapies at central and distal locations of SOBP was calculated with respect to 6 MV photons. IgGconjugated GNRs (iGNRs) were used as controls in all experiments.

Results:

cGNRs demonstrated significant radiosensitization when compared to iGNRs for 6MV photons (1.14 vs 1.04), 100 MeV protons (1.19 vs 1.04), and 160 MeV protons (1.17 vs 1.04). While RBE10 for proton beams at the center of SOBP revealed similar effects for both 100 and 160 MeV (RBE10=1.39 vs 1.38; p>0.05), enhanced radiosensitization was observed at the distal SOBP with 100 MeV beams demonstrating greater effect than 160 MeV beams (RBE10=1.79 vs 1.6; p<0.05).

Conclusion:

EGFR‐targeting GNRs significantly enhance the RBE of protons well above the accepted 1.1 value. The enhanced RBE observed for lower energy protons (100 MeV) and at the distal SOBP suggests that low energy components may play a role in the observed radiosensitization effect. This strategy holds promise for clinical translation and could evolve as a paradigm‐changing approach in the field of radiation oncology.

The UT MD Anderson Cancer Center's Institutional Research Grant funding to P. Diagaradjane
SU‐F‐T‐671: Effects of Collimator Material On Proton Minibeams
Authors: Lee E , Eagle J , Sandison G , Cao N , Stewart R , Marsh S , Meyer J
Published: July 2016

Purpose:

To investigate the dosimetric effects of collimator material on spatially modulated proton minibeams (pMBRT). Methods: pMBRT holds promise to exhibit shallow depth normal‐tissue sparing effects similar to synchrotron based microbeams while also retaining potential for uniform dose distributions for tumor targets. TOPAS Monte Carlo simulations were performed for a 5cm thick multislit collimator with 0.3mm slits and 1mm center‐to‐center spacing for a 50.5MeV proton minibeam while varying collimator material between brass, tungsten, and iron. The collimator was placed both “flush” at the water phantom surface and at 5cm distance to investigate the effects on surface dose, peak‐to‐valley‐dose‐ratio (PVDR) and neutron contribution.

Results:

For flush placement, the neutron dose at the phantom surface for the tungsten collimator was approximately 20% higher than for brass and iron. This was not reflected in the overall surface dose, which was comparable for all materials due to the relatively low neutron contribution of <0.1%. When the collimator was retracted, the overall neutron contribution was essentially identical for all three collimators. Surface dose dropped by ∼40% for all collimator materials with air gap compared to being flush with the phantom surface. This surface dose reduction was at the cost of increase in valley dose for all collimator materials due to increased angular divergence of the mini‐beams at the surface and their consequent geometric penumbra at depth. When the collimator was placed at distance from the phantom surface the PVDR decreased. The peak‐to‐entrance‐dose ratio was highest for the iron collimator with 5cm air gap. Conclusion: The dosimetric difference between the collimator materials is minimal despite the relatively higher neutron contribution at the phantom surface for the tungsten collimator when placed flush. The air gap between the collimator and phantom surface strongly influences all dosimetry parameters due to the influence of scatter on the narrow spatial modulation.