Conformal Plans Research Articles

The effect of slice thickness on target and organs at risk volumes, dosimetric coverage and radiobiological impact in IMRT planning.

Dose-volume histogram (DVH) has become an important tool for evaluation of radiation outcome as reflected from many clinical protocols. While dosimetric accuracy in treatment planning system (TPS) is well quantified, the variability in volume estimation is uncertain due to reconstruction algorithm that is investigated in this study. In addition, the impact of dose distribution and tumor control probability (TCP) were also investigated with CT slice thickness for IMRT planning. A water phantom containing various objects with accurately known volume ranging from 1 to 100 cm(3) was scanned with 1, 2, 3, 5, and 10 mm slice thickness. The CT data sets were sent to Eclipse TPS for contour delineation and volume estimation. The data were compared with known volume for the estimation of error in the volume of each structure. IMRT Plans were generated on phantom containing four objects with different slice thickness (1-5 mm) to calculate TCP. ICRU-83-recommended dose points such as D 2%, D 50%, D 98%, as well as homogeneity and conformity index were also calculated. The variability of volumes with CT slice thickness was significant especially for small volume structures. A maximum error of 92% was noticed for 1 cm(3) volume of object with 10 mm slice thickness, whereas it was ~19% for 1 mm slice thickness. For 2 and 3 cm(3) objects, the maximum error of 99% was noticed with 10 mm slice thickness and ~60% with 5 mm. The differences are smaller for larger volumes with a cutoff at about 20 cm(3). The calculated volume of the objects is a function of reconstruction algorithm and slice thickness. The PTV mean dose and TCP decreased with increasing slice thickness. Maximum variation of ~5% was noticed in mean dose and ~2% in TCP with change in slice thickness from 1 to 5 mm. The relative decrease in target volume receiving 95% of the prescribed dose is ~5% with change in slice thickness from 1 to 5 mm. The homogeneity index increases up to 163% and conformity index decreases by 4% between 1 and 5 mm slice thickness, producing highly inhomogeneous and least conformal treatment plan. Estimation of a volume is dependent on CT slice thickness and the contouring algorithm in a TPS. During commissioning of TPS and for all clinical protocols, evaluation of volume should be included to provide the limit of accuracy in DVH from TPS, especially for small objects. A smaller slice thickness provides superior dosimetry with improved TCP. Thus, the smallest possible slice thickness should be used for IMRT planning, especially when smaller structures are present.

Unintended irradiation of internal mammary chain – Is that enough?

AimTo evaluate the unintentional coverage of the internal mammary chain (IMC) with tangential fields irradiation to the breast, and its relation with the type of surgery employed. BackgroundThe dose distribution in regions adjacent to the treatment targets (mammary gland or chest wall), with incidental irradiation of the IMC, could translate into clinical benefit, due to the proximity of these regions. Materials and methodsOne hundred and twelve consecutive conformal radiotherapy plans were correlating the average dose to the IMC with the type of surgery employed, the extent of disease and the irradiation techniques. ResultsThe mean doses to IMC after modified radical mastectomy (MRM), modified radical mastectomy with immediate reconstruction (MRM+R), and breast conservative surgery (BCS) were 30.34Gy, 30.26Gy, and 18.67Gy, respectively. Significant differences were identified between patients who underwent MRM or MRM+R over BCS (p=0.01 and 0.003, respectively), but not between MRM and MRM+R (p=0.88). Mean doses to IMC were greater in patients with T3–T4 tumors when compared with more initial stages (≤T2) (p=0.0096). The lymph node involvement also correlated with higher average doses to IMC (node positive: 26.1Gy×node negative: 17.8Gy, p=0.0017). ConclusionsThe moderate dose level to the IMC in the unintentional irradiation scenario seems to be insufficient to treat the subclinical disease, although it could have an impact in patients undergoing mastectomy.

P0033 Significance of patterns of failure after treatment with curative intent in adenocarcinoma of stomach

Background Improved overall and relapse-free survival with adjuvant loco-regional radiotherapy augmented by concurrent chemotherapy using fluorinated pyrimidines has been shown in adenocarcinoma of stomach. Adjacent critical organs can be spared without compromising dose to target using 3D conformational radiotherapy, and intensity-modulated radiotherapy has failed to document any added advantage. The aim of this retrospective analysis was to assess outcome and patterns of failure in all patients with non-metastatic adenocarcinoma of stomach, including gastro-oesophageal junction, who received adjuvant postoperative chemoradiotherapy between 2006 and 2013. Methods Electronic medical records and radiotherapy records were retrieved and analysed. Patterns of failure were evaluated. The primary endpoint was overall survival. Findings There were 73 cases. Male to female ratio was 57:16 (78.1%:21.9%). Median age of presentation was 57 years (range 27–73). Median follow up was 52.27 months. 50.7% of cases had poorly differentiated tumours. Pathological stage distribution was as follows: stage IA 1.4%, stage IB 8.2%, stage IIA 19.2%, stage IIB 26%, stage IIIA 20.5%, stage IIIB 19.2%, and stage IIIC 5.5%. All patients had undergone total or partial gastrectomies. Concurrent chemotherapy was based on fluorouracil. Radiotherapy was planned using 3D-conformal, six-field, mono-isocentric techniques, and a dose of 4500 cGy in 25 fractions was delivered. Median overall survival was 27.7 months (95% confidence interval (CI) 15.48–39.92). Patients with poorly differentiated tumours had median overall survival of 19.2 months ( p = 0.0592). Pathological stage was inversely related to overall survival. For stage I tumours, 5-year overall survival was 83.33%, and for stage IIIC tumours, median overall survival was 8.53 months ( p = 0.0344). Significantly poor overall survival was associated with recurrence ( p p p = 0.0170; and distant: p = 0.002). Interpretation Loco-regional control correlates with overall survival, stressing the need for aggressive treatment. Whether an escalation in radiation dose in this era of conformal planning would improve local control and overall survival needs to be explored and requires robust data from randomised controlled trials.

Definitive radiotherapy with concurrent oncothermia for stage IIIB non-small-cell lung cancer: A case report.

Hyperthermia enhances the susceptibility of tumors to radiotherapy (RT) and chemotherapy. Oncothermia, also known as electro-hyperthermia, is a new treatment modality developed to overcome the problems of traditional hyperthermia by selectively delivering energy to the malignant tissues. The present study reports the outcome of combined oncothermia and RT in a 75-year-old patient with stage IIIB non-small-cell lung cancer (NSCLC). Due to the advanced age and the performance status of the patient, the combination of systemic chemotherapy and RT was deemed infeasible; therefore, the patient instead decided to undergo oncothermia concurrently with definitive RT. The RT was administered at a dose of 64.8 Gy in 36 fractions using a three-dimensional conformal plan technique. Oncothermia was started concomitantly with RT and was performed for 60 min per session, two sessions per week, for a total of 12 sessions. No severe toxicities developed, with the exception of mild odynophagia, which resolved soon after the treatments. Follow-up computed tomography showed complete tumor response, and the patient was alive with no evidence of the disease 18 months after the completion of the treatment. In conclusion, the present case report suggests that oncothermia combined with RT, with the former possessing radiosensitizing potential and no additional toxicities, may be a promising alternative for advanced-age and/or frail patients with locally advanced NSCLC.

SU-E-T-562: Motion Tracking Optimization for Conformal Arc Radiotherapy Plans: A QUASAR Phantom Based Study

Purpose: This study is to use plan parameters optimization (Dose rate, collimator angle, couch angle, initial starting phase) to improve the performance of conformal arc radiotherapy plans with motion tracking by increasing the plan performance score (PPS). Methods: Two types of 3D conformal arc plans were created based on QUASAR respiratory motion phantom with spherical and cylindrical targets. Sinusoidal model was applied to the MLC leaves to generate motion tracking plans. A MATLAB program was developed to calculate PPS of each plan (ranges from 0–1) and optimize plan parameters. We first selected the dose rate for motion tracking plans and then used simulated annealing algorithm to search for the combination of the other parameters that resulted in the plan of the maximal PPS. The optimized motion tracking plan was delivered by Varian Truebeam Linac. In-room cameras and stopwatch were used for starting phase selection and synchronization between phantom motion and plan delivery. Gaf-EBT2 dosimetry films were used to measure the dose delivered to the target in QUASAR phantom. Dose profiles and Truebeam trajectory log files were used for plan delivery performance evaluation. Results: For spherical target, the maximal PPS (PPSsph) of the optimized plan was 0.79: (Dose rate: 500MU/min, Collimator:more » 90°, Couch: +10°, starting phase: 0.83π). For cylindrical target, the maximal PPScyl was 0.75 (Dose rate: 300MU/min, Collimator: 87°, starting phase: 0.97π) with couch at 0°. Differences of dose profiles between motion tracking plans (with the maximal and the minimal PPS) and 3D conformal plans were as follows: PPSsph=0.79: %ΔFWHM: 8.9%, %Dmax: 3.1%; PPSsph=0.52: %ΔFWHM: 10.4%, %Dmax: 6.1%. PPScyl=0.75: %ΔFWHM: 4.7%, %Dmax: 3.6%; PPScyl=0.42: %ΔFWHM: 12.5%, %Dmax: 9.6%. Conclusion: By achieving high plan performance score through parameters optimization, we can improve target dose conformity of motion tracking plan by decreasing total MLC leaf travel distance and leaf speed.« less

SU‐E‐T‐625: Potential for Reduced Radiation Induced Toxicity for the Treatment of Inoperable Non‐Small‐Cell Lung Cancer Using RapidArc Planning

Purpose:To investigate the feasibility of using RapidArc (RA) treatment planning to reduce irradiation volume of normal lung and other organs at risk (OARs) in the treatment of inoperable non‐small‐cell lung cancer (NSCLC) patients.Methods:A retrospective treatment planning and delivery study was performed to compare target coverage and the volumes of the normal lung, spinal cord, heart and esophagus on 4D‐CT scan above their dose tolerances delivered by RA vs. IMRT for ten inoperable NSCLC patients (Stage I‐IIIB). RA plans consisted of either one‐full or two‐partial co‐planar arcs used to treat 95% of the planning target volume (PTV) with 6MV beam to a prescription of 66Gy in 33 fractions. IMRT plans were generated using 5–7 co‐planar fields with 6MV beam. PTV coverage, dose‐volume histograms, homogeneity/conformity indices (CI), total number of monitor units(MUs), beam‐on time and delivery accuracy were compared between the two treatment plans.Results:Similar target coverage was obtained between the two techniques. RA (CI=1.02) provided more conformal plans without loss of homogeneity compared to IMRT plans (CI=1.12). Compared to IMRT, RA achieved a significant median dose reduction in V10 (3%), V20 (8%), and mean lung dose (3%) on average, respectively. On average, V5 was comparable between the two treatment plans. RA reduced mean esophagus (6%), mean heart (18%), and maximum spinal cord dose (7%), on average, respectively. Total number of MUs and beam‐on time were each reduced almost by a factor of 2 when compared to IMRT‐patient comfort, reduced intra‐fraction‐motion and leakage dose. The average IMRT and RA QA pass rate was about 98% for both types of plans for 3%/3mm criterion.Conclusion:Compared to IMRT plans, RA provided not only comparable target coverage, but also improved conformity, treatment time, and significant reduction in irradiation of OARs. This may potentially allow for target dose escalation without increase in normal tissue toxicity.

SU‐F‐BRD‐12: When Does Pencil Beam Scanning Become Superior to Passive Scattered Proton Therapy for Pediatric Head and Neck Cancers?

Purpose:To investigate the dosimetric benefits of pencil beam scanning (PBS) compared with passive scattered (PS) proton therapy for treatment of pediatric head&neck patients as a function of the PBS spot size and explore the advantages of using apertures in PBS.Methods:Ten pediatric patients with head&neck cancers treated by PS proton therapy at our institution were retrospectively selected. The histologies included rhabdomyosarcoma, ependymoma, astrocytoma, craniopharyngioma and germinoma. The prescribed dose ranged from 36 to 54 Gy(RBE). Five PBS plans were created for each patient using variable spot size (average sigma at isocenter) and choice of beam specific apertures: (1) 10mm spots, (2) 10mm spots with apertures, (3) 6mm spots, (4) 6mm spots with apertures, and (5) 3mm spots. The plans were optimized for intensity modulated proton therapy (IMPT) with no single beam uniformity constraints. Dose volume indices as well as equivalent uniform dose (EUD) were compared between PS and PBS plans.Results:Although target coverage was clinically adequate for all cases, the plans with largest (10mm) spots provide inferior quality compared with PS in terms of dose to organs‐at‐risk (OAR). However, adding apertures to these plans ensured lower OAR dose than PS. The average EUD difference between PBS and PS plans over all patients and organs at risk were (1) 2.5%, (2) −5.1%, (3) ‐5%, (4) −7.8%, and (5) −9.5%. As the spot size decreased, more conformal plans were achieved that offered similar target coverage but lower dose to the neighboring healthy organs, while alleviating the need for using apertures.Conclusion:The application of PBS does not always translate to better plan qualities compared to PS depending on the available beam spot size. We recommend that institutions with spot size larger than ∼6mm at isocenter consider using apertures to guarantee clinically comparable or superior dosimetric efficacy to PS treatments.

TU-AB-201-07: Image Guided Endorectal HDR Brachytherapy Using a Compliant Balloon Applicator

Purpose: High dose rate endorectal brachytherapy is an option to deliver a focal, high-dose radiotherapy to rectal tumors for patients undergoing non-operative management. We investigate a new multichannel, MR compatible applicator with a novel balloon-based design to provide improved treatment geometry. We report on the initial clinical experience using this applicator. Methods: Patients were enrolled on an IRB-approved, dose-escalation protocol evaluating the use of the anorectal (AR-1) applicator (Ancer Medical, Hialeah, FL), a multichannel applicator with two concentric balloons. The inner balloon supports 8 source lumens; the compliant outer balloon expands to separate the normal rectal wall and the source lumens, yet deforms around a firm, exophytic rectal mass, leading to dose escalation to tumor while sparing normal rectum. Under general anesthesia, gold fiducial markers were inserted above and below the tumor, and the AR applicator was placed in the rectum. MRI-based treatment plans were prepared to deliver 15 Gy in 3 weekly fractions to the target volume while sparing healthy rectal tissue, bladder, bowel and anal muscles. Prior to each treatment, CBCT/Fluoroscopy were used to place the applicator in the treatment position and confirm the treatment geometry using rigid registration of the CBCT and planning MRI. After registration of the applicator images, positioning was evaluated based on the match of the gold markers. Results: Highly conformal treatment plans were achieved. MR compatibility of the applicator enabled good tumor visualization. In spite of the non-rigid nature of the applicators and the fact that a new applicator was used at each treatment session, treatment geometry was reproducible to within 2.5 mm. Conclusions: This is the first report on using the AR applicator in patients. Highly conformal plans, confidence in MRI target delineation, in combination with reproducible treatment geometry provide encouraging feedback for continuation with dose escalation in these patients.

SU‐E‐T‐787: Utility of the Two Candidate 192‐Ir and 169‐Yb HDR Sources for Use with a Novel Direction Modulated Brachytherapy Tandem Applicator for Cervical Cancer Treatment

Purpose:A novel tungsten alloy shielded, MRI‐compatible, direction modulated brachytherapy (DMBT) concept tandem applicator, which enables unprecedented intensity modulation, was used to evaluate treatment plan quality improvement over a conventional tandem. The utility of the 192‐Ir and 169‐Yb HDR sources, for use with the DMBT applicator, was evaluated.Methods:The total diameter of the DMBT tandem applicator is 6.0 mm, which consists of 5.4‐mm diameter tungsten alloy and 0.3 mm thick plastic sheath. The tandem has 6 symmetric peripheral 1.3‐mm diameter grooves for the source to travel. MCNPX v.2.6 was used to simulate the 192‐Ir and 169‐Yb sources inside the DMBT applicator. First, TG‐43 source parameters were evaluated. Second, 3D dose matrix with 1 mm3 resolution were imported into an in‐house‐coded inverse optimization treatment planning program to obtain optimal plans for 19 clinical cases. All plans were compared with the standard tandem and ring plans. Prescription dose was 15.0 Gy. All plans were normalized to receive the same HRCTV D90.Results:Generally, the DMBT tandem (and ring) plans were better than the conventional tandem and ring plans for 192‐Ir and 169‐Yb HDR sources. The mean data of D2cc for bladder, rectum, and sigmoid were 11.65±2.30 Gy, 7.47±3.05 Gy, and 9.84±2.48 Gy for Ir‐192 DMBT tandem, respectively. These data for Yb‐169 were 11.67±2.26 Gy, 7.44±3.02 Gy, and 9.83±2.38 Gy, respectively. The HR‐CTV D98 and V100 were 16.37±1.86 Gy and 97.37 ± 1.92 Gy for Ir‐192 DMBT, respectively. The corresponding values for Yb‐169 were 16.43±1.86 Gy, and 97.51 ± 1.91 Gy. Plans with the 169‐Yb source generally produced more favorable results where V100 increased by 13.65% while D2cc across all OARs reduced by 0.54% compared with the 192‐Ir plans.Conclusion:For the DMBT tandem applicator, 169‐Yb source seems to produce more directional beams resulting in increased intensity modulation capacity, thus resulting in more conformal plans

SU‐E‐T‐331: Dosimetric Impact of Multileaf Collimator Leaf Width On Stereotactic Radiosurgery (SRS) RapidArc Treatment Plans for Single and Multiple Brain Metastases

Purpose:To determine the effects of multileaf collimator (MLC) leaf width on normal‐brain‐tissue doses and dose conformity of SRS RapidArc treatment plans for brain tumors.Methods:Ten patients with 24 intracranial tumors (seven with 1–2 and three with 4–6 lesions) were planned using RapidArc for both Varian Millennium 120 MLC (5 mm leaf width) and high definition (HD) MLC (2.5 mm leaf width). Between 2 and 8 arcs were used with two full coplanar arcs and the rest non‐coplanar half arcs. 6 MV beams were used and plans were optimized with a high priority to the Normal Tissue Objective (to achieve dose conformity and sharp dose fall‐off) and normal brain tissue. Calculation was done using AAA on a 1 mm grid size. The prescription dose ranged from 14–22 Gy. Plans were normalized such that 99% of the target received the prescription dose. Identical beam geometries, optimizations, calculations, and normalizations were used for both plans. Paddick Conformity Index (PCI), V4, V8 and V12 Gy for normal brain tissue and Integral Dose were used for analysis.Results:In all cases, HD MLC plans performed better in sparing normal brain tissue, achieving a higher PCI with a lower Integral Dose. The average PCI for all 24 targets was 0.75±0.23 and 0.70±0.23 (p ≤0.0015) for HD MLC and Millennium MLC plans, respectively. The average ratio of normal brain doses for Millennium MLC to HD MLC plans was 1.30±0.16, 1.27±0.15, and 1.31±0.18 for the V4, V8, and V12, respectively. The differences in normal brain dose for all criteria were statistically significant with p‐value < 0.02. On average Millennium MLC plans had a 16% higher integral dose than HD MLC plans.Conclusion:Significantly better dose conformity with reduced volume of normal brain tissue and integral dose was achieved with HD MLC plans compared to Millennium MLC plans.

SU‐E‐T‐640: Proton Modulated Arc Therapy Using Scanned Pencil Beams

Purpose:Rotational proton radiotherapy would be an interesting treatment modality if it proves to produce dose distributions that conform to the target as well or better than currently available treatment modalities, while reducing the dose to the surrounding organs at risk. A treatment planning study is presented, showing how this objective can be achieved using a single or small number of scanned mono‐energetic pencil beams delivered while the proton gantry rotates at the same time that they may deliver faster and more biologically effective treatments than current proton modalities.Methods Proton treatment using single‐ (SFO) and multi‐field optimized (MFO) PBS plans are compared with PMAT plans in phantoms of different shape and uniformity as well as on a brain case. The method followed in ECLIPSE to produce PMAT plans is presented and the feasibility is discussed. Results Table 1 shows that the conformity and uniformity of the PMAT plans is of similar order than those of the SFO and MFO plans. However, in the brain case, the DVHs shown in Figure 1 indicate a significant reduction of the dose to the OARs when PMAT is used. Similarly, the use of PMAT increases the LET in the treatment area, which could translate into a more biologically effective treatment.Conclusions PMAT could provide dose distributions as conformal as current PBS plans but could also offer faster beam delivery and more biologically effective proton plans.

SU‐C‐BRB‐05: Investigation of Conformal Arc Therapy Utilizing Newly Designed Cobalt 60 Machine

Purpose:A new modernized design for cobalt 60 (Co‐60) machines is being developed with a ring type gantry. In this study we investigate the beneficial outcome of the new design for conformal arc therapy for various clinical sites. The new modality was evaluated based on isodose distributions and dose volume histograms as compared to 6MV photon beams from conventional linear accelerators.Methods:Computed tomographic images of seven different SBRT patients were selected from our patient database. All of these cases were previously planned on the Eclipse treatment planning system. New plans for these patients were generated with a modified conformal arc technique using both 6MV and Co‐60 beams. The conformal arc was created by the delivery of treatment fields conformal to the target cross‐section at every 5 or 10 degrees. The field shape was modified or turned off when it initially passed through a critical structure. Monte Carlo codes, MCBEAM and MCPLAN, were used for the machine head simulation and phantom/patient dose calculation, respectively. In the new Co‐60 machine design, the source‐to‐isocenter distance was 60cm and the treatment head included the Co‐60 source, primary collimator, jaws and MLC.Results:For all cases investigated, conformal arc plans utilizing Co‐60 beams achieved similar conformity (mean conformity index=1.19) comparing to 6MV photon beams. Isodose distributions were tailored similarly around the PTV; both Co‐60 and 6MV plans met our clinical acceptance criteria for the target coverage, and the maximum and minimum target doses. The DVH for the Co‐60 plans showed slightly lower doses to the critical structures although the differences were small in most cases.Conclusion:There were no significant dosimetric differences between 6MV and Co‐60 plans. Our results confirmed that this new Co‐60 design could be a cost‐effective machine for advanced radiotherapy due to its low cost, low maintenance and high up time.

SU-E-J-93: Development of Pre-Contoured Human Model Library in DICOM-RT Format for the Epidemiological Study of the Radiotherapy Patients

Purpose: Prior to 3D conformal radiation therapy planning, patient anatomy information was mostly limited to 2D beams-eye-view from the conventional simulator. To analyze the outcomes of such treatments for radiation late effects, 3D computational human models are often used in commercial treatment planning systems (TPSs). However, several underlying difficulties such as time-consuming manual delineation procedures of a large number of structures in the model have always limited its applications. Primary objective of this work was to develop a human model library for the epidemiological study by converting 3D-surface model organs to DICOM-RT format (DICOM-RT structure) using an in-house built software. We converted the ICRP reference human models to DICOM-RT models, which can be readily adopted for various dose calculations. Methods: MATLAB based code were utilized to convert the contour drawings extracted in text-format from the 3D graphic-tool, Rhinoceros into DICOM-RT structure format for 50 different organs of each model using a 16GB dual-core processor. The conversion periods were measured for each DICOM-RT models, and the reconstructed structure volumes were validated against the original 3D-surface models in the TPS. Ten reference hybrid whole-body models (8-pediatric and 2-adults) were automatically processed to create DICOM-RT computational human model library. Results: Mean contour conversion period was found to be 580 (N=2) and 394.5 (N=8) seconds for 50 organs in the adult and pediatric models respectively. A good agreement for large organs (NRMSD <1.0%) and small organs (NRMSD <7.7%) was also observed between the original volumes and corresponding DICOM-RT structure volumes of the organs. Conclusion: The ICRP reference human models were converted into DICOM-RT format to support the epidemiological study using a large cohort of conventional radiotherapy patients. Due to its DICOM-compatibility, the library may be implemented to many other different applications. We also expect to develop the library by including additional models in future.

SU-E-T-293: Dosimetric Analysis of Microscopic Disease in SBRT for Lung Cancers

Purpose: To evaluate the dosimetry of microscopic disease (MD) region of lung cancer in stereotactic body radiation therapy (SBRT). Methods: For simplicity, we assume organ moves along one dimension. The probability distribution function of tumor position was calculated according to the breathing cycle. The dose to the MD region was obtained through accumulating the treatment planning system calculated doses at different positions in a breathing cycle. A phantom experiment was then conducted to validate the calculated results using a motion phantom (The CIRS ‘Dynamic’ Thorax Phantom). The simulated breathing pattern used a cos4(x) curve with an amplitude of 10mm. A 3-D conformal 7-field plan with 6X energy was created and the dose was calculated in the average intensity projection (AIP) simulation CT images. Both films (EBT2) and optically stimulated luminescence (OSL) detectors were inserted in the target of the phantom to measure the dose during radiation delivery (Varian Truebeam) and results were compared to planning dose parameters. Results: The Gamma analysis (3%/3mm) between measured dose using EBT2 film and calculated dose using AIP was 80.5%, indicating substantial dosimetric differences. While the Gamma analysis (3%/3mm) between measured dose using EBT2 and accumulated dose using 4D-CT was 98.9%, indicating the necessity of dose accumulation using 4D-CT. The measured doses using OSL and theoretically calculated doses using probability distribution function at the corresponding position were comparable. Conclusion: Use of static dose calculation in the treatment planning system could substantially underestimate the actually delivered dose in the MD region for a moving target. Funding Supported by NSFC, No.81372436

SU‐E‐T‐451: Hybrid‐VMAT: A Novel Technique Combining VMAT and 3D in Planning Whole Breast Radiotherapy with a Simultaneously‐Integrated Boost (WBRT+SIB)

Purpose:The RTOG 1005 trial offered a hypofractionated arm in delivering WBRT+SIB. Traditionally, treatments were planned at our institution using field‐in‐field (FiF) tangents with a concurrent 3D conformal boost. With the availability of VMAT, it is possible that a hybrid VMAT‐3D planning technique could provide another avenue in treating WBRT+SIB.Methods:A retrospective study of nine patients previously treated using RTOG 1005 guidelines was performed to compare FiF+3D plans with the hybrid technique. A combination of static tangents and partial VMAT arcs were used in base‐dose optimization. The hybrid plans were optimized to deliver 4005cGy to the breast PTVeval and 4800cGy to the lumpectomy PTVeval over 15 fractions. Plans were optimized to meet the planning goals dictated by RTOG 1005.Results:Hybrid plans yielded similar coverage of breast and lumpectomy PTVs (average D95 of 4013cGy compared to 3990cGy for conventional), while reducing the volume of high dose within the breast; the average D30 and D50 for the hybrid technique were 4517cGy and 4288cGy, compared to 4704cGy and 4377cGy for conventional planning. Hybrid plans increased conformity as well, yielding CI95% values of 1.22 and 1.54 for breast and lumpectomy PTVeval volumes; in contrast, conventional plans averaged 1.49 and 2.27, respectively. The nearby organs at risk (OARs) received more low dose with the hybrid plans due to low dose spray from the partial arcs, but all hybrid plans did meet the acceptable constraints, at a minimum, from the protocol. Treatment planning time was also reduced, as plans were inversely optimized (VMAT) rather than forward optimized.Conclusion:Hybrid‐VMAT could be a solution in delivering WB+SIB, as plans yield very conformal treatment plans and maintain clinical standards in OAR sparing. For treating breast cancer patients with a simultaneously‐integrated boost, Hybrid‐VMAT offers superiority in dosimetric conformity and planning time as compared to FIF techniques.

SU‐E‐T‐567: Neutron Dose Equivalent Evaluation for Pencil Beam Scanning Proton Therapy with Apertures

Purpose:To determine the neutron contamination from the aperture in pencil beam scanning during proton therapy.Methods:A Monte Carlo based proton therapy research platform TOPAS and the UF‐series hybrid pediatric phantoms were used to perform this study. First, pencil beam scanning (PBS) treatment pediatric plans with average spot size of 10 mm at iso‐center were created and optimized for three patients with and without apertures. Then, the plans were imported into TOPAS. A scripting method was developed to automatically replace the patient CT with a whole body phantom positioned according to the original plan iso‐center. The neutron dose equivalent was calculated using organ specific quality factors for two phantoms resembling a 4‐ and 14‐years old patient.Results:The neutron dose equivalent generated by the apertures in PBS is 4–10% of the total neutron dose equivalent for organs near the target, while roughly 40% for organs far from the target. Compared to the neutron dose equivalent caused by PBS without aperture, the results show that the neutron dose equivalent with aperture is reduced in the organs near the target, and moderately increased for those organs located further from the target. This is due to the reduction of the proton dose around the edge of the CTV, which causes fewer neutrons generated in the patient.Conclusion:Clinically, for pediatric patients, one might consider adding an aperture to get a more conformal treatment plan if the spot size is too large. This work shows the somewhat surprising fact that adding an aperture for beam scanning for facilities with large spot sizes reduces instead of increases a potential neutron background in regions near target.Changran Geng is supported by the Chinese Scholarship Council (CSC) and the National Natural Science Foundation of China (Grant No. 11475087).

SU‐E‐T‐183: Clinical Quality Assurance Workflow for Dynamic Tumor Tracking Radiation Dose Delivery

Purpose:One of the most important aspects of implementation of new treatment modalities is an ‘end‐to‐end’ verification of the treatment process. Radiation treatment based on dynamic tracking of a tumor is highly patient‐specific, therefore, special attention should be paid to quality assurance of the treatment delivery. Our goal was to design the clinical workflow that ensures accurate delivery of the planned dose using the Dynamic Target Tracking option of VeroTM (BrainLab,MHI) linac.Methods:A patient simulation is designed to include a pre‐treatment session to verify whether the system can reliably track the motion of the implanted marker and build the 4D model of the target motion. The external surrogate and target motion patterns are recorded in the ExactracTM log files. In this work, a spectrum of custom marker and external surrogate motion trajectories closely resembling the patient specific motion patterns was used. 1mm thick/11mm long VisicoilTM marker was placed 15 and 20mm from the center of the spherical tissue equivalent target (centroid to centroid distance) in the 4D motion phantom (CIRSTM). 3D conformal (3 mm block margin) SBRT plans were delivered to 2 moving targets in the phantom: 1) 20mm diameter target that allows ion chamber dose measurement and 2) 25mm target that allows using film to measure CAX dose (GafchromicTM EBT3 used). The measured dose was compared to the iPlanTM TPS results using MonteCarlo algorithm (1% variance, Dose‐to‐water).Results:On average, film shows 98.9% pass using gamma criterion for 2% and 2mm DTA, 94.3% match for 2% and 1 mm DTA, 98% pass for 1% and 2 mm DTA however only 88% points passing for 1% and 1 mm DTA. Ion chamber measurements agreed with the calculation within 1.5%.Conclusion:The clinical QA workflow was designed for SBRT delivery using real‐time tumor tracking on VeroTM linac.

SU‐F‐BRD‐02: Incorporating Uncertainty Analysis in Plan Comparison of VMAT, Double Scattering Proton Plan and IMPT for Lung Irradiation

Purpose:We investigate the effect of residual setup and motion errors in lung irradiation for VMAT, double scattering (DS) proton beams and spot scanning (IMPT) in a case study.Methods:The CT image and contour sets of a lung patient treated with 6 MV VMAT is re‐planned with DS as well as IMPT subject to the same constraints; V20(lung), V10(lung) and V5(lung)&lt; 15%, 20% and 25% respectively, V20(heart)&lt;25% and V100%(PTV)≥95%. In addition, uncertainty analysis in the form of isocenter shifts (±1–3mm) was incorporated in the DVH calculations to assess the plan robustness.Results:Only the IMPT plan satisfies all the specified constraints. The 3D‐conformal DS proton plan is able to achieve better sparing of the lung and heart dose compared to VMAT. For the lung, V20, V10 and V5 are 13%, 19% and 25% respectively for IMPT, 18%, 23% and 30% respectively for DS, and 20%, 30% and 42% respectively for VMAT. For heart: 0.6% for IMPT, 2.4% for DS and 30% for VMAT. When incorporating isocenter shifts in DVH calculations, the maximum changes in V20, V10 and V5 for lung are 14%, 21% and 28% respectively for IMPT. The corresponding max changes are19%, 24% and 32% respectively for DS, and 22%, 32% and 44% respectively for VMAT. The largest change occurs in the PTV coverage. For IMPT, V100%(PTV) varies between 88–96%, while V100%(PTV) for VMAT suffers a larger change compared to DS (Δ=5.5% vs 3.3%).Conclusion:While only IMPT satisfies the stringent dose‐volume constraints for the lung irradiation, it is not as robust as the 3D conformal DS plan. DS also has better sparing in lung and heart compared to VMAT and similar PTV coverage. By including isocenter shifts in dose‐volume calculations in treatment planning of lung, DS appears to be more robust than VMAT.

SU‐E‐T‐505: Inter‐Comparison of Clinical Plans Created On Truebeam with Trilogy and Clinac

PurposeThe purpose of this study is to investigate the dosimetric differences in clinical treatment plans when machine parameters are switched from Varian Truebeam to older Varian models and vice‐versa. This is used to determine the clinically‐safe limits to directly transfer patients from Truebeam to Trilogy or Clinac.MethodsThirty‐two patients with cancer of different treatment sites such as lung, head‐and‐neck, prostate and breast were studied. The clinically approved IMRT, VMAT or 3D conformal treatment plans were delivered either on Truebeam or Clinac/Trilogy. Keeping the monitor units, fraction size and other machine parameters the same, the plans were recomputed on a different Varian linear accelerator (e.g., Truebeam plans were recomputed on Trilogy and vice‐versa). The plans comparison was done using the target D98 and normal organ D2, D10, D20, D30, D50 and D70 dose metrics.ResultsThe perfraction dose‐difference in the PTV‐D98 for all 32 patients varied from 0.01 to −0.06 Gy with median being −0.04 Gy. For a 2Gy/fraction treatment course, this would be maximum PTV‐D98 dose‐difference of 6 cGy/fraction or 30 cGy/5fractions. For organs‐at‐risks the maximum per‐fraction dosedifference in D2, D5, D10, D20, D30, D50 and D70 between Truebeam and Trilogy/Clinac plans varied from 0.06 to −0.07 Gy with median being with — 0.02 Gy. The 3D‐CRT plans had relatively lower dose‐difference in comparison to IMRT and VMAT plans. The dose computed on Truebeam was systematically lower than the dose computed on Trilogy/ Clinac.Conclusionsfor conventional fractionation schedules, assuming the maximum uncertainty of &lt;2%, it is clinically safe to switch the treatment machine for 5 fractions. For hypo‐fractionated treatments with higher dose per‐fraction, plans may need to be revisited before switching the linear accelerator from Truebeam to Clinac/Trilogy or vice‐versa.

SU‐E‐T‐573: Normal Tissue Dose Effect of Prescription Isodose Level Selection in Lung Stereotactic Body Radiation Therapy

Purpose:To evaluate dose fall‐off in normal tissue for lung stereotactic body radiation therapy (SBRT) cases planned with different prescription isodose levels (IDLs), by calculating the dose dropping speed (DDS) in normal tissue on plans computed with both Pencil Beam (PB) and Monte‐Carlo (MC) algorithms.Methods:The DDS was calculated on 32 plans for 8 lung SBRT patients. For each patient, 4 dynamic conformal arc plans were individually optimized for prescription isodose levels (IDL) ranging from 60% to 90% of the maximum dose with 10% increments to conformally cover the PTV. Eighty non‐overlapping rind structures each of 1mm thickness were created layer by layer from each PTV surface. The average dose in each rind was calculated and fitted with a double exponential function (DEF) of the distance from the PTV surface, which models the steep‐ and moderate‐slope portions of the average dose curve in normal tissue. The parameter characterizing the steep portion of the average dose curve in the DEF quantifies the DDS in the immediate normal tissue receiving high dose. Provided that the prescription dose covers the whole PTV, a greater DDS indicates better normal tissue sparing. The DDS were compared among plans with different prescription IDLs, for plans computed with both PB and MC algorithms.Results:For all patients, the DDS was found to be the lowest for 90% prescription IDL and reached a highest plateau region for 60% or 70% prescription. The trend was the same for both PB and MC plans.Conclusion:Among the range of prescription IDLs accepted by lung SBRT RTOG protocols, prescriptions to 60% and 70% IDLs were found to provide best normal tissue sparing.