B Gill

Monte Carlo based, patient-specific RapidArc QA using Linac log files

PURPOSE A Monte Carlo (MC) based QA process to validate the dynamic beam delivery accuracy for Varian RapidArc (Varian Medical Systems, Palo Alto, CA) using Linac delivery log files (DynaLog) is presented. Using DynaLog file analysis and MC simulations, the goal of this article is to (a) confirm that adequate sampling is used in the RapidArc optimization algorithm (177 static gantry angles) and (b) to assess the physical machine performance [gantry angle and monitor unit (MU) delivery accuracy]. METHODS Ten clinically acceptable RapidArc treatment plans were generated for various tumor sites and delivered to a water-equivalent cylindrical phantom on the treatment unit. Three Monte Carlo simulations were performed to calculate dose to the CT phantom image set: (a) One using a series of static gantry angles defined by 177 control points with treatment planning system (TPS) MLC control files (planning files), (b) one using continuous gantry rotation with TPS generated MLC control files, and (c) one using continuous gantry rotation with actual Linac delivery log files. Monte Carlo simulated dose distributions are compared to both ionization chamber point measurements and with RapidArc TPS calculated doses. The 3D dose distributions were compared using a 3D gamma-factor analysis, employing a 3%/3 mm distance-to-agreement criterion. RESULTS The dose difference between MC simulations, TPS, and ionization chamber point measurements was less than 2.1%. For all plans, the MC calculated 3D dose distributions agreed well with the TPS calculated doses (gamma-factor values were less than 1 for more than 95% of the points considered). Machine performance QA was supplemented with an extensive DynaLog file analysis. A DynaLog file analysis showed that leaf position errors were less than 1 mm for 94% of the time and there were no leaf errors greater than 2.5 mm. The mean standard deviation in MU and gantry angle were 0.052 MU and 0.355 degrees, respectively, for the ten cases analyzed. CONCLUSIONS The accuracy and flexibility of the Monte Carlo based RapidArc QA system were demonstrated. Good machine performance and accurate dose distribution delivery of RapidArc plans were observed. The sampling used in the TPS optimization algorithm was found to be adequate.

A Monte Carlo model of the Varian IGRT couch top for RapidArc QA

The objectives of this study are to evaluate the effect of couch attenuation on quality assurance (QA) results and to present a couch top model for Monte Carlo (MC) dose calculation for RapidArc treatments. The IGRT couch top is modelled in Eclipse as a thin skin of higher density material with a homogeneous fill of foam of lower density and attenuation. The IGRT couch structure consists of two longitudinal sections referred to as thick and thin. The Hounsfield Unit (HU) characterization of the couch structure was determined using a cylindrical phantom by comparing ion chamber measurements with the dose predicted by the treatment planning system (TPS). The optimal set of HU for the inside of the couch and the surface shell was found to be respectively -960 and -700 HU in agreement with Vanetti et al (2009 Phys. Med. Biol. 54 N157-66). For each plan, the final dose calculation was performed with the thin, thick and without the couch top. Dose differences up to 2.6% were observed with TPS calculated doses not including the couch and up to 3.4% with MC not including the couch and were found to be treatment specific. A MC couch top model was created based on the TPS geometrical model. The carbon fibre couch top skin was modelled using carbon graphite; the density was adjusted until good agreement with experimental data was observed, while the density of the foam inside was kept constant. The accuracy of the couch top model was evaluated by comparison with ion chamber measurements and TPS calculated dose combined with a 3D gamma analysis. Similar to the TPS case, a single graphite density can be used for both the thin and thick MC couch top models. Results showed good agreement with ion chamber measurements (within 1.2%) and with TPS (within 1%). For each plan, over 95% of the points passed the 3D gamma test.

Monte Carlo calculation of dose distribution in early stage NSCLC patients planned for accelerated hypofractionated radiation therapy in the NCIC-BR25 protocol.

The dosimetric consequences of plans optimized using a commercial treatment planning system (TPS) for hypofractionated radiation therapy are evaluated by re-calculating with Monte Carlo (MC). Planning guidelines were in strict accordance with the Canadian BR25 protocol which is similar to the RTOG 0236 and 0618 protocols in patient eligibility and total dose, but has a different hypofractionation schedule (60 Gy in 15 fractions versus 60 Gy in 3 fractions). A common requirement of the BR25 and RTOG protocols is that the dose must be calculated by the TPS without tissue heterogeneity (TH) corrections. Our results show that optimizing plans using the pencil beam algorithm with no TH corrections does not ensure that the BR25 planning constraint of 99% of the PTV receiving at least 95% of the prescription dose would be achieved as revealed by MC simulations. This is due to poor modelling of backscatter and lateral electronic equilibrium by the TPS. MC simulations showed that as little as 75% of the PTV was actually covered by the 95% isodose line. The under-dosage of the PTV was even more pronounced if plans were optimized with the TH correction applied. In the most extreme case, only 23% of the PTV was covered by the 95% isodose.

A novel 3D‐printed phantom insert for 4D PET/CT imaging and simultaneous integrated boost radiotherapy

Purpose: To construct a 3D‐printed phantom insert designed to mimic the variable PET tracer uptake seen in lung tumor volumes and a matching dosimetric insert to be used in simultaneous integrated boost (SIB) phantom studies, and to evaluate the design through end‐to‐end tests. Methods: A set of phantom inserts was designed and manufactured for a realistic representation of gated radiotherapy steps from 4D PET/CT scanning to dose delivery. A cylindrical phantom (Φ80 × 120 mm) holds inserts for PET/CT scanning. The novel 3D printed insert dedicated to 4D PET/CT mimics high PET tracer uptake in the core and low uptake in the periphery. This insert is a variable density porous cylinder (Φ44.5 × 70.0 mm), ABS‐P430 thermoplastic, 3D printed by fused deposition modeling an inner (Φ11 × 42 mm) cylindrical void. The square pores (1.8 × 1.8 mm2 each) fill 50% of outer volume, resulting in a 2:1 PET tracer concentration ratio in the void volume with respect to porous volume. A matching cylindrical phantom insert is dedicated to validate gated radiotherapy. It contains eight peripheral holes and one central hole, matching the location of the porous part and the void part of the 3D printed insert, respectively. These holes accommodate adaptors for Farmer‐type ion chamber and cells vials. End‐to‐end tests were designed for imaging, planning, and dose measurements. Results: End‐to‐end test were performed from 4D PET/CT scanning to transferring data to the planning system, target volume delineation, and dose measurements. 4D PET/CT scans were acquired of the phantom at different respiratory motion patterns and gating windows. A measured 2:1 18F‐FDG concentration ratio between inner void and outer porous volume matched the 3D printed design. Measured dose in the dosimetric insert agreed well with planned dose on the imaging insert, within 3% for the static phantom and within 5% for most breathing patterns. Conclusions: The novel 3D printed phantom insert mimics variable PET tracer uptake typical of tumors. Obtained 4D PET/CT scans are suitable for segmentation and treatment planning and delivery in SIB gated treatments. Our experiments demonstrate the feasibility of this set of phantom inserts serving as end‐to‐end quality‐assurance phantoms of SIB radiotherapy.

Rapid Prototyping, Design and Early Testing of a Novel Device for Supine Positioning of Large Volume or Pendulous Breasts in Radiotherapy

Here we describe the development of a novel device for breast positioning in supine radiotherapy that reduces breast sag and skin folds for patients with large or pendulous breasts. The overall aim of this work is to provide a practical and robust means of reducing high grade skin toxicity (moist desquamation) which tends to occur in skin folds. Participants with breast cup size D or greater were recruited to this ethics board approved prototype design study. Brassiere size, cup size, breast diameter, body mass index, height, weight, skin folds and torso dimensions were measured. Participants were positioned in treatment position on a breast board, with arms above the head and skin folds were identified and measured. 3D optical surface imaging provided initial design ideas and a rapid prototyping process using 3D printing was employed to arrive at a suitable design. The final clinical device consists of a curved carbon fibre breast support scoop suspended from a rigid frame that is compatible with commercially available breast boards. In addition to reducing skin folds, the device better positions the breast on the chest wall to help minimize the volume of normal tissue being irradiated and facilitates rapid setup. We present results of preliminary testing of the device, including dose buildup incurred by the carbon fibre scoop, skin fold reduction data and treatment planning data from CT simulations with and without the device. Surface dose with the device in place remains less than 80% of the prescription dose to the breast. Skin folds were reduced and reductions in irradiated volumes of lung and body were achieved compared with clinical plans without the supportive device. The novel breast support shows great potential to address a long-standing problem for a significant population of patients undergoing radiotherapy for breast cancer.

MO‐FG‐BRA‐05: Dosimetric and Radiobiological Validation of Respiratory Gating in Conventional and Hypofractionated Radiotherapy of the Lung: Effect of Dose, Dose Rate, Gating Window and Breathing Pattern

PURPOSE to evaluate the dosimetric and radiobiological consequences from having different gating windows, dose rates, and breathing patterns in gated VMAT lung radiotherapy. METHODS A novel 3D-printed moving phantom with central high and peripheral low tracer uptake regions was 4D FDG-PET/CT-scanned using ideal, patient-specific regular, and irregular breathing patterns. A scan of the stationary phantom was obtained as a reference. Target volumes corresponding to different uptake regions were delineated. Simultaneous integrated boost (SIB) 6 MV VMAT plans were produced for conventional and hypofractionated radiotherapy, using 30-70 and 100% cycle gating scenarios. Prescribed doses were 200 cGy with SIB to 240 cGy to high uptake volume for conventional, and 800 with SIB to 900 cGy for hypofractionated plans. Dose rates of 600 MU/min (conventional and hypofractionated) and flattening filter free 1400 MU/min (hypofractionated) were used. Ion chamber measurements were performed to verify delivered doses. Vials with A549 cells placed in locations matching ion chamber measurements were irradiated using the same plans to measure clonogenic survival. Differences in survival for the different doses, dose rates, gating windows, and breathing patterns were analyzed. RESULTS Ion chamber measurements agreed within 3% of the planned dose, for all locations, breathing patterns and gating windows. Cell survival depended on dose alone, and not on gating window, breathing pattern, MU rate, or delivery time. The surviving fraction varied from approximately 40% at 2Gy to 1% for 9 Gy and was within statistical uncertainty relative to that observed for the stationary phantom. CONCLUSIONS Use of gated VMAT in PET-driven SIB radiotherapy was validated using ion chamber measurements and cell survival assays for conventional and hypofractionated radiotherapy.

SU‐E‐T‐52: A Monte Carlo Model of the Varian IGRT Couch for RapidArc

Purpose: To evaluate the impact of the Varian IGRT treatment couch on quality assurance results and to present a Monte Carlomodel of the IGRT couch that can be included on both a phantom and patient CT dataset. Methods: Three clinical RapidArc treatment plans were generated for various tumor sites. The correct set of Hounsfield Units for each part of the couch structure was determined experimentally. For each patient three verification plans were produced, one including the thick model of the couch, one including the thin model of the couch and one without any couch included. Monte Carlo dose calculations were performed in a water equivalent phantom for each of these plans. Various values of the physical densities for the couch shell were used. Results were compared to measurements using a calibrated Farmer ionization chamber with an active volume of 0.6 cm3. Comparison of RapidArc and Monte Carlo 3D doses was performed using a 3 dimensional Gamma‐factor analysis with a 3%/3mm DTA criteria. Results: Good agreement was observed between chamber measurements and the treatment planning system for each verification plans. Differences as high as 2.6% was observed between the calculated dose by the planning system with and without the couch. The optimal physical density to model the couch shell was determined to be 0.65g/cm3 giving the best agreement with both chamber measurements and the planning system with Gamma values below 1 in over 95% of the points considered for all plans. Conclusions: This work indicates the possibility to accurately model the Varian IGRT couch for Monte Carlo RapidArc simulations and to apply it on patient CT dataset.

SU‐GG‐T‐486: Sparing of Lung Function Using Perfusion SPECT Guided IMRT Treatment Planning for Lung Cancer Patients

Purpose: To develop a method for improving functional lung sparing utilizing perfusion SPECT information and Monte Carlo(MC)dose calculation in IMRToptimization of lungcancerradiotherapy.Methods and Materials: 99mTc‐macroaggregated albumin (MAA) SPECT scans were acquired before radiotherapy (RT) for five non‐small cell lungcancer patients treated with RT (60Gy / 30 fx). Perfusion SPECTimages were reconstructed with attenuation, scatter correction and rigidly registered with planning CT afterwards. Beamlets generated from MC simulation were incorporated into direct aperture optimization (DAO) of static gantry step and shoot IMRTtreatment planning. For each patient, three RT plans were generated. 1. Clinical plan: 3D‐CRT plan generated in vendors software which is clinically delivered. 2. DVH driven plan: IMRT plan generated using DAO and MCdose calculation without SPECT guidance. Conventional DVH constraints for targets and OARs were used in the treatment planning, 3. SPECT driven plan: IMRT plan generated using DAO and MCdose calculation with SPECT guidance. SPECT weighted mean dose (SWMD) and Equivalent uniform dose (EUD) constraints were incorporated into the objective function. SWMD constraints were applied to ipsilateral and contralateral lungs respectively as the metric of lung function sparing. EUD was adopted to optimize planning target volume (PTV) dose coverage. Results: 1. Compared to clinical plan, same target dose coverage was achieved in both DVH driven and SPECT driven plans. 95% iso‐dose line covered more than 97% of PTV in both plans. 2. Compared to DVH driven plans, in SPECT driven plans, V5 and V20 were reduced by ∼8% and ∼3% respectively, SWMD were reduced by ∼1Gy. Thus superior sparing of both lung function and volume was achieved. Conclusion: Comparing to conventional DVH driven IMRT plans, superior lung sparing of both anatomical and functional volumes can be achieved in SPECT driven IMRT planning based on EUD and SWMD.

Poster — Wed Eve—46: Quantitative Evaluation on the Accuracy of Image Registration Methods in SPECT Guided Radiation Therapy for Lung Cancer Patients

Purpose: To quantitatively evaluate the accuracy of several SPECT/CT image registration methods in recent studies and its impact on the functional lung volume segmentation in SPECT guided radiation therapy (RT) treatment planning.Methods and Materials: Five lungcancer patients were consented to have a perfusion SPECT scan with 99mTc‐macroaggregated albumin. During the scan, a low resolution CTimage was acquired using the SPECT/CT scanner. This CT scan was co‐registered to the patients planning CT scan through four rigid and deformable image registration programs (rigid registration, skin/lung control points based registration and B‐spline deformable registration). After the CT to CT co‐registration, original SPECT reconstructions were warped and co‐registered to the planning CT scan. The functional lung volumes were segmented from each deformed SPECT using 10, 20, …, 90% of maximum pixel value as a threshold. The differences in the size and contours of each functional volume were calculated. Results: Based on the evaluation of registered CTimages, the result from B‐spline registration demonstrated the smallest intensity difference. Using the warped SPECTimages obtained from this registration method as a reference, the smallest difference in the size and contour of functional volumes was found using rigid registration. In the point‐based registrations, a better result was found when the control points were placed on lung volume instead of body contour. Conclusion: Apply B‐spline based image registration method in SPECT‐guided RT studies was shown to be accurate. Point‐based image registration using skin markers with a standalone SPECTscanner was found least accurate.

SU‐FF‐J‐136: The Impact of Attenuation and Scatter Correction On the SPECT Guided Radiation Therapy for Lung Cancer Patients: Comparison of SPECT Weighted Mean Dose and Functional Lung Segmentation

Purpose: To investigate the impact of different types of image reconstruction and attenuation/scatter (A/S) correction on the calculation of dosimetric indices proposed to be used for Single Photon Emission Computed Tomography(SPECT) guided dose escalation in lungcancer patients. Methods and Materials:SPECTlung perfusion scans were obtained for nine lungcancer patients using 99mTc‐macroaggregated albumin. Four image sets were reconstructed from each scan: one using a vendor provided ordered subsets expectation maximization (OSEM) algorithm, two quantitative SPECTreconstructions using OSEM methods with different types of A/S corrections and the fourth an OSEM reconstruction without any A/S correction. SPECT weighted mean dose (SWMD), dose function histogram, and functional lung volume have been calculated from dose distributions and regional perfusion maps. To investigate the dependence of SWMDs on gantry angle, twelve equally spaced co‐planar open field radiation beams delivering the same MU were centered on the PTV. Three field sizes, 5×5, 7.5×7.5 and 10×10cm2 were considered. SWMDs were calculated for each field and reconstruction. Functional lung volumes were segmented in each reconstruction using 10, 20, …, 90% of maximum SPECT uptake as a threshold. Results: SWMDs calculated from reconstructions without A/S correction showed more than 5% average difference compared to those with corrections. With A/S corrections, more consistent SWMDs were found in all the three OSEM reconstructions (average difference ∼2%). However, a large variation was observed between segmented functional lung volumes and the V20 of these volumes in all four reconstructions. The difference between the volumes reached over 50% regardless of whether A/S correction was applied in the reconstruction.Conclusion: Functional volume segmentation is sensitive to the type of A/S correction. In contrast, SPECT weighted mean dose calculation produces more consistent results and appears to be a more robust choice for clinical outcome analysis and SPECT guided treatment planning.