J. Orban de Xivry

Experimental assessment of proton dose calculation accuracy in inhomogeneous media

PURPOSE Proton therapy with Pencil Beam Scanning (PBS) has the potential to improve radiotherapy treatments. Unfortunately, its promises are jeopardized by the sensitivity of the dose distributions to uncertainties, including dose calculation accuracy in inhomogeneous media. Monte Carlo dose engines (MC) are expected to handle heterogeneities better than analytical algorithms like the pencil-beam convolution algorithm (PBA). In this study, an experimental phantom has been devised to maximize the effect of heterogeneities and to quantify the capability of several dose engines (MC and PBA) to handle these. METHODS An inhomogeneous phantom made of water surrounding a long insert of bone tissue substitute (1×10×10 cm3) was irradiated with a mono-energetic PBS field (10×10 cm2). A 2D ion chamber array (MatriXX, IBA Dosimetry GmbH) lied right behind the bone. The beam energy was such that the expected range of the protons exceeded the detector position in water and did not attain it in bone. The measurement was compared to the following engines: Geant4.9.5, PENH, MCsquare, as well as the MC and PBA algorithms of RayStation (RaySearch Laboratories AB). RESULTS For a γ-index criteria of 2%/2mm, the passing rates are 93.8% for Geant4.9.5, 97.4% for PENH, 93.4% for MCsquare, 95.9% for RayStation MC, and 44.7% for PBA. The differences in γ-index passing rates between MC and RayStation PBA calculations can exceed 50%. CONCLUSION The performance of dose calculation algorithms in highly inhomogeneous media was evaluated in a dedicated experiment. MC dose engines performed overall satisfactorily while large deviations were observed with PBA as expected.

Dose distribution for gynecological brachytherapy with dose accumulation between insertions: Feasibility study

PURPOSE For gynecological treatments, it is standard to acquire CT images and preferably also MR images before each treatment to calculate the dose of the day. The dose of the complete treatment is calculated by adding the dose metrics of each fraction. It makes the conservative assumption that the same part of the organs at risk always receives the highest dose. The dose calculated this way often limits the prescription dose or the target coverage. We investigated the use of deformable image registration (DIR) as an alternative method to assess the cumulative dose for a treatment course. METHODS AND MATERIALS Rigid registration is preformed on CT images, followed by DIR. DIR can be based either solely on the three-dimensional images or combined with organ contours. To improve DIR in the pelvic region with low CT contrast, we propose (1) using contours drawn on CT or (2) modifying artificially the contrast in certain volumes. The dose matrix from fraction_n (n > 1) is deformed using a calculated deformation field. RESULTS The use of the contrast-enhanced images or of contour information helps to guide the DIR. However, because of the very high dose gradients involved in brachytherapy, the uncertainty on the accumulated dose remains of the order of 5-10%. Even for good contour matching, a small local error in the deformation can have significant consequences for the dose distribution. CONCLUSIONS Using DIR, based on image features and contours, allows to accumulate the dose from different brachytherapy fractions. A robust validation procedure should be developed.

TH‐CD‐BRA‐04: Assessing How Stochastic CT Noise Can Lead to Systematic Proton Range Errors

Purpose: To demonstrate that the discontinuous nature of the CT number to stopping power ratio (SPR) calibration curve, combined with the presence of uncorrelated zero-mean Gaussian CT noise, leads to non-negligible and tissue-dependent systematic errors in SPRs and proton range, typically not taken into account in usual safety margins for proton therapy. Methods: Increased systematic errors with noise standard deviation have first been observed in proton range Monte-Carlo simulations with stoichiometric calibrations, whereas only zero-mean random errors were expected. Their existence has then been proved analytically for arbitrary calibration curves and material distributions along the proton path and validated through continuous slowing down approximation (CSDA) simulations. Their importance relative to the other sources of uncertainty has then been estimated in head-and-neck, lung, and pelvis patient data for multiple beam orientations. CT noise has first been reduced using a double-pass median filtering approach and a Gaussian noise has then been added to obtain total standard deviations between 10 to 40 HU. Results: This study provides close form equations for the systematic error and uncertainty on SPR and proton range due to uncorrelated noise. They have shown to accurately match CSDA simulation results with realistic calibration curves and material distributions. Depending on the tissue distribution and the position of the discontinuities along the curve the resulting effect on range varies but has shown never to cancel out completely as opposed to common beliefs. The analysis performed on patient data with clinical calibration curves has confirmed that fact with estimated systematic range errors of 0.2–0.5% and uncertainties (4 σ) between 0.5 and 1% with typical CT noise levels. Conclusion: A new source of SPR and range systematic errors has been highlighted and proved not to be negligible compared to the 3.5% uncertainty reference value used for safety margin design This study is linked to a public partnership between UCL and IBA funded by the Walloon region under convention number 1017266 and 1217662

SU-F-BRD-15: Quality Correction Factors in Scanned Or Broad Proton Therapy Beams Are Indistinguishable

Purpose: The IAEA TRS-398 code of practice details the reference conditions for reference dosimetry of proton beams using ionization chambers and the required beam quality correction factors (kQ). Pencil beam scanning (PBS) requires multiple spots to reproduce the reference conditions. The objective is to demonstrate, using Monte Carlo (MC) calculations, that kQ factors for broad beams can be used for scanned beams under the same reference conditions with no significant additional uncertainty. We consider hereafter the general Alfonso formalism (Alfonso et al, 2008) for non-standard beam. Methods: To approach the reference conditions and the associated dose distributions, PBS must combine many pencil beams with range modulation and shaping techniques different than those used in passive systems (broad beams). This might lead to a different energy spectrum at the measurement point. In order to evaluate the impact of these differences on kQ factors, ion chamber responses are computed with MC (Geant4 9.6) in a dedicated scanned pencil beam (Q_pcsr) producing a 10×10cm2 composite field with a flat dose distribution from 10 to 16 cm depth. Ion chamber responses are also computed by MC in a broad beam with quality Q_ds (double scattering). The dose distribution of Q _pcsr matches the dose distribution of Q_ds. k_(Q_pcsr,Q_ds) is computed for a 2×2×0.2cm3 idealized air cavity and a realistic plane-parallel ion chamber (IC). Results: Under reference conditions, quality correction factors for a scanned composite field versus a broad beam are the same for air cavity dose response, k_(Q_pcsr,Q_ds) =1.001±0.001 and for a Roos IC, k_(Q_pcsr,Q_ds) =0.999±0.005. Conclusion: Quality correction factors for ion chamber response in scanned and broad proton therapy beams are identical under reference conditions within the calculation uncertainties. The results indicate that quality correction factors published in IAEA TRS-398 can be used for scanned beams in the SOBP of a high-energy proton beam. Jefferson Sorriaux is financed by the Walloon Region under the convention 1217662. Jefferson Sorriaux is sponsored by a public-private partnership IBA - Walloon Region

SU-E-T-464: On the Equivalence of the Quality Correction Factor for Pencil Beam Scanning Proton Therapy

PURPOSE In current practice, most proton therapy centers apply IAEA TRS-398 reference dosimetry protocol. Quality correction factors (kQ) take into account in the dose determination process the differences in beam qualities used for calibration unit and for treatment unit. These quality correction factors are valid for specific reference conditions. TRS-398 reference conditions should be achievable in both scattered proton beams (i.e. DS) and scanned proton beams (i.e. PBS). However, it is not a priori clear if TRS-398 kQ data, which are based on Monte Carlo (MC) calculations in scattered beams, can be used for scanned beams. Using TOPAS-Geant4 MC simulations, the study aims to determine whether broad beam quality correction factors calculated in TRS-398 can be directly applied to PBS delivery modality. METHODS As reference conditions, we consider a 10×10×10 cm3 homogeneous dose distribution delivered by PBS system in a water phantom (32/10 cm range/modulation) and an air cavity placed at the center of the spread-out-Bragg-peak. In order to isolate beam differences, a hypothetical broad beam is simulated. This hypothetical beam reproduces exactly the same range modulation, and uses the same energy layers than the PBS field. Ion chamber responses are computed for the PBS and hypothetical beams and then compared. RESULTS For an air cavity of 2×2×0.2 cm3 , the ratio of ion chamber responses for the PBS and hypothetical beam qualities is 0.9991 ± 0.0016. CONCLUSION Quality correction factors are insensitive to the delivery pattern of the beam (broad beam or PBS), as long as similar dose distributions are achieved. This investigation, for an air cavity, suggests that broad beam quality correction factors published in TRS-398 can be applied for scanned beams. J. Sorriaux is financially supported by a public-private partnership involving the company Ion Beam Applications (IBA).

Combined influence of CT random noise and HU-RSP calibration curve nonlinearities on proton range systematic errors

Proton range random and systematic uncertainties are the major factors undermining the advantages of proton therapy, namely, a sharp dose falloff and a better dose conformality for lower doses in normal tissues. The influence of CT artifacts such as beam hardening or scatter can easily be understood and estimated due to their large-scale effects on the CT image, like cupping and streaks. In comparison, the effects of weakly-correlated stochastic noise are more insidious and less attention is drawn on them partly due to the common belief that they only contribute to proton range uncertainties and not to systematic errors thanks to some averaging effects. A new source of systematic errors on the range and relative stopping powers (RSP) has been highlighted and proved not to be negligible compared to the 3.5% uncertainty reference value used for safety margin design. Hence, we demonstrate that the angular points in the HU-to-RSP calibration curve are an intrinsic source of proton range systematic error for typical levels of zero-mean stochastic CT noise. Systematic errors on RSP of up to 1% have been computed for these levels. We also show that the range uncertainty does not generally vary linearly with the noise standard deviation. We define a noise-dependent effective calibration curve that better describes, for a given material, the RSP value that is actually used. The statistics of the RSP and the range continuous slowing down approximation (CSDA) have been analytically derived for the general case of a calibration curve obtained by the stoichiometric calibration procedure. These models have been validated against actual CSDA simulations for homogeneous and heterogeneous synthetical objects as well as on actual patient CTs for prostate and head-and-neck treatment planning situations.

PO-0783: Design of cone-beam CT for proton therapy gantry

Purpose/Objective: Cone-beam CT is widely used in radiation therapy proving tremendous interest for patient positioning and daily monitoring of anatomical modifications. Moreover, the use of volumetric imaging can contribute to a more accurate location of the target volume which is important in proton therapy (PT) where sharp dose gradients are used. Despite this potential advantage, integrating a CBCT on a PT gantry has not already been done and poses additional challenges such as potentially larger geometrical deformations compared to a LINAC (Table 1). In this paper, we describe how the first CBCT has been implemented in a PT gantry.The general design, choice of hardware, and software implementation are outlined and results on a phantom demonstrate the possibilities of the system.

SU-E-J-125: Classification of CBCT Noises in Terms of Their Contribution to Proton Range Uncertainty

PURPOSE This study assesses the potential use of CBCT images in adaptive protontherapy by estimating the contribution of the main sources of noise and calibration errors to the proton range uncertainty. METHODS Measurements intended to highlight each particular source have been achieved by adapting either the testbench configuration, e.g. use of filtration, fan-beam collimation, beam stop arrays, phantoms and detector reset light, or the sequence of correction algorithms including water precorrection. Additional Monte-Carlo simulations have been performed to complement these measurements, especially for the beam hardening and the scatter cases. Simulations of proton beams penetration through the resulting images have then been carried out to quantify the range change due to these effects. The particular case of a brain irradiation is considered mainly because of the multiple effects that the skull bones have on the internal soft tissues. RESULTS On top of the range error sources is the undercorrection of scatter. Its influence has been analyzed from a comparison of fan-beam and full axial FOV acquisitions. In this case, large range errors of about 12 mm can be reached if the assumption is made that the scatter has only a constant contribution over the projection images. Even the detector lag, which a priori induces a much smaller effect, has been shown to contribute for up to 2 mm to the overall error if its correction only aims at reducing the skin artefact. This last result can partially be understood by the larger interface between tissues and bones inside the skull. CONCLUSION This study has set the basis of a more systematical analysis of the effect CBCT noise on range uncertainties based on a combination of measurements, simulations and theoretical results. With our method, even more subtle effects such as the cone-beam artifact or the detector lag can be assessed. SBR and JOR are financed by iMagX, a public-private partnership between the region Wallone of Belgium and IBA under convention #1217662.

SU-E-T-435: Development and Commissioning of a Complete System for In-Vivo Dosimetry and Range Verification in Proton Therapy

PURPOSE In-vivo dose and beam range verification in proton therapy could play significant roles in proton treatment validation and improvements. Invivo beam range verification, in particular, could enable new treatment techniques one of which, for example, could be the use of anterior fields for prostate treatment instead of opposed lateral fields as in current practice. We have developed and commissioned an integrated system with hardware, software and workflow protocols, to provide a complete solution, simultaneously for both in-vivo dosimetry and range verification for proton therapy. METHODS The system uses a matrix of diodes, up to 12 in total, but separable into three groups for flexibility in application. A special amplifier was developed to capture extremely small signals from very low proton beam current. The software was developed within iMagX, a general platform for image processing in radiation therapy applications. The range determination exploits the inherent relationship between the internal range modulation clock of the proton therapy system and the radiological depth at the point of measurement. The commissioning of the system, for in-vivo dosimetry and for range verification was separately conducted using anthropomorphic phantom. EBT films and TLDs were used for dose comparisons and range scan of the beam distal fall-off was used as ground truth for range verification. RESULTS For in-vivo dose measurement, the results were in agreement with TLD and EBT films and were within 3% from treatment planning calculations. For range verification, a precision of 0.5mm is achieved in homogeneous phantoms, and a precision of 2mm for anthropomorphic pelvic phantom, except at points with significant range mixing. CONCLUSION We completed the commissioning of our system for in-vivo dosimetry and range verification in proton therapy. The results suggest that the system is ready for clinical trials on patient.

TH‐C‐144‐09: Workflow of the in Vivo Range Verification in Proton Therapy

PURPOSE A practical and effective range verification technique for proton treatments is the current need to harness the entire benefits of proton therapy. We developed a new technique to compute the proton beam range in the double scattering mode using the unique time-dose patterns produced by the steps in the range modulator wheel.We present the integration of the range verification system for treatments of prostate cancer in the AP field configuration with the standard proton therapy workflow. We also present the status of the latest detector design, the various steps of the clinical workflow and the integration of the logical modules for a safe and effective range guided treatment of prostate cancer with AP field. METHODS The method uses a matrix of diodes as dosimeters designed to be inserted in the rectum using a rectal balloon. The diodes measure the time-dose patterns for a range-check beam. An analysis routine calculates the WEPL at each diode position and evaluates if the signals are range mixed. The obtained WEPL values are compared with treatment planning for beam range verification and correction and the treatment proceeds. During the treatment, the same detectors are used to monitor the dose to the rectum. The standard workflow for proton therapy is modified to adapt for this range verification procedure by adding the above processes. RESULTS The validation study on a pelvic phantom proved that the range could be verified with an accuracy of 2 mm with just 0.2 % of the fractionated dose. This workflow enables treatment of prostate cancer albeit sparing the rectum. CONCLUSION The range verification system will improve treatment quality for prostate cancers using AP fields. With the results from clinical trials, we hope this simple method will find uses in other proton treatments where a range verification system would be advantageous.