Roberto Righetto

Comparison between a built-in “dual side” chest imaging device and a standard “single side” CR

An integrated readout computed radiography system (Fuji XU-D1) incorporating dual-side imaging plates (ST-55BD) was analyzed in terms of modulation transfer function (MTF), noise power spectrum (NPS), and detective quantum efficiency (DQE) for standard beam qualities RQA 9 and RQA 5. NPS and DQE were assessed using a detector entrance air kerma consistent with clinical practice for chest radiography. Similar investigation was performed on a standard reader (Fuji FCR 5000) using single-side imaging plates (ST-VI). Negligible differences were found between the MTFs of the two imaging systems for RQA 9, whereas for RQA 5 the single-side system exhibited slightly superior MTF. Regarding noise response, the dual-side system turned out to be better performing for both beam qualities over a wide range of frequencies. For RQA 9, at 8 microGy, the DQE of the dual-side system was moderately higher over the whole frequency range, whereas for RQA 5, at 10 microGy, significant improvement was found at low- and midrange frequencies. As an example, at 1 cycle/mm, the following improvements in the DQE of the dual-side system were observed: +22% (RQA 9, at 8 microGy), +50% (RQA 9, at 30 microGy), and +45% (RQA 5, at 10 microGy).

Supine craniospinal irradiation in pediatric patients by proton pencil beam scanning

BACKGROUND AND PURPOSE Proton therapy is the emerging treatment modality for craniospinal irradiation (CSI) in pediatric patients. Herein, special methods adopted for CSI at proton Therapy Center of Trento by pencil beam scanning (PBS) are comprehensively described. MATERIALS AND METHODS Twelve pediatric patients were treated by proton PBS using two/three isocenters. Special methods refer to: (i) patient positioning in supine position on immobilization devices crossed by the beams; (ii) planning field-junctions via the ancillary-beam technique; (iii) achieving lens-sparing by three-beams whole-brain-irradiation; (iv) applying a movable-snout and beam-splitting technique to reduce the lateral penumbra. Patient-specific quality assurance (QA) program was performed using two-dimensional ion chamber array and γ-analysis. Daily kilovoltage alignment was performed. RESULTS PBS allowed to obtain optimal target coverage (mean D98%>98%) with reduced dose to organs-at-risk. Lens sparing was obtained (mean D1∼730cGyE). Reducing lateral penumbra decreased the dose to the kidneys (mean Dmean<600cGyE). After kilovoltage alignment, potential dose deviations in the upper and lower junctions were small (average 0.8% and 1.2% respectively). Due to imperfect modeling of range shifter, QA showed better agreements between measurements and calculations at depths >4cm (mean γ>95%) than at depths<4cm. CONCLUSIONS The reported methods allowed to effectively perform proton PBS CSI.

Technical Note : A direct ray-tracing method to compute integral depth dose in pencil beam proton radiography with a multilayer ionization chamber

PURPOSE To introduce a fast ray-tracing algorithm in pencil proton radiography (PR) with a multilayer ionization chamber (MLIC) for in vivo range error mapping. METHODS Pencil beam PR was obtained by delivering spots uniformly positioned in a square (45 × 45 mm2 field-of-view) of 9 × 9 spots capable of crossing the phantoms (210 MeV). The exit beam was collected by a MLIC to sample the integral depth dose (IDDMLIC). PRs of an electron-density and of a head phantom were acquired by moving the couch to obtain multiple 45 × 45 mm2 frames. To map the corresponding range errors, the two-dimensional set of IDDMLIC was compared with (i) the integral depth dose computed by the treatment planning system (TPS) by both analytic (IDDTPS) and Monte Carlo (IDDMC) algorithms in a volume of water simulating the MLIC at the CT, and (ii) the integral depth dose directly computed by a simple ray-tracing algorithm (IDDdirect) through the same CT data. The exact spatial position of the spot pattern was numerically adjusted testing different in-plane positions and selecting the one that minimized the range differences between IDDdirect and IDDMLIC. RESULTS Range error mapping was feasible by both the TPS and the ray-tracing methods, but very sensitive to even small misalignments. In homogeneous regions, the range errors computed by the direct ray-tracing algorithm matched the results obtained by both the analytic and the Monte Carlo algorithms. In both phantoms, lateral heterogeneities were better modeled by the ray-tracing and the Monte Carlo algorithms than by the analytic TPS computation. Accordingly, when the pencil beam crossed lateral heterogeneities, the range errors mapped by the direct algorithm matched better the Monte Carlo maps than those obtained by the analytic algorithm. Finally, the simplicity of the ray-tracing algorithm allowed to implement a prototype procedure for automated spatial alignment. CONCLUSIONS The ray-tracing algorithm can reliably replace the TPS method in MLIC PR for in vivo range verification and it can be a key component to develop software tools for spatial alignment and correction of CT calibration.

Single-energy intensity modulated proton therapy.

In this note, an intensity modulated proton therapy (IMPT) technique, based on the use of high single-energy (SE-IMPT) pencil beams, is described.The method uses only the highest system energy (226 MeV) and only lateral penumbra to produce dose gradient, as in photon therapy. In the study, after a preliminary analysis of the width of proton pencil beam penumbras at different depths, SE-IMPT was compared with conventional IMPT in a phantom containing titanium inserts and in a patient, affected by a spinal chordoma with fixation rods.It was shown that SE-IMPT has the potential to produce a sharp dose gradient and that it is not affected by the uncertainties produced by metal implants crossed by the proton beams. Moreover, in the chordoma patient, target coverage and organ at risk sparing of the SE-IMPT plan resulted comparable to that of the less reliable conventional IMPT technique. Robustness analysis confirmed that SE-IMPT was not affected by range errors, which can drastically affect the IMPT plan.When accepting a low-dose spread as in modern photon techniques, SE-IMPT could be an option for the treatment of lesions (e.g. cervical bone tumours) where steep dose gradient could improve curability, and where range uncertainty, due for example to the presence of metal implants, hampers conventional IMPT.

Water equivalent thickness of immobilization devices in proton therapy planning – Modelling at treatment planning and validation by measurements with a multi-layer ionization chamber

PURPOSE To investigate the range errors made in treatment planning due to the presence of the immobilization devices along the proton beam path. METHODS The measured water equivalent thickness (WET) of selected devices was measured by a high-energy spot and a multi-layer ionization chamber and compared with that predicted by treatment planning system (TPS). Two treatment couches, two thermoplastic masks (both un-stretched and stretched) and one headrest were selected. At TPS, every immobilization device was modelled as being part of the patient. The following parameters were assessed: CT acquisition protocol, dose-calculation grid-sizes (1.5 and 3.0mm) and beam-entrance with respect to the devices (coplanar and non-coplanar). Finally, the potential errors produced by a wrong manual separation between treatment couch and the CT table (not present during treatment) were investigated. RESULTS In the thermoplastic mask, there was a clear effect due to beam entrance, a moderate effect due to the CT protocols and almost no effect due to TPS grid-size, with 1mm errors observed only when thick un-stretched portions were crossed by non-coplanar beams. In the treatment couches the WET errors were negligible (<0.3mm) regardless of the grid-size and CT protocol. The potential range errors produced in the manual separation between treatment couch and CT table were small with 1.5mm grid-size, but could be >0.5mm with a 3.0mm grid-size. In the headrest, WET errors were negligible (0.2mm). CONCLUSIONS With only one exception (un-stretched mask, non-coplanar beams), the WET of all the immobilization devices was properly modelled by the TPS.

Clinical Pencil Beam Scanning: Present and Future Practices

Pencil beam scanning (PBS) is the most advanced beam delivery technology in particle therapy nowadays. After a pioneering phase, PBS is rapidly becoming available on a larger scale worlwide, and is expected to be the standard beam delivery technique in the future to come. The characterization of a PBS isocentric gantry involves a number of validation tests both at the hardware level (e.g. mechanical isocentricity of gantry and patient positioning system) and at the beam geometry level (e.g. spot size, shape and positional accuracy as a function of gantry angle and energy). A beam model is then generated in the treatment planning systems (TPS), and an extensive validation is needed, from simple geometries to heterogenous phantoms mimicking a patient. Last but not least, planning techniques ensuring plan robustness with respect to setup error and range uncertainties should be implemented in order to minimize the difference between planned and delivered dose distribution.

Registration of pencil beam proton radiography data with X‐ray CT

Purpose: Proton radiography seems to be a promising tool for assessing the quality of the stopping power computation in proton therapy. However, range error maps obtained on the basis of proton radiographs are very sensitive to small misalignment between the planning CT and the proton radiography acquisitions. In order to be able to mitigate misalignment in postprocessing, the authors implemented a fast method for registration between pencil proton radiography data obtained with a multilayer ionization chamber (MLIC) and an X‐ray CT acquired on a head phantom. Methods: The registration was performed by optimizing a cost function which performs a comparison between the acquired data and simulated integral depth‐dose curves. Two methodologies were considered, one based on dual orthogonal projections and the other one on a single projection. For each methodology, the robustness of the registration algorithm with respect to three confounding factors (measurement noise, CT calibration errors, and spot spacing) was investigated by testing the accuracy of the method through simulations based on a CT scan of a head phantom. Results: The present registration method showed robust convergence towards the optimal solution. For the level of measurement noise and the uncertainty in the stopping power computation expected in proton radiography using a MLIC, the accuracy appeared to be better than 0.3° for angles and 0.3 mm for translations by use of the appropriate cost function. The spot spacing analysis showed that a spacing larger than the 5 mm used by other authors for the investigation of a MLIC for proton radiography led to results with absolute accuracy better than 0.3° for angles and 1 mm for translations when orthogonal proton radiographs were fed into the algorithm. In the case of a single projection, 6 mm was the largest spot spacing presenting an acceptable registration accuracy. Conclusions: For registration of proton radiography data with X‐ray CT, the use of a direct ray‐tracing algorithm to compute sums of squared differences and corrections of range errors showed very good accuracy and robustness with respect to three confounding factors: measurement noise, calibration error, and spot spacing. It is therefore a suitable algorithm to use in the in vivo range verification framework, allowing to separate in postprocessing the proton range uncertainty due to setup errors from the other sources of uncertainty.

EP-1691: A planning approach for lens sparing proton craniospinal irradiation in pediatric patients

S789 ________________________________________________________________________________ majority of cases surpassed all optimal dose constraints demonstrating the high quality of the planning technique. The incorporation of deep inspiration breath hold (DIBH) ensured doses to the heart were exceptionally low; mean heart dose for left breast cases averaged 1.4Gy for both treatment options. As neither technique has proven superior, the significantly reduced treatment times associated with VMAT make this a more desirable option to implement clinically.

PO-0914: Adjustment of CT calibration in presence of titanium implants by pencil beam proton radiography

S441 ________________________________________________________________________________ After acquisition, each image was coupled to a navigator signal and assigned to a respiratory bin with either phase or amplitude binning. A complete 4D MRI consisted of 110 assigned image states (10 bins, 11 slices). For phase binning, bins are determined by dividing each endexhale peak to peak position into evenly distributed bins. For amplitude binning, bins were determined according to the navigator based breathing amplitude range. The range was defined per volunteer and divided into bins. The minima and maxima were the mean values of end-inhale and end-exhale amplitudes, respectively. The two strategies were used to reconstruct 4D MRI images for 5 volunteers (4 female, mean age 30 years) obtained on a 3T scanner. The position and superior–inferior (SI) motion of the diaphragm were quantified by registering the diaphragm to the begin-inhale image of a series (bin 1). Sorting images into respiratory bins often resulted in multiple images assigned to the same state. From this set, the image with the median diaphragm position was selected for 4D MRI reconstruction. Sometimes, when no images could be assigned to a state, an incomplete 4D MRI resulted. The 4DMRIs were evaluated on data completeness (filled states of 4D MRI data set) and intra-bin variation of diaphragm position (mean standard deviation (SD) and maximum SD). The variation was calculated over all bins from 3 central slices covering the largest diaphragm motion.

SU-G-TeP2-13: Patient-Specific Reduction of Range Uncertainties in Proton Therapy by Proton Radiography with a Multi-Layer Ionization Chamber

PURPOSE The conversion from Hounsfield units (HU) to stopping powers is a major source of range uncertainty in proton therapy (PT). Our contribution shows how proton radiographs (PR) acquired with a multi-layer ionization chamber in a PT center can be used for accurate patient positioning and subsequently for patient-specific optimization of the conversion from HU to stopping powers. METHODS A multi-layer ionization chamber was used to measure the integral depth-dose (IDD) of 220 MeV pencil beam spots passing through several anthropomorphic phantoms. The whole area of interest was imaged by repositioning the couch and by acquiring a 45×45 mm2 frame for each position. A rigid registration algorithm was implemented to correct the positioning error between the proton radiographs and the planning CT. After registration, the stopping power map obtained from the planning CT with the calibration curve of the treatment planning system was used together with the water equivalent thickness gained from two proton radiographs to generate a phantom-specific stopping power map. RESULTS Our results show that it is possible to make a registration with submillimeter accuracy from proton radiography obtained by sending beamlets separated by more than 1 mm. This was made possible by the complex shape of the IDD due to the presence of lateral heterogeneities along the path of the beam. Submillimeter positioning was still possible with a 5 mm spot spacing. Phantom specific stopping power maps obtained by minimizing the range error were cross-verified by the acquisition of an additional proton radiography where the phantom was positioned in a random but known manner. CONCLUSION Our results indicate that a CT-PR registration algorithm together with range-error based optimization can be used to produce a patient-specific stopping power map. Sylvain Deffet reports financial funding of its PhD thesis by Ion Beam Applications (IBA) during the confines of the study and outside the submitted work. Francois Vander Stappen reports being employed by Ion Beam Applications (IBA) during the confines of the study and outside the submitted work.