D.C. Weber

Effect of Anatomic Changes on Pencil Beam Scanned Proton Dose Distributions for Cranial and Extracranial Tumors

PURPOSEnToxa0estimate the frequency and impact of anatomic changes on the delivered dose in pencil beam scanning proton therapy, to assess the need for repeat CT scanning and adaptive replanning.nnnMETHODS AND MATERIALSnA total of 730 patients treated at Paul Scherrer Institut between 2007 and 2014 were included in this study, for which the number of patients who had control CT scans and who were replanned as a result of anatomic changes was analyzed. For those that were replanned, the nominal dose distributions (originally optimized on the planning CT scan) were recalculated on the replanning CT scan and differences evaluated using standard dose metrics for planning target volumes and clinical target volumes and organs at risk (OARs).nnnRESULTSnControl CT studies were acquired for 244 patients (33.5%), and replanning was deemed clinically necessary for 40 (16%) of these (5.5% of the total cohort). The OARs and target dose differences between the nominal and recalculated dose distributions were found to be strongly dependent on the subgroup of patients. Nevertheless, dose differences were found to be ≤ 5% for 88% of all analyzed OARs, and planning target volume/clinical target volume V95% was reduced byxa0≤5% in 87%/90% of cases.nnnCONCLUSIONSnDespite anatomic variations, clinically delivered plans have been found to be robust to anatomic changes, with replanning being deemed necessary in only a small number of cases. However, because the dosimetric effect of such changes can be quite large for some cases, they have to be monitored and evaluated on an individual basis.

Treatment log files as a tool to identify treatment plan sensitivity to inaccuracies in scanned proton beam delivery.

Dose distributions delivered at Gantry 2 at the Paul Scherrer Institut (PSI) can be reconstructed on the patient anatomy based on machine log files. With the present work, the dependency of the log file calculation on the planning optimization technique and on other planning parameters, such as field direction and tumour size, has been investigated. Interestingly, and despite the typically higher modulation of Intensity Modulated Proton Therapy (IMPT) plans, the results for both Single Field Uniform Distribution and IMPT approaches have been found to be similar. In addition, complex fields with steep in-field dose gradients, such as Simultaneous Integrated Boost, and with couch movements in between the delivery, also resulted in good agreement between planned and reconstructed doses. Nevertheless, highly modulated plans can have regions of larger local dose deviations and attention should therefore be paid during the planning stage to the location of isolated, highly weighted pencil beams. We propose also, that further effort should be invested in order to predict field robustness to delivery fluctuations before the clinical delivery of the plan as part of the plan specific Quality Assurance.

Range resolution and reproducibility of a dedicated phantom for proton PBS daily quality assurance

PURPOSEnWedge phantoms coupled with a CCD camera are suggested as a simple means to improve the efficiency of quality assurance for pencil beam scanning (PBS) proton therapy, in particular to verify energy/range consistency on a daily basis. The method is based on the analysis of an integral image created by a pencil beam (PB) pattern delivered through a wedge. We have investigated the reproducibility of this method and its dependence on setup and positional beam errors for a commercially available phantom (Sphinx®, IBA Dosimetry) and CCD camera (Lynx®, IBA Dosimetry) system.nnnMATERIAL AND METHODSnThe phantom includes 4 wedges of different thickness, allowing verification of the range for 4 energies within one integral image. Each wedge was irradiated with a line pattern of clinical energies (120, 150, 180 and 230MeV). The equipment was aligned to the isocenter using lasers, and the delivery was repeated for 5 consecutive days, 4 times each day. Range was computed using the myQA software (IBA Dosimetry) and inter- and intra-setup uncertainty were calculated. Dependence of range on energy was investigated delivering the same pencil beam pattern but with energy variations in steps of ±0.2MeV for all the nominal energies, up to ±1.0MeV. Possible range uncertainties, caused by setup and positional errors, were then simulated including inclination of the phantom, pencil beam and couch shifts.nnnRESULTSnIntra position setup (based on in-room laser system) shows a maximum in plane difference within 1.5mm. Range reproducibility (standard deviation) was less than 0.14mm. Setup and beam errors did not affect significantly the results, except for a vertical shift of 10mm which leads to an error in the range computation.nnnCONCLUSIONnTaking into account different day-to-day setup and beam errors, day-to-day determination of range has been shown to be reproducible using the proposed system.

The impact of pencil beam scanning techniques on the effectiveness and efficiency of rescanning moving targets

Therapeutic pencil beams are typically scanned using one of the following three techniques: spot scanning, raster scanning or line scanning. While providing similar dose distributions to the target, these three techniques can differ significantly in their delivery time sequence. Thus, we can expect differences in effectiveness and time efficiency when trying to mitigate interplay effects using rescanning. At the Paul Scherrer Institute, we are able to irradiate treatment plans using either of the three delivery techniques. Hence, we can compare them directly with identical underlying machine parameters such as energy switching time or minimum/maximum beam current. For this purpose, we selected three different liver targets, optimized plans for spots, and converted them to equivalent raster and line scanning plans. In addition to the scanning technique, we varied the underlying motion curve, starting phase, prescription dose and rescanning strategy, which resulted in a total of 1584 4D dose calculations and 49 measurements. They indicate that rescanning becomes effective when achieving a high number of rescans for every dose element. Fixed minimum spot weights for spot and raster scanning machines often hamper this. By introducing adaptive scaling of the beam current within iso-energy layers for line scanning, we can flexibly lower the minimum weight whenever required and achieve higher rescanning capability. Averaged over all scenarios studied, volumetric rescanning is significantly more effective than layered provided the same number of rescans are applied. Fast lateral scanning contributes to the efficiency of rescanning. We observed that in any given time window, we can always perform more rescans using raster or line scanning compared to spot scanning irradiations. Thus, we conclude that line scanning represents a promising technique for rescanning by combining both effectiveness and efficiency.

Alternatives to patient specific verification measurements in proton therapy: a comparative experimental study with intentional errors

Patient specific verification (PSV) measurements for pencil beam scanning (PBS) proton therapy are resource-consuming and necessitate substantial beam time outside of clinical hours. As such, efforts to safely reduce the PSV-bottleneck in the clinical work-flow are of great interest. Here, capabilities of current PSV methods to ensure the treatment integrity were investigated and compared to an alternative approach of reconstructing the dose distribution directly from the machine control- or delivery log files with the help of an independent dose calculation (IDC). Scenarios representing a wide range of delivery or work-flow failures were identified (e.g. error in spot position, air gap or pre-absorber setting) and machine files were altered accordingly. This yielded 21 corrupted treatment files, which were delivered and measured with our clinical PSV protocol. IDC machine- and log file checks were also conducted and their sensitivity at detecting the errors compared to the measurements. Although some of the failure scenarios induced clinically relevant dose deviations in the patient geometry, the PSV measurement protocol only detected one out of 21 error scenarios. However, 11 and all 21 error scenarios were detected using dose reconstructions based on the log and machine files respectively. Our data suggests that, although commonly used in particle therapy centers, PSV measurements do a poor job detecting data transfer failures and imperfect delivery machine performance. Machine- and log-file IDCs have been shown to successfully detect erroneous work-flows and to represent a reliable addition to the QA procedure, with the potential to replace PSV.

PO-0850: Interplay effect quantification of PBS lung tumour proton therapy with various fractionation schemes

Material and Methods: For two example lung tumour cases (I and II), three-field 3D plans were calculated on a patient specific range-adapted ITV (rITV) using a spot spacing of 4mm orthogonal to the beam directions. 4D dose calculations were performed, simulating three different fractionation treatments with schemes of (A)2.5Gyx35fx, (B)5Gyx10fx and (C)13.5Gyx3fx, based on machine and delivery parameters of the Varian ProBeam system (lateral scanning speed of 5/20 mm/ms and energy switching time of 700 ms with layer-wise optimized dose rates). 1xto 10xlayered and volumetric rescanning was simulated to mitigate residual motion effects. The final dose distributions for fractioned treatments were obtained by superposition and normalization of the 4D dose distributions of each field and each fraction with random starting phases sampled from 4DCT (10 different phases with 100 random starts). We used homogeneity index (HI:D5-D95) in the CTV to quantify the resultant 4D dose distributions within the target, while for the normal lung (both lungs minus CTV), V20, mean lung dose (MLD) and D2 were compared.

PO-0805: Proton radiography for the clinical commissioning of the new Gantry2 head support at PSI

In figure a comparison of TPS and MC planar dose distribution with 2DQA measurements is shown. In our protocol, if the passing rate (PR) is above 95% the field is accepted. If it is between 95 and 90% a justification must be added to the QA report to flag the field as accepted. A passing rate below 90% makes the field unacceptable. In the graph 27 fields belonging to 10 patients are analysed. MC has a PR always greater than 95% for every depth showing a good agreement with measurements. TPS results are always in the “grey” area between 90 and 95%. The execution time of a 2DQA with an array of ICs takes almost 1 hour and half; simulations, that can be performed in parallel, take 11 minutes on average.

Poster: Physics track: Dose measurementsPO-0857: Are the dosimetric verification results of spot scanned IMPT fields dependent on field specific parameters?

PO-0856 A retrospective study of image guided adaptive radiation therapy in prostate cancer M. Guasp Tortajada, R. García-Mollá, L. Vidueira-Martínez, N. De Marco-Blancas, J. Bonaque-Alandí, M. AlbertAntequera, A. Santos-Serra, J. López-Tarjuelo Consorcio Hosptialario Provincial de Castellon, Servicio de Radiofísica y Protección Radiológica, Castellón de la plana, Spain Consorcio Hosptialario Provincial de Castellón, Servicio de Oncología Radioterápica, Castellón de la plana, Spain