A static beam delivery device for fast scanning proton arc-therapy

Arc-therapy is a dose delivery technique regularly applied in photon radiation therapy, and is currently subject of great interest for proton therapy as well. In this technique, proton beams are aimed at a tumor from different continuous ranges of incident directions (so called 'arcs'). This technique can potentially yield a better dose conformity around the tumor and a very low dose in the surrounding healthy tissue. Currently, proton-arc therapy is performed by rotating a proton gantry around the patient, adapting the normally used dose-delivery method to the arc-specific motion of the gantry. Here we present first results from a feasibility study of the conceptual design of a new static fast beam delivery device/system for proton-arc therapy, which could be used instead of a gantry. In this novel concept, the incident angle of proton beams can be set rapidly by only changing field strengths of small magnets. This device eliminates the motion of the heavy gantry and related hardware. Therefore, a reduction of the total treatment time is expected. In the feasibility study presented here, we concentrate on the concept of the beam transport. Based on several simple, but realistic assumptions and approximations, proton tracking calculations were performed in a 3D magnetic field map, to calculate the beam transport in this device and to investigate and address several beam-optics challenges. We propose and simulate corresponding solutions and discuss their outcomes. To enable the implementation of some usually applied techniques in proton therapy, such as pencil beam scanning, energy modulation and beam shaping, we present and discuss our proposals. Here we present the concept of a new idea to perform fast proton arc-scanning and we report on first results of a feasibility study. Based on these results, we propose several options and next steps in the design.

Outcomes, Prognostic Factors and Salvage Treatment for Recurrent Chordoma After Pencil Beam Scanning Proton Therapy at the Paul Scherrer Institute

AIMS

The outcome of chordoma patients with local or distant failure after proton therapy is not well established. We assessed the disease-specific (DSS) and overall survival of patients recurring after proton therapy and evaluated the prognostic factors affecting DSS.

MATERIALS AND METHODS

A retrospective analysis was carried out of 71 recurring skull base (n = 36) and extracranial (n = 35) chordoma patients who received adjuvant proton therapy at initial presentation (n = 42; 59%) or after post-surgical recurrence (n = 29; 41%). The median proton therapy dose delivered was 74 GyRBE (range 62-76). The mean age was 55 ± 14.2 years and the male/female ratio was about one.

RESULTS

The median time to first failure after proton therapy was 30.8 months (range 3-152). Most patients (n = 59; 83%) presented with locoregional failure only. There were only 12 (17%) distant failures, either with (n = 5) or without (n = 7) synchronous local failure. Eight patients (11%) received no salvage therapy for their treatment failure after proton therapy. Salvage treatments after proton therapy failure included surgery, systemic therapy and additional radiotherapy in 45 (63%), 20 (28%) and eight (11%) patients, respectively. Fifty-three patients (75%) died, most often from disease progression (47 of 53 patients; 89%). The median DSS and overall survival after failure was 3.9 (95% confidence interval 3.1-5.1) and 3.4 (95% confidence interval 2.5-4.4) years, respectively. On multivariate analysis, extracranial location and late failure (≥31 months after proton therapy) were independent favourable prognostic factors for DSS.

CONCLUSION

The survival of chordoma patients after a treatment failure following proton therapy is poor, particularly for patients who relapse early or recur in the skull base. Although salvage treatment is administered to most patients with uncontrolled disease, they will ultimately die as a result of disease progression in most cases.

Factors influencing the performance of patient specific quality assurance for pencil beam scanning IMPT fields

PURPOSE

A detailed analysis of 2728 intensity modulated proton therapy (IMPT) fields that were clinically delivered to patients between 2007 and 2013 at Paul Scherrer Institute (PSI) was performed. The aim of this study was to analyze the results of patient specific dosimetric verifications and to assess possible correlation between the quality assurance (QA) results and specific field metrics.

METHODS

Dosimetric verifications were performed for every IMPT field prior to patient treatment. For every field, a steering file was generated containing all the treatment unit information necessary for treatment delivery: beam energy, beam angle, dose, size of air gap, nuclear interaction (NI) correction factor, number of range shifter plates, number of Bragg peaks (BPs) with their position and weight. This information was extracted and correlated to the results of dosimetric verification of each field which was a measurement of two orthogonal profiles using an orthogonal ionization chamber array in a movable water column.

RESULTS

The data analysis has shown more than 94% of all verified plans were within defined clinical tolerances. The differences between measured and calculated dose depend critically on the number of BPs, total thickness of all range shifter plates inserted in the beam path, and maximal range. An increase of the dose difference was observed with smaller number of BPs (i.e., smaller tumor) and smaller ranges (i.e., superficial tumors). The results of the verification do not depend, however, on the prescribed dose, NI correction, or the size of the air gap. There is no dependency of the transversal and longitudinal spot position precision on the beam angle. The value of NI correction depends on the number of spots and number of range shifter plates.

CONCLUSIONS

The presented study has shown that the verification method used at Centre for Proton Therapy at Paul Scherrer Institute is accurate and reproducible for performing patient specific QA. The results confirmed that the dose discrepancy is dependent on the size and location of the tumor.

Assessing the quality of proton PBS treatment delivery using machine log files: comprehensive analysis of clinical treatments delivered at PSI Gantry 2

Pencil beam scanning (PBS) proton therapy requires the delivery of many thousand proton beams, each modulated for position, energy and monitor units, to provide a highly conformal patient treatment. The quality of the treatment is dependent on the delivery accuracy of each beam and at each fraction. In this work we describe the use of treatment log files, which are a record of the machine parameters for a given field delivery on a given fraction, to investigate the integrity of treatment delivery compared to the nominal planned dose. The dosimetry-relevant log file parameters are used to reconstruct the 3D dose distribution on the patient anatomy, using a TPS-independent dose calculation system. The analysis was performed for patients treated at Paul Scherrer Institute on Gantry 2, both for individual fields and per series (or plan), and delivery quality was assessed by determining the percentage of voxels in the log file dose distribution within  +/-  1% of the nominal dose. It was seen that, for all series delivered, the mean pass rate is 96.4%. Furthermore, this work establishes a correlation between the delivery quality of a field and the beam position accuracy. This correlation is evident for all delivered fields regardless of individual patient or plan characteristics. We have also detailed further usefulness of log file analysis within our clinical workflow. In summary, we have highlighted that the integrity of PBS treatment delivery is dependent on daily machine performance and is specifically highly correlated with the accuracy of beam position. We believe this information will be useful for driving machine performance improvements in the PBS field.

A treatment planning comparison of 3D conformal therapy, intensity modulated photon therapy and proton therapy for treatment of advanced head and neck tumours

BACKGROUND AND PURPOSE

In this work, the potential benefits and limitations of different treatment techniques, based on mixed photon-electron beams, 3D conformal therapy, intensity modulated photons (IM) and protons (passively scattered and spot scanned), have been assessed using comparative treatment planning methods in a cohort of patients presenting with advanced head and neck tumours.

MATERIAL AND METHODS

Plans for five patients were computed for all modalities using CT scans to delineate target volume (PTV) and organs at risk (OAR) and to predict dose distributions. The prescribed dose to the PTV was 54 Gy, whilst the spinal cord was constrained to a maximum dose of 40.5 Gy for all techniques. Dose volume histograms were used for physical and biological evaluation, which included equivalent uniform dose (EUD) calculations.

RESULTS

Excluding the mixed photon-electron technique, PTV coverage was within the defined limits for all techniques, with protons providing significantly improved dose homogeneity, resulting in correspondingly higher EUD results. For the spinal cord, protons also provided the best sparing with maximum doses as low as 17 Gy. Whilst the IM plans were demonstrated to be significantly superior to non-modulated photon plans, they were found to be inferior to protons for both criteria. A similar result was found for the parotid glands. Although they are partially included in the treated volume there is a clear indication that protons, and to a lesser extent IM photons, could play an important role in preserving organ functionality with a consequent improvement of the patient's quality of life.

CONCLUSIONS

For advanced head and neck tumours, we have demonstrated that the use of IM photons or protons both have the potential to reduce the possibility of spinal cord toxicity. In addition, a substantial reduction of dose to the parotid glands through the use of protons enhances the interest for such a treatment modality in cases of advanced head and neck tumours. However, in terms of target coverage, the use of 3D conformal therapy, although somewhat inferior in quality to protons or IM photons, has been shown to be a reasonable alternative to the more advanced techniques. In contrast, the conventional technique of mixed photon and electron fields has been shown to be inferior to all other techniques for both target coverage and OAR involvement.

[Single-slice and multi-slice computerized tomography: dosimetric comparison with diagnostic reference dose levels]

PURPOSE

The absorbed dose during clinical examinations of the head, thorax, abdomen and pelvis performed with a single-slice CT scanner and a new multi-slice CT system was measured and compared. Technical parameters, defined at installation and memorized on the two CT machines relate to a standard-sized patient and were considered the reference standard. Our experimental data were also been compared with the Diagnostic Reference Levels (D.L. 26/5/2000 n.187, Annex V).

MATERIAL AND METHODS

We compared the performance of a multi-slice GE LightSpeed QX/i ADVANTAGE to that of a single-slice GE ProSpeed SX. The radiation beam profiles were measured at isocenter using a phosphor plate. Dose measurements were performed, according to the EUR 16262 EN Guidelines, with a 10-cm long CT pencil ionisation chamber and two PMMA phantoms (CEI EN 61223-2-6) for head and body respectively.

RESULTS

The obtained (normalised and weighted) computed tomographic dose index (nCTDIW) values were systematically higher for the multi-slice system (up to 36%) and the dose-length product (DLP) values on the multi-slice scanner exceeded the equivalent single-slice DLP values. The values were, however, always lower than DRLs, except in the case of the head multi-slice protocol, the technical parameters of which need to be improved. Our results allowed moreover to calibrate the automatic dose evaluation system of the multi-slice system, which systematically underestimated DLP values.

DISCUSSION AND CONCLUSIONS

The comparison showed that the multi-slice scanner delivers a higher dose compared to the single-slice scanner. This is due to the radiation beam profile which is wider than the total active detector width, to the shorter focal spot-to-isocenter distance and to the effective scan length, which is longer than the nominal irradiated volume because the reconstruction algorithm of a multi-slice helical CT image requires the projection data from all detector rows. Nevertheless, the technology of new CT systems equipped with a multiple row detectors array can improve the protection of the patient thanks to very short irradiation time (less than 1 s) and reduced current values. In order to optimize the dose to the patient some acquisition parameters have been adjusted for head examinations.