M Wolanski

Supine proton beam craniospinal radiotherapy using a novel tabletop adapter

To develop a device that allows supine craniospinal proton and photon therapy to the vast majority of proton and photon facilities currently experiencing limitations as a result of couch design issues. Plywood and carbon fiber were used for the development of a prototype unit. Once this was found to be satisfactory after all design issues were addressed, computer-assisted design (CAD) was used and carbon fiber tables were built to our specifications at a local manufacturer of military and racing car carbon fiber parts. Clinic-driven design was done using real-time team discussion for a prototype design. A local machinist was able to construct a prototype unit for us in <2 weeks after the start of our project. Once the prototype had been used successfully for several months and all development issues were addressed, a custom carbon fiber design was developed in coordination with a carbon fiber manufacturer in partnership. CAD methods were used to design the units to allow oblique fields from head to thigh on patients up to 200 cm in height. Two custom-designed carbon fiber craniospinal tabletop designs now exist: one long and one short. Four are in successful use in our facility. Their weight tolerance is greater than that of our robot table joint (164 kg). The long unit allows for working with taller patients and can be converted into a short unit as needed. An affordable, practical means of doing supine craniospinal therapy with protons or photons can be used in most locations via the use of these devices. This is important because proton therapy provides a much lower integral dose than all other therapy methods for these patients and the supine position is easier for patients to tolerate and for anesthesia delivery. These units have been successfully used for adult and pediatric supine craniospinal therapy, proton therapy using oblique beams to the low pelvis, treatment of various spine tumors, and breast-sparing Hodgkins therapy.

Dosimetric characteristics of a single use MOSFET dosimeter for in vivo dosimetry in proton therapy

PURPOSE Entrance dose (or skin dose) is an important part of patient quality assurance in external beam radiation therapy. However, entrance dose verification in proton beam is not routinely performed. In this study, the OneDose single use MOSFET detector system for in vivo dosimetry measurement in proton therapy is investigated. METHODS Using a solid water phantom, several fundamental dosimetric characteristics of the OneDose system are studied with a proton beam: The reproducibility (consistency) of the dosimeter, the linearity with dose and dose rate, energy dependence, directional dependence, LET dependence, and fading (delay readout with time) is studied. RESULTS OneDose detectors show dose and dose rate linearity but exhibit pronounced energy dependence at depth and a large variation in dose response with LET. On the other hand, the detector response remain relatively constant (within 3%) at surface over a wide range of energies. There is also a slight angular dependence (about 2%) up to 60 degrees angle of incidence. However, detector orientation such that incidence along the long axis of the detector should be avoided as the proton beam will have to traverse a large amount of the copper backing. Since most in vivo dosimetry involves entrance dose measurement, the OneDose at surface appears to be well suited for such application. OneDose exhibits small intrabatch variation (< or = 2% at one SD) indicating that it is only necessary to calibration a few detectors from each batch. The interbatch variation is generally within 3%. CONCLUSIONS The small detector size and its relatively flexible design of OneDose allow dose measurement to be performed on a curved surface or in small cavities that is otherwise difficult with the conventional diode detectors. The slight drawback in its angular dependence can be easily handled by angular dependence table. However, since OneDose is a single use detector, the intra-batch consistency must be verified before the remaining detectors from the same batch could be used for in vivo dosimetry. It is advisable that the detectors from the same batch be taken for the same application to reduce the dosimetric uncertainty. For detectors from different batches, inter-batch consistency should also be verified to obtain reliable results. OneDose provides an opportunity to measure in vivo dose with proton beam within acceptable clinical criterion of +/- (5.0%-6.5%).

Technique for sparing previously irradiated critical normal structures in salvage proton craniospinal irradiation

BackgroundCranial reirradiation is clinically appropriate in some cases but cumulative radiation dose to critical normal structures remains a practical concern. The authors developed a simple technique in 3D conformal proton craniospinal irradiation (CSI) to block organs at risk (OAR) while minimizing underdosing of adjacent target brain tissue.MethodsTwo clinical cases illustrate the use of proton therapy to provide salvage CSI when a previously irradiated OAR required sparing from additional radiation dose. The prior radiation plan was coregistered to the treatment planning CT to create a planning organ at risk volume (PRV) around the OAR. Right and left lateral cranial whole brain proton apertures were created with a small block over the PRV. Then right and left lateral “inverse apertures” were generated, creating an aperture opening in the shape of the area previously blocked and blocking the area previously open. The inverse aperture opening was made one millimeter smaller than the original block to minimize the risk of dose overlap. The inverse apertures were used to irradiate the target volume lateral to the PRV, selecting a proton beam range to abut the 50% isodose line against either lateral edge of the PRV. Together, the 4 cranial proton fields created a region of complete dose avoidance around the OAR. Comparative photon treatment plans were generated with opposed lateral X-ray fields with custom blocks and coplanar intensity modulated radiation therapy optimized to avoid the PRV. Cumulative dose volume histograms were evaluated.ResultsTreatment plans were developed and successfully implemented to provide sparing of previously irradiated critical normal structures while treating target brain lateral to these structures. The absence of dose overlapping during irradiation through the inverse apertures was confirmed by film. Compared to the lateral X-ray and IMRT treatment plans, the proton CSI technique improved coverage of target brain tissue while providing the least additional radiation dose to the previously irradiated OAR.ConclusionsProton craniospinal irradiation can be adapted to provide complete sparing of previously irradiated OARs. This technique may extend the option of reirradiation to patients otherwise deemed ineligible for further radiotherapy due to prior dose to critical normal structures.

Dosimetric comparison between proton and photon beams in the moving gap region in cranio-spinal irradiation (CSI)

Abstract Purpose. To investigate the moving gap region dosimetry in proton beam cranio-spinal irradiation (CSI) to provide optimal dose uniformity across the treatment volume. Material and methods. Proton beams of ranges 11.6 cm and 16 cm are used for the spine and the brain fields, respectively. Beam profiles for a 30 cm snout are first matched at the 50% level (hot match) on the computer. Feathering is simulated by shifting the dose profiles by a known distance two successive times to simulate a 2 × feathering scheme. The process is repeated for 2 mm and 4 mm gaps. Similar procedures are used to determine the dose profiles in the moving gap for a series of gap widths, 0–10 mm, and feathering step sizes, 4–10 mm, for a Varian iX 6MV beam. The proton and photon dose profiles in the moving gap region are compared. Results. The dose profiles in the moving gap exhibit valleys and peaks in both proton and photon beam CSI. The dose in the moving gap for protons is around 100% or higher for 0 mm gap, for both 5 and 10 mm feathering step sizes. When the field gap is comparable or larger than the penumbra, dose minima as low as 66% is obtained. The dosimetric characteristics for 6 MV photon beams can be made similar to those of the protons by appropriately combining gap width and feathering step size. Conclusion. The dose in the moving gap region is determined by the lateral penumbras, the width of the gap and the feathering step size. The dose decreases with increasing gap width or decreasing feathering step size. The dosimetric characteristics are similar for photon and proton beams. However, proton CSI has virtually no exit dose and is beneficial for pediatric patients, whereas with photon beams the whole lung and abdomen receive non-negligible exit dose.

A sector-integration method for dose/MU calculation in a uniform scanning proton beam.

An accurate, simple and time-saving sector integration method for calculating the proton output (dose/monitor unit, MU) is presented based on the following treatment field parameters: aperture shape, aperture size, measuring position, beam range and beam modulation. The model is validated with dose/MU values for 431 fields previously measured at our center. The measurements were obtained in a uniform scanning proton beam with a parallel plate ionization chamber in a water phantom. For beam penetration depths of clinical interest (6-27 cm water), dose/MU values were measured as a function of spread-out Bragg peak (SOBP) extent and aperture diameter. First, 90 randomly selected fields were used to derive the model parameters, which were used to compute the dose/MU values for the remaining 341 fields. The min, max, average and the standard deviation of the difference between the calculated and the measured dose/MU values of the 341 fields were used to evaluate the accuracy and stability, for different energy ranges, aperture sizes, measurement positions and SOBP values. The experimental results of the five different functional sets showed that the calculation model is accurate with calculation errors ranging from -2.4% to 3.3%, and 99% of the errors are less than +/-2%. The accuracy increases with higher energy, larger SOBP and bigger aperture size. The average error in the dose/MU calculation for small fields (field size <25 cm(2)) is 0.31 +/- 0.96 (%).

SU-E-T-496: Dosimetric Comparison between Protons and Photons in the Field Junction in Craniospinal Irradiation (CSI)

Purpose: In this study, we investigated the dosimetry in the moving gap region in proton CSI for various combinations of field junction widths and feathering step sizes. We have also compared the dosimetry in the field junction between proton and photon beams. Methods: Dose profiles for two proton ranges, 11.6cm and 16cm, both with 10cm SOBP for the 30cm snout are used. Feathering of the junction is simulated by shifting the profiles by two successive steps. Three junction widths (0, 2, 4mm) and two feathering step sizes (5, 10 mm) are investigated. Similar simulations (but also include larger gap widths due to larger penumbra) are performed for 6 and 15 MV x‐ rays. Dose profiles at the field junctions are then compared for the proton and photon fields for various gaps and feathering step sizes. Results: For protons, full dose is achieved in the field junction for 0mm gap for both 5mm and 10mm featherings. However, with even a 2mm gap, the dose falls to below 90% in the moving gap due to the steep proton penumbra. For photons, even 4mm gap still results in >90% dose in the moving gap due to the large penumbra and the ‘tail’ in the profiles. Despite the different dose characteristics, it is possible to produce similar dose profiles in the moving gap for proton and photon fields by varying the gap and the feathering sizes for photon fields. Conclusion: Full dose can be safely achieved in the junction by ‘hot‐matching’ the spine and the brain fields in proton CSI and feathering the junctions. The steep proton penumbra results in a large dose gradient even with 2mm gap between fields. It is possible to produce similar dosimetry in the junction with photon CSI. The difference is that there is no exit dose with protons.

SU‐E‐T‐500: Immobilization Device for Supine Proton Craniospinal Irradiation

Purpose: A prototype immobilization board has been developed and implemented for proton therapy of the craniospinal axis at the IU Health Proton Therapy Center. This innovation is important because proton beam CSI spares the majority of the thoracic and abdominal organs, avoiding much of the acute and long term toxicity outside of the craniospinal axis associated with standard radiation therapy techniques. Proton CSI provides a significantly lower integral dose than otherwise achievable. The delivery of supine craniospinal irradiation (CSI) is a challenge due to the limitations of the treatment couches in routine clinical use. This is particularly true for protonradiation which requires the patient be reproducibly placed on a smoothly varying (“edgeless”) surface. Methods: Since the largest field available to us is 30 cm × 30 cm and field matching is required, the immobilization board attaches to a standard treatment couch mounted on a robotic patient positioner which moves precisely to cover all areas of the body above the mid‐femur for patients up to 200 cm. The weight tolerance of the production board exceeds the weight tolerance of the robot (360 pounds). Results: We discuss this immobilization device, and our experience treating pediatric and adult patients in the supine position with and without anesthesia. Conclusions: Anticipated modifications to the design of the immobilization board will increase access to the majority of the patients body, including beams from oblique posterior angles. The cost effective device can be adapted to any standard tabletop (including photon units).

SU‐GG‐T‐336: Effect of Treatment and Beam Parameters on Surface Dose in Proton Beam Therapy

Purpose: The advantage of lower skin dose in protonbeam therapy may result in less radiation‐related side effects which are typically seen in hypo‐fractionated conventional external beam therapies. In this study, we evaluate the surface dose (SD) in proton therapy as a function of various beam parameters. Materials & Methods: SD is defined as the ratio of absorbed dose on CAX at surface to that at the middle of the Spread‐Out Bragg Peak (SOBP). SD in proton therapy is affected by several parameters: energy ( E ), SOBP , source‐to‐surface distance ( SSD ), air gap ( g ), field radius ( r ), material thickness upstream of surface besides air ( t ), atomic number of medium ( Z ), beam angle relative to surface (q), and nozzle type ( N ). The parameters, t, Z and q are not included in this study.. Results: Giving a proton range in water of 27 cm, SD rises from 30 to 90% with SOBP increasing from 0 to17cm. At high energy, SD is 5% higher in the uniform scanning (US) nozzle than in double scattering (DS) nozzle. This can be explained by the large difference in source‐to‐axis distances in US and DS nozzles (250 cm vs 320 cm). SSD and g have minimal impact on SD for r > 2.5cm. For small fields ( r < 2.5cm), SD increases significantly with field size decreases. Within 15mm of the surface there is a small but pronounced 2% buildup in protonbeam at high energies, which may be partially due to secondary electrons, secondary protons and heavy charged particles. Conclusions: In a clinical setting, SD is most significantly affected by SOBP extent, followed by field size, SAD, and beamenergy. In general, SD ranges from 50 to 95%. It is important to clearly understand and minimize SD in Proton therapy, especially in hypo‐fractioned treatments.

SU‐GG‐T‐473: Dose Uncertainty Due to High‐Z Materials in Clinical Proton Beam Therapy

Purpose: In proton therapy, high‐Z materials, such as dental alloys, sternal reconstruction plates, prosthesis, and metallic ports, can introduce significant dose perturbations. Our objective is to quantify the high‐Z induced dosimetry uncertainty in clinical proton beams. Method and Materials: Dose perturbations from one titanium vascular port and a steel injection port of a breast expander were studied. An extended CT‐electron density (ED) curve for MVCT was obtained with an RMI CT phantom and metal plates (Al, Sn, Ti, Pb). Measurements taken with a 2D ion chamber array placed at different depths downstream from the high‐Z‐solid water interface were compared with dose calculations on the XiO treatment planning system based on both the MVCT and kVCT images. The Monte Carlo code FLUKA was used to verify accuracy of inhomogeneity corrections in the pencil beam algorithm. Dose perturbation factor (DPF) was defined as the ratio of the doses with and without the high‐Z material. Results: For MVCT, the CT‐ED relationship is linear from lung to lead. There are considerable dose enhancement (>10%) near the high‐Z interface due to secondary electrons from the metallic port. DPF as large as 20% was observed within the spread‐out Bragg peak. MVCT images provided more accurate delineation of the metallic object compared to kVCT, which tends to overestimate the water equivalent thickness of the metal object, resulting in shallower proton depth than its actual value. The DPF calculated from MVCT planning agrees with the measured results within 10%. Results from Monte Carlo calculations are comparable to results from XiO although there are small differences. Conclusions: Understanding dose uncertainty induced by high‐Z material is very important in proton therapy. MVCT based treatment planning may be preferred with an extended CT‐ED curve. Difference between measured and calculated dose distribution shall be quantified.

SU‐GG‐T‐474: Feasibility Study of MVCT Imaging Guided Adaptive Proton Therapy for Head and Neck Cancers

Purpose: Extending MVCTs potential for daily patient position and anatomy verification, we investigated the feasibility of using MVCT to assess the daily delivered dose for head and neck patients treated with proton beams. Method and Materials: A Rando head phantom was scanned on a Philips Brilliance Big Bore CTscanner to obtain the planning kVCT images. The corresponding MVCT images were acquired on a helical Tomotherapy unit. To establish CT‐electron density curves, a Gammex RMI 465 electron density phantom was scanned on both kVCT and MVCT scanners. A typical 4‐field plan was created for the phantom and dose distribution was calculated using pencil beam algorithm from the XIO treatment planning system. After MVCT and kVCT images manually registered, kVCT‐based treatment plan with structure contours and beam configuration including beam shaping devices was mapped to MVCT images for dose re‐computation. Dose map, isodose curves, dose profiles, gamma maps, and dose‐volume histograms were used for dose comparison. Results: Although the noise level in MVCT images is higher than that with the comparabl kVCT the dose distribution computed on MVCT image is comparable to the dose distribution computed on the kVCT images. The dose comparison showed an agreement of higher than 97% between the two plans given the criteria of 3% dose difference and 3 mm distance to agreement. The biggest dose difference between two plans occurred around the periphery of the target. There was negligible difference in the DVH comparison. Conclusions: MVCT images acquired for daily setup verification in image‐guidedproton therapy are feasible for assessment of daily deliveredproton dose distributions with accuracy comparable to that of kVCT‐based dose calculation. The original treatment plan based on kVCT can be transferred to the daily MVCT image set to evaluate the actual dose distribution for adaptive proton therapy.