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Title: SU-E-T-42: A Method to Determine the Optimal Proton Scanned Beam Angle to Ensure Robust Treatment Planning

Abstract

Purpose: The ability of pencil beam scanning (PBS) to deliver highly conformal dose distributions may be affected by patient- and physics-related uncertainties. In clinical practice, selection of proton beam angles is determined qualitatively. This study investigates whether an optimal proton PBS beam angle could be quantitatively determined to ensure robust planning for pelvic targets. Methods: PBS beam angles were optimized based on two independent criteria; shortest and most homogeneous path from the patient surface to the distal edge of the target. The beam angle optimization criteria for gantry angles between 90°-270° were quantified in 10° increments for each ray, calculated as the straight line distance from the surface of the skin to the CTV’s distal edge. The goal was to minimize the path length of a proton PBS beam from the patient surface to the distal edge of the CTV, relative to the entry point, while minimizing HU inhomogeneity along the ray. HU homogeneity (i.e. HU variation) was quantitatively defined as the standard deviation of the average intra-ray HU intensity distribution of the rays comprising a single beam. This method was validated relative to inter-fraction changes on ten consecutive, locally advanced, rectal cancer patients, who underwent an average 4 verificationmore » CTs. The displacement of the 95–98% isodose lines was determined from forward calculated dose distributions on verification CTs. Results: The posterior beam (180°) had the average shortest path length, 132.7±17.2mm, and the most homogenous path, 31.9±4.3HU. The 95–98% isodose lines from all plans verified our path length to within 2.3±1.2% and HU homogeneity to within 1.2±0.5%. Conclusion: The proposed optimization algorithm determined the posterior beam dose distribution as the most robust relative to inter-fraction variation for large pelvic targets treated with PBS and was validated via verification CT for our patient cohort. Future work will focus on further algorithm development.« less

Authors:
; ;  [1]
  1. University of Pennsylvania, Philadelphia, PA (United States)
Publication Date:
OSTI Identifier:
22545174
Resource Type:
Journal Article
Resource Relation:
Journal Name: Medical Physics; Journal Volume: 42; Journal Issue: 6; Other Information: (c) 2015 American Association of Physicists in Medicine; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
60 APPLIED LIFE SCIENCES; ALGORITHMS; COMPUTERIZED TOMOGRAPHY; NEOPLASMS; OPTIMIZATION; PATIENTS; PLANNING; PROTON BEAMS; RADIATION DOSE DISTRIBUTIONS; RADIATION DOSES; RECTUM; SKIN; VERIFICATION

Citation Formats

Kiely, J Blanco, White, B, and Both, S. SU-E-T-42: A Method to Determine the Optimal Proton Scanned Beam Angle to Ensure Robust Treatment Planning. United States: N. p., 2015. Web. doi:10.1118/1.4924403.
Kiely, J Blanco, White, B, & Both, S. SU-E-T-42: A Method to Determine the Optimal Proton Scanned Beam Angle to Ensure Robust Treatment Planning. United States. doi:10.1118/1.4924403.
Kiely, J Blanco, White, B, and Both, S. Mon . "SU-E-T-42: A Method to Determine the Optimal Proton Scanned Beam Angle to Ensure Robust Treatment Planning". United States. doi:10.1118/1.4924403.
@article{osti_22545174,
title = {SU-E-T-42: A Method to Determine the Optimal Proton Scanned Beam Angle to Ensure Robust Treatment Planning},
author = {Kiely, J Blanco and White, B and Both, S},
abstractNote = {Purpose: The ability of pencil beam scanning (PBS) to deliver highly conformal dose distributions may be affected by patient- and physics-related uncertainties. In clinical practice, selection of proton beam angles is determined qualitatively. This study investigates whether an optimal proton PBS beam angle could be quantitatively determined to ensure robust planning for pelvic targets. Methods: PBS beam angles were optimized based on two independent criteria; shortest and most homogeneous path from the patient surface to the distal edge of the target. The beam angle optimization criteria for gantry angles between 90°-270° were quantified in 10° increments for each ray, calculated as the straight line distance from the surface of the skin to the CTV’s distal edge. The goal was to minimize the path length of a proton PBS beam from the patient surface to the distal edge of the CTV, relative to the entry point, while minimizing HU inhomogeneity along the ray. HU homogeneity (i.e. HU variation) was quantitatively defined as the standard deviation of the average intra-ray HU intensity distribution of the rays comprising a single beam. This method was validated relative to inter-fraction changes on ten consecutive, locally advanced, rectal cancer patients, who underwent an average 4 verification CTs. The displacement of the 95–98% isodose lines was determined from forward calculated dose distributions on verification CTs. Results: The posterior beam (180°) had the average shortest path length, 132.7±17.2mm, and the most homogenous path, 31.9±4.3HU. The 95–98% isodose lines from all plans verified our path length to within 2.3±1.2% and HU homogeneity to within 1.2±0.5%. Conclusion: The proposed optimization algorithm determined the posterior beam dose distribution as the most robust relative to inter-fraction variation for large pelvic targets treated with PBS and was validated via verification CT for our patient cohort. Future work will focus on further algorithm development.},
doi = {10.1118/1.4924403},
journal = {Medical Physics},
number = 6,
volume = 42,
place = {United States},
year = {Mon Jun 15 00:00:00 EDT 2015},
month = {Mon Jun 15 00:00:00 EDT 2015}
}
  • Purpose: In prostate proton radiotherapy, three fiducial markers are used for patient daily alignment. However fiducial alignment can change beamline heterogeneity in proton therapy. The purpose of this study is to determine the difference in fiducial and boney anatomy alignment for patient treatment. Methods and materials: Prostate cancer patients who received proton treatment were included in this study. 3 fiducial markers were implanted before the initial CT. All the patients were re-CT’d every 2 weeks to check the fiducial marker position reproducibility as well as dosimetric consistence of target coverage. In geometry study, re-CT were fused to the initial CTmore » base on the boney anatomy and the average fiducial marker displacement was measured the centers of the fiducials. Dosimetrically, the initial plan was recalculated directly to re-CT image set based on the boney alignment and fiducial alignment to determine the difference from daily treatment. Prostate coverage and hotspots were evaluated using the dose to 98% of the CTV (D98) and dose to 2% (D2), respectively. Results: The shift from the initial 6 patient CT image sets resulted in an average change in the fiducial location of 5.70 +/− 3 mm. Dosimetric comparison from a single patient revealed that differences from the planned dose resulted from both boney and fiducial alignment. Planned clinical treatment volume coverage resulted in a D98 of 70.44Gy and D2 of 70.84Gy compared to a D98 of 70.13Gy and D2 70.94Gy for boney alignment and a D98 of 70.08Gy and D2 71.18Gy for fiducial alignment respectively. Conclusion: This study demonstrates that with boney anatomy alignment there is little change to CTV coverage and only slightly worse CTV coverage and hotspot production with fiducial alignment. An increase patient cohort and further investigation is necessary to determine the whether boney alignment can help better control dose heterogeneity.« less
  • Purpose: To propose a proton pencil beam scanning (PBS) cranial spinal irradiation (CSI) treatment planning technique robust against patient roll, isocenter offset and proton range uncertainty. Method: Proton PBS plans were created (Eclipse V11) for three previously treated CSI patients to 36 Gy (1.8 Gy/fractions). The target volume was separated into three regions: brain, upper spine and lower spine. One posterior-anterior (PA) beam was used for each spine region, and two posterior-oblique beams (15° apart from PA direction, denoted as 2PO-15) for the brain region. For comparison, another plan using one PA beam for the brain target (denoted as 1PA)more » was created. Using the same optimization objectives, 98% CTV was optimized to receive the prescription dose. To evaluate plan robustness against patient roll, the gantry angle was increased by 3° and dose was recalculated without changing the proton spot weights. On the re-calculated plan, doses were then calculated using 12 scenarios that are combinations of isocenter shift (±3mm in X, Y, and Z directions) and proton range variation (±3.5%). The worst-case-scenario (WCS) brain CTV dosimetric metrics were compared to the nominal plan. Results: For both beam arrangements, the brain field(s) and upper-spine field overlap in the T2–T5 region depending on patient anatomy. The maximum monitor unit per spot were 48.7%, 47.2%, and 40.0% higher for 1PA plans than 2PO-15 plans for the three patients. The 2PO-15 plans have better dose conformity. At the same level of CTV coverage, the 2PO-15 plans have lower maximum dose and higher minimum dose to the CTV. The 2PO-15 plans also showed lower WCS maximum dose to CTV, while the WCS minimum dose to CTV were comparable between the two techniques. Conclusion: Our method of using two posterior-oblique beams for brain target provides improved dose conformity and homogeneity, and plan robustness including patient roll.« less
  • Purpose: To study the cross-field and depth dose profiles of spot-scanned pencil beam configurations for the treatment of ocular tumors and to compare their performance to a simulated scattered beam. Methods: Dose distributions in a cubic water phantom were compared for beams that passed through a final 24mm diameter aperture to deposit maximum dose at 2.4cm depth. The pencil-beam spots formed a hexagonally-packed ring with a center-to-center spacing of 4mm. The protons exited the nozzle with energy 95.5MeV, traversed a 4.5cm water-equivalent range shifter, and travelled either 42.5cm or 100cm to the phantom surface. The aperture-to-phantom distance (APD) was 5.7cmmore » to allow room for eye-tracking hardware. A configuration with APD=0 was also tested. The scattered beam was generated with energy 159MeV, passed through 127mm of Lexan, exited the final aperture, and travelled 5.7cm to the phantom surface. This latter configuration is comparable to the MGH single scattered beamline. All beams were modelled with TOPAS1.0-beta6 compiled with GEANT4.9.6p2. Results: The modeled scattered beam produced a distal fall-off along the central axis of zd90%-zd10%=3.6mm. For the pencil beam, the zd90%-zd10% was 1.6mm in all configurations. The scattered beam's cross-field penumbra at depth of maximum dose was r90%- r10%=1.9mm. For the spot-scanned configuration with the range-shifter-tophantom distance (RsPD) of 100cm, similar cross-field profiles were achieved with r90%-r10%=2.0mm. At shorter RsPD of 42.5cm, the crossfield penumbras were 5.6mm and 7.7mm for APD=0cm and APD=5.7cm, respectively. Conclusion: For proton treatments employing a range shifter, the cross-field and central axis dose profiles depend on the quality of the original beam, the size of the range shifter, the distance from the range shifter exit to the patient, and the distance from the final aperture to the patient. A spot-scanned pencil beam configuration can achieve cross-field penumbras equal to a scattered beam and superior distal gradients.« less
  • Purpose: A single-room proton system, the Mevion S250, was introduced into the arena of proton radiotherapy by Mevion Medical Systems. The first unit was installed and operates at the S. Lee Kling Proton Therapy Center at Barnes-Jewish Hospital. The objective of this abstract is to report the system's beam characteristics and Eclipse commissioning. Methods: Commissioning data were acquired for modelling longitudinal fluence, virtual source position, effective source position, source size and Bragg peaks in Eclipse. Stoichiometric CT calibration was generated via ICRU44 human. Spread-out Bragg peaks (SOBP) were measured with Parallel Plate Chamber and profiles with solid state detector formore » model validation. Heterogeneity effects were measured with bone and lung inserts in the beam line. RT dose was computed in a virtual water phantom, and exported from Eclipse to compare with measurements at various depths and axis. SOBPs were fine-tuned with partial shining correction and entry correction to match measurements. Output factor was measured for each individual field with an ADCL ion chamber in a water tank and fitted to a polynomial function to cross-check the monitor unit verification. Results: Ranges of all 24 options were measured within ±1mm tolerance. Modulations met a ±1mm or ±2% tolerance. SOBP flatness met a ±3% tolerance. Distal fall off (80%-20%) were measured between 6mm and 7mm for all options. Virtual source positions varied between 177cm and 195cm, decreasing with field size and range. SOBP generated by Eclipse agreed with measurements within ±3% in the entry region, and ±1%/±1mm in other regions. Sanity check for output achieved 5% accuracy in 98% of cases. Conclusion: The commissioning of the first Mevions S250 proton therapy system met specifications. The unit has been put in clinical operation since 12/17/2013.« less
  • Purpose: To evaluate density inhomogeneities which can effect dose distributions for real-time image gated spot-scanning proton therapy (RGPT), a dose calculation system, using treatment planning system VQA (Hitachi Ltd., Tokyo) spot position data, was developed based on Geant4. Methods: A Geant4 application was developed to simulate spot-scanned proton beams at Hokkaido University Hospital. A CT scan (0.98 × 0.98 × 1.25 mm) was performed for prostate cancer treatment with three or four inserted gold markers (diameter 1.5 mm, volume 1.77 mm3) in or near the target tumor. The CT data was read into VQA. A spot scanning plan was generatedmore » and exported to text files, specifying the beam energy and position of each spot. The text files were converted and read into our Geant4-based software. The spot position was converted into steering magnet field strength (in Tesla) for our beam nozzle. Individual protons were tracked from the vacuum chamber, through the helium chamber, steering magnets, dose monitors, etc., in a straight, horizontal line. The patient CT data was converted into materials with variable density and placed in a parametrized volume at the isocenter. Gold fiducial markers were represented in the CT data by two adjacent voxels (volume 2.38 mm3). 600,000 proton histories were tracked for each target spot. As one beam contained about 1,000 spots, approximately 600 million histories were recorded for each beam on a blade server. Two plans were considered: two beam horizontal opposed (90 and 270 degree) and three beam (0, 90 and 270 degree). Results: We are able to convert spot scanning plans from VQA and simulate them with our Geant4-based code. Our system can be used to evaluate the effect of dose reduction caused by gold markers used for RGPT. Conclusion: Our Geant4 application is able to calculate dose distributions for spot scanned proton therapy.« less