Kenneth P. Gall

Computer‐assisted positioning of radiotherapy patients using implanted radiopaque fiducials

For certain external beam radiotherapy procedures, precise alignment of patients with the treatment beam is essential for good treatment outcome. A method has been developed for quickly achieving precise patient alignment with the aid of stereoscopically located fiducial markers. The alignment algorithm is developed from standard rigid body mechanics using closed form solutions, obviating the need for iterative fitting methods. The technique is implemented with a digitizing tablet and plane film radiographs. The accuracy of alignment with this method in phantom studies is better than 1 mm and 1 deg relative to CT data. The repeatability of positioning is 0.5 mm (standard deviation) and 0.36 deg (standard deviation).

A new miniature x-ray device for interstitial radiosurgery: dosimetry.

A miniature, battery operated 40 kV x-ray device has been developed for the interstitial treatment of small tumors ( < 3 cm diam) in humans. X rays are emitted from the tip of a 10 cm long, 3 mm diameter probe that is stereotactically inserted into the tumor. The beam, characterized by half-value layer (HVL), spectrum analysis, and isodose contours, behaves essentially as a point isotropic source with an effective energy of 20 keV at a depth of 10 mm in water. The absolute output from the device was measured using a parallel plate ionization chamber, modified with a platinum aperture. The dose rate in water determined from these chamber measurements was found to be nominally 150 cGy/min at a distance of 10 mm for a beam current of 40 microA and voltage of 40 kV. The dose in water falls off approximately as the third power of the distance. To date, 14 patients have been treated with this device in a phase I clinical trial.

The measurement of proton stopping power using proton-cone-beam computed tomography

A cone-beam computed tomography (CT) system utilizing a proton beam has been developed and tested. The cone beam is produced by scattering a 160 MeV proton beam with a modifier that results in a signal in the detector system, which decreases monotonically with depth in the medium. The detector system consists of a Gd2O2S:Tb intensifying screen viewed by a cooled CCD camera. The Feldkamp-Davis-Kress cone-beam reconstruction algorithm is applied to the projection data to obtain the CT voxel data representing proton stopping power. The system described is capable of reconstructing data over a 16 x 16 x 16 cm3 volume into 512 x 512 x 512 voxels. A spatial and contrast resolution phantom was scanned to determine the performance of the system. Spatial resolution is significantly degraded by multiple Coulomb scattering effects. Comparison of the reconstructed proton CT values with x-ray CT derived proton stopping powers shows that there may be some advantage to obtaining stopping powers directly with proton CT. The system described suggests a possible practical method of obtaining this measurement in vivo.

A precision cranial immobilization system for conformal stereotactic fractionated radiation therapy

PURPOSE Conformal radiotherapy has been shown to benefit from precision alignment of patient target to therapy beam (1, 6, 13). This work describes an optimized immobilization system for the fractionated treatment of intracranial targets. A study of patient motion demonstrates the high degree of immobilization which is available. METHODS AND MATERIALS A system using dental fixation and a thermoplastic mask that relocates on a rigid frame is described. The design permits scanning studies using computed tomography (CT) and magnetic resonance imaging (MR), conventional photon radiotherapy, and high precision stereotactic proton radiotherapy to be performed with minimal repositioning variation. Studies of both intratreatment motion and daily setup reliability are performed on patients under treatment for paranasal sinus carcinoma. Multiple radiographs taken during single treatments provide the basis for a three-dimensional (3D) motion analysis. Additionally, studies of orthogonal radiographs used to setup for proton treatments and verification port films from photon treatments are used to establish day to day patient position variation in routine use. RESULTS Net 3D patient motion during any treatment is measured to be 0.9 +/- 0.4 mm [mean +/- standard deviation (SD)] and rotation about any body axis is 0.14 +/- 0.67 degrees (mean +/- SD). Day-to-day setup accuracy to laser marks is limited to 2.3 mm (mean) systematic error and 1.6 mm (mean) random error. CONCLUSION We conclude that the most stringent immobilization requirements of 3D conformal radiotherapy adjacent to critical normal structures can be met with a high precision system such as the one described here. Without the use of pretreatment verification, additional developments in machine and couch design are needed to assure that patient repositioning accuracy is comparable to the best level of patient immobility achievable.

Dosimetric results from a feasibility study of a novel radiosurgical source for irradiation of intracranial metastases

PURPOSE A feasibility study addressing the role of a new miniature x-ray device, the Photon Radiosurgery System (PRS), for interstitial radiosurgical treatment of intracranial metastatic neoplasms, was conducted at our institution. To gain insight into the role of PRS vis-à-vis other currently available radiosurgical treatment modalities, dosimetric comparisons of Linac Radiosurgery and proton beam therapy were performed in the treatment of a small approximately spherical metastasis. METHODS AND MATERIALS The photon radiosurgery system is a miniature, battery operated, high-voltage x-ray generator that produces low-energy x-rays with an effective energy of 10-20 keV emanating from the tip of a probe stereotactically inserted into small tumors (< 3 cm in diameter) in humans. Patients, 18 years or older, with supratentorial mass lesions less than 3 cm in diameter were eligible if they were likely to survive their systemic cancer and be capable of self-care for more than 4 months. Patients were ineligible if presenting with infratentorial lesions, contraindications for biopsy, or receipt of chemotherapy or radiotherapy within 4 weeks were ineligible. RESULTS Fourteen patients with metastatic supratentorial lesions were treated from December 1992 to December 1993 for metastatic tumors to the brain. Single doses of 10-20 Gy were delivered to spherical targets of 10 to 35 mm in diameter. Treatment, including biopsy, pathologic review and radiation treatment, generally took less than 3 h. One patient, later found to have an ischemic stroke, developed a small hemorrhage from the biopsy that preceded interstitial irradiation. There were no other complications. Median survival was 10 months. Three locally recurrent lesions failed at 3.5, 4, and 10 months after treatment. All patients had stable or improved Karnofsky status for 2 weeks to 21 months after treatment. The PRS dosimetry appears at least as good as that obtained using 6 MV Linac or 160 MeV protons. Analyses of dose-volume histograms comparing the volumes of normal CNS tissue irradiated employing each of the respective modalities suggest a small sparing of normal tissue with PRS, as opposed to linac or protons, in this patient population with small, approximately spherical tumors. CONCLUSIONS The PRS device provides a unique cost and time efficient procedure for providing interstitial radiation therapy immediately following histologic confirmation of malignancy in patients undergoing biopsy of intracranial lesions. The PRS treatment appears safe, and preliminary data suggest no evidence of treatment-related morbidity within the life span of the selected patient population. When treating small, spherical lesions, PRS appears to offer a modest dosimetric advantage over Linac or proton beam therapy in sparing normal tissue. These encouraging results have prompted a Phase II trial that is currently underway. Further efforts are necessary in the design of a clinically relevant trial addressing the role of fractionated external beam radiation therapy with boost vs. PRS treatment with WBRT in the treatment of single metastases.

Comparison of proton and x‐ray conformal dose distributions for radiosurgery applications

Highly focused dose distributions for radiosurgery applications are successfully achieved using either multiple static high-energy particle beams or multiple-arc circular x-ray beams from a linac. It has been suggested that conformal x-ray techniques using dynamically shaped beams with a moving radiation source would offer advantages compared to the use of only circular beams. It is also thought that, generally, charged particle beams such as protons offer dose deposition advantages compared to x-ray beams. A comparison of dose distributions was made between a small number of discrete proton beams, multiple-arc circular x-ray beams, and conformal x-ray techniques. Treatment planning of a selection of radiosurgery cases was done for these three techniques. Target volumes ranged from 1.0-25.0 cm3. Dose distributions and dose volume histograms of the target and surrounding normal brain were calculated. The advantages and limitations of each technique were primarily dependent upon the shape and size of the target volume. In general, proton dose distributions were superior to x-ray distributions; both shaped proton and shaped x-ray beams delivered dose distributions which were more conformal than x-ray techniques using circular beams; and the differences between all proton and x-ray distributions were negligible for the smallest target volumes, and greatest for the larger target volumes.

Two-dimensional dose distribution of a miniature x-ray device for stereotactic radiosurgery.

The Photon Radiosurgery System is a miniature x-ray device developed for the treatment of small intracranial neoplasms. The x-rays are generated at the tip of a 10-cm-long, 3-mm-diam probe with a nearly isotropic distribution. Results from measurements of the two-dimensional dose distribution around the x-ray source are presented using two methods: (1) dose measurement with an ionization chamber and a water phantom system and (2) dose measurement with radiochromic film and a solid water phantom. The shape of the two angular dose distributions in the axial plane agree with each other to with approximately 10% and the dose at 10 mm from the source, orthogonal to the probe axis, was about 20% lower than at the same distance along the axis. The relative dose difference of 20% corresponds to a change in distance from the source of +/- 0.3 mm at 10 mm. It is shown that the anisotropy of radiation distribution in the axial plane can be improved to approximately 10% by adjusting the electron beam with a 12% reduction in the overall radiation output.

Proton dosimetry in bone using electron spin resonance

We have studied the ESR response of proton-irradiated (in vitro) bone. The ESR response as a function of proton (E = 105 MeV) dose to bone was linear from 0 to 50 Gy and similar to the photon (E = 6 MV) dose response. The ESR depth response (Bragg) curve was depressed as compared to a depth-response curve determined with a parallel plate ionization chamber (PPIC). There was a short-term ESR signal fade in the Bragg peak region, likely attributable to the organic component in bone. We are continuing to investigate these latter two effects.

Alanine EPR dosimeter response in proton therapy beams.

We report a series of measurements directed to assess the suitability of alanine as a mailable dosimeter for dosimetry quality assurance of proton radiation therapy beams. These measurements include dose-response of alanine at 140 MeV, and comparison of response vs energy with a parallel plate ionization chamber. All irradiations were made at the Harvard Cyclotron Laboratory, and the dosimeters were read at NIST. The results encourage us that alanine could be expected to serve as a mailable dosimeter with systematic error due to differential energy response no greater than 3% when doses of 25 Gy are used.

ESR dosimetry of human cortical bone irradiated by a therapeutic proton beam

Dosimetry based on the electron spin resonance (ESR) technique was used to study human cortical bone irradiated by a therapeutic proton beam. Bone samples were irradiated with 160-MeV protons and, for comparison, 6-MV photons, Additional samples were held aside as controls. The ESR peak-to-peak heights (PPH) for various doses were studied. A Bragg curve was generated by irradiating bone samples at various depths and measuring the ESR response. A companion Bragg curve was also generated via parallel plate ion chamber (PPIC) measurements. The proton-to-photon dose-response was 0.806/spl plusmn/0.034 (+1 s.d.) in the Bragg plateau. This ratio is lower than previously reported and is likely attributable to an intermediate-length (year,) decay component. In the Bragg curve analysis, an iterative technique for sub-millimeter sample depth-correction was employed. The Bragg peak position and height of the ESR data differed from the PPIC data. The ESR Bragg peak shift was -0.6 mm. The ratio of the ESR-to-PPIC Bragg peak height was 0.804/spl plusmn/0.029 (+1 s.d.), for a normalization in the Bragg plateau region. In conclusion, the difference in the peak position and the peak value of the Bragg curve may, in part, be from detector size effects, as noted in the very interesting work of Bichsel (1995). A direct comparison, however, can not be made because of differences in the two studies. The role of differential proton stopping-power effects in the bone response is discussed.