Stereotactic radiosurgery for recurrent malignant gliomas.

PURPOSE: To evaluate the role of stereotactic radiosurgery in the management of recurrent malignant gliomas. PATIENTS AND METHODS: We treated 35 patients with large (median treatment volume, 28 cm3) recurrent tumors that had failed to respond to conventional treatment. Twenty-six patients (74%) had glioblastomas multiforme (GBM) and nine (26%) had anaplastic astrocytomas (AA). RESULTS: The mean time from diagnosis to radiosurgery was 10 months (range, 1 to 36), from radiosurgery to death, 8.0 months (range, 1 to 23). Twenty-one GBM (81%) and six AA (67%) patients have died. The actuarial survival time for all patients was 21 months from diagnosis and 8 months from radiosurgery. Twenty-two of 26 patients (85%) died of local or marginal failure, three (12%) of noncontiguous failure, and one (4%) of CSF dissemination. Age (P = .0405) was associated with improved survival on multivariate analysis, and age (P = .0110) and Karnofsky performance status (KPS) (P = .0285) on univariate analysis. Histology, treatment volume, and treatment dose were not significant variables by univariate analysis. Seven patients required surgical resection for increasing mass effect a mean of 4.0 months after radiosurgery, for an actuarial reoperation rate of 31%. Surgery did not significantly influence survival. At surgery, four patients had recurrent tumor, two had radiation necrosis, and one had both tumor and necrosis. The actuarial necrosis rate was 14% and the pathologic findings could have been predicted by the integrated logistic formula for developing symptomatic brain injury. CONCLUSION: Stereotactic radiosurgery appears to prolong survival for recurrent malignant gliomas and has a lower reoperative rate for symptomatic necrosis than does brachytherapy. Patterns of failure are similar for both of these techniques.

1995 survey of physics teaching efforts in radiation oncology residency programs.

A physics teaching survey was constructed and sent to the 83 radiation oncologist training programs. The survey requested program information regarding size, staffing, curriculum, lab/rotation programs, organization, requirements, instructor makeup, teaching materials, and board certification examination results. The surveys were sent to the physicist responsible for the physics program. Forty-nine (59%) institutions returned completed surveys, of which 43 (88%) were university-associated programs, and 27 (55%) were 4-year programs. On average, there were two residents/year. Most programs (39) taught physics exclusively during the first year (PG2). Some programs taught different subjects (or levels) to different year residents. Radiation dosimetry, treatment planning, and brachytherapy constituted nearly half of the teaching hours. On average the total classroom time expended by physicists was 61.4 h/year with a range of 24-118 h. The mean for laboratory/demonstration time was 27 h/year with 18 programs providing none. Physics orientation/rotations ranged from 1 to 480 h with a mean of 170 h for a physics rotation taking place in year 2 (PG3). Mandatory attendance was 80% for first-year residents and decreased in later years. Homework was assigned in 76% of the programs, and 65% of the programs were graded. The primary instructors averaged 18.2 years of experience, and the majority were ABR/ABMP certified. Khan's textbook was the most prevalent resource for most subjects. No correlation could be made between teaching hours and ABR physics percentile scoring. The survey results reveal enormous differences in national teaching efforts.

Evaluation of eye shields made of tungsten and aluminum in high-energy electron beams.

PURPOSE: To protect the lens and cornea of the eye when treating the eyelid with electrons, we designed a tungsten and aluminum eye shield that protected both the lens and cornea, and also limited the amount of backscatter to the overlying eyelid when using electron beam therapy. METHODS AND MATERIALS: Custom curved tungsten eye shields, 2 mm and 3 mm thick, were placed on Kodak XV film on 8 cm polystyrene and irradiated to evaluate the transmission through the shields. To simulate the thickness of the eyelid and to hold the micro-TLDs, an aquaplast mold was made to match the curvature of the eye shields. Backscatter was measured by placing the micro-TLDs on the beam entrance side to check the dose to the underside of the eyelid. Measurements were done with no aluminum, 0.5, and 1.0 mm of aluminum on top of the tungsten eye shields. The measurements were repeated with 2- and 3-mm flat pieces of lead to determine both the transmission and the backscatter dose for this material. RESULTS: Tungsten proved to be superior to lead for shielding the underlying structures and for reducing backscatter. At 6 MeV, a 3-mm flat slab of tungsten plus 0.5 mm of aluminum, resulted in .042 Gy under the shield when 1.00 Gy is delivered to dmax. At 6 MeV for a 3-mm lead plus 0.5-mm aluminum, .046 Gy was measured beneath the shield, a 9.5% decrease with the tungsten. Backscatter was also decreased from 1.17 to 1.13 Gy, a 4% decrease, when using tungsten plus 0.5 mm of aluminum vs. the same thickness of lead. Measurements using 9 MeV were performed in the same manner. With 3 mm tungsten and 0.5 mm of aluminum, at 3 mm depth the dose was .048 Gy compared to .079 Gy with lead and aluminum (39% decrease). Additionally, the backscatter dose was 3% less using tungsten. Simulating the lens dose 3 mm beyond the shield for the 2-mm and 3-mm custom curved tungsten eye shields plus 0.5 mm of aluminum was .030 and .024 Gy, respectively, using 6 MeV (20% decrease). Using 9-MeV electrons, the dose 3 mm beyond the shield was .048 Gy for the 2-mm shield and .029 Gy for the 3-mm shield (40% decrease). Backscatter was not further decreased using thicker tungsten. With a 6-MeV beam, using the 2-mm or 3-mm custom tungsten eye shields plus 0.5 mm of aluminum, the backscattered doses were 1.03 and 1.02 Gy, respectively. The backscatter dose with 9 MeV was 1.06 Gy using the 2-mm custom shield plus 0.5 mm aluminum and 1.05 Gy with a 3-mm custom shield plus 0.5 mm aluminum. There was very little difference in backscatter dosage under the eyelid using 0.5 vs. 1.0 mm of aluminum. Therefore, for patient comfort, we recommend using 0.5 mm of aluminum. CONCLUSIONS: Tungsten is superior to lead as a material for eye shields due to its higher density and lower atomic number (Z). Using 6- and 9-MeV electrons, tungsten provides the necessary protection for the lens and cornea of the eye and decreases the amount of backscatter to the eyelid above the shield.

Evaluation of dose variation during total skin electron irradiation using thermoluminescent dosimeters.

PURPOSE: To determine acceptable dose variation using thermoluminescent dosimeters (TLD) in the treatment of Mycosis Fungoides with total skin electron beam (TSEB) irradiation. METHODS AND MATERIALS: From 1983 to 1993, 22 patients were treated with total skin electron beam therapy in the standing position. A six-field technique was used to deliver 2 Gy in two days, treating 4 days per week, to a total dose of 35 to 40 Gy using a degraded 9 MeV electron beam. Thermoluminescent dosimeters were placed on several locations of the body and the results recorded. The variations in these readings were analyzed to determine normal dose variation for various body locations during TSEB. RESULTS: The dose to flat surfaces of the body was essentially the same as the dose to the prescription point. The dose to tangential surfaces was within +/- 10% of the prescription dose, but the readings showed much more variation (up to 24%). Thin areas of the body showed large deviations from the prescription dose along with a large amount of variation in the readings (up to 22%). Special areas of the body, such as the perineum and eyelid, showed large deviations from the prescription dose with very large (up to 40%) variations in the readings. DISCUSSION: The TLD results of this study will be used as a quality assurance check for all new patients treated with TSEB. The results of the TLDs will be compared with this baseline study to determine if the delivered dose is within acceptable ranges. If the TLD results fall outside the acceptable limits established above, then the patient position can be modified or the technique itself evaluated.

Total skin electron arc irradiation using a reclined patient position.

Several techniques for the treatment of the total skin of the patient using electron beams have been described in the literature. However, most techniques presuppose that the patient is capable of maintaining a standing position for the duration of the treatment. For patients either weakened by disease or those suffering from a loss of limbs, this is often an unrealistic expectation. We will describe a total skin electron irradiation technique that allows the patient to remain in a reclined position without sacrificing dose uniformity. This technique uses two symmetric +/- 48 degrees arc electron beams to provide a field uniformity of +/- 5% over a range of approximately 250 cm X 45 cm. Six patient positions are used to provide a uniform dose around the periphery of the patient. A description of the treatment technique along with details of the dosimetry are given.

Maintaining accuracy in stereotactic radiosurgery.

PURPOSE: To provide the manufacture's specification for the base phantom of a commercially available stereotactic radiosurgery system so that its accuracy can be confirmed, and to describe a calibration device that allows the accuracy of the base phantom to be verified quickly and on a routine basis. Modifications to the target pointer system that make matching the pointer tips easier and less likely to damage the pointer tips are also described. METHODS AND MATERIALS: In stereotactic radiosurgery, spatial accuracy is the key factor for successful dose delivery. With some commercially available systems, this accuracy depends on the accuracy of the base phantom coordinate system, how closely the tip of the target pointer can be matched to the tip of the base phantom pointer, and how accurately the coordinates set on the isocentric subsystem match those set on the base phantom. Two major problems, usually overlooked when evaluating system accuracy are, first, the base phantom, which establishes the stereotactic coordinate system, is assumed to be completely accurate. This is a dangerous assumption because the base phantom is used frequently for routine patient treatments and for standard quality assurance tests. To exacerbate the problem, no independent device is provided with stereotactic systems to check the accuracy of the base phantom. Second, the accuracy of the isocenter coordinates set on the head support stand depends upon how closely the target pointer and the base phantom pointer can be aligned. The hardware provided with the system is difficult to use and easily leads to damage of the pointer tips. RESULTS: In this work, we provide the manufacturer's specifications for a popular stereotactic system, describe a device that can be used to check quickly and easily the accuracy of the base phantom, and describe a modification to the transfer pointer system that allows the pointer tips to be more easily aligned with reduced possibility of damage to the pointer tips. CONCLUSION: The methods and apparatus described in this paper should be useful to anyone using a base phantom for testing radiosurgery accuracy.

Single dose versus fractionated stereotactic radiotherapy for recurrent high-grade gliomas.

PURPOSE: To evaluate the efficacy of stereotactic radiotherapy (SRT) in patients with recurrent high-grade gliomas by comparing two different treatment regimens, single dose or fractionated radiotherapy. METHODS AND MATERIALS: Between April 1991 and January 1998, 71 patients with recurrent high-grade gliomas were treated with SRT. Forty-six patients (65%) were treated with single dose radiosurgery (SRS) and 25 patients (35 %) with fractionated stereotactic radiotherapy (FSRT). For the SRS group, the median radiosurgical dose of 17 Gy was delivered to the median of 50% isodose surface (IDS) encompassing the target. For the FSRT group, the median dose of 37.5 Gy in 15 fractions was delivered to the median of 85% IDS. RESULTS: Actuarial median survival time was 11 months for the SRS group and 12 months for the FSRT group (p = 0.3, log-rank test). Variables predicting longer survival were younger age (p = 0.006), lower grade (p = 0.0006), higher Karnofsky Performance Scale (KPS) (p = 0.0005), and smaller tumor volume (p = 0.02). Patients in the SRS group had more favorable prognostic factors, with median age of 48 years, KPS of 70, and tumor volume of 10 ml versus median age of 53 years, KPS of 60, and tumor volume of 25 ml in the FSRT group. Late complications developed in 14 patients in the SRS group and 2 patients in the FSRT group (p<0.05). CONCLUSION: Given that FSRT patients had comparable survival to SRS patients, despite having poorer pretreatment prognostic factors and a lower risk of late complications, FSRT may be a better option for patients with larger tumors or tumors in eloquent structures. Since this is a nonrandomized study, further investigation is needed to confirm this and to determine an optimal dose/fractionation scheme.

A survey of radiation oncologists regarding their radiation physics instruction.

The American Association of Physicists in Medicine, Committee on Training of Radiologists conducted a survey of radiation oncologists requesting information regarding their radiation oncology physics training. General questions were asked of the oncologist regarding their radiation oncology practice such as number of oncologists, number of new patients treated, and the size and type of facility in which the practice is located. The oncologist also responded to questions regarding their educational background. The survey requested the radiation oncologists to answer questions regarding the adequacy and importance of their training in specific areas of radiation physics. The responders indicated that the importance of most physics topics in their clinical practice corresponded to the level of their understanding. The survey indicated that for most radiation oncologists their physics instruction was an important and interesting part of their residency program.

An afterloading brachytherapy device utilizing thermoplastic material.

An afterloading brachytherapy device for treatment of residual cancer in an enucleated orbit with two cesium-137 sources was designed using a thermoplastic material, Aquaplast. The device consists of a face-mask support held in place with elastic bands around the head and an acrylic afterloading applicator. The device is very easy to make, holds the sources firmly in place, allows full mobility of the patient, and gives excellent dose distribution to the target area. It was easily tolerated by a 7-year-old child during the 50 h of treatment.

Radiation treatment decreases bone cancer pain, osteolysis and tumor size.

Radiotherapy is the cornerstone of palliative treatment for primary bone cancer in animals and metastatic bone cancer in humans. However, the mechanism(s) responsible for pain relief after irradiation is unknown. To identify the mechanism through which radiation treatment decreases bone cancer pain, the effect of radiation on mice with painful bone cancer was studied. Analysis of the effects of a 20-Gy treatment on localized sites of painful bone cancers was performed through assessments of animal behavior, radiographs and histological analysis. The findings indicated that radiation treatment reduced bone pain and supported reduced cancer burden and reduced osteolysis as mechanisms through which radiation reduces bone cancer pain.

The response characteristics of a newly designed plane-parallel ionization chamber in high-energy photon and electron beams.

A new plane-parallel ionization chamber has been designed by Attix to overcome the shortcoming of previous commercially available parallel-plate ionization chambers for dosimetry in high-energy photon and electron beams in radiation oncology. This investigation details the performance characteristics of this new, commercially available plane-parallel chamber. The magnitude of the polarity effect in high-energy electron beams is shown to be less than 1% while the polarity effect in high-energy photon beams is lower than several other plane-parallel ionization chambers. The over response of the chamber in the buildup region of normally incident high-energy photon beams is less than 1% for 6- and 24-MV x rays while the response of the new chamber to obliquely incident x-ray beams was affected much less by the angle of beam incidence than the other chambers tested. These superior response characteristics are primarily due to the construction characteristics of the collecting electrode arrangement. The Attix chamber, with a wall diameter (w) of 40 mm and a plate separation (s) of 1 mm, has an aspect ratio, (w/s), of 40. This exceeds the previously reported design criterion of w/s > or = 25 required to properly measure surface and buildup dose in either conventional therapy beams or in beams that are highly contaminated.

Compensating filter design using radiographic stereo shift information.

This paper describes a method of designing 3-dimensional compensating filters for radiation therapy using photon beams. A radiopaque grid is placed on the patient surface and stereo shift radiographs are taken of the treatment area. With the aid of a computer, tissue deficit information is calculated. Isothickness lines are plotted for the different missing tissue thicknesses and lead sheets with proper magnification are cut from these plots and assembled into the final tissue compensator.

A mathematical expression for %DD accurate from Co-60 to 24 MV.

Obtaining accurate %DD values for routine treatment calculations is essential in radiation therapy. Many papers have presented expressions to calculate this or related parameters but these expressions often required that the parameters needed by the equation be determined for each individual treatment unit. This paper presents an expression that calculates %DD values with a mean-square accuracy of approximately 1.0% versus measured values. The expression is applicable to beam energies ranging from Co-60 to 24 MV, field sizes from 4 X 4 to 40 X 40 cm2, and depths from 1 cm deeper than dmax to 30 cm. The only information required by this expression that is machine specific is the ionization ratio.

Measurement of dose in the buildup region using fixed-separation plane-parallel ionization chambers.

Accurate measurement of dose at the surface of a phantom and in the buildup region is a difficult task but one that is important for the proper treatment of patients. The instruments of choice for these measurements are extrapolation chambers but few institutions have these instruments at their disposal. As a result, fixed-separation plane-parallel ionization chambers are most commonly used for this purpose. Recent papers have re-emphasized the inaccuracies in the measurement of dose in the buildup region of normally incident photon beams when using fixed-separation plane-parallel ionization chambers. Data for Co-60, 6-, 10-, 18-, and 24-MV photon beams are presented that show the magnitude of this over response in the buildup region for several commercially available plane-parallel ionization chambers versus results obtained using both an extrapolation chamber and LiF thermoluminescent detectors. Differences in the percent depth dose at the surface of a phantom of greater than 19% were found for one of the chambers. All chambers over responded in the buildup region to some degree based upon their internal dimensions. The appropriateness of published corrections for these chambers is evaluated and guidelines for the accurate measurement of dose in the buildup region are presented.

Plane-parallel ionization chamber response in the buildup region of obliquely incident photon beams.

Fixed-separation plane-parallel ionization chambers have been shown to overestimate the dose in the buildup region of normally incident high-energy photon beams. This work shows that these ionization chambers exhibit an even greater over-response in the buildup region of obliquely incident photon beams. This over-response at oblique incidence is greatest at the surface of the phantom and increases with increasing angle of beam incidence. In addition, the magnitude of the over-response depends on field size, beam energy, and chamber construction. This study shows that plane-parallel ionization chambers can over-respond by more than a factor of 2.3 at the phantom surface for obliquely incident high-energy photon fields.

The polarity effect for commercially available plane-parallel ionization chambers.

The polarity effect was investigated for three different commercially available plane-parallel ionization chambers: the Memorial Pipe chamber, the Victoreen/Nuclear Associates model 30-329 chamber manufactured by PTW, Frieburg, and the Capintec PS-033 thin-window ionization chamber. The primary study was the polarity effect versus depth below the phantom surface for 6-, 10-, 18-, and 24-MV x-ray beams, and 9- and 22-MeV electron beams. The polarity effect in the region of nonelectronic equilibrium that exists at the interface of two dissimilar materials, polystyrene and aluminum, was investigated as well as the effects of field size. For the group of plane-parallel ionization chambers that we studied, we found a polarity effect of only 1%-2% for electron beams at the depth of dmax. At depths greater than dmax, the polarity effect for electrons increased and was as high as 4.5% for some chambers. When used in the buildup region of high-energy photon beams, these same chambers exhibited up to a 30% difference in collected charge between one polarity and the other. This effect and its relationship to physical chamber characteristics is discussed.

Evaluation of electronic portal imaging device for missing tissue compensator design and verification.

The purpose of this investigation is to determine if electronic portal imaging devices (EPIDs) can be used for the design and verification of compensating filters. In order to do this, we investigated the operating characteristics of a commercially available EPID and the variation in transmitted dose for various measurement situations. We performed four initial tests to determine the EPID response specific to compensator situations. The tests determined EPID response to variable patient SSDs, different gantry angles, positions of an inhomogeneity within a phantom, and the sensitivity variation of different parts of the imager. After these tests, we determined the attenuation functions relating EPID response to phantom thickness for various phantom materials. With these functions, we tested simple compensation situations to demonstrate that missing tissue compensators can both be designed and verified using EPIDs.

Evaluation of a model-based treatment planning system for dose computations in the kilovoltage energy range.

The ability to determine dose distribution and calculate organ doses from diagnostic x rays has become increasingly important in recent years because of relatively high doses in interventional radiology and cardiology procedures. In an attempt to determine the dose from both diagnostic and orthovoltage x rays, we have used a commercial treatment planning system (Pinnacle, ADAC Laboratories, Milpitas, CA) to calculate the doses in phantoms from kilovoltage x rays. The planning system's capabilities for dose computation have been extended to lower energies by the addition of energy deposition kernels in the 20-110 keV range and modeling of the 60, 80, 100, and 120 kVp beams using the system. We compared the dose calculated by the system with that measured using thermoluminescent dosimeters (TLDs) placed in various positions within several phantoms. The phantoms consisted of a cubical solid water phantom, the solid water phantom with added lung and bone inhomogeneities, and the Rando anthropomorphic phantom. Using Pinnacle, a treatment plan was generated using CT scans of each of these phantoms and point doses at the positions of TLD chips were calculated. Comparisons of measured and computed values show an average difference of less than 2% within materials of atomic number less than and equal to that of water. The algorithm, however, does not produce accurate results in and around bone inhomogeneities and underestimates attenuation of x rays by bone by an average of 145%. A modification to the CT number-to-density conversion table used by the system resulted in significant improvements in the dose calculated to points beyond bone.

Lung dose calculations at kilovoltage x-ray energies using a model-based treatment planning system.

The determination of the dose to organs from diagnostic x rays has become important because of reports of radiation injury to patients from fluoroscopically guided interventional procedures. We have modified a convolution/superposition-based treatment planning system to compute the dose distribution for kilovoltage beams. We computed lung doses using this system and compared them to those calculated using the CDI3 organ dose calculation program. We also computed average lung doses from a simulated radiofrequency ablation procedure and compared our results to published doses for a similar procedure. Doses calculated using this system were an average of 20% lower for AP beams and 7% higher for PA beams than those obtained using CDI3. The ratio of the average dose to the lungs to the skin dose from the simulated ablation procedure ranged from 25% higher to 15% lower than that determined by other authors. Our results show that a treatment planning system designed for use in the megavoltage energy range can be used for calculating organ doses in the diagnostic energy range. Our doses compare well with those previously reported. Differences are partly due to variations in experimental techniques. Using a three-dimensional (3-D) treatment planning system to calculate dose also allows us to generate dose volume histograms (DVH) and compute normal tissue complication probabilities (NTCP) for diagnostic procedures.

Generation and use of photon energy deposition kernels for diagnostic quality x rays.

Accurately determining the dose from low energy x rays is becoming increasingly important. This is especially so because of high doses in interventional radiology procedures and also because of the desire to model accurately the dose around low energy brachytherapy sources. Various methods to estimate the dose from specific procedures are available but they only give a general idea of the true dose to various organs. The use of sophisticated three-dimensional (3D) dose deposition algorithms designed originally for radiation therapy treatment planning can be extended to lower photon energy regions. The majority of modern 3D treatment planning systems use a variation of the convolution algorithm to calculate dose distributions. This could be extended into the diagnostic energy range with the availability of lower energy deposition kernels ( < 100 keV). We have used version four of the Electron Gamma Shower (EGS4) system of Monte Carlo codes to generate photon energy deposition kernels in the energy range of 20-110 keV and have implemented them in a commercial 3D treatment planning system (Pinnacle, ADAC Laboratories, Milpitas, CA). The kernels were generated using the "SCASPH" EGS4 user code by selecting the appropriate transport parameters suitable for the relative low energy of the incident photons. The planning system was subsequently used to model diagnostic quality beams and to calculate depth dose and cross profile curves. Comparisons of the calculated curves have been made with measurements performed in a homogeneous water phantom.