TLD measurements of the relative output factors for the Leksell Gamma Knife

The accuracy of the output factor directly affects the accuracy of dose delivery during patient treatment. At our Gamma Knife center, annual output factor measurements have been carried out by using a high accuracy TLD technique, characterized by the group annealing and sorting (GAS) procedure. For each collimator exposure, one to five LiF Thermoluminescent Dosimeter (TLD) 1 x 1 x 1 mm3 cubes were used, depending upon the collimator size, and the process was repeated until approximately 10 TLDs had been exposed for each collimator. Transit radiation dose accumulated during the motion of the treatment couch was measured for each collimator helmet, and the result was subtracted from the uncorrected TLD dose measurements. The mean values of the output factors for the 14, 8 and 4 mm collimators from last 5 years were 0.985 ¿ 0.001, 0.948 ¿ 0.005 and 0.833 ¿ 0.007, respectively, relative to the 18 mm collimator. These measured relative output factors are virtually identical to the values recommended by the manufacturer for the 14 mm and 8 mm collimators. On the other hand, the output factor for the 4 mm collimator was approximately 4.1% larger than that recommended by the manufacturer. The significance of these findings is discussed.

Monitor unit calculation for an intensity modulated photon field by a simple scatter-summation algorithm

An important issue in intensity modulated radiation therapy (IMRT) is the verification of the monitor unit (MU) calculation of the planning system using an independent procedure. Because of the intensity modulation and the dynamic nature of the delivery process, the problem becomes much more involved than that in conventional radiation therapy. In this work, a closed formula for MU calculation is derived. The approach is independent of the specific form of leaf sequence algorithms. It is straightforward to implement the procedure using a simple computer program. The approach is illustrated by a simplified example and is demonstrated by a few CORVUS (NOMOS Corporation, Sewickley, PA) treatment plans. The results indicate that it is robust and suitable for IMRT MU verification.

Radial dose distribution, dose to water and dose rate constant for monoenergetic photon point sources from 10 keV to 2 MeV:EGS4 Monte Carlo model calculation

A comprehensive set of dose distributions from monoenergetic photon-emitting isotropic point sources in a medium can be used as a reference database for the dosimetry of photon emitter sources in that medium. Data of this type for water over the photon energy range from 15 keV to 2 MeV have been published based on calculations using a one-dimensional photon transport model. The present work, based on a previously published EGS4 Monte Carlo code, updates the classic data set of Berger and provides more extensive calculations than previously available. Air kerma strength per unit photon emission rate from an isotropic point emitter is obtained as a function of energy using published data for mass energy absorption coefficients. The TG-43 dose rate constant for water as a function of energy is calculated for monoenergetic photon emitters as the ratio of dose rate to water at 1 cm to air kerma strength for unit photon emission rate. Results for the radial dose distribution agree well with the data of Berger between 40 and 400 keV. For energies > or =500 keV, a previously undescribed buildup region for the radial dose function is identified. Thickness of the buildup region ranges from 1 mm at 500 keV to 8 mm at 2 MeV. Between 15 and 30 keV, the radial dose function within a few millimeters of the emitter is calculated to be 4%-5% higher than values derived from Berger's data. The maximum dose rate constant for monoenergetic photon emitters occurs at an energy of 60 keV, and has the value 1.355 cGy h(-1)U(-1), where U is the unit of air kerma strength, 1 microGy m2 h(-1). This would correspond to the maximum hypothetical dose rate constant for a brachytherapy photon source emitting photons of energy < or =2 MeV.

Dosimetric comparison of three photon radiosurgery techniques for an elongated ellipsoid target

PURPOSE

To examine the dosimetric differences among three radiosurgery techniques: gamma knife, linac multiple arcs, and conformally-shaped static fields.

METHODS AND MATERIALS

A simulated target was taken to be a prolate ellipsoid, 25 mm in diameter, 35 mm in length, centrally located in a three-dimensional (3D) model of a patient head taken from MR images. Single isocenter linac treatment plans were developed, 9 portals for the static shaped field technique, and a 7-arc plan for the multiple arc method. A total of 13 isocenters with 3 different collimators were used in the gamma knife plan.

RESULTS

At dose levels from 25% to 50% of the reference dose, multiple arc and shaped-field plans treated a greater volume than the gamma knife plan. The linac plans, however, delivered the dose more homogeneously across the target volume as compared to the gamma knife plan. For the dose levels between 50-100%, the shaped fields and gamma knife plan have a similar dose distribution, and treated slightly less volume than the multiple arc plan.

CONCLUSION

For a target of limited volume and essentially any shape, one can obtain closely conformal dosimetry with the gamma knife. For a regular-shaped target, the single isocenter multiple arc technique gives a more homogenous dose distribution within the target. Static shaped fields offer an alternative radiosurgery technique, with dosimetry similar to the multiple arc method, applicable to targets of any shape.

TLD dose measurement: a simplified accurate technique for the dose range from 0.5 cGy to 1000 cGy

A simplified TLD technique characterized by high precision and reproducibility of dose measurement is presented. One hundred eighty LiF TLD rods 1 mm diam x 3 mm length as obtained from the manufacturer were annealed for 1 h at 400 degrees C followed immediately by 2 h at 105 degrees C. After exposure to a dose of 1 Gy of 4 MV x rays, TLDs were annealed for 15 min at 105 degrees C, then read out. TLDs were then sorted into five groups, ranging from 26 to 50 rods each with approximately equal sensitivity after correcting for the drift in the sensitivity of the TLD reader during the readout session. Maintaining group identity, the TLDs were again annealed, irradiated and read out. Fewer than 10% of the TLDs were removed from each group because the corrected readings differed from the respective group mean by more than 3.5%. The standard deviation of the readout was approximately 1.5% within each group. The planchet heater was not flushed with nitrogen gas. Various tests were performed to assess the stability of the group sorting technique and the linearity of TLD dose response. After reannealing, five TLDs were randomly drawn from one of the presorted groups, and subjected to various dose of 4 MV radiation over the range from 0.5 to 1000 cGy. This resulted in an average readout standard deviation of 1.2%. Response per unit dose was almost flat over the range from 0.5 cGy to 100 cGy, and increased by 15% over the range from 100 cGy to 1000 cGy. TLD sensitivity was affected by the duration of the anneal, but was virtually independent of the various time delays between irradiation, prereadout anneal, and readout. The group annealing and sorting (GAS) procedure provides a simple, reliable, precise, convenient, and accurate method for TLD measurements.

Prealbumin measurement as a screening tool for protein calorie malnutrition in emergency hospital admissions: a pilot study

OBJECTIVE

To examine the effectiveness of prealbumin measurement as a routine diagnostic test for protein calorie malnutrition (PCM) in emergency admissions.

DESIGN

Cohort study.

SETTING

Canadian tertiary care hospital.

PATIENTS

A total of 147 consecutive patients admitted through the emergency department, excluding psychiatric admissions, for whom a serum sample was sent to the chemistry laboratory.

OUTCOME MEASURES

(1) Albumin and prealbumin levels to diagnose PCM; (2) length of stay (LOS), in-hospital mortality, and nutritional supplementation in patients with PCM; and (3) cost analysis of using prealbumin level as an admission screening test for PCM.

RESULTS

Twenty-four per cent of the patients had at least mild PCM (prealbumin level < 160 mg/L). Albumin level (< 35 g/L) had a sensitivity of 50% and specificity of 95% in identifying PCM. The mean LOS in the patients with PCM was 16 (standard deviation 18) days, compared with 8 (SD 12) days in the patients without PCM (p < 0.0002). The in-hospital mortality rate was 17% in patients with PCM compared with 4% in patients without PCM (p < 0.02). Only 42% of patients with PCM received nutritional supplementation. Cost analysis of screening with prealbumin level projected a saving of $414 per patient screened.

CONCLUSIONS

PCM is underdiagnosed in current clinical practice and is associated with an increased LOS and mortality rate. Prealbumin is a biochemical marker that could be used as a cost-effective screening test to identify patients with PCM who may benefit from nutritional supplementation.

Gamma knife radiosurgery for metastatic melanoma: an analysis of survival, outcome, and complications

OBJECTIVE

Although the mainstays for treatment of metastatic brain disease have been surgery and/or external beam radiation therapy, an increasing number of patients are being referred for stereotactic radiosurgery as the primary intervention for their intracranial pathological abnormalities. The lack of efficacy and cognitive and behavioral consequences of whole brain irradiation have prompted clinicians to select patients for alternative therapies. This study analyzes the effectiveness of Leksell gamma unit therapy for metastatic melanoma to the brain.

METHODS

We present our experience with 59 Leksell gamma unit treatment sessions in 45 consecutive patients who presented with metastatic melanoma to the brain. Five of these procedures were performed as salvage therapy for patients who needed second radiosurgical treatment for new lesions that were remote from the previous targets and were not included in the overall analyses.

RESULTS

The population included 78% male patients. The mean patient age was 53 years (age range, 24-80 yr). The mean time from diagnosis of primary melanoma to discovery of brain metastasis was 43 months (median, 27.5 mo; range, 1-180 mo). At the time of diagnosis of brain disease, 35.5% of the patients (16 of 45 patients) had neurological symptoms, 77.7% (35 of 45 patients) had known visceral metastases, and 11.1% (5 of 45 patients) had seizure disorders. Eighty-six percent of the lesions (80 of 93 lesions) were cortical, 12% (11 of 93 lesions) were cerebellar, 1% (1 of 93 lesions) were pontine, and 1% (1 of 93 lesions) were thalamic. Fifty-seven percent of the sessions (31 of 54 sessions) were performed for a single lesion, 24.1% (13 of 54 sessions) for two lesions, 9.2% (5 of 54 sessions) for three lesions, 7.4% (4 of 54 sessions) for four lesions, and 1.8% (1 of 54 sessions) for five lesions. The mean treatment volume was 5.6 cc, with a mean prescription of 21.6 Gy to the 56.0% mean isodose line. The median survival time of the patients in our population, using Kaplan-Meier curves, was 43 months from the time of diagnosis of primary melanoma (range, 3-180 mo) and 8 months (range, 1-20 mo) from the time of gamma knife treatment. Complications included seizures within 24 hours of the procedure in four patients, with transient nausea and vomiting in three patients, transient worsening of preprocedure paresis responsive to steroids in three patients, and increased confusion in one patient. All 45 patients were located for follow-up (mean follow-up duration, 1 yr). After gamma knife treatment, 78% of the patients (35 of 45 patients) experienced either improved or stable neurological symptomatology before death or at the time of the latest follow-up examination. There were 26 deaths (58%). The cause of death was determined to be neurological in only 2 of 45 patients (7.7%). Follow-up magnetic resonance images revealed a 97% local tumor control rate of gamma knife-treated lesions, with 28% radiographic disappearance (9 of 32 cases). Six patients developed new lesions remote from radiosurgical targets and underwent second procedures.

CONCLUSION

Although metastatic melanoma to the brain continues to have a foreboding prognosis for long-term survival, gamma knife radiosurgery seems to be a relatively safe, noninvasive, palliative therapy, halting or reversing neurological progression in 77.8% of treated patients (35 of 45 patients). The survival rate matches or exceeds those previously reported for surgery and other forms of radiotherapy. Only 7.7% of the patients in our study population who died as a result of metastatic melanoma (2 of 26 patients) died as a result of neurological disease. The routine use of therapeutic level antiseizure medication is emphasized, considering the findings of our review.

Conformal episcleral plaque therapy

PURPOSE

Episcleral plaque therapy (EPT) with sealed 125I sources is widely used in the treatment of choroidal melanoma. In EPT, as elsewhere in radiotherapy, concern for normal tissue tolerance has frequently been a dose-limiting factor. The concept of conformal therapy, which seeks to improve dose homogeneity within the tumor and greatly reduce the dose to uninvolved structures may provide a solution to this problem. Radioactive sources are typically distributed uniformly over the surface of an episcleral plaque and are sometimes offset slightly from the scleral surface to reduce the dose to the sclera relative to the apex and prescribed therapeutic margin at the tumor base. Nevertheless, it is not uncommon for scleral dose to exceed the dose to the apex of intermediate to tall tumors by a factor of 4 or more. The availability of low-energy sealed sources such as 125I prompted the development of gold-backed plaques to shield noninvolved periocular tissues. The concept of shielding can be extended to include collimation of individual sources. The potential advantages of individual source collimation include reduced scleral dose, more homogeneous tumor dose, and superior shielding of adjacent normal structures such as the fovea as compared to previous plaque designs.

METHODS AND MATERIALS

A three-dimensional treatment-planning system has been extended to design a plaque that incorporates individually collimated 125I sources. Thermoluminescent dosimetry (TLD) and radiochromic film were used to compare calculated dose-rate distributions with measured dose rates in an acrylic phantom.

RESULTS

Calculations predict that source collimation in the form of a "slotted" gold plaque will achieve the purposes of the study. The collimating effect of the slots is demonstrated qualitatively using radiochromic film, and the accuracy of the calculation is demonstrated quantitatively with TLD.

CONCLUSION

The episcleral plaque described in this report is simpler to assemble than previous plaque designs. It produces a more homogeneous dose distribution in the tumor, reduces scleral dose by up to 50% as compared to conventional designs, and significantly reduces radiation dose to uninvolved structures adjacent to the plaque.

Extracranial radiation doses in patients undergoing gamma knife radiosurgery

OBJECTIVE

To determine extracranial doses in patients undergoing gamma knife radiosurgery and identify component sources of the extracranial doses using phantom measurements.

METHODS

The lateral canthi, thyroid, sternum, and midpelvis region were monitored in 104 unselected patients during their gamma knife treatments using thermoluminescent dosimetry. Measured doses were normalized to integral dose, equivalent time (which is defined in relation to the activity of the cobalt-60 sources), and collimator size to correlate radiation doses with these parameters. A phantom was constructed from a polystyrene sphere as a model of the head adjacent to thoracic and pelvic body sections from a commercial humanoid phantom.

RESULTS

On average, 18 minutes of equivalent time and five isocenters were required to achieve the prescribed dose coverage. The median prescribed dose was 18 Gy. For the lateral canthi, thyroid, sternum, and pelvis, the median doses were 24, 20, 21, and 4 cGy, respectively. Normalization to equivalent time and collimator size was superior to other techniques. Phantom measurements supported the results from patient measurements and further refined estimates of component doses to extracranial sites.

CONCLUSION

Doses to extracranial sites ranged from 1.5% of the prescribed dose for the lateral canthi to 0.2% for the pelvis. Doses to the sternum and pelvis were proportional to the duration of irradiation. Scatter radiation contributed more than 50% of the dose to the canthi and thyroid. Leakage radiation typically contributed 80 to 90% of the dose to the sternum and pelvis. Radiation during patient couch transit contributed little to the doses at the measured extracranial sites.

Episcleral plaque thermoradiotherapy of posterior uveal melanomas

Episcleral plaque radiotherapy is a widely applied treatment for selected patients with uveal melanomas. This treatment is well tolerated but may produce severe late radiation complications resulting in decreased visual acuity that reduces the attractiveness of conservative therapy. The purpose of this study was to access if the addition of episcleral hyperthermia decreases late radiation complications through radiation dose reduction while maintaining high incidence of local tumor control. In a 3-year period, episcleral plaque thermoradiotherapy was given to 25 patients with uveal melanoma in a Phase I study. The mean tumor height was 6.2 mm and the mean tumor basal area was 173 mm(2). The mean radiation dose given to the tumor apex was 72.2 Gy and the mean hyperthermia temperature, given once for 45 min, was 43.5 degrees C. Of the 25 patients treated, 22 (88%) showed tumor height reduction, 2 (8%) showed no change, and 1 (4%) had an increase in tumor height. At the last follow-up (range, 20-68 months; mean, 31.2 months), a 43% mean tumor height reduction was recorded (p = 0.0002). Of the 22 patients initially showing tumor regression, 2 (9%) had subsequent tumor progression. At least ambulatory vision (>5/200) was maintained by 20 (80%) patients. Severe complications, including hemorrhagic retinal detachment and a large vitreous hemorrhage, were seen in 2 (8%) patients early in this Phase I study. The treatment program was well tolerated by the study patients. Severe late treatment toxicity was sharply reduced by limiting the mean scleral temperature to < or equal to 44 degrees C. This study employing 30% lower radiation doses, showed tumor regression in the majority of patients. Longer follow-up is needed to assess long-term treatment efficacy and late treatment complications.

Mechanical malfunction of the Leksell Gamma knife during patient treatment

During the course of a patient treatment with a North American U-type gamma unit, the remote hydraulic valve controlling the direction of couch motion failed to change state. The couch, helmet and patient remained in treatment position after the expiration of treatment time for one of the target shots. No unusual equipment warning indications had been observed prior to the malfunction. The gamma unit was new, having been used to treat approximately 20 patients since it had begun to be used clinically 11 weeks previously. This specific situation was not addressed in our posted Emergency Procedures, which dealt explicitly with loss of electrical power, and loss of hydraulic pressure. In the present case, the hydraulic gauges indicated full pressure. After attempts to disengage the patient remotely proved unsuccessful, personnel entered the room. The table clutch at the foot of the couch was operated to disengage the couch/helmet assembly from its docked position. While this was not mentioned in our emergency procedures, the act had the effect of causing the cobalt-60 sources to be misaligned with the collimator apertures, thereby immediately terminating the patient treatment. This also had the unanticipated effect of substantially reducing radiation leakage exposure rate next to the couch near the tunnel opening. The patient was released from the helmet trunnions using a manufacturer-supplied long-handled special Allen key. The key was used conventionally, to release the trunnion locking mechanism, and also unconventionally to force a separation of a trunnion from the docking slot on the patient head frame. The patient was then removed from the tunnel by sliding out the pad on which she was lying. Anesthesiology personnel accompanied the patient out of the room. The unit functioned properly upon the replacement of the valve by manufacturer service personnel the next day. The patient returned for completion of treatment 1 week later. There were only minor changes to the overall patient dosimetry as a result of the malfunction. Personnel exposures were very low. The malfunction was reported to State authorities, who conducted an investigation, that was in turn followed up by an investigation by the Nuclear Regulatory Commission.

Gamma unit facility: concept genesis, architectural design and practical realization

The physical creation of a gamma unit facility requires the development of a broad-perspective multidisciplinary plan. The primary goal is radiosurgical treatment of intracranial lesions in a functional environment. The practical realization of a facility optimally designed for patient treatment is dependent on factors which include the facility setting, architectural goals, radiation safety requirements, and patient and medical team needs. This necessitates combined intellectual resources from neurosurgery, radiation oncology and physics, anesthesia, radiology, nursing, administration, and architectural and engineering teams. We undertook the development of a gamma unit facility which optimized the ergonomics and efficiency of patient evaluation, care and treatment, given the instrument requirements. This general plan based on our experience can be used for the development of other gamma unit facilities.

Surgery with adjuvant irradiation in patients with pathologic stage C adenocarcinoma of the prostate

BACKGROUND

In recent years, the routine use of prostate-specific antigen (PSA) to detect cancer of the prostate (CaP) early has renewed the controversy regarding radiotherapy versus radical prostatectomy as the superior definitive treatment. Radiotherapy alone has been reported to result in a high incidence of local recurrence, whereas on the other hand surgical treatment has resulted in a high incidence of microscopic residual tumor. The purpose of this study was to review our treatment results with radical prostatectomy followed by planned courses of postoperative irradiation in patients with pathologic Stage (PS) C disease.

METHODS

From 1972 to 1989, 95 patients with CaP with PS C tumors were treated with radical prostatectomy and bilateral pelvic lymphadenectomy. Pathologic stage distribution was: C1 in 26 (27%), C2 in 37 (39%), and C3 in 32 (34%) patients. The median follow-up was 6 years. All 95 study patients received postoperative pelvic irradiation as the only adjuvant treatment. Radiotherapy treated volume included the prostatic fossa and its immediate vicinity. The RT dose ranged from 33 Gy to 61.8 Gy (median, 45 Gy).

RESULTS

The overall 5- and 10-year actuarial survival rates were 94% and 73%, respectively, with the 5 and 10 year disease specific survival of 98% and 91%, respectively. Clinical and/or prostate specific antigen recurrence was 31% at 5 years and 44% at 10 years. Prostate specific antigen elevation without clinical evidence of recurrent disease was recorded in 26 (27%) patients. Seminal vesicle involvement (C3) and high Gleason's score (8-10) were the most important factors predicting recurrence. Of the 95 patients treated, 2 had pelvic recurrence alone and 1 had local and distant metastatic disease. Radiotherapy was well tolerated with no clinically important morbidity.

CONCLUSION

Based on this experience, moderate dose adjuvant radiotherapy after radical prostatectomy in patients with PS C CaP is recommended.

Single isocenter treatment planning for homogeneous dose delivery to nonspherical targets in multiarc linear accelerator radiosurgery

PURPOSE

Conventional radiosurgery refers to single isocenter focused beam irradiation of small intracranial targets with a single collimator. Conventional radiosurgery is characterized by spherical-shaped isodose surfaces. Nonspherical targets require a different approach to avoid exposing a large volume of nontarget brain tissue to high dose, particularly for lesions larger than 25 mm. Multiple isocenters are frequently used to treat nonspherical large targets, but multiple isocenter treatments are associated with a relative dose inhomogeneity of approximately 100% within the target volume, and may be correlated with an increased rate of complications. The feasibility of conformally treating elongated targets to an approximately homogeneous dose using a single isocenter methodology will be demonstrated.

METHODS AND MATERIALS

A prolate ellipsoid of revolution, 25 mm in diameter, 35 mm in length, positioned at five representative locations in a head phantom, was used as a target model. The alignment of the target was taken to be parallel in turn to each of the three principal axes of the head model (A/P, R/L, and C/C). Dose conformation is achieved by nonuniform arc weighting, selective limitation of the extent of individual arcs, and the use of different collimator apertures for the different arcs in accordance with the aperture size required to encompass the target for that arc. Treatment plans were selected based on considerations of dose-volume histograms and conformation of the 80% dose surface with the surface of the target. The goal was that the minimum target dose would not be less than 80% of the maximum dose.

RESULTS

Practical treatment plans for which the minimum target dose exceeded 80% were obtained for the three orthogonal orientations of the target for the five target locations. Plan parameters were essentially independent of the target position, dependent only on target orientation. The 80% isodose contour surface enclosed on average 2.8 cm3 larger volume (range: 2.1-3.9 cm3) than the prescribed 11.45 cm3 target. The minimum dose to the target ranged from 80.1 to 84.5%, and the average dose to the target was 94.26%. The 80-to-50% dose volume enclosed an average of 14.8 cm3 of nontarget volume (range: 12.7-15.9 cm3). Average deviation in the principal planes of the 80% isodose lines from the surface of the target volume was 0.95 mm for the 25 mm dimension (range 0.0 to +1.9 mm) and 0.86 mm (range 0.0 to + 2.4 mm) for the 35 mm dimension.

CONCLUSION

Standardized single isocenter treatment plans with the isocenter at the center of the target can achieve good conformation of the dose distribution to targets elongated along any of the principal axes, and located anywhere in the brain.

Comparison of radiation dosimetry in water and in solid phantom materials for I-125 and Pd-103 brachytherapy sources: EGS4 Monte Carlo study

A model based on the EGS4 Monte Carlo system is developed for calculating dose rate to water from an embedded low-energy brachytherapy source, given measurement data of dose rate to water within a water-substitute solid phantom for a source of given strength. The EGS4-based model is used to calculate point source dose rate distributions per unit source strength for water and for several species of solid phantoms. As a model for the dose rate distribution to an infinitesimal water inhomogeneity embedded in the solid phantom, the Monte Carlo is used to calculate dose rate to a thin spherical shell of water contained within the solid phantom at various distances centered on the source. Comparing this distribution to that calculated for a homogeneous water phantom, one can derive correction factors to determine dose rate to homogeneous water medium from measurements with water dose-calibrated detectors within the solid phantom. These factors are calculated for polymethylmethacrylate (PMMA or acrylic), solid water (WT1), and RW-1, a material optimized for low-energy dosimetry, with photon spectra from Pd-103 and from two commercial models of I-125 seed used as input. For model 6711 I-125 seeds at 1 cm in PMMA and WT1, the calculated ratios of dose rate to water to dose rate to water in the solid phantom are 0.893 and 1.038, respectively. Applying these factors to published, apparently divergent measurements of the specific dose rate constant based on measurements in PMMA and WT1 phantoms, the specific dose rate constant inferred for the model 6711 seed are found to be in excellent agreement: 0.878 cGy h-1U-1 (1.115 cGy- cm2h-1mCi-1) from the mean of the WT1 measurements and 0.879 cGy h-1U-1 from the PMMA measurements. Among the phantoms studied for Pd-103 and I-125, measurements in RW-1 would most closely approximate those for water.