Bilateral epidural extension of thoracic capillary vertebral (intraosseous) hemangioma mimicking spinal meningioma.

We describe a rare case of vertebra (intraosseous) hemangioma with bilateral and symmetrical epidural extension causing cord compression in a 24-year-old woman. The epidural component was isointense to cord on both T1 and T2 sequences, and enhanced markedly and homogenously following gadolinium administration. The gradual in onset and progressive nature with the typical enhancing pattern lead the neurosurgeon to the more common diagnosis of spinal meningioma. Epidural extension of vertebral hemangiomas causing cord compression is rarely reported. Review of literatures reveal that cases that have been reported are of unilateral extension into epidural space and of cavernous type. This is the first case report of capillary vertebral (intraossous) hemangioma with bilateral extension through both intervetebral foramen into the epidural space causing myelopathy.

A review on the clinical implementation of respiratory-gated radiation therapy.

Respiratory-gated treatment techniques have been introduced into the radiation oncology practice to manage target or organ motions. This paper will review the implementation of this type of gated treatment technique where the respiratory cycle is determined using an external marker. The external marker device is placed on the abdominal region between the xyphoid process and the umbilicus of the patient. An infrared camera tracks the motion of the marker to generate a surrogate for the respiratory cycle. The relationship, if any, between the respiratory cycle and the movement of the target can be complex. The four-dimensional computed tomography (4DCT) scanner is used to identify this motion for those patients that meet three requirements for the successful implementation of respiratory-gated treatment technique for radiation therapy. These requirements are (a) the respiratory cycle must be periodic and maintained during treatment, (b) the movement of the target must be related to the respiratory cycle, and (c) the gating window can be set sufficiently large to minimise the overall treatment time or increase the duty cycle and yet small enough to be within the gate. If the respiratory-gated treatment technique is employed, the end-expiration image set is typically used for treatment planning purposes because this image set represents the phase of the respiratory cycle where the anatomical movement is often the least for the longest time. Contouring should account for tumour residual motion, setup uncertainty, and also allow for deviation from the expected respiratory cycle during treatment. Respiratory-gated intensity-modulated radiation therapy (IMRT) treatment plans must also be validated prior to treatment. Quality assurance should be performed to check for positional changes and the output in association with the motion-gated technique. To avoid potential treatment errors, radiation therapist (radiographer) should be regularly in-serviced and made aware of the need to invoke the gating feature when prescribed for selected patients.

Commissioning and quality assurance for MLC-based IMRT.

The commissioning and quality assurance (QA) associated with the implementation of linear accelerator multileaf collimator (MLC)-based intensity-modulated radiation therapy (IMRT) at the University of Nebraska Medical Center are described. Our MLC-based IMRT is implemented using the PRIMUS linear accelerator interface through the IMPAC record and verification system to the CORVUS treatment planning system. The "step-and-shoot" technique is used for this MLC-based IMRT. Commissioning process requires the verification of predefined parameters available on the CORVUS and the collection of some machine data. The machine data required are output factor in air and output factor in phantom, and percent depth dose for a number of field sizes. In addition, inplane and crossplane dose profiles of 4 x 4 cm and 20 x 20 cm field sizes and diagonal dose profiles of a large field size have to be measured. Validation of connectivity and dose model includes the use of uniform intensity bar strips, triangular-shaped nonuniform intensity bar strip, and N-shaped target. QA procedure follows the recommendation of the AAPM Task Group No. 40 report. In addition, the leaf position accuracy and reproducibility of the MLC should be checked at regular intervals. The dose validation is implemented through the hybrid plan where the patient beam parameters are applied to a flat phantom. Independent dose calculation method is used to confirm the dose delivery plan and data input to the CORVUS.

Independent dose calculations for the corvus MLC IMRT.

Two independent dose calculation methods have been explored to validate MLC-based IMRT plans from the NOMOS CORVUS system. After the plan is generated on the CORVUS planning system, the beam parameters are imported into an independent workstation. The beam parameters consist of intensity maps at each gantry angle. In addition, CT scans of the patient are imported into the independent workstation to obtain the external contour of the patient. The coordinate system is defined relative to the alignment point chosen in the CORVUS plan. The 2 independent calculation methods are based on a pencil beam kernel convolution and a Clarkson-type differential scatter summation, respectively. The pencil beam data for a 1 x 1-cm beam, as formed by the multileaf collimator, were measured for the 6-MV photon beam from a Siemens PRIMUS linear accelerator using film dosimetry. In the pencil beam method, the dose at a point is calculated using the depth and off-axis distance from a given pencil beam, corrected for beam intensity. The scatter summation method used the conversion of measured depth dose data into scatter maximum ratios. In this method, the differential scatter from each pencil beam is corrected for the beam intensity. Isodose distributions were generated using the independent dose calculations and compared to the CORVUS plans. Although isodose distributions from both methods show good agreement with the CORVUS plan, our implementation of the differential scatter summation approach seems more favorable. The 2 independent dose calculation algorithms are described in this paper.

Leaf sequencing techniques for MLC-based IMRT.

The nonuniform fields required by intensity-modulation radiation therapy (IMRT) can be delivered using conventional multileaf collimators (MLC) as beam modulators. In MLC-based IMRT, the nonuniform field is initially converted into an intensity map represented as a matrix of beam intensities. The intensity map is then decomposed into a series of subfields or segments of uniform intensities. Although there are many ways of segmenting the beam intensity matrix, a resulting subfield is only deliverable if it satisfies the constraints imposed by the MLC. These constraints exist as a result of the design of the MLC. The simplest constraint of the MLC is that its pairs of leaves can only move in and out in one dimension. Additional constraints include collision of opposing leaves and the need to match the tongue-and-groove to reduce interleaf leakage. The practical aspect of MLC-based IMRT requires that an optimized algorithm decomposes the nonuniform field into the least number of segments and therefore reduces the delivery time. This paper examines the static use and the dynamic use of MLCs to perform MLC-based IMRT.

Independent dose calculations for the PEACOCK System.

An independent dose calculation method has been developed to validate intensity-modulated radiation therapy (IMRT) plans from the NOMOS PEACOCK System. After the plan is generated on the CORVUS planning system, the beam parameters are imported into an independent workstation. The beam parameters consist of intensity maps at each gantry angle and each arc position. In addition, CT scans of the patient are imported into the independent workstation to obtain the external contour of the patient. The coordinate system is defined relative to the alignment point chosen in the CORVUS plan. The independent calculation uses the pencil beam data viz tissue maximum ratio (TMR) and beam profiles for a single 1 x 0.8-cm beamlet formed by the NOMOS multileaf intensity-modulating collimator (MIMiC) leaf. The pencil beam data were measured for the 6-MV photon beam from Siemens PRIMUS linear accelerator using film dosimetry. The dose at a point is calculated using the depth and off-axis distance from a given pencil beam, corrected for its beam intensity. Isodose distributions are generated using the independent dose calculations and compared to the CORVUS plans. Isodose distributions show good agreement with the CORVUS plans for a number of clinical cases. The independent dose calculation algorithm is described in this paper.

Comparative study between IMRT with NOMOS BEAK and linac-based radiosurgery in the treatment of intracranial lesions.

A comparative study was undertaken to examine intracranial irradiation using intensity-modulation radiation therapy (IMRT) and linear accelerator-based radiosurgery. The IMRT was examined using the Peacock system with a BEAK attachment. A clinical case involving a metastatic brain lesion, treated with 3 radiosurgery isocenters, was planned for IMRT. The radiosurgery was planned using the Leibinger planning system. The IMRT was planned using the CORVUS planning system. The CORVUS planning system uses an inverse planning algorithm, a recent development in radiotherapy. Isodose distributions and dose volume histograms were generated and compared. Analysis of the dosimetry shows that the dose conformity and homogeneity within the target using the RTOG guidelines are superior for IMRT. The advantages of IMRT using inverse planning system include the ease of planning and execution of treatment, especially for cases that involve concave targets that require multiple isocenters using radiosurgery.

Commissioning of Peacock System for intensity-modulated radiation therapy.

The Peacock System was introduced to perform tomographic intensity-modulated radiation therapy (IMRT). Commissioning of the Peacock System included the alignment of the multileaf intensity-modulating collimator (MIMiC) to the beam axis, the alignment of the RTA device for immobilization, and checking the integrity of the CRANE for indexing the treatment couch. In addition, the secondary jaw settings, couch step size, and transmission through the leaves were determined. The dosimetric data required for the CORVUS planning system were divided into linear accelerator-specific and MIMiC-specific. The linear accelerator-specific dosimetric data were relative output in air, relative output in phantom, percent depth dose for a range of field sizes, and diagonal dose profiles for a large field size. The MIMiC-specific dosimetric data were the in-plane and cross-plane dose profiles of a small and a large field size to derive the penumbra fit. For each treatment unit, the Beam Utility software requires the data be entered into the CORVUS planning system in modular forms. These modules were treatment unit information, angle definition, configuration, gantry and couch angles range, dosimetry, results, and verification plans. After the appropriate machine data were entered, CORVUS created a dose model. The dose model was used to create known simple dose distribution for evaluation using the verification tools of the CORVUS. The planned doses for phantoms were confirmed using an ion chamber for point dose measurement and film for relative dose measurement. The planning system calibration factor was initially set at 1.0 and will be changed after data on clinical cases are acquired. The treatment unit was released for clinical use after the approval icon was checked in the verification plans module.

Immobilization devices for intensity-modulated radiation therapy (IMRT).

Three-dimensional conformal radiation therapy (3DCRT) and intensity-modulated radiation therapy (IMRT) plans show radiation dose distribution that is highly conformal to the target volume. The successful clinical implementation of these radiotherapy modalities requires precise positioning of the target to avoid a geographical miss. Effective reduction in target positional inaccuracies can be achieved with the proper use of immobilization devices. This paper reviews some of the immobilization devices that have been used and/or have the potential of being used for IMRT. The immobilization devices being reviewed include stereotactic frame, Talon system, thermoplastic molds, Alpha Cradles, and Vac-Lok system. The implementation of these devices at various anatomical sites is discussed.

Quality assurance procedures for the Peacock system.

The Peacock system is the product of technological innovations that are changing the practice of radiotherapy. It uses dynamic beam modulation technique and inverse planning algorithm, both of which are new methodologies, to perform intensity-modulation radiation therapy (IMRT). The quality assurance (QA) procedure established by Task Group No. 40 did not adequately consider these emerging modalities. A review of literature indicates that published articles on QA procedures concentrate primarily on the verification of dose delivered to phantom during commissioning of the system and dose delivered to phantom before treating patients. Absolute dose measurements using ion chambers and relative dose measurements using film dosimetry have been used to verify delivered doses. QA on equipment performance and equipment safety is limited. This paper will discuss QA on equipment performance, equipment safety, and patient setup reproducibility.

Comparison of posterior fossa and tumor bed boost in medulloblastoma.

To quantify the difference between the area of brain irradiated using the posterior fossa boost (PFB) and tumor bed boost (TBB) in medulloblastoma, we studied 15 simulation radiographs of patients treated in our institution from 1990 and 1999. The PFB was compared with the TBB, which was defined as the tumor bed plus 2-cm margin as demonstrated by postoperative magnetic resonance imaging. The PFB field treated a mean area of 9.43 cm2 more brain than the TBB. In 3 patients (20%), the area of the brain in the TBB was larger than the PFB. In 11 patients (73.3%), the PFB field had more than 10% more brain than the TBB. The cochlea was in the PFB and TBB field in all patients. In more than two thirds of patients, the area of brain irradiated with the PFB was at least 10% greater than the TBB. Future studies are needed to determine whether the TBB can replace the PFB in patients with medulloblastoma.

Review of dosimetric functions for meterset calculations.

Mathematical expressions used to calculate doses in a patient, based on data measured in a phantom, have to be simple, understandable, and reliable to minimize possible calculational error. In light of this concern, this paper reviews the dosimetric functions used in meterset calculations to determine the treatment times or monitor units for a prescribed dose. The dosimetric functions are the percent depth dose, the tissue-air ratio, the tissue-phantom ratio, and the inverse square law. This review examined the definition of the dosimetric functions, the inter-relationships among the dosimetric functions, and the mathematical expressions used in meterset calculations for nonstandard source-to-surface distances in a phantom.

Dosimetric evaluation of abutted fields using asymmetric collimators for treatment of head and neck.

PURPOSE: The objective of this study was to reevaluate the dose nonuniformity of abutted fields defined using asymmetric collimators and one isocenter for treatment of the head and neck region. METHODS AND MATERIALS: Bilateral parallel-opposed fields abutted to the anterior field at one isocenter were implemented in the treatment of head and neck. The effect of digital display tolerance can produce dose nonuniformity at the junction of the abutted fields. The amount of dose nonuniformity was quantified using both mathematical summation of dose profiles and by direct measurement of doses at the junction of the two abutted fields. The dose nonuniformity was obtained by irradiating the superior part of a film using bilateral parallel-opposed fields and the inferior part by an anterior field with a gap or an overlap. Dose profiles were taken at the depth of maximum dose for the anterior field across the abutted fields. The dose nonuniformity was determined for the case where the asymmetric jaw was set at -2 mm, -1 mm, 0, +1 mm, and +2 mm from the beam central axis. RESULTS: The dose at the junction increases systematically as the abutment of the fields changes from a gap to an overlap. The dose nonuniformity with 1-mm gap and 1-mm overlap is about 15% underdose and overdose, respectively. CONCLUSION: Imperfect abutment of split fields due to digital display tolerance (+/-1 mm) of asymmetric collimator can cause an underdose or overdose of 15% of the delivered dose.

Dosimetric assessment of nonperfectly abutted fields using asymmetric collimators.

The abutment of adjacent fields has been facilitated through the use of asymmetric collimators. Conceptually, the abutment yields a perfectly uniform dose distribution across the junction, provided the asymmetric jaw is set precisely at the beam central axis. However, the asymmetric jaw has an associated tolerance, which can cause the abutment to be misaligned. This study examined the dose distribution at the junction of nonperfectly abutted fields. The abutment of fields was carried out using an asymmetric collimation of 5 x 10 cm, with an asymmetric jaw positioned at the beam central axis. A film was initially exposed using this field with the collimator set at 90 degrees. The collimator was then rotated 180 degrees and the same film was exposed for the second time to create the field abutment. Positioning the asymmetric jaw with respect to the beam central axis set the amount of gap and overlap between the abutted fields. The dose distribution was measured for asymmetric jaw positioning of -2, -1, 0, + 1, and +2 mm from the beam central axis. In addition, the dose distribution was also computed mathematically by summing the 2 dose profiles with defined gap or overlap. A field mismatch of +/-1 mm would result in a dose nonuniformity of 17%, and a +/-2 mm mismatch would produce a 35% dose nonuniformity.

Radiation field simulation of gynecologic malignancies: localization of the cervix and vagina with a flexible vaginal localizer contrast tampon.

The authors evaluated a flexible vaginal localizer contrast tampon for radiation therapy simulation. In 51 patients, the degree of cervical or vaginal cuff displacement secondary to the contrast tampon was evaluated by comparing simulation radiographs (with tampon) and initial portal radiographs (without tampon). The same comparisons were made on the radiographs obtained in 25 control subjects who underwent simulation without a tampon. Mean displacement in the group who underwent simulation with a tampon was minimal (< or = 5 mm in each direction) and similar to that in the control group. This technique provides reliable cervical and vaginal cuff localization.

Review of AAPM Task Group No. 43 recommendations on interstitial brachytherapy sources dosimetry. American Association of Physicists in Medicine.

In 1995, the American Association of Physicists in Medicine (AAPM) Task Group No. 43 (TG-43) published its recommendations on the dosimetry of interstitial brachytherapy sources. The report recommended the use of a new dose calculation formalism based on measured quantities. The formalism in modular form permits the computation of doses in two dimensions for 103Pd, 125I, and 192Ir sources. The TG-43 dose calculation formalism introduced new and updated quantities such as air kerma strength, dose rate constant, radial dose function, anisotropy function and anisotropy factor. The dose rate obtained using the TG-43 dose calculation formalism and updated source dosimetry data can be expected to be different from some of the currently used systems by as much as 17%. For the same treatment and implementing the TG-43 dosimetry with point source approximation, the widely prescribed dose of 160 Gy for 125I permanent implants using model 6711 sources changes to 144 Gy. In addition to the dose calculation formalism, TG-43 report also stated that the air kerma strength provided by NIST is estimated to be approximately 7-10% higher than it should be, due to low energy photon contamination for 125I. This difference has not been accounted for in the TG-43 report.

Dosimetric considerations of water-based bolus for irradiation of extremities.

The dosimetry of high-energy photon beams in the treatment of superficial lesions occurring in extremities was examined. Large parallel-opposed fields with different photon beam energies were used. The extremity was immersed in water contained in a commercially available plastic wastebasket. The water bolus serves to even out the surface irregularities of the extremities and to remove the skin sparing effect. A polystyrene block was placed at the floor of the wastebasket to ensure that the extremity was encompassed in the radiation fields. The photon beam energies considered were 4 MV, 6 MV, 10 MV, and 24 MV. The results show that the dose distributions are more homogeneous with higher photon beam energies. The isodose lines are more constricted at mid-plane for low energy photon beams. Higher energy photon beams, 10 MV and up would be preferable for the treatment of superficial lesions of the extremities immersed in water bolus contained in a typical wastebasket size.

Energy dependence of a new solid state diode for low energy photon beam dosimetry.

Any newly introduced radiation dosimeters must be evaluated for clinical use. This paper reports an evaluation of the energy response of a newly fabricated solid state diode for low-energy photon beam dosimetry. The diode, which has minimal buildup, is one of five models designed for in vivo patient dosimetry. Measurements were made using this diode for x-ray beams with kilovoltage potential from 20 to 100 kVp from a superficial x-ray treatment unit. In addition, measurements were also made using calibrated ion chamber designed for low-energy x-ray beams. A cone size of 20 x 20 cm was used and measurements were taken with diode and ion chamber placed at 35 cm from the source. The energy response was determined by taking the ratio of the diode measurement to the ion chamber measurement. The results show that the response of this new diode increases as the quality of the x-ray beam increases at low energy range; however, the percent difference of the detector response between the 70 kVp and 100 kVp x-ray beam is less than 1%, suggesting that this detector can be used as a dosimetry tool for the typical superficial x-ray treatments.