Jason W. Sohn

Characterization of a commercial multileaf collimator used for intensity modulated radiation therapy.

The characteristics of a commercial multileaf collimator (MLC) to deliver static and dynamic multileaf collimation (SMLC and DMLC, respectively) were investigated to determine their influence on intensity modulated radiation therapy (IMRT) treatment planning and quality assurance. The influence of MLC leaf positioning accuracy on sequentially abutted SMLC fields was measured by creating abutting fields with selected gaps and overlaps. These data were also used to measure static leaf positioning precision. The characteristics of high leaf-velocity DMLC delivery were measured with constant velocity leaf sequences starting with an open field and closing a single leaf bank. A range of 1-72 monitor units (MU) was used providing a range of leaf velocities. The field abutment measurements yielded dose errors (as a percentage of the open field max dose) of 16.7+/-0.7% mm(-1) and 12.8+/-0.7% mm(-1) for 6 MV and 18 MV photon beams, respectively. The MLC leaf positioning precision was 0.080+/-0.018 mm (single standard deviation) highlighting the excellent delivery hardware tolerances for the tested beam delivery geometry. The high leaf-velocity DMLC measurements showed delivery artifacts when the leaf sequence and selected monitor units caused the linear accelerator to move the leaves at their maximum velocity while modulating the accelerator dose rate to deliver the desired leaf and MU sequence (termed leaf-velocity limited delivery). According to the vendor, a unique feature to their linear accelerator and MLC is that the dose rate is reduced to provide the correct cm MU(-1) leaf velocity when the delivery is leaf-velocity limited. However, it was found that the system delivered roughly 1 MU per pulse when the delivery was leaf-velocity limited causing dose profiles to exhibit discrete steps rather than a smooth dose gradient. The root mean square difference between the steps and desired linear gradient was less than 3% when more than 4 MU were used. The average dose per MU was greater and less than desired for closing and opening leaf patterns, respectively, when the delivery was leaf-velocity limited. The results indicated that the dose delivery artifacts should be minor for most clinical cases, but limit the assumption of dose linearity when significantly reducing the delivered dose for dosimeter characterization studies or QA measurements.

Results of linear accelerator-based stereotactic radiosurgery for recurrent and newly diagnosed acoustic neuromas

Stereotactic radiosurgery (SRS) is used to treat acoustic neuromas, but additional information is needed to firmly establish its safety and efficacy. We review our experience over 7 years treating 29 consecutive patients with a modified linear accelerator (linac) SRS system. Between August 1989 and October 1995, 29 patients with a median age of 67 years (range 26 to 83) underwent linac SRS treatment. Twenty‐five patients had unilateral acoustic neuromas, and four patients with neurofibromatosis type II had bilateral vestibular schwannoma. Eligibility criteria for SRS were recurrent tumors (n = 9), age >65 (n = 16), or patient preference (n = 6). Follow‐up magnetic resonance imaging scans were performed on all patients. The most common presenting symptoms were hearing impairment (18 patients) and gait difficulties (17 patients). Ten patients were deaf in the affected ear prior to treatment. Doses to the periphery of the tumor ranged from 800 to 2,400 cGy (median 1,600 cGy) prescribed to the 50% to 80% isodose line (median 80%). After a median radiographic follow‐up of 49 months (range 4 to 110 months), 11 tumors were smaller, 17 were stable, and one had evidence of progression (at 41 months). The 5‐year local disease control rate (Kaplan‐Meier estimate) was 94%. Acute complications were minimal, with only two patients experiencing nausea and vomiting after the procedure. Long‐term complications included new or progressive trigeminal and facial nerve deficits with estimated 5‐year incidences of 15% and 32%, respectively. Subjective hearing reduction or loss occurred in 14 (74%) of the 19 patients who had useful hearing prior to treatment. Five patients died from unrelated causes. These results suggest that linac SRS provides excellent short‐term tumor control rates. Since there was a high risk of cranial nerve neuropathy, we do not recommend using only computed tomography‐based planning and high prescription doses. Int. J. Cancer (Radiat. Oncol. Invest.) 90, 145–151 (2000).

Deconvolution of detector size effect for small field measurement

Parametrization of the small fields employed in stereotactic applications is a painstaking process involving extensive film dosimetry to achieve acceptable beam edge definition. Use of cylindrical or spherical detectors for profile measurements would simplify data acquisition but add a volume averaging artifact to beam edge definition. We demonstrate a simple approach to unfolding the chamber size artifact from measured small beam profiles using typical cylindrical chambers. In comparison with film measurements we have found good agreement when the detector response function is deconvoluted from the measured profiles, although the amount of correction needed is fairly minimal for the detectors studied.

A method for delivering accurate and uniform radiation dosages to the head and neck with asymmetric collimators and a single isocenter

PURPOSEnTo investigate the use of asymmetric collimators and a single isocenter for delivering a uniform, accurate dose of radiation to the head, neck, and supraclavicular lymph nodes.nnnMETHODS AND MATERIALSnA linear accelerator with a pair of asymmetric collimators is required for this technique. An isocenter was placed at the junction of the lateral head and neck fields and the anterior supraclavicular field. The asymmetric collimators were set longitudinally, by collimator rotation if necessary. The collimators split the radiation beam to all portals. Dose uniformity was measured at the junction with films in solid-water phantoms.nnnRESULTSnFilm dosimetry showed a uniform dose at the junction without hot or cold regions. A digital display tolerance of +/- 1.0 mm for a field size maintained an acceptable uniform dose (+/- 5% dose variation) at the junction. The single isocenter and asymmetric collimators reduced field setup time by half. No table rotation was required to match fields.nnnCONCLUSIONnThe asymmetric collimators lead to easy and accurate patient setup. The absence of the trapezoid effect resulted in the complete coverage of the submandibular and cervical nodes without any hot spots.

INDICATIONS FOR EXCLUDING THE SEMINAL VESICLES WHEN TREATING CLINICALLY LOCALIZED PROSTATIC ADENOCARCINOMA WITH RADIOTHERAPY ALONE

PURPOSEnThe indications for treating the seminal vesicles (SV) in patients with clinically localized carcinoma of the prostate are controversial. We sought to define subgroups of patients in whom coverage could be avoided, using pretreatment prostate specific antigen (PSA) values and the Gleason score. Because the rectum is the major dose-limiting structure, we also measured the extent of rectal sparing achieved by excluding the SV from external beam treatment fields.nnnMETHODS AND MATERIALSnWe retrospectively studied lateral x-ray simulation films of 43 patients treated with standard four-field radiotherapy and dose-volume histograms of eight patients treated with conformal radiotherapy. The rectal surface areas were measured and the volumes were calculated including and excluding the SV. The pathology reports of patients treated with radical prostatectomy alone between 1987 and 1993 were reviewed. Patients without preoperative PSA levels or biopsy Gleason scores, or who received neoadjuvant hormonal therapy were excluded. Of the 368 remaining patients, 66 (18%) had preoperative PSA levels < or = 4, 172 (47%) had PSA levels 4-10, and 130 (35%) had PSA levels > 10. The Gleason score was < or = 6 in 269 (73%), and 99 (27%) had a score > or = 7.nnnRESULTSnThe reduction in the total irradiated rectal areas to full doses when the SV were excluded ranged from 5 to 67% in individual patients (median, 44%). The median reduction in the irradiated rectal volumes to 50% of the prescribed dose, as determined by dose-volume histograms, was 51% (range: 37-76%). The median reduction in bladder volumes was 9% (range: 6%-15%). The incidence of SV involvement was 19% (70 out of 368). Patients with normal PSA levels (< or = 4 ng/mL) had a 3% (2 out of 66) rate of SV involvement. Patients with PSA levels between 4-10 ng/mL had a 8% (10 out of 126) rate of SV involvement. All other cases had at least 20% rate of SV involvement, with a combined rate of 33%. Using a cutoff of 15%, two low risk groups were identified: all patients with PSA levels < or = 4, and patients with PSA 4-10 and Gleason score < or = 6. These constituted 52% (192 out of 368) of all patients. Overall, the low-risk patients had a 6% (12 out of 192) incidence of SV involvement vs. 33% (58 out of 176) for the high risk patients (p < or = .001).nnnCONCLUSIONSnExcluding the SV from the treatment field can significantly reduce (40%-50%) the volume of irradiated rectum. Our data confirm that pretreatment PSA levels and Gleason scores can be effectively used to define subgroups of patients in whom SV irradiation can be avoided. We propose excluding the SV in all patients with PSA levels < or = 4, and patients with PSA levels 4-10 and a Gleason score < or = 6.

TOWARD AUTOMATED QUALITY ASSURANCE FOR INTENSITY- MODULATED RADIATION THERAPY

PURPOSEnTo investigate whether high-quality, relatively inexpensive, document and transparency scanners used as densitometers are sufficiently quantitative for routine quality assurance (QA).nnnMETHODS AND MATERIALSnThe scanner we investigated used a linear amplifier, digitizing gray-scale images to 12-bit resolution with a user-selected spatial resolution of 0.170 mm(2) pixels. To reduce Newtons rings artifacts, the standard glass platen was replaced by glass with an antireflective coating. Conversion of reading to transmission was conducted by permanently placing a calibrated photographic step tablet on the scanner platen. After conversion to light transmission, a zero-phase two-dimensional Wiener filter was used to reduce pixel-to-pixel signal variation. Light-scatter artifacts were removed by deconvolution of a measured light-spread kernel. The light-spread kernel artifacts were significant along the scanners detector axis, but were insignificant along the scanning axis.nnnRESULTSnPixel-to-pixel noise was better than 2% for optical densities, ranging from 0.4 to 2.0 and 0 to 2.7 for the unfiltered and filtered images, respectively. The document scanning system response was compared against a confocal scanning laser densitometer. A series of IMRT dose distribution and dose calibration film sets were scanned using the two scanners, and the measured dose was compared. The maximum mean and standard deviation of the measured dose difference between the document scanner and confocal scanner was 1.48% and 1.06%, respectively.nnnCONCLUSIONnWhile the document scanners are not as flexible as dedicated film densitometers, these results indicate that, using the intensity and scatter corrections, the system provides accurate and precise measurements up to an optical density of 2.0, sufficient for routine IMRT film QA. For some film types, this requires the reduction in monitor units to limit the dose delivered to the film. The user must be cautious that the delivered IMRT dose is scaled appropriately. This inexpensive and accurate system is being integrated into an automated QA program.

Three-dimensional conformal pancreas treatment: comparison of four- to six-field techniques.

PURPOSEnWe compare practical conformal treatment approaches to pancreatic cancer using 6 and 18 MV photons and contrast those approaches against standard techniques.nnnMETHODS AND MATERIALSnA four-field conformal technique for treating pancreas cancer has been developed using nonopposed 18 MV photons. This approach has been extended to 6 MV photon application by the addition of one to two fields. These techniques have been optimized to increase sparing of normal liver and bowel, compared with opposed-field methods, to improve patient tolerance of high doses. In this study we compare these techniques in a simulated tumor model in a cylindrical phantom. Dose-volume analysis is used to quantify differences between the conformal, nonopposed techniques with conformal, opposed field methods. This model is also used to evaluate the effect of 1-2 cm setup errors on dose-volume coverage.nnnRESULTSnDose-volume analysis demonstrates that five-to-six field conformal treatments using 6 MV photons provides similar or better dose coverage and normal tissue sparing characteristics as an optimized 18 MV, four-field approach when 1-2 cm margins are included for setup uncertainty. All approaches using nonopposed beam geometry provide significant reduction in the volume of tissue encompassed by the 30-50% isodose surfaces, as compared with four-field box techniques.nnnCONCLUSIONSnThree-dimensional (3D) conformal treatments can be designed that significantly improve dose-volume characteristics over conventional treatment designs without costing unacceptable amounts of machine time. Further, deep intraabdominal sites can be adequately accessed and treated on intermediate energy machines with a relatively moderate increase in machine time.

Dose-volume histogram analysis of techniques for irradiating pituitary adenomas

PURPOSEnThree-dimensional treatment planning was performed to evaluate three standard coplanar irradiation techniques (two-field parallel-opposed, three-field, and 110 degrees bilateral arcs), the 330 degrees single rotational arc, and a four noncoplanar arc technique for the treatment of pituitary adenomas. We sought to identify the optimal technique for minimizing the dose delivered to the normal tissues around the pituitary gland.nnnMETHODS AND MATERIALSnContours of the pituitary tumor and normal tissues were traced onto computed axial tomography (CT) scans and reconstructed in three dimensions using a three-dimensional planning system. A total dose of 45 Gy was delivered to the pituitary lesion with the five techniques using 6 MV and 18 MV photons, and dose-volume histograms were generated.nnnRESULTSnThe 18 MV photons delivered a lower dose to the temporal lobe than did the 6 MV photons in the two-field technique, but this advantage was not evident for the other techniques. The three-field technique improved dose distribution throughout the temporal lobes with low doses being delivered to the frontal lobe. The bilateral arc and the 330 degrees arc techniques were superior to stationary two- and three-fields techniques for sparing the temporal lobes. The four noncoplanar arc technique delivered less doses to the temporal and frontal lobes than did the other techniques. However, the lens dose (3.6 Gy/25 fractions) was higher compared to the other techniques.nnnCONCLUSIONnAnalysis of the dose-volume histograms shows the various dosimetric advantages and disadvantages of the five techniques. Based upon individual considerations, including the patients age and medical history, one can decide the optimal technique for treatment.

Differential dosing of prostate and seminal vesicles using dynamic multileaf collimation

Abstract Purpose: We have investigated the potential of applying different doses to the prostate (PTV2) and prostate/seminal vesicles (PTV1) using multileaf collimation (MLC) for intensity modulated radiation therapy (IMRT). Current dose-escalation studies call for treatment of the PTV1 to 54 Gy in 27 fractions followed by 20 Gy minimum to the PTV2. A daily minimum PTV dose of 2 Gy using a 7-field technique (4 obliques, opposed laterals, and an ant-post field) is delivered. This requires monitor unit calculations, paper and electronic chart entry, and quality assurance for a total of 14 fields. The goal of MLC IMRT is to improve efficiency and deliver superior dose distributions. Acceptance testing and commissioning of the dynamic MLC (DMLC) option on a dual-energy accelerator was accomplished. Most of the testing was performed using segmental MLC (SMLC) IMRT with stop-and-shoot sequences built within the dynamic mode of the DMLC. Methods and Materials: The MLC IMRT fields were forward planned using a three-dimensional treatment planning system. The 14 fields were condensed to 7 SMLC IMRT fields with two segments each. In this process, steps were created by moving the leaves to the reduced field positions. No dose ( Results: The acceptance testing and commissioning demonstrated that the leaves move to programmed positions accurately and in a timely manner. We did find an ∼1 mm offset of the set leaf position and radiation edge (50%) due to the curved-end nature and calibration limitations. The 7-field SMLC IMRT treatment duplicated the 14-field static plan dose distribution with variations no greater than 1.5%. Conclusions: The MLC IMRT approach will improve efficiency because the number of electronic and chart entries has decreased by a factor of 2. Portal images are able to capture the initial and final MLC segments. The question of differential daily dose to the prostate and seminal vesicles remains.

UNIFORM IRRADIATION OF THE CRANIOSPINAL AXIS WITH A PENUMBRA MODIFIER AND AN ASYMMETRIC COLLIMATOR

PURPOSEnA technique is described that uses an independent, asymmetric collimator and a penumbra modifier to uniformly administer radiation over the craniospinal axis.nnnMETHOD AND MATERIALSnAn isocenter is used at the junction of the cranial and the upper spinal fields. These fields are defined by a single isocenter at spinal cord depth and an independent, asymmetric collimator. From the isocenter, the cranial and upper spinal fields are extended 1 cm inferiorly and superiorly, respectively. A modifier provides a 2-cm wide penumbra at the central axis of the beam (overlapping region) and is attached to a wedge tray in an accessory slot. This modifier allows the fields to be matched so a uniform dose is delivered over the isocenter and junction. Dose distribution was measured with an anthropomorphic head-and-neck wax phantom that included the seventh cervical vertebrae. A film was placed in a coronal cut at the spinal cord level.nnnRESULTSnThe administered dose varied less than 10% through the craniospinal axis. Reproducibility with portal films has been very good. Advantages include dose homogeneity, easy reproducibility, and decreased setup time.nnnCONCLUSIONnThis single-field isocentric technique allows more uniform irradiation of the craniospinal axis than do previously described techniques.