Deep inspiration breath hold to reduce irradiated heart volume in breast cancer patients.

PURPOSE: To evaluate the use of deep inspiration breath hold (DIBH) during tangential breast radiation therapy as a means of reducing irradiated cardiac volume. METHODS AND MATERIALS: The Active Breathing Control (ABC) device designed at William Beaumont Hospital, Michigan was used to quantify the potential benefit of radiation delivery during DIBH for five left-sided breast cancer patients. This device initiates a breath hold at a predefined, reproducible lung volume. For each patient, two CT scans were acquired with and without breath hold, and virtual simulation was performed for regular tangent and wide-tangent techniques. The resulting dose-volume histograms were calculated, and the volume of heart irradiated to 25 Gy or more was assessed. RESULTS: The influence of ABC on irradiated heart volumes varied considerably among the five patients. Three patients with substantial cardiac volume in the treatment field during normal respiration showed a significant dose-volume histogram reduction when deep inspiration was applied, with decreases in the heart volume receiving 25 Gy of more than 40 cc observed. For one patient, deep inspiration reduced irradiated cardiac volumes only with the wide-tangent technique, while one patient showed no substantial irradiated volume decrease. CONCLUSION: A DIBH technique during tangential breast irradiation has the potential to significantly decrease irradiated cardiac volume for suitably selected patients. The magnitude of the impact of the breath hold application depends on patient anatomy, lung capacity, and pulmonary function.

Computed tomographic simulation of craniospinal fields in pediatric patients: improved treatment accuracy and patient comfort.

PURPOSE: To reduce the time required for planning and simulating craniospinal fields through the use of a computed tomography (CT) simulator and virtual simulation, and to improve the accuracy of field and shielding placement. METHODS AND MATERIALS: A CT simulation planning technique was developed. Localization of critical anatomic features such as the eyes, cribriform plate region, and caudal extent of the thecal sac are enhanced by this technique. Over a 2-month period, nine consecutive pediatric patients were simulated and planned for craniospinal irradiation. Four patients underwent both conventional simulation and CT simulation. Five were planned using CT simulation only. The accuracy of CT simulation was assessed by comparing digitally reconstructed radiographs (DRRs) to portal films for all patients and to conventional simulation films as well in the first four patients. RESULTS: Time spent by patients in the CT simulation suite was 20 min on average and 40 min maximally for those who were noncompliant. Image acquisition time was <10 min in all cases. In the absence of the patient, virtual simulation of all fields took 20 min. The DRRs were in agreement with portal and/or simulation films to within 5 mm in five of the eight cases. Discrepancies of > or =5 mm in the positioning of the inferior border of the cranial fields in the first three patients were due to a systematic error in CT scan acquisition and marker contouring which was corrected by modifying the technique after the fourth patient. In one patient, the facial shield had to be moved 0.75 cm inferiorly owing to an error in shield construction. CONCLUSIONS: Our analysis showed that CT simulation of craniospinal fields was accurate. It resulted in a significant reduction in the time the patient must be immobilized during the planning process. This technique can improve accuracy in field placement and shielding by using three-dimensional CT-aided localization of critical and target structures. Overall, it has improved staff efficiency and resource utilization.

The influence of patient geometry on the selection of chest wall irradiation techniques in post-mastectomy breast cancer patients.

BACKGROUND AND PURPOSE: To evaluate three chest wall (CW) irradiation techniques: wide tangential photon beams, direct appositional electron field and electron arc therapy with regards to target coverage and normal tissue tolerance. MATERIALS AND METHODS: Thirty-two post-mastectomy breast cancer patients were planned using three CW irradiation techniques. Computed tomography (CT) simulation was done on all patients and clinical target, heart and lung volumes were contoured. For each technique, dose distributions and dose-volume histograms (DVH) were calculated. Pass/fail criteria were applied based on volumetric target and critical structure dose coverage. Passing criteria for target was 95% of target receiving 95% of dose using a standard dose of 50 Gy/25 fractions, for heart

Buildup region of high-energy x-ray beams in radiosurgery.

The depths of dose maxima of radiosurgical x-ray beams increase with the field diameter in the range from 10-30 mm. Furthermore, the degree of the increase is proportional to the photon beam energy. This behavior is in contrast to that of large radiotherapeutic fields, where the depths of dose maxima decrease with increasing field size. The surface doses, depths of dose maxima, and collimator scatter factors for radiosurgical beams with energies between 6 and 18 MV are presented. It is shown that the change in dmax for radiosurgical beams is a result of photon scatter in the phantom rather than in the collimator. Monte Carlo simulations of the radiosurgical beams are used to separate the total dose into two components: the primary dose and the scattered dose. Calculations indicate that buildup region characteristics are governed by the primary dose deposition in the medium. The scattered dose has a minimal effect on the total dose deposited in media by radiosurgical beams.

Buildup region and depth of dose maximum of megavoltage x-ray beams.

The depth of dose maximum, dmax, of megavoltage x-ray beams was studied as a function of beam energy and field size for 6-, 10-, and 18-MV x-ray beams and field sizes ranging from 1 x 1 to 30 x 30 cm2. For a given beam energy, dmax increases rapidly with increasing field size at small fields, reaches a maximum around 5 x 5 cm2 and then gradually decreases with increasing field size for large fields. Monte Carlo simulations combined with measurements verified that the effect observed at small field sizes is caused by in-phantom scatter, while at large fields the effect is due to scatter contamination of the primary beam from the linac head. A comparison between the dmax behavior of flattened beams to that of unflattened beams indicates that the dmax decrease at large fields for flattened beams is caused mainly by contamination electrons which are produced in the flattening filter and further scattered by collimator jaws and air.

Calculation of x-ray spectra for radiosurgical beams.

As conformal radiosurgery using micromultileaf collimators gains feasibility, dose calculation algorithms based on Monte Carlo or convolution techniques may become necessary. These require radiosurgical x-ray spectra. The most accurate method currently available to estimate clinical radiosurgery spectra is the Monte Carlo method. In this study the EGS4 Monte Carlo system was used to simulate the thick target of a 6 MV linear accelerator used for radiosurgery in our center. The calculated spectrum was attenuated through any significant mass thickness of material downstream from the target. The attenuated thick-target spectral distributions calculated both with and without the flattening filter were compared to the attenuated, thin target spectrum based on the small angle Schiff analytical spectrum calculated for the same target and attenuator material, as well as with a published spectrum from a full Monte Carlo simulation of a treatment head with a flattener in place. The Schiff spectrum neglects contributions from lower-energy scattered electrons that significantly degrade the quality of the beam. The flattener is removed from our accelerator during radiosurgery to increase the dose rate to approximately 750 cGy/min for a 10 x 10 cm2 field at the depth of dose maximum. This leaves a substantial fluence of photons below 1 MeV that are not observed in published spectra calculated for accelerators with flattening filters. Removal of the flattening filter has a measurable effect on the central axis depth dose, reducing the percentage dose at 10 cm depth from 59.2% to 54.3% for a 10 mm diam field. Radiosurgical off-axis ratios and percentage depth dose distributions calculated from these spectra with the EGS4 Monte Carlo code were compared to measured data. Measured and calculated dose distributions both with and without flattener were in good agreement. The dose distributions were found to be insensitive to the differences in the various calculated spectral distributions. Thus, although the attenuated Schiff spectrum is significantly harder than the clinical beam, it is adequate for dose calculations of radiosurgical beams.

Cylindrical dose distributions in pseudodynamic rotation radiosurgery: an experimental study.

A linac-based radiosurgical technique is reported which produces cylindrical isodose distributions covering cylindrical targets of arbitrary orientations within the patient's head. The technique uses rectangular collimators and 4 degrees of freedom: gantry and couch rotation, as defined by a previously known dynamic rotation technique, collimator rotation, and collimator length adjustment. The relationship between the four parameters is derived and because of its complexity, the cylindrical dynamic rotation technique is introduced as a pseudodynamic technique. For cylindrical targets, the cylindrical pseudodynamic technique is comparison to the standard spherical technique produces considerable dose savings to healthy tissue surrounding the target and gives a similar or better dose fall-off outside the target.

A halo-ring technique for fractionated stereotactic radiotherapy.

Stereotactic radiosurgery has become established as an effective treatment modality for certain non-malignant brain diseases such as arteriovenous malformations. This paper describes an extension of our linear accelerator-based radiosurgical technique to fractionated treatment of intracranial disease. The fractionated stereotactic radiotherapy technique expands the use of the modality by sparing normal cells within the treatment volume thus improving the therapeutic ratio. The first treatment is given using a stereotactic frame both for target localization and patient immobilization. The frame is then removed and subsequent treatments use a standard neurosurgical halo-ring for patient immobilization. The halo-ring is left in place on the skull for the duration of the course of treatment. Thus the physical requirements for fractionation pertain firstly to the patient immobilization and target localization using the halo-ring and secondly to the stringent quality assurance procedures required to maintain spatial accuracy under these new conditions. We describe a sensitive and effective technique for checking the rotational beam parameters and collimator alignment which we use immediately prior to treatment to ensure adequate accuracy of dose delivery to the target volume.

An asymmetric half-wedged field technique for the treatment of inferiorly extended head and neck cancers.

Adequate dose coverage of the target volume of head and neck cancer patients can be difficult to achieve with lateral opposing fields when the patient's neck is short and the disease involves the lower cervical region. Rotation of the couch away from the side of the gantry, such that the beams are angled towards the inferior aspect of the patient, can result in a better dose distribution. However, if the neck separation is large, unacceptable dose inhomogeneities will still exist. The conventional approach in these situations is to treat with parallel opposed lateral fields junctioned with anterior/posterior opposed low neck fields. We present a variation on the couch rotation technique as an alternative. Asymmetric jaws and half-wedged fields are used to obtain a homogeneous dose within an inferiorly extended target volume. The advantages of this technique are a simple set up and the capability of adding spinal cord shielding without shielding midline primary tumour.