International Journal of Radiation Oncology*Biology*Physics
Physics ContributionQuality assurance of serial tomotherapy for head and neck patient treatments
Introduction
A commercial serial tomotherapy planning and delivery system1was recently installed and commissioned in our clinic (1). The system is used to deliver conformal dose distributions using serial tomotherapy, and includes a treatment-planning system and computer-driven tertiary dy-namic multileaf collimator (DMLC)2. The DMLC consists of 40 leaves, arranged in two parallel adjacent banks of 20 leaves each, and is oriented so that a line connecting the projected leaf centers is parallel to the gantry rotation plane. The leaves are pneumatically driven and move in a direction normal to the gantry rotation plane, each projecting on our accelerator to a field size (bixel) of 0.84 × 1.0 cm2. The beam is modulated by opening and closing leaves during arc delivery so that, over a narrow gantry angle range (in our case, 5°), the fraction of fluence required from a bixel is equal to the fraction of gantry angle that the leaf is open. The dose calculation algorithm has been described by Wu et al. (2) and Low et al. (3).
Quality assurance (QA) procedures for treatment planning and delivery of traditional conformal therapy have been previously described 4, 5. However, there are significant differences between conventional three-dimensional (3D) conformal therapy and serial tomotherapy treatment deliveries. In conventional therapy, the radiation dose is delivered using portals with high fluence gradients only at portal boundaries. The influence of spatial uncertainties, such as patient movement, can be accounted for by adding a margin to the known and suspected tumor to ensure that the tumor will receive an adequate dose 6, 7. For serial tomotherapy, the radiation dose is delivered by abutting sequential treated volumes, and patient positioning errors of as little as 1 mm can cause dose errors of 10% within the target (8).
The QA of serial tomotherapy dose distributions is more complex than for traditional external beam therapy. For traditional therapy, patient treatments are generated by a set of fixed portals, differing by well-defined degrees of freedom, such as entry angle, portal shape, and intensity. QA of dose calculation and delivery is conducted by definition and verification of a set of experiments encompassing the degrees of freedom encountered in clinical practice (4). The smooth behavior of fixed portal dosimetry as a function of these degrees of freedom allows the definition of a sparse set of test plans to serve as an indication of dose calculation and delivery accuracy for most clinical cases. These dose distributions can be verified using common dosimetry tools and techniques. After a representative selection of beams has been verified, the QA dedicated to individual patients will normally consist of verification of portal geometry, collimator, couch, and gantry angles, source-to–surface distances, and beam modification devices, such as compensating filters. Most of these checks can be conducted by the radiation therapist delivering the treatment and require little added time or effort. The use of record-and–verify systems aids in the efficiency and thoroughness of treatment-specific QA.
Unlike traditional therapy, with serial tomotherapy, no treatment geometry subsets can be used to verify the dose calculation and treatment delivery for a wide range of treatments. Although the dose distribution of an individual bixel can be accurately verified, the large number of bixels used in arc-based serial tomotherapy limits the utility of simple geometric patient-specific verifications. Therefore, a different approach is needed (9).
The accuracy of the intensity-modulated radiation therapy (IMRT) dose distribution calculation algorithm and treatment delivery system was evaluated recently by Low et al. (10), using cubic phantoms (11) and geometric target volumes and critical structures. Ionization chambers, film, and thermoluminescent dosimeters (TLDs) were used. Ionization chamber measurements were used to compare to single-point dose calculations and were found to agree within a standard deviation of 1.0% for regions within the high-dose levels. Ionization chamber measurements also showed an agreement of 1.2% in low-dose regions, averaging 30% of the maximum delivered dose. Spatial localization of the dose distributions was evaluated by using radiographic film placed in the transverse and coronal orientations. The measured and calculated high-dose gradient regions were compared to determine dose distribution localization accuracy. With one exception, the calculated and measured dose distributions agreed within 2 mm.
Verellen et al. (9) described the clinical efficacy of the same tomotherapy system. Patient immobilization and positioning were evaluated using an initial image obtained with a diagnostic x-ray beam and subsequent portal films obtained using the DMLC with all leaves open. Although intertreatment motion was evaluated, no films were obtained to monitor intratreatment motion. Dosimetric verification was conducted using test treatment plans on anthropomorphic phantoms, with spatial positioning of the dose distribution determined using radiographic film and the dose accuracy verified using alanine detectors. Results indicated excellent agreement (discrepancies within 1%) between calculated and measured absolute doses, but doses were measured only for target regions. Doses outside the target were estimated based on TLD measurements placed on the surface of a single patient.
Previous unpublished studies of the dosimetric accuracy of the Peacock treatment planning and delivery system have also been conducted by Grant et al. 12, 13, 14 and Butler et al. (15). We describe the process used to verify the delivered dose to 9 head and neck patients. Additional system QA procedures were implemented to verify sensitive geometric parameters. Patient positioning was verified by comparison of portal films against independently generated digitally reconstructed radiographs. Dose distributions were evaluated for each patient using homogeneous cubic phantoms, with radiographic films used to identify the dose distribution localization, and ionization chambers and TLDs for dose measurements. Doses were evaluated both in high- and low-dose regions. Emphasis was placed on the independent determination of the spatial localization of the measured and calculated dose distributions.
Section snippets
Daily tests
The DMLC was inserted into the slot reserved for the blocking tray assembly hardware. Two retractable steel pins were mounted to the DMLC mounting tray that mated to two holes drilled into the collimator head assembly. When the DMLC was placed in the slot, the pins were inserted into the holes, providing a reproducible reference location. During initial installation, the DMLC was positioned such that the center of the DMLC open field was coincident with the accelerator beam central axis.
System and dose delivery
Figure 2a shows the index abutment film obtained using the nominal index spacing of from 1.64 through 1.72 cm. The sensitivity of the optical density of the overlap and underlap regions on the index spacing is evident. Figure 2 shows images of index films obtained directly before and after the couch bearings were cleaned. A demonstrable improvement was noted after the couch bearings were cleaned, with a maximum motion of > 0.4 mm and ≤ 0.4 mm before and after cleaning, respectively.
Table 2
Discussion and conclusions
The additional physical QA procedures have proven to be useful in identifying potential geometric misalignments of either the patient or the DMLC. In particular, the couch motion tests have more than once determined that the couch longitudinal bearings had become unacceptably dirty and required cleaning. Although the couch motion tests were manually conducted, the high sensitivity of the test allowed a manual inspection to determine if the couch motion was within the selected tolerance of 0.4
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