On the use of the MLC dosimetric leaf gap as a quality control tool for accurate dynamic IMRT delivery.

PURPOSE: MLC leaf gap consistency is critical for the accurate delivery of dynamic IMRT plans. It is estimated that a systematic MLC leaf gap change of 0.6 mm will result in a 2% change to the equivalent uniform dose to a clinical target volume for a typical head and neck sliding window (SW) IMRT plan. The aim of this work is to use the measured dosimetric leaf gap (DLG) to verify the dosimetric reproducibility of dynamically delivered SW IMRT plans. This study focuses on Varian linacs equipped with the 120 Millennium MLC and the Eclipse treatment planning system (TPS), but can be extended to other linac/MLC/TPS combination. METHODS: An ionization chamber, a diode array, and an electronic portal imaging device (EPID) were used to assess the DLG in zero (central axis), one, and two dimensions, respectively. The DLG for zero and two dimensions was derived from measurements of SW fields of decreasing width (2, 1.5, 1, and 0.5 cm). The DLG in one dimension was measured directly from a single SW sweeping across a linear diode array. This one-dimensional DLG measurement was based on the full width at half maximum (FWHM) of the dose rate versus time spectrum. RESULTS: The DLG derived from ion chamber measurements at central axis agrees to within 0.1 mm, with the DLG measured directly from the FWHM of dose rate versus time spectrum. The measured DLG depends on the control points used for the MLC SW fields. When two control points were used, the DLG measured at central axis showed an increase of 0.6 mm with respect to the same measurements performed using three or more control points. The two-dimensional distribution of DLG obtained using the EPID identified leaf gap errors as small as +/- 0.2 mm in isolated areas away from central axis. CONCLUSIONS: Comprehensive measurements of the DLG in 0D, 1D, and 2D provide an accurate assessment of DLG value required during TPS commissioning. These DLG measurements can also be used as a quality control tool to quantify changes of the MLC calibration and leaf gap consistency, which is critical for the accurate delivery of dynamically delivered SW IMRT plans.

Angular dependence of the output of a kilovoltage X-ray therapy unit.

During the recommissioning of a Philips RT-250 kilovoltage X-ray unit, unexpected output variations with tube head rotation (cross-plane) and tube head tilt (in-plane) were observed. The measured output showed an increase of up to 7.3% relative to the neutral position (0? in-plane and 0? cross-plane) over the possible range of angles of in-plane rotation for 75 kVp (half-value layer, HVL = 1.84 mm Al). A less pronounced but noticeable output change (with respect to the neutral position) was observed for cross-plane rotation reaching 2% for the 225 kVp beam (HVL = 0.90 mm Cu). This output variation was observed while manually adjusting the current to maintain constancy according to the current meter gauge. In order to address the observed output dependence with head orientation, the dose rate monitor chamber of the kilovoltage unit was calibrated to monitor the beam output in real time. The dose rate was manually adjusted to maintain a constant dose rate (in r/min) as displayed on the r/min gauge. This approach resulted in maintaining beam output for the 75 kVp and the 225 kVp beams within +/- 2% for the in-plane angle variation and +/- 0.5% for the cross-plane angle variation. A daily output check that includes ion chamber-based measurements at the neutral position and an in-plane angle of 45? has been implemented using the constant dose rate approach to monitor the stability of the X-ray beams. As a result of the output variations with in/cross-plane rotation, the quality control (QC) procedures that are typically used for clinical setup have been modified to test the stability of the beams under the non-neutral positioning of the X-ray tube.

Small bowel dose parameters predicting grade ≥ 3 acute toxicity in rectal cancer patients treated with neoadjuvant chemoradiation: an independent validation study comparing peritoneal space versus small bowel loop contouring techniques.

PURPOSE: To determine whether volumes based on contours of the peritoneal space can be used instead of individual small bowel loops to predict for grade ≥3 acute small bowel toxicity in patients with rectal cancer treated with neoadjuvant chemoradiation therapy. METHODS AND MATERIALS: A standardized contouring method was developed for the peritoneal space and retrospectively applied to the radiation treatment plans of 67 patients treated with neoadjuvant chemoradiation therapy for rectal cancer. Dose-volume histogram (DVH) data were extracted and analyzed against patient toxicity. Receiver operating characteristic analysis and logistic regression were carried out for both contouring methods. RESULTS: Grade ≥3 small bowel toxicity occurred in 16% (11/67) of patients in the study. A highly significant dose-volume relationship between small bowel irradiation and acute small bowel toxicity was supported by the use of both small bowel loop and peritoneal space contouring techniques. Receiver operating characteristic analysis demonstrated that, for both contouring methods, the greatest sensitivity for predicting toxicity was associated with the volume receiving between 15 and 25 Gy. CONCLUSION: DVH analysis of peritoneal space volumes accurately predicts grade ≥3 small bowel toxicity in patients with rectal cancer receiving neoadjuvant chemoradiation therapy, suggesting that the contours of the peritoneal space provide a reasonable surrogate for the contours of individual small bowel loops. The study finds that a small bowel V15 less than 275 cc and a peritoneal space V15 less than 830 cc are associated with a less than 10% risk of grade ≥3 acute toxicity.

FracMod: a computational tool for assessing IMRT field modulation.

The authors develop and investigate a user-friendly computational tool (FracMod) to quantify modulation complexity in planned IMRT fields. FracMod comprises a graphical user interface and variogram fractal dimension (FD) analysis tool developed by the authors using MATLAB(®), and made freely available at http://www.medphysfiles.com/index.php. FracMod is investigated for its ability to identify overly-modulated dynamic IMRT fields designed for prostatic carcinoma treatments. A set of 5 prostate alone plans and 5 prostate plus pelvic node plans were used to choose FD cut-points that ensure no false positives in distinguishing between moderate and high field modulation. IMRT quality control (QC) was performed on all the treatment fields using Varian(®) Portal Dosimetry and MapCHECK™. Receiver operating characteristic analysis was used to quantitatively compare the classification performance of FD and the number of monitor units (MUs). The effect of dose rate on the average leaf pair opening (ALPO) and the number of MUs delivered was also investigated. The variogram FD performed better than the number of MUs in identifying overly-modulated fields. FD thresholds >2.15 for prostate alone and >2.20 for prostate plus pelvic nodes correctly identified 75% and 100% of the high modulation fields, respectively, with no false positives. With appropriate cut-points, MapCHECK™ identified the most highly modulated IMRT fields, whereas Varian(®) Portal Dosimetry could not. As expected, ALPO decreases with increasing modulation and increasing dose rate. FracMod is a user-friendly tool that allows one to accurately quantify and identify overly-modulated IMRT fields at the treatment planning stage before they are sent for patient-specific QC.