Implementation of FFT convolution and multigrid superposition models in the FOCUS RTP system

In radiotherapy treatment planning, convolution/superposition algorithms currently represent the best practical approach for accurate photon dose calculation in heterogeneous tissues. In this work, the implementation, accuracy and performance of the FFT convolution (FFTC) and multigrid superposition (MGS) algorithms are presented. The FFTC and MGS models use the same 'TERMA' calculation and are commissioned using the same parameters. Both models use the same spectra, incorporate the same off-axis softening and base incident lateral fluence on the same measurements. In addition, corrections are explicitly applied to the polyenergetic and parallel kernel approximations, and electron contamination is modelled. Spectra generated by Monte Carlo (MC) modelling of treatment heads are used. Calculations using the MC spectra were in excellent agreement with measurements for many linear accelerator types. To speed up the calculations, a number of calculation techniques were implemented, including separate primary and scatter dose calculation, the FFT technique which assumes kernel invariance for the convolution calculation and a multigrid (MG) acceleration technique for the superposition calculation. Timing results show that the FFTC model is faster than MGS by a factor of 4 and 8 for small and large field sizes, respectively. Comparisons with measured data and BEAM MC results for a wide range of clinical beam setups show that (a) FFTC and MGS doses match measurements to better than 2% or 2 mm in homogeneous media; (b) MGS is more accurate than FFTC in lung phantoms where MGS doses are within 3% or 3 mm of BEAM results and (c) FFTC overestimates the dose in lung by a maximum of 9% compared to BEAM.     

A detailed evaluation of TomoDirect 3DCRT planning for whole-breast radiation therapy

The goal of this work was to develop planning strategies for whole-breast radiotherapy (WBRT) using TomoDirect three-dimensional conformal radiation therapy (TD-3DCRT) and to compare TD-3DCRT with conventional 3DCRT and TD intensity-modulated radiation therapy (TD-IMRT) to evaluate differences in WBRT plan quality. Computed tomography (CT) images of 10 women were used to generate 150 WBRT plans, varying in target structures, field width (FW), pitch, and number of beams. Effects on target and external maximum doses (EMD), organ-at-risk (OAR) doses, and treatment time were assessed for each parameter to establish an optimal planning technique. Using this technique, TD-3DCRT plans were generated and compared with TD-IMRT and standard 3DCRT plans. FW 5.0 cm with pitch = 0.250 cm significantly decreased EMD without increasing lung V20 Gy. Increasing number of beams from 2 to 6 and using an additional breast planning structure decreased EMD though increased lung V20 Gy. Changes in pitch had minimal effect on plan metrics. TD-3DCRT plans were subsequently generated using FW 5.0 cm, pitch = 0.250 cm, and 2 beams, with additional beams or planning structures added to decrease EMD when necessary. TD-3DCRT and TD-IMRT significantly decreased target maximum dose compared to standard 3DCRT. FW 5.0 cm with 2 to 6 beams or novel planning structures or both allow for TD-3DCRT WBRT plans with excellent target coverage and OAR doses. TD-3DCRT plans are comparable to plans generated using TD-IMRT and provide an alternative to conventional 3DCRT for WBRT.     

Breast conservation surgery and interstitial brachytherapy in the management of locally recurrent carcinoma of the breast: the Allegheny General Hospital experience

PURPOSE: To evaluate lumpectomy followed by interstitial brachytherapy as an acceptable salvage therapy for women who have developed localized recurrence of breast cancer after conservation surgery and postoperative external radiotherapy. METHODS AND MATERIALS: Between 1/1998 and 10/2006, 21 patients with T0 or T1 in-breast recurrence of carcinoma were offered interstitial low-dose rate brachytherapy after tumor re-excision as an alternative to salvage mastectomy. All patients had failed lumpectomy followed by standard postoperative external beam radiotherapy (range, 5000-6040cGy) as treatment for the initial breast carcinoma. Seven recurred as ductal carcinoma in situ, 2 as infiltrating lobular carcinoma, and 12 as recurrent invasive carcinoma. The recurrent tumors were excised with final margins of resection free of residual disease per National Surgical Adjuvant Breast and Bowel Project definition. Tumor bed implantation was then carried out with an interstitial technique using (192)Ir with the target volume consisting of the tumor bed plus a minimum 1.0-cm clinical margin. The required minimum dose delivered to the target volume was 4500-5000cGy (range, 4500-5530). RESULTS: Twenty of 21 patients were free of local disease with a median observation time of 40 months (range, 3-69). The single patient who developed a second local recurrence was treated successfully with simple mastectomy. Two patients succumbed to systemic disease at 17 and 24 months after salvage implant therapy. One patient developed a contralateral breast cancer. Cosmetic results defined by the National Surgical Adjuvant Breast and Bowel Project cosmesis scale were acceptable. One patient developed a localized seroma requiring multiple needle aspirations before complete resolution. Two patients developed localized skin breakdown in the tumor bed. One healed after 6 months of conservative treatment. The other healed 9 months later with Grade II cosmesis. This patient also developed a concurrent postoperative wound infection. CONCLUSIONS: Repeat lumpectomy followed by brachytherapy is feasible and may be an acceptable alternative to salvage mastectomy in patients who locally fail conservation breast therapy; however, longer followup and greater patient numbers may be needed to better define the role of salvage brachytherapy.     

Dosimetric and technical aspects of intraoperative I-125 brachytherapy for stage I non-small cell lung cancer

Initial treatment outcome data from our institution for stage I non-small cell lung cancer (NSCLC) patients have shown that sublobar resection in combination with iodine-125 (I-125) brachytherapy is associated with recurrence rates of 2.0%, compared to 18.6% with sublobar resection alone. In this work, the technical and dosimetric aspects required to execute this procedure from the radiation oncology perspective as well as an analysis of the dose distributions of patients treated with this technique are presented. In this treatment technique, I-125 seeds in vicryl suture are embedded into vicryl mesh and surgically inserted providing a 2.0 cm margin on each side of the resection staple line. A nomogram is developed to determine the suture spacing in the vicryl mesh, as a function of seed activity in order to deliver 120 Gy at a distance of 0.5 cm above and below the seed array. Post-operative dosimetry consists of a CT-based planning and dose volume analysis. Dose distributions, dose volume histograms and mean dose data for lung are analysed in a group of patients. Dosimetric results show significant lung sparing with only a small volume of lung irradiated for all patients with mean lung dose values ranging from 1.5 Gy to 5.4 Gy. Lung brachytherapy with I-125 at the time of sublobar resection is a highly conformal option of dose delivery for stage I NSCLC patients with compromised physiologic reserve. Patient-related toxicity clinically measured by loss of pulmonary function and radiation-induced pneumonitis have not been linked to this procedure.     

Monitoring tumor motion with on-line mega-voltage cone-beam computed tomography imaging in a cine mode

Accurate daily patient localization is becoming increasingly important in external-beam radiotherapy (RT). Mega-voltage cone-beam computed tomography (MV-CBCT) utilizing a therapy beam and an on-board electronic portal imager can be used to localize tumor volumes and verify the patient's position prior to treatment. MV-CBCT produces a static volumetric image and therefore can only account for inter-fractional changes. In this work, the feasibility of using the MV-CBCT raw data as a fluoroscopic series of portal images to monitor tumor changes due to e.g. respiratory motion was investigated. A method was developed to read and convert the CB raw data into a cine. To improve the contrast-to-noise ratio on the MV-CB projection data, image post-processing with filtering techniques was investigated. Volumes of interest from the planning CT were projected onto the MV-cine. Because of the small exposure and the varying thickness of the patient depending on the projection angle, soft-tissue contrast was limited. Tumor visibility as a function of tumor size and projection angle was studied. The method was well suited in the upper chest, where motion of the tumor as well as of the diaphragm could be clearly seen. In the cases of patients with non-small cell lung cancer with medium or large tumor masses, we verified that the tumor mass was always located within the PTV despite respiratory motion. However for small tumors the method is less applicable, because the visibility of those targets becomes marginal. Evaluation of motion in non-superior-inferior directions might also be limited for small tumor masses. Viewing MV-CBCT data in a cine mode adds to the utility of MV-CBCT for verification of tumor motion and for deriving individualized treatment margins.     

Comparison of mega-voltage cone-beam computed tomography prostate localization with online ultrasound and fiducial markers methods

The online image-guided localization data from 696 ultrasound (US), 598 mega-voltage cone-beam computed tomography (MV-CBCT), and 393 seed markers (SMs) couch alignments for patients undergoing intensity modulation radiotherapy of the prostate were analyzed. Daily US, MV-CBCT and SM images were acquired for 19, 17 and 12 patients, respectively, after each patient was immobilized in a vacuum cradle and setup to skin markers as the center of mass. The couch shifts applied in the lateral (left-right/LR), vertical (anterior-posterior/AP), and longitudinal (superior-inferior/SI) directions, along with the magnitude of the three-dimensional (3D) shift vector, were analyzed and compared for all three methods. The percentage of shifts larger than 5 mm in all directions was also compared. Clinical target volume-planning target volume (CTV-to-PTV) expansion margins were estimated based on the localization data with US, CB, and SM image guidance. Results show the US data have greater variability. Systematic and random shifts were -1.2 +/- 6.8 mm (LR), -2.8 +/- 5.1 mm (SI) and -1.0 +/- 5.9 mm (AP) for US, 1.0 +/- 3.9 mm (LR), -1.3 +/- 2.5 mm (SI) and -0.3 +/- 3.9 mm (AP) for CB, and -1.0 +/- 3.4 mm (LR), 0.0 +/- 3.4 mm (SI) and 0.5 +/- 4.1 mm (AP) for SM. The mean 3D shift distance was larger using US (8.8 +/- 6.2 mm) compared to CB and SM (5.3 +/- 3.4 mm and 5.2 +/- 3.7 mm, respectively). The percentage of US shifts larger than 5 mm were 34%, 31%, and 38% in the LR, SI, and AP directions, respectively, compared to 18%, 6%, and 16% for CB and 14%, 10%, and 20% for SM. MV-CBCT and SM localization data suggest a different distribution of prostate center-of-mass shifts with smaller variability, compared to US. The online MV-CBCT and SM image-guidance data show that for treatments that do not include daily prostate localization, one can use a CTV-to-PTV margin that is 4 mm smaller than the one suggested by US data, hence allowing more rectum and bladder sparing and potentially improving the therapeutic ratio.     

Rotational setup errors in pediatric stereotactic radiation therapy

PurposeStereotactic radiation therapy (SRT) is an increasingly commonly used technique in children. The use of image guidance increases the ability to accurately position patients. With our robotic couch, rotational errors that can be corrected are limited to approximately 3 degrees. Given this limitation, we reviewed the rotational setup errors in our pediatric brain tumor population.Methods and MaterialsWe reviewed the rotational corrections for all pediatric (age ≤ 21 years old) patients treated at our facility from 2009 to 2011. We compared children < 5 years old treated to children between 5 and 21 years old (≥ 5 years old). Also, we analyzed the effect of steroid use and trends in rotational errors over the treatment period in each age group.ResultsThe mean pitch, roll, and yaw rotational setup errors for younger children are ? 0.70 ± 2.60 degrees, ? 0.06 ± 1.89 degrees, and 0.69 ± 2.42 degrees, respectively; for children ≥ 5 years old, they are 0.46 ± 2.09 degrees, ? 0.06 ± 1.89 degrees, and 0.69 ± 2.42 degrees, respectively. The mean pitch corrections are larger for children < 5 years old (P < .001) and the variance of the pitch, roll, and yaw corrections are all larger for children < 5 years old (P < .001). The frequency of rotational errors above 3 degrees for pitch, roll, and yaw is 21.7%, 10.6%, and 20.9% for children < 5 years old, and 15.6%, 2.1%, and 13.8% for children ≥ 5 years old. In both age groups, pitch and roll corrections were larger for children treated with steroids.ConclusionsRotational errors in our pediatric population occur more frequently than previously reported, and are more common in younger children and in children treated with steroids. These rotational set up errors may not be fully correctable due to mechanical and safety limitations. We have altered our planning and treatment process to better account for rotational errors in children receiving SRT.     

Investigation of Simple IMRT Delivery Techniques for Non-Small Cell Lung Cancer Patients with Respiratory Motion Using 4DCT

Techniques for generating simplified IMRT treatment plans for treating non-small cell lung cancer (NSCLC) patients with respiratory motion were investigated. To estimate and account for respiratory motion, 4-dimensional computed tomography (4DCT) datasets from 5 patients were used to design 5-field 6-MV ungated step-and-shoot intensity modulated radiotherapy (IMRT) plans delivering a dose of 66 Gy to the planning target volume (PTV). For each patient, 2 plans were generated using the mean intensity and the maximum intensity of 10 CT datasets from different breathing phases. The plans also utilized different margins around the clinical target volume/internal target volume (CTV/ITV) to account for tumor motion. To reduce the treatment time and ensure accurate dose delivery to moving targets, the number of intensity levels was minimized while maintaining dose coverage to PTV and minimizing dose to organs at risk (OARs). Dose-volume histograms (DVHs), dosimetric metrics, and outcome probabilities were evaluated for all plans. Plans using the averaged CT image dataset were inferior, requiring larger margins around the PTV, with a maximum of 1.5 cm, to ensure coverage of the tumor, and therefore increased the dose to OARs located in proximity of the tumor. The plans based on superimposed CT image datasets achieved full coverage of the tumor, while allowing tight margins around the PTV and minimizing the dose to OARs. A small number of intensity-levels (3 to 5), resulting in IMRT plans with a total of 13 to 30 segments, were sufficient for homogeneous PTV coverage, without affecting the sparing of OARs. In conclusion, a technique involving treatment planning with the superimposed CT scans of all respiratory phases, and the application of IMRT with only a small number of segments was feasible despite significant tumor motion; however, greater patient numbers are needed to support the statistical significance of the results presented in this work.