S. Comi

Automatic Segmentation and Online virtualCT in Head-and-Neck Adaptive Radiation Therapy

PURPOSE The purpose of this work was to develop and validate an efficient and automatic strategy to generate online virtual computed tomography (CT) scans for adaptive radiation therapy (ART) in head-and-neck (HN) cancer treatment. METHOD We retrospectively analyzed 20 patients, treated with intensity modulated radiation therapy (IMRT), for an HN malignancy. Different anatomical structures were considered: mandible, parotid glands, and nodal gross tumor volume (nGTV). We generated 28 virtualCT scans by means of nonrigid registration of simulation computed tomography (CTsim) and cone beam CT images (CBCTs), acquired for patient setup. We validated our approach by considering the real replanning CT (CTrepl) as ground truth. We computed the Dice coefficient (DSC), center of mass (COM) distance, and root mean square error (RMSE) between correspondent points located on the automatically segmented structures on CBCT and virtualCT. RESULTS Residual deformation between CTrepl and CBCT was below one voxel. Median DSC was around 0.8 for mandible and parotid glands, but only 0.55 for nGTV, because of the fairly homogeneous surrounding soft tissues and of its small volume. Median COM distance and RMSE were comparable with image resolution. No significant correlation between RMSE and initial or final deformation was found. CONCLUSION The analysis provides evidence that deformable image registration may contribute significantly in reducing the need of full CT-based replanning in HN radiation therapy by supporting swift and objective decision-making in clinical practice. Further work is needed to strengthen algorithm potential in nGTV localization.

Salvage image-guided intensity modulated or stereotactic body reirradiation of local recurrence of prostate cancer

OBJECTIVE To retrospectively evaluate external beam reirradiation (re-EBRT) delivered to the prostate/prostatic bed for local recurrence, after radical or adjuvant/salvage radiotherapy (RT). METHODS 32 patients received re-EBRT between February 2008 and October 2013. All patients had clinical/radiological local relapse in the prostate or prostatic bed and no distant metastasis. re-EBRT was delivered with selective RT technologies [stereotactic RT including CyberKnife(TM) (Accuray, Sunnyvale, CA); image-guidance and intensity-modulated RT etc.]. Toxicity was evaluated using the Radiation Therapy Oncology Group/European Organization for Research and Treatment of Cancer criteria. Biochemical control was assessed according to the Phoenix definition (NADIR + 2 ng ml(-1)). RESULTS Acute urinary toxicity: G0, 24 patients; G1, 6 patients; G2, 2 patients. Acute rectal toxicity: G0, 28 patients; G1, 2 patients; and G2, 1 patient. Late urinary toxicity (evaluated in 30 cases): G0, 23 patients; G1, 6 patients; G2, 1 patient. Late renal toxicity: G0, 25 patients; G1, 5 patients. A mean follow-up of 21.3 months after re-EBRT showed that 13 patients were free of cancer, 3 were alive with biochemical relapse and 12 patients were alive with clinically evident disease. Four patients had died: two of disease progression and two of other causes. CONCLUSION re-EBRT using modern technology is a feasible approach for local prostate cancer recurrence offering 2-year tumour control in about half of the patients. Toxicity of re-EBRT is low. Future studies are needed to identify the patients who would benefit most from this treatment. ADVANCES IN KNOWLEDGE Our series, based on experience in one hospital alone, shows that re-EBRT for local relapse of prostate cancer is feasible and offers a 2-year cure in about half of the patients.

Salvage Stereotactic Body Radiotherapy for Isolated Lymph Node Recurrent Prostate Cancer: Single Institution Series of 94 Consecutive Patients and 124 Lymph Nodes

Background The purpose of the study was to evaluate the prostate serum antigen (PSA) response, local control, progression‐free survival (PFS), and toxicity of stereotactic body radiotherapy (SBRT) for lymph node (LN) oligorecurrent prostate cancer. Patients and Methods Between May 2012 and October 2015, 124 lesions were treated in 94 patients with a median dose of 24 Gy in 3 fractions. Seventy patients were treated for a single lesion and 25 for > 1 lesion. In 34 patients androgen deprivation (AD) was combined with SBRT. We evaluated biochemical response according to PSA level every 3 months after SBRT: a 3‐month PSA decrease from pre‐SBRT PSA of more than 10% identified responder patients. In case of PSA level increase, imaging was performed to evaluate clinical progression. Toxicity was assessed every 6 to 9 months after SBRT. Results Median follow‐up was 18.5 months. In 13 patients (14%) Grade 1 to 2 toxicity was reported without any Grade 3 to 4 toxicity. Biochemical response, stabilization, and progression were observed in 64 (68%), 10 (11%), and 20 (21%) of 94 evaluable patients. Clinical progression was observed in 31 patients (33%) after a median time of 8.1 months. In‐field progression occurred in 12 lesions (9.7%). Two‐year local control and PFS rates were 84% and 30%, respectively. Age older than 75 years correlated with better biochemical response rate. Age older than 75 years, concomitant AD administered up to 12 months, and pelvic LN involvement correlated with longer PFS. Conclusion SBRT is safe and offers good in‐field control. At 2 years after SBRT, 1 of 3 patients is progression‐free. Further investigation is warranted to identify patients who benefit most from SBRT and to define the optimal combination with AD. Micro‐Abstract Stereotactic body radiotherapy is being investigated in nodal oligometastatic prostate cancer recurrences as an alternative to systemic treatment. This approach yields excellent in‐field control and a low toxicity profile. In selected cases, this approach might also defer palliative androgen deprivation therapy.

Geometric and dosimetric accuracy and imaging dose of the real-time tumour tracking system of a gimbal mounted linac

PURPOSE To suggest a comprehensive testing scheme to evaluate the geometric and dosimetric accuracy and the imaging dose of the VERO dynamic tumour tracking (DTT) for its clinical implementation. METHODS Geometric accuracy was evaluated for gantry 0° and 90° in terms of prediction (EP), mechanical (EM) and tracking (ET) errors for sinusoidal patterns with 10 and 20 mm amplitudes, 2-6 s periods and phase shift up to 1 s and for 3 patient patterns. The automatic 4D model update was investigated simulating changes in the breathing pattern during treatment. Dosimetric accuracy was evaluated with gafchromic films irradiated in static and moving phantom with and without DTT. The entrance skin dose (ESD) was assessed using a solid state detector and gafchromic films. RESULTS The RMS of EP, EM, and ET were up to 0.8, 0.5 and 0.9 mm for all non phased-shifted motion patterns while for the phased-shifted ones, EP and ET increased to 2.2 and 2.6 mm. Up to 4 updates are necessary to restore a good correlation model, according to type of change. For 100 kVp and 1 mA s X-ray beam, the ESD per portal due to 20 s fluoroscopy was 16.6 mGy, while treatment verification at a frequency of 1 Hz contributed with 4.2 mGy/min. CONCLUSIONS The proposed testing scheme highlighted that the VERO DTT system tracks a moving target with high accuracy. The automatic update of the 4D model is a powerful tool to guarantee the accuracy of tracking without increasing the imaging dose.

Dosimetric characterization of 3D printed bolus at different infill percentage for external photon beam radiotherapy

BACKGROUND AND PURPOSE 3D printing is rapidly evolving and further assessment of materials and technique is required for clinical applications. We evaluated 3D printed boluses with acrylonitrile butadiene styrene (ABS) and polylactide (PLA) at different infill percentage. MATERIAL AND METHODS A low-cost 3D printer was used. The influence of the air inclusion within the 3D printed boluses was assessed thoroughly both with treatment planning system (TPS) and with physical measurements. For each bolus, two treatment plans were calculated with Monte Carlo algorithm, considering the computed tomography (CT) scan of the 3D printed bolus or modelling the 3D printed bolus as a virtual bolus structure with a homogeneous density. Depth dose measurements were performed with Gafchromic films. RESULTS High infill percentage corresponds to high density and high homogeneity within bolus material. The approximation of the bolus in the TPS as a homogeneous material is satisfying for infill percentages greater than 20%. Measurements performed with PLA boluses are more comparable to the TPS calculated profiles. For boluses printed at 40% and 60% infill, the discrepancies between calculated and measured dose distribution are within 5%. CONCLUSIONS 3D printing technology allows modulating the shift of the build-up region by tuning the infill percentage of the 3D printed bolus in order to improve superficial target coverage.

Set-up errors in head and neck cancer patients treated with intensity modulated radiation therapy: Quantitative comparison between three-dimensional cone-beam CT and two-dimensional kilovoltage images

OBJECTIVES To compare the patient set-up error detection capabilities of three-dimensional cone beam computed tomography (3D-CBCT) and two-dimensional orthogonal kilovoltage (2D-kV) techniques. METHODS 3D-CBCT and 2D-kV projections were acquired on 29 head-and-neck (H&N) patients undergoing Intensity Modulated Radiotherapy (IMRT) on the first day of treatment (time 0) and after the delivery of 40 Gy and 50 Gy. Set-up correction vectors were analyzed after fully automatic image registration as well as after revision by radiation oncologists. The dosimetric effects of the different sensitivities of the two image guidance techniques were assessed. RESULTS A statistically significant correlation among detected set-up deviations by the two techniques was found along anatomical axes (0.60 < ρ < 0.72, p < 0.0001); no correlation was found for table rotation (p = 0.41). No evidence of statistically significant differences between the indications provided along the course of the treatment was found; this was also the case when full automatic versus manually refined correction vectors were compared. The dosimetric effects analysis revealed slight statistically significant differences in the median values of the maximum relative dose to mandible, spinal cord and its 5 mm Planning Organ at Risk Volume (0.95%, 0.6% and 2.45%, respectively), with higher values (p < 0.01) observed when 2D-kV corrections were applied. CONCLUSION A similar sensitivity to linear set-up errors was observed for 2D-kV and 3D-CBCT image guidance techniques in our H&N patient cohort. Higher rotational deviations around the table vertical axis were detected by the 3D-CBCT with respect to the 2D-kV method, leading to a consistent better sparing of organs at risk.

Radiotherapy of Hodgkin and Non-Hodgkin Lymphoma: A Nonrigid Image-Based Registration Method for Automatic Localization of Prechemotherapy Gross Tumor Volume.

Purpose: To improve the contouring of clinical target volume for the radiotherapy of neck Hodgkin/non-Hodgkin lymphoma by localizing the prechemotherapy gross target volume onto the simulation computed tomography using [18F]-fluorodeoxyglucose positron emission tomography/computed tomography. Material and Methods: The gross target volume delineated on prechemotherapy [18F]-fluorodeoxyglucose positron emission tomography/computed tomography images was warped onto simulation computed tomography using deformable image registration. Fifteen patients with neck Hodgkin/non-Hodgkin lymphoma were analyzed. Quality of image registration was measured by computing the Dice similarity coefficient on warped organs at risk. Five radiation oncologists visually scored the localization of automatic gross target volume, ranking it from 1 (wrong) to 5 (excellent). Deformable registration was compared to rigid registration by computing the overlap index between the automatic gross target volume and the planned clinical target volume and quantifying the V95 coverage. Results: The Dice similarity coefficient was 0.80 ± 0.07 (median ± quartiles). The physicians’ survey had a median score equal to 4 (good). By comparing the rigid versus deformable registration, the overlap index increased from a factor of about 4 and the V95 (percentage of volume receiving the 95% of the prescribed dose) went from 0.84 ± 0.38 to 0.99 ± 0.10 (median ± quartiles). Conclusion: This study demonstrates the impact of using deformable registration between prechemotherapy [18F]-fluorodeoxyglucose positron emission tomography/computed tomography and simulation computed tomography, in order to automatically localize the gross target volume for radiotherapy treatment of patients with Hodgkin/non-Hodgkin lymphoma.

PO-0995: Evaluation of VMAT-RapidArc, IMRT-VERO and proton-RT for a hypofractionated scheme of prostate cancer treatment

Conclusions: Incident analysis, task analyses, cognitive models and existing HRA databases have been used to build the qualitative taxonomic structure for the HRA method. These taxonomies are a first step to support the systematic analysis of potential errors and influencing factors. Next steps: Complete characterization of GTTs and PIFs using domain expert opinions and the adapted cognitive models. Develop the model for estimating error probabilities and test the HRA method to assess patient safety at a specific RT center.

Can the Day 0 CT-scan predict the post-implant scanning? Results from 136 prostate cancer patients

PURPOSE Post-implant CT-scanning is an essential part of permanent prostate brachytherapy. However, the evaluation of post-implant CT dosimetry is not straightforward due to the edema that can modify the dose to the prostate and to the organs at risk. The aim of this study is to evaluate the impact of the timing of the post-implant CT-scan on the dosimetric results and to verify if the Day 0 scan findings can predict Day 50 scanning. METHODS 136 consecutive patients who received monotherapy with I-125 implants were selected for this study. Two sets of 8 dosimetric quality parameters corresponding to 2 different CT-scans (Day 0 and Day 50) were calculated and compared. The dosimetric parameters included are the percentage volume of the post-implant prostate receiving 80%, 100% and 150% of the prescribed dose, the doses covering 80% and 90% of the prostate volume and the Dose Homogeneity Index. The values of the dose covering 1cm3 of the rectum and urethra were assessed. RESULTS All the dosimetric parameters of the Day 50 were higher than those of the Day 0 scan. Linear functions were obtained that calculate D90 and V100 values at Day 50 based on the Day 0 findings. Rectal and urethral parameters tended to be underestimated on Day 0 CT-scan relative to Day 50 based dosimetry. CONCLUSIONS Predicting the Day 50 dosimetry from the Day 0 scan could be a possible alternative to a Day 50 scan only in specific situations, but with a degree of uncertainty in the predicted values.

PO-0848: Image-guided lung stereotactic treatments with the vero system

· A thin plate spline deformation, adapting the original fluence0 into fluenceX. In contrast to Ref. marker positions are used instead of organ contours. · An adapted leaf motion, calculated in Eclipse (Varian Medical Systems, Palo Alto, CA). For the dosimetric step ray tracing is used to calculate the radiological path length d, from the source to each marker. Next, the tissue to phantom ratio, TPR(0.5,d), is calculated using a fixed 0.5cm field size. The TPRplanX / TPRplan0-median over the different markers rescales the number of monitor units of each beam. For validation a 5 beam sliding window IMRT plan is optimized for the TG119 prostate structures. The phantom is extended with 4 markers and bony anatomy. The initial plan0 delivers 77Gy(2.2Gy/Fr). Fractions are simulated by applying translations and isotropic scaling using literature values (Table 1). The isotropic expansion is derived from the shrinkage factor, and is used to evaluate the robustness of our approach.