Urethra-Sparing Stereotactic Body Radiation Therapy for Prostate Cancer: Quality Assurance of a Randomized Phase 2 Trial.

PURPOSE: To present the radiation therapy quality assurance results from a prospective multicenter phase 2 randomized trial of short versus protracted urethra-sparing stereotactic body radiation therapy (SBRT) for localized prostate cancer. METHODS AND MATERIALS: Between 2012 and 2015, 165 patients with prostate cancer from 9 centers were randomized and treated with SBRT delivered either every other day (arm A, n = 82) or once a week (arm B, n = 83); 36.25 Gy in 5 fractions were prescribed to the prostate with (n = 92) or without (n = 73) inclusion of the seminal vesicles (SV), and the urethra planning-risk volume received 32.5 Gy. Patients were treated either with volumetric modulated arc therapy (VMAT; n = 112) or with intensity modulated radiation therapy (IMRT; n = 53). Deviations from protocol dose constraints, planning target volume (PTV) homogeneity index, PTV Dice similarity coefficient, and number of monitor units for each treatment plan were retrospectively analyzed. Dosimetric results of VMAT versus IMRT and treatment plans with versus without inclusion of SV were compared. RESULTS: At least 1 major protocol deviation occurred in 51 patients (31%), whereas none was observed in 41. Protocol violations were more frequent in the IMRT group (P < .001). Furthermore, the use of VMAT yielded better dosimetric results than IMRT for urethra planning-risk volume D98% (31.1 vs 30.8 Gy, P < .0001), PTV D2% (37.9 vs 38.7 Gy, P < .0001), homogeneity index (0.09 vs 0.10, P < .0001), Dice similarity coefficient (0.83 vs 0.80, P < .0001), and bladder wall V50% (24.5% vs 33.5%, P = .0001). To achieve its goals volumetric modulated arc therapy required fewer monitor units than IMRT (2275 vs 3378, P <.0001). The inclusion of SV in the PTV negatively affected the rectal wall V90% (9.1% vs 10.4%, P = .0003) and V80% (13.2% vs 15.7%, P = .0003). CONCLUSIONS: Protocol deviations with potential impact on tumor control or toxicity occurred in 31% of patients in this prospective clinical trial. Protocol deviations were more frequent with IMRT. Prospective radiation therapy quality assurance protocols should be strongly recommended for SBRT trials to minimize potential protocol deviations.

Urethra-sparing stereotactic body radiotherapy for prostate cancer: how much can the rectal wall dose be reduced with or without an endorectal balloon?

BACKGROUND: This is a dosimetric comparative study intended to establish appropriate low-to-intermediate dose-constraints for the rectal wall (Rwall) in the context of a randomized phase-II trial on urethra-sparing stereotactic body radiotherapy (SBRT) for prostate cancer. The effect of plan optimization on low-to-intermediate Rwall dose and the potential benefit of an endorectal balloon (ERB) are investigated. METHODS: Ten prostate cancer patients, simulated with and without an ERB, were planned to receive 36.25Gy (7.25Gyx5) to the planning treatment volume (PTV) and 32.5Gy to the urethral planning risk volume (uPRV). Reference plans with and without the ERB, optimized with respect to PTV and uPRV coverage objectives and the organs at risk dose constraints, were further optimized using a standardized stepwise approach to push down dose constraints to the Rwall in the low to intermediate range in five sequential steps to obtain paired plans with and without ERB (Vm1 to Vm5). Homogeneity index for the PTV and the uPRV, and the Dice similarity coefficient (DSC) for the PTV were analyzed. Dosimetric parameters for Rwall including the median dose and the dose received by 10 to 60% of the Rwall, bladder wall (Bwall) and femoral heads (FHeads) were compared. The monitor units (MU) per plan were recorded. RESULTS: Vm4 reduced by half D30%, D40%, D50%, and Dmed for Rwall and decreased by a third D60% while HIPTV, HIuPRV and DSC remained stable with and without ERB compared to Vmref. HIPTV worsened at Vm5 both with and without ERB. No statistical differences were observed between paired plans on Rwall, Bwall except a higher D2% for Fheads with and without an ERB. CONCLUSIONS: Further optimization to the Rwall in the context of urethra sparing prostate SBRT is feasible without compromising the dose homogeneity to the target. Independent of the use or not of an ERB, low-to-intermediate doses to the Rwall can be significantly reduced using a four-step sequential optimization approach.

Setup accuracy of the Novalis ExacTrac 6DOF system for frameless radiosurgery.

PURPOSE: Stereotactic radiosurgery using frame-based positioning is a well-established technique for the treatment of benign and malignant lesions. By contrast, a new trend toward frameless systems using image-guided positioning techniques is gaining mainstream acceptance. This study was designed to measure the detection and positioning accuracy of the ExacTrac/Novalis Body (ET/NB) for rotations and to compare the accuracy of the frameless with the frame-based radiosurgery technique. METHODS AND MATERIALS: A program was developed in house to rotate reference computed tomography images. The angles measured by the system were compared with the known rotations. The accuracy of ET/NB was evaluated with a head phantom with seven lead beads inserted, mounted on a treatment couch equipped with a robotic tilt module, and was measured with a digital water level and portal films. Multiple hidden target tests (HTT) were performed to measure the overall accuracy of the different positioning techniques for radiosurgery (i.e., frameless and frame-based with relocatable mask or invasive ring, respectively). RESULTS: The ET/NB system can detect rotational setup errors with an average accuracy of 0.09° (standard deviation [SD] 0.06°), 0.02° (SD 0.07°), and 0.06° (SD 0.14°) for longitudinal, lateral, and vertical rotations, respectively. The average positioning accuracy was 0.06° (SD 0.04°), 0.08° (SD 0.06°), and 0.08° (SD 0.07°) for longitudinal, lateral and vertical rotations, respectively. The results of the HTT showed an overall three-dimensional accuracy of 0.76 mm (SD 0.46 mm) for the frameless technique, 0.87 mm (SD 0.44 mm) for the relocatable mask, and 1.19 mm (SD 0.45 mm) for the frame-based technique. CONCLUSIONS: The study showed high detection accuracy and a subdegree positioning accuracy. On the basis of phantom studies, the frameless technique showed comparable accuracy to the frame-based approach.

Single fraction versus fractionated linac-based stereotactic radiotherapy for vestibular schwannoma: a single-institution experience.

PURPOSE: To evaluate and compare outcomes for patients with vestibular schwannoma (VS) treated in a single institution with linac-based stereotactic radiosurgery (SRS) or by fractionated stereotactic radiotherapy (SRT). METHODS AND MATERIALS: One hundred and nineteen patients (SRS = 78, SRT = 41) were treated. For both SRS and SRT, beam shaping is performed by a mini-multileaf collimator. For SRS, a median single dose of 12.5 Gy (range, 11-14 Gy), prescribed to the 80% isodose line encompassing the target, was applied. Of the 42 SRT treatments, 32 treatments consisted of 10 fractions of 3-4 Gy, and 10 patients received 25 sessions of 2 Gy, prescribed to the 100% with the 95% isodose line encompassing the planning target volume. Mean largest tumor diameter was 16.6 mm in the SRS and 24.6 mm in the SRT group. Local tumor control, cranial nerve toxicity, and preservation of useful hearing were recorded. Any new treatment-induced cranial nerve neuropathy was scored as a complication. RESULTS: Median follow-up was 62 months (range, 6-136 months), 5 patients progressed, resulting in an overall 5-year local tumor control of 95%. The overall 5-year facial nerve preservation probability was 88% and facial nerve neuropathy was statistically significantly higher after SRS, after prior surgery, for larger tumors, and in Koos Grade ≥3. The overall 5-year trigeminal nerve preservation probability was 96%, not significantly influenced by any of the risk factors. The overall 4-year probability of preservation of useful hearing (Gardner-Robertson score 1 or 2) was 68%, not significantly different between SRS or SRT (59% vs. 82%, p = 0.089, log rank). CONCLUSION: Linac-based RT results in good local control and acceptable clinical outcome in small to medium-sized vestibular schwannomas (VSs). Radiosurgery for large VSs (Koos Grade ≥3) remains a challenge because of increased facial nerve neuropathy.

Geometric accuracy of a novel gimbals based radiation therapy tumor tracking system.

PURPOSE: VERO is a novel platform for image guided stereotactic body radiotherapy. Orthogonal gimbals hold the linac-MLC assembly allowing real-time moving tumor tracking. This study determines the geometric accuracy of the tracking. MATERIALS AND METHODS: To determine the tracking error, an 1D moving phantom produced sinusoidal motion with frequencies up to 30 breaths per minute (bpm). Tumor trajectories of patients were reproduced using a 2D robot and pursued with the gimbals tracking system prototype. Using the moving beam light field and a digital-camera-based detection unit tracking errors, system lag and equivalence of pan/tilt performance were measured. RESULTS: The system lag was 47.7 ms for panning and 47.6 ms for tilting. Applying system lag compensation, sinusoidal motion tracking was accurate, with a tracking error 90% percentile E(90%)<0.82 mm and similar performance for pan/tilt. Systematic tracking errors were below 0.14 mm. The 2D tumor trajectories were tracked with an average E(90%) of 0.54 mm, and tracking error standard deviations of 0.20 mm for pan and 0.22 mm for tilt. CONCLUSIONS: In terms of dynamic behavior, the gimbaled linac of the VERO system showed to be an excellent approach for providing accurate real-time tumor tracking in radiation therapy.

Prospective, risk-adapted strategy of stereotactic body radiotherapy for early-stage non-small-cell lung cancer: results of a Phase II trial.

PURPOSE: Validation of a prospective, risk-adapted strategy for early-stage non-small-cell lung cancer (NSCLC) patients treated with stereotactic body radiotherapy (SBRT). METHODS AND MATERIALS: Patients with a T1-3N0M0 (American Joint Committee on Cancer 6th edition) NSCLC were accrued. Using the Radiation Therapy Oncology Group definition, patients were treated to a total dose of 60,Gy in three fractions for peripherally located lesions and four fractions for centrally located lesions. The primary endpoint was toxicity, graded according to the Radiation Therapy Oncology Group acute and late morbidity scoring system, and the National Cancer Institute Common Terminology Criteria for Adverse Events Version 3.0. Secondary endpoints were local control and survival. RESULTS: A total of 40 patients were included, 17 with a centrally located lesion. The lung toxicity-free survival estimate at 2 years was 74% and was related to the location (central vs. peripheral) and the size of the target volume. No dose volumetric parameters could predict the occurrence of lung toxicity. One patient died because of treatment-related toxicity. The 1-year and 2-year local progression-free survival estimates were 97% and 84%, respectively, and were related to stage (T1 vs. T2) related (p = 0.006). Local failure was not more frequent for patients treated in four fractions. The 1-year local progression-free survival estimate dropped below 80% for lesions with a diameter of more than 4 cm. CONCLUSION: The proposed risk-adapted strategy for both centrally and peripherally located lesions showed an acceptable toxicity profile while maintaining excellent local control rates. The correlation between local control and tumor diameter calls for the inclusion of tumor stage as a variable in future study design.

The effect of tomotherapy imaging beam output instabilities on dose calculation.

A radiotherapy treatment plan is based on an anatomical 'snapshot' of the patient acquired during the preparation stage using a kVCT (kilovolt computed tomography) scanner. Anatomical changes will occur during the treatment course, in some cases requiring a new treatment plan to deliver the prescribed dose. With the introduction of 3D volumetric on-board imaging devices, it became feasible to use the produced images for dose recalculation. However, the use of these on-board imaging devices in clinical routine for the calculation of dose depends on the stability of the images. In this study the validation of tomotherapy MVCT (megavolt computed tomography) produced images, for the purpose of dose recalculation by the Planned Adaptive software, has been performed. To investigate the validity of MVCT images for dose calculation, a treatment plan was created based on kVCT-acquired images of a solid water phantom. During a period of 4 months, MVCT images of the phantom have been acquired and were used by the planned adaptive software to recalculate the initial kVCT-based dose on the MVCT images. The influence of the adapted IVDTs (image value-to-density tables) has been investigated as well as the effect of image acquisition with or without preceding airscan. Output fluctuations and/or instabilities of the imaging beam result in MV images of different quality yielding different results when used for dose calculation. It was shown that the output of the imaging beam is not stable, leading to differences of nearly 3% between the original kV-based dose and the recalculated MV-based dose, for solid water only. MVCT images can be used for dose calculation purposes bearing in mind that the output beam is liable to fluctuations. The acquisition of an IVDT together with the MVCT image set, that is going to be used for dose calculation, is highly recommended.

An in-house developed resettable MOSFET dosimeter for radiotherapy.

The purpose of this note is to report the feasibility and clinical validation of an in-house developed MOSFET dosimetry system and describe an integrated non-destructive reset procedure. Off-the-shelf MOSFETs are connected to a common PC using an 18 bit/analogue-input and 16 bit/output data acquisition card. A reading algorithm was developed defining the zero-temperature-coefficient point (ZTC) to determine the threshold voltage. A wireless interface was established for ease of use. The reset procedure consists of an internal circuit generating a local heating induced by an electrical current. Sensitivity has been investigated as a function of bias voltage (0-9 V) to the gate. Dosimetric properties have been evaluated for 6 MV and 15 MV clinical photon beams and in vivo benchmarking was performed against thermoluminescence dosimeters (TLD) for conventional treatments (two groups of ten patients for each energy) and total body irradiation (TBI). MOSFETS were pre-irradiated with 20 Gy. Sensitivity of 0.08 mV cGy(-1) can be obtained for 200 cGy irradiations at 5 V bias voltage. Ten consecutive measurements at 200 cGy yield a SD of 2.08 cGy (1.05%). Increasing the dose in steps from 5 cGy to 1000 cGy yields a 1.00 Pearson correlation coefficient and agreement within 2.0%. Dose rate dependence (160-800 cGy min(-1)) was within 2.5%, temperature dependence within 2.0% (25-37 degrees C). A strong angular dependence has been observed for gantry incidences exceeding +/-30 degrees C. Dose response is stable up to 50 Gy (saturation occurs at approximately 90 Gy), which is used as threshold dose before resetting the MOSFET. An average measured-over-calculated dose ratio within 1.05 (SD: 0.04) has been obtained in vivo. TBI midplane-dose assessed by entrance and exit dose measurements agreed within 1.9% with ionization chamber in phantom, and within 1.0% with TLD in vivo. An in-house developed resettable MOSFET-based dosimetry system is proposed. The system has been validated and is currently used for in vivo entrance dose measurement in clinical routine for simple (open field) treatment configurations.

Dosimetric assessment of static and helical TomoTherapy in the clinical implementation of breast cancer treatments.

BACKGROUND AND PURPOSE: Investigation of the use of TomoTherapy and TomoDirect versus conventional radiotherapy for the treatment of post-operative breast carcinoma. This study concentrates on the evaluation of the planning protocol for the TomoTherapy and TomoDirect TPS, dose verification and the implementation of in vivo dosimetry. MATERIALS AND METHODS: Eight patients with different breast cancer indications (left/right tumor, axillary nodes involvement (N+)/no nodes (N0), tumorectomy/mastectomy) were enrolled. TomoTherapy, TomoDirect and conventional plans were generated for prone and supine positions leading to six or seven plans per patient. Dose prescription was 42Gy in 15 fractions over 3weeks. Dose verification of a TomoTherapy plan is performed using TLDs and EDR2 film inside a home-made wax breast phantom fixed on a rando-alderson phantom. In vivo dosimetry was performed with TLDs. RESULTS: It is possible to create clinically acceptable plans with TomoTherapy and TomoDirect. TLD calibration protocol with a water equivalent phantom is accurate. TLD verification with the phantom shows measured over calculated ratios within 2.2% (PTV). An overresponse of the TLDs was observed in the low dose regions (<0.1Gy). The film measurements show good agreement for high and low dose regions inside the phantom. A sharp gradient can be created to the thoracic wall. In vivo dosimetry with TLDs was clinically feasible. CONCLUSIONS: The TomoTherapy and TomoDirect modalities can deliver dose distributions which the radiotherapist judges to be equal to or better than conventional treatment of breast carcinoma according to the organ to be protected.

Treatment delivery time optimization of respiratory gated radiation therapy by application of audio-visual feedback.

PURPOSE: The feasibility to use visually guided voluntary breath-hold with and without audio assistance to reduce the total treatment time was evaluated. MATERIALS AND METHODS: Patients referred for gated SBRT received hypofractionation schedules for lung or liver treatments. The patients were treated with the Novalis system (BrainLAB AG, Feldkirchen, Germany) and IGRT was performed with ExacTrac5.0/NovalisBody allowing gated irradiation. Video glasses, used for visual feedback to guide voluntary breath-hold, allowed additional audio assistance during treatment. The technique was applied for 25 patients of whom 9 were treated in free breathing, 7 had only visual feedback and another 9 had both audio and visual feedback. RESULTS: The delivery time of gated treatment during free breathing had an average value of 1.7 min/100 MU (SD 0.6 min/100 MU). The introduction of visual feedback reduced the average delivery time to 1.4 min/100 MU (SD 0.4 min/100 MU). The treatments with additional audio assistance indicated a significant reduction (p=0.004) of the average delivery time to 0.9 min/100 MU (SD 0.2 min/100 MU). CONCLUSION: The introduction of visually guided voluntary breath-hold with audio assistance led to treatment times for gated radiation therapy approaching conformal beam delivery times, which made gated treatments applicable in conventional treatment time slots.

Assessment of intrafractional movement and internal motion in radiotherapy of rectal cancer using megavoltage computed tomography.

PURPOSE: The aim of this study was to provide estimates of setup and internal margins of patients treated for rectal carcinoma using helical tomotherapy and to assess possible margin adaptations. Using helical tomotherapy, highly conformal dose distributions can be created, and the integrated megavoltage computed tomography (MVCT) modality allows very precise daily patient positioning. In clinical protocols, however, margins originating from traditional setup procedures are still being applied. This work investigates whether this modality can aid in redefining treatment margins. METHODS AND MATERIALS: Ten patients who were treated with tomotherapy underwent MVCT scanning before and after 10 treatments. Using automatic registration the necessary setup margin was investigated by means of bony landmarks. Internal margins were assessed by delineating and describing the mesorectal movement. RESULTS: Based on bony landmarks, movement of patients during treatments was limited to 2.45 mm, 1.99 mm, and 1.09 mm in the lateral, longitudinal, and vertical direction, respectively. Systematic errors were limited to <1 mm. Measured movement of the mesorectal space was -1.6 mm (+/- 4.2 mm) and 0.1 mm (+/- 4.0 mm) for left and right lateral direction. In the antero-posterior direction, mean shifts were -2 mm (+/- 6.8 mm) and -0.4 mm (+/- 3.8 mm). Mean shifts in the cranio-caudal direction were respectively -3.2 mm (+/- 5.6 mm) and -3.2 mm (+/- 6.8 mm). CONCLUSIONS: The use of the integrated MVCT on the tomotherapy system can minimize the setup margin for rectal cancer, and can also be used to adequately describe the internal margin allowing for direct treatment margin adaptation.

Innovations in image-guided radiotherapy.

The limited ability to control for the location of a tumour compromises the accuracy with which radiation can be delivered to tumour-bearing tissue. The resultant requirement for larger treatment volumes to accommodate target uncertainty restricts the radiation dose because more surrounding normal tissue is exposed. With image-guided radiotherapy (IGRT) these volumes can be optimized and tumoricidal doses can be delivered, achieving maximal tumour control with minimal complications. Moreover, with the ability of high-precision dose delivery and real-time knowledge of the target volume location, IGRT has initiated the exploration of new indications for radiotherapy, some of which were previously considered infeasible.

A feasibility study of image-guided hypofractionated conformal arc therapy for inoperable patients with localized non-small cell lung cancer.

We treated 36 cases of stage I/II non-small cell lung cancer in inoperable patients. Treatments were planned to a total isocenter dose of 60Gy (8x7.5Gy) based on a dynamic field shaping arc, employing one arc to span as much area as possible and if needed additional weighted segments. The 2 year infield progression free probability is 65%. Disease-specific survival is 75% at 2 years. No patients experienced grade 3-4 toxicity.

An assessment of the use of skin flashes in helical tomotherapy using phantom and in-vivo dosimetry.

BACKGROUND AND PURPOSE: In helical tomotherapy the nature of the optimizing and planning systems allows the delivery of dose on the skin using a build-up compensating technique (skin flash). However, positioning errors or changes in the patient's contour can influence the correct dosage in these regions. This work studies the behavior of skin-flash regions using phantom and in-vivo dosimetry. MATERIALS AND METHODS: The dosimetric accuracy of the tomotherapy planning system in skin-flash regions is checked using film and TLD on phantom. Positioning errors are induced and the effect on the skin dose is investigated. Further a volume decrease is simulated using bolus material and the results are compared. RESULTS: Results show that the tomotherapy planning system calculates dose on skin regions within 2 SD using TLD measurements. Film measurements show drops of dose of 2.8% and 26% for, respectively, a 5mm and 10mm mispositioning of the phantom towards air and a dose increase of 9% for a 5mm shift towards tissue. These measurements are confirmed by TLD measurements. A simulated volume reduction shows a similar behavior with a 2.6% and 19.4% drop in dose, measured with TLDs. CONCLUSION: The tomotherapy system allows adequate planning and delivery of dose using skin flashes. However, exact positioning is crucial to deliver the dose at the exact location.

Assessment of secondary patient motion induced by automated couch movement during on-line 6 dimensional repositioning in prostate cancer treatment.

BACKGROUND AND PURPOSE: The purpose of this study is to assess retrospectively secondary patient motion induced by 6D patient setup correction. MATERIALS AND METHODS: For 104 patients, treated with Novalis, 6D setup correction prior to treatment was performed by ExacTrac5.0/NovalisBody in combination with the Robotic Tilt Module mounted underneath the Exact Couch top. This 6D correction might induce additional setup errors due to patient reaction against the rotations. To evaluate induced secondary motion, the 6D setup correction is verified and evaluated with respect to the tolerance limits. RESULTS: The majority of measured secondary motions are found within the tolerance limits. Detected secondary motions are mostly found in longitudinal shifts and lateral rotations, and mainly found in only 1 dimension during the same verification. The verifications indicate that the patient population can be divided into a group that hardly moves and a group that moves throughout all 6D setup corrections. The patient's behavior can be predicted by the evaluation of the first five fractions as none of the patients demonstrate a learning curve during the treatment. CONCLUSIONS: 6D setup correction does not induce secondary motion for the majority of the patients and can therefore be applied for all treatment indications.

Six dimensional analysis with daily stereoscopic x-ray imaging of intrafraction patient motion in head and neck treatments using five points fixation masks.

The safety margins used to define the Planning Target Volume (PTV) should reflect the accuracy of the target localization during treatment that comprises both the reproducibility of the patient positioning and the positional uncertainty of the target, so both the inter- and intrafraction motion of the target. Our first aim in this study was to determine the intrafraction motion of patients immobilized with a five-point thermoplastic mask for head and neck treatments. The five-point masks have the advantage that the patient's shoulders as well as the cranial part of the patient's head is covered with the thermoplastic material that improves the overall immobilization of the head and neck region of the patient. Thirteen patients were consecutively assigned to use a five-point thermoplastic mask. The patients were positioned by tracking of infrared markers (IR) fixed to the immobilization device and stereoscopic x-ray images were used for daily on-line setup verification. Repositioning was carried out prior to treatment as needed; rotations were not corrected. Movements during treatment were monitored by real-time IR tracking. Intrafraction motion and rotation was supplementary assessed by a six-degree-of-freedom (6-D) fusion of x-ray images, taken before and after all 385 treatments, with DRR images generated from the planning CT data. The latter evaluates the movement of the patient within the thermoplastic mask independent from the mask movement, where IR tracking evaluates the movement of the mask caused by patient movement in the mask. These two movements are not necessarily equal to each other. The maximum intrafraction movement detected by IR tracking showed a shift [mean (SD; range)] of -0.1(0.7; 6.0), 0.1(0.6; 3.6), -0.2(0.8;5.5) mm in the vertical, longitudinal, and lateral direction, respectively, and rotations of 0.0(0.2; 1.6), 0.0(0.2; 1.7) and 0.2(0.2; 2.4) degrees about the vertical, longitudinal, and lateral axis, respectively. The standard deviations and ranges found with the 6-D fusion demonstrate intrafraction patient displacements of -0.5(1.2; 7.4), 0.3(0.7; 5.3), 0.0(0.7; 5.7) mm in the vertical, longitudinal, and lateral direction, respectively, and rotations of -0.1(0.6; 4.1), 0.1(0.7; 8.3) and -0.2(0.8; 8.2) degrees about the vertical, longitudinal, and lateral axis, respectively. The 6-D fusions are considerably larger (p < 0.05) than detected by IR tracking. This indicates that the external marker tracking underestimates the magnitude of the actual intrafraction motion and rotation of the patient. The intrafraction motion detected for the patients immobilized with a conventional thermoplastic mask was relatively large. The feasibility to reduce this intrafraction movement by the application of alternative five-point thermoplastic mask types was evaluated as a second aim of this study. The preliminary results showed a clear reduction in the range, being an indication for the random movements, of both the intrafraction shift and rotation for both alternative mask types. The 6-D fusion is found a useful tool for a fast evaluation of the actual patient's intrafraction shift and rotation and shows the latter is not negligible and needs to be taken into account additional to the initial setup accuracy when determining the PTV margin.

Importing measured field fluences into the treatment planning system to validate a breathing synchronized DMLC-IMRT irradiation technique.

BACKGROUND AND PURPOSE: Recalculating dose distributions using measured IMRT fluence fields imported into the treatment planning system (TPS) to evaluate the technical feasibility of a prototype developed for breathing synchronized irradiation. PATIENTS AND METHODS: DMLC-IMRT fluence patterns acquired on radiographic film, generated by the linac in non-gated and gated mode, have been imported into the TPS. The effect of dose blurring and possible interplay between organ motion and leaf motion, and the efficacy of a breathing synchronized irradiation technique (an adapted version of a commercially available image-guidance system: NOVALIS BODY/ExacTrac4.0, BrainLAB AG) have been evaluated using radiographic film mounted to a simple phantom simulating a breathing pattern of 16 cycles per minute and covering a distance of 4 cm to obtain the resulting fluence maps. Two situations have been investigated to illustrate this principle: (a) a tumor located close to the diaphragm to assess the influence of organ motion on the dose to the target volume as well as to the gastro-intestinal tract that presents a high risk at intersecting with the beam during the breathing cycle. (b) A mediastinal lesion requiring complicated fluence patterns. RESULTS: Importing measured fluence maps yielded highly disturbed reconstructed dose distributions in case of the non-gated delivery with the phantom in motion (both orthogonal and parallel to the leaf direction), whereas the measurements from the static (film fixed in space) and the gated delivery showed good agreement with the original theoretical dose distribution. These findings have been confirmed by the dose-volume histograms, corresponding tumor control probabilities, conformity index and dose heterogeneity values. The normal tissue complication probabilities investigated in this study seem to be affected to a lesser degree, which concurs with the observation that the motion effects result in a dose spread in the direction of motion. The applied breathing synchronization technique introduced an increased treatment time with a factor 3-4. CONCLUSIONS: The use of measured fluence fields, delivered by the linac in non-gated and gated mode, as imported fluence maps for the treatment planning system is an interesting quality assurance tool and revealed the dramatic impact of dose blurring and interplay between DMLC-IMRT dose delivery and organ motion, as well as the potential of breathing synchronization to resolve this issue. The possible advantage of breathing synchronized irradiation is compromised with an increased treatment time.

A simple theoretical verification of monitor unit calculation for intensity modulated beams using dynamic mini-multileaf collimation.

A spreadsheet based program is presented to perform an independent Monitor Unit (MU) calculation verification for the Quality Assurance (QA) of Intensity Modulated Radiation Therapy (IMRT) using Dynamic MultiLeaf Collimation (DMLC). The computed dose value is compared to the planned dose by calculating the percent dose difference per Intensity Modulated Beam (IMB) and absolute dose difference per IMB. The proposed acceptability levels are +/-5.0% or +/-2.0 cGy for the percent dose difference per IMB and the absolute dose difference per IMB, respectively. For percent dose difference per treatment, an acceptability level of +/-2.0% is proposed. The presented program is considered adequate for checking the treatment plans calculated for IMRT treatments using DMLC as a part of the QA procedure.

Dosimetric evaluation of partially overlapping intensity modulated beams using dynamic mini-multileaf collimation.

The dose distribution resulting from partially overlapping intensity modulated beams (IMBs) assigned to different isocenters for the treatment of the same planning target volume (PTV) was evaluated. These partially overlapping IMBs are used in static intensity modulated radiation therapy (IMRT) treatments with the Novalis system using the mini-MultiLeaf Collimator (mini-MLC) in Dynamic MultiLeaf Collimation (DMLC) mode. The resultant dose distribution was verified dosimetrically for a cylindrical target defined in a homogeneous cubic phantom. The phantom positioning can introduce dose nonuniformities in the resultant dose distribution by nonperfect positioning of the isocenters in accordance with each other. The dose inhomogeneities are quantified mathematically by summation of the dose profiles of the used IMBs and experimentally by measurement of the resulting dose profiles with radiographic film and thermoluminescent detectors (TLD). The mathematical estimation of the resulting dose profile of the treatment with a perfect positioning of the isocenters showed a good agreement with the planned dose profile. The magnitude of the maximum dose inhomogeneities introduced by the simulated supplementary shifts between the isocenters decreases by -8.54% mm(-1) as the shift changes from -0.30 +/- 0.10 cm to +0.30 +/- 0.10 cm. The TLD measurements showed a similar variation of the magnitude of the maximum dose inhomogeneities: -8.77% mm(-1). The amount of dose variation was underestimated with the radiographic film measurements, which showed a variation of -7.17% mm(-1). The film measurements demonstrated that the magnitude of the introduced maximum dose inhomogeneities did not alter significantly throughout the PTV. The approach of using partially overlapping IMBs assigned to different isocenters to enlarge the treatment region introduces smaller dose inhomogeneities in the resultant dose distribution than when abutting treatment fields are used. The resultant dose distribution of this treatment technique is less sensitive to positioning errors of the used treatment isocenters.

Quality assurance of a system for improved target localization and patient set-up that combines real-time infrared tracking and stereoscopic X-ray imaging.

BACKGROUND AND PURPOSE: The aim of this study is to investigate the positional accuracy of a prototype X-ray imaging tool in combination with a real-time infrared tracking device allowing automated patient set-up in three dimensions. MATERIAL AND METHODS: A prototype X-ray imaging tool has been integrated with a commercially released real-time infrared tracking device. The system, consisting of two X-ray tubes mounted to the ceiling and a centrally located amorphous silicon detector has been developed for automated patient positioning from outside the treatment room prior to treatment. Two major functions are supported: (a) automated fusion of the actual treatment images with digitally reconstructed radiographs (DRRs) representing the desired position; (b) matching of implanted radio opaque markers. Measurements of known translational (up to 30.0mm) and rotational (up to 4.0 degrees ) set-up errors in three dimensions as well as hidden target tests have been performed on anthropomorphic phantoms. RESULTS: The system's accuracy can be represented with the mean three-dimensional displacement vector, which yielded 0.6mm (with an overall SD of 0.9mm) for the fusion of DRRs and X-ray images. Average deviations between known translational errors and calculations varied from -0.3 to 0.6mm with a standard deviation in the range of 0.6-1.2mm. The marker matching algorithm yielded a three-dimensional uncertainty of 0.3mm (overall SD: 0.4mm), with averages ranging from 0.0 to 0.3mm and a standard deviation in the range between 0.3 and 0.4mm. CONCLUSIONS: The stereoscopic X-ray imaging device integrated with the real-time infrared tracking device represents a positioning tool allowing for the geometrical accuracy that is required for conformal radiation therapy of abdominal and pelvic lesions, within an acceptable time-frame.