The role of image guidance in respiratory gated radiotherapy

Clinical practicality and patient performance for surface-guided automated VMAT gating for DIBH breast cancer radiotherapy

BACKGROUND AND PURPOSE

To evaluate the performance of automated surface-guided gating for left-sided breast cancer with DIBH and VMAT.

MATERIALS AND METHODS

Patients treated in the first year after introduction of DIBH with VMAT were retrospectively considered for analysis. With automated surface-guided gating the beam automatically switches on/off, if the surface region of interest moved in/out the gating tolerance (±3 mm, ±3°). Patients were coached to hold their breath as long as comfortably possible. Depending on the patient's preference, patients received audio instructions during treatment delivery. Real-time positional variations of the breast/chest wall surface with respect to the reference surface were collected, for all three orthogonal directions. The durations and number of DIBHs needed to complete dose delivery, and DIBH position variations were determined. To evaluate an optimal gating window threshold, smaller tolerances of ±2.5 mm, ±2.0 mm, and ±1.5 mm were simulated.

RESULTS

525 fractions from 33 patients showed that median DIBH duration was 51 s (range: 30-121 s), and median 4 DIBHs per fraction were needed to complete VMAT dose delivery. Median intra-DIBH stability and intrafractional DIBH reproducibility approximated 1.0 mm in each direction. No large differences were found between patients who preferred to perform the DIBH procedure with (n = 21) and without audio-coaching (n = 12). Simulations demonstrated that gating window tolerances could be reduced from ±3.0 mm to ±2.0 mm, without affecting beam-on status.

CONCLUSION

Independent of the use of audio-coaching, this study demonstrates that automated surface-guided gating with DIBH and VMAT proved highly efficient. Patients' DIBH performance far exceeded our expectations compared to earlier experiences and literature. Furthermore, gating window tolerances could be reduced.

Medical X-band linear accelerator for high-precision radiotherapy

PURPOSE

Recently, high-precision radiotherapy systems have been developed by integrating computerized tomography or magnetic resonance imaging to enhance the precision of radiotherapy. For integration with additional imaging systems in a limited space, miniaturization and weight reduction of the linear accelerator (linac) system have become important. The aim of this work is to develop a compact medical linac based on 9.3 GHz X-band RF technology instead of the S-band RF technology typically used in the radiotherapy field.

METHODS

The accelerating tube was designed by 3D finite-difference time-domain and particle-in-cell simulations because the frequency variation resulting from the structural parameters and processing errors is relatively sensitive to the operating performance of the X-band linac. Through the 3D simulation of the electric field distribution and beam dynamics process, we designed an accelerating tube to efficiently accelerate the electron beam and used a magnetron as the RF source to miniaturize the entire linac. In addition, a side-coupled structure was adopted to design a compact linac to reduce the RF power loss. To verify the performance of the linac, we developed a beam diagnostic system to analyze the electron beam characteristics and a quality assurance (QA) experimental environment including 3D lateral water phantoms to analyze the primary performance parameters (energy, dose rate, flatness, symmetry, and penumbra) The QA process was based on the standard protocols AAPM TG-51, 106, 142 and IAEA TRS-398.

RESULTS

The X-band linac has high shunt impedance and electric field strength. Therefore, even though the length of the accelerating tube is 37 cm, the linac could accelerate an electron beam to more than 6 MeV and produce a beam current of more than 90 mA. The transmission ratio is measured to be approximately 30% ~ 40% when the electron gun operates in the constant emission region. The percent depth dose ratio at the measured depths of 10 and 20 cm was approximately 0.572, so we verified that the photon beam energy was matched to approximately 6 MV. The maximum dose rate was measured as 820 cGy/min when the source-to-skin distance was 80 cm. The symmetry was smaller than the QA standard and the flatness had a higher than standard value due to the flattening filter-free beam characteristics. In the case of the penumbra, it was not sufficiently steep compared to commercial equipment, but it could be compensated by improving additional devices such as multileaf collimator and jaw.

CONCLUSIONS

A 9.3 GHz X-band medical linac was developed for high-precision radiotherapy. Since a more precise design and machining process are required for X-band RF technology, this linac was developed by performing a 3D simulation and ultraprecision machining. The X-band linac has a short length and a compact volume, but it can generate a validated therapeutic beam. Therefore, it has more flexibility to be coupled with imaging systems such as CT or MRI and can reduce the bore size of the gantry. In addition, the weight reduction can improve the mechanical stiffness of the unit and reduce the mechanical load.

The Development of Technology for Effective Respiratory-Gated Irradiation Using an Image-Guided Small Animal Irradiator

The development of image-guided small animal irradiators represents a significant improvement over standard irradiators by enabling preclinical studies to mimic radiotherapy in humans. The ability to deliver tightly collimated targeted beams, in conjunction with gantry or animal couch rotation, has the potential to maximize tumor dose while sparing normal tissues. However, the current commercial platforms do not incorporate respiratory gating, which is required for accurate and precise targeting in organs subject to respiration related motions that may be up to the order of 5 mm in mice. Therefore, a new treatment head assembly for the Xstrahl Small Animal Radiation Research Platform (SARRP) has been designed. This includes a fast X-ray shutter subsystem, a motorized beam hardening filter assembly, an integrated transmission ionization chamber to monitor beam delivery, a kinematically positioned removable beam collimator and a targeting laser exiting the center of the beam collimator. The X-ray shutter not only minimizes timing errors but also allows beam gating during imaging and treatment, with irradiation only taking place during the breathing cycle when tissue movement is minimal. The breathing related movement is monitored by measuring, using a synchronous detector/lock-in amplifier that processes diffuse reflectance light from a modulated light source. After thresholding of the resulting signal, delays are added around the inhalation/exhalation phases, enabling the "no movement" period to be isolated and to open the X-ray shutter. Irradiation can either be performed for a predetermined time of X-ray exposure, or through integration of a current from the transmission monitor ionization chamber (corrected locally for air density variations). The ability to successfully deliver respiratory-gated X-ray irradiations has been demonstrated by comparing movies obtained using planar X-ray imaging with and without respiratory gating, in addition to comparing dose profiles observed from a collimated beam on EBT3 radiochromic film mounted on the animal's chest. Altogether, the development of respiratory-gated irradiation facilitates improved dose delivery during animal movement and constitutes an important new tool for preclinical radiation studies. This approach is particularly well suited for irradiation of orthotopic tumors or other targets within the chest and abdomen where breathing related movement is significant.

Setup error and motion during deep inspiration breath-hold breast radiotherapy measured with continuous portal imaging

BACKGROUND

The position and residual motion of the chest wall of breast cancer patients during treatment in deep inspiration breath-hold (DIBH) were investigated.

MATERIAL AND METHODS

The study included 58 left-sided breast cancer patients treated with DIBH three-dimensional (3D) conformal radiotherapy in 15 or 25 fractions. The DIBH levels were monitored using an external marker block placed on the chest, either shifted 5 cm to the right at the level of the xiphoid process (Group 1, 27 consecutive patients) or placed medially on the inferior part of the sternum (Group 2, 31 consecutive patients). At every third treatment fraction, continuous portal images were acquired. The time-resolved chest wall position during treatment was compared with the planned position to determine the inter-fraction setup errors and the intra-fraction motion of the chest wall.

RESULTS

The DIBH compliance was 95% during both recruitment periods. A tendency of smaller inter-fraction setup errors and intra-fraction motion was observed for group 2 (medial marker block position). However, apart from a significantly reduced inter-field random shift (σ = 1.7 mm vs. σ = 0.9 mm, p = 0.005), no statistically significant differences between the groups were found. In a combined analysis, the group mean inter-fraction setup error was M = - 0.1 mm, with random and systematic errors of σ = 1.7 mm and Σ = 1.4 mm. The group mean inter-field shift was M = 0.0 (σ = 1.3 mm and Σ = 1.1 mm) and the group mean standard deviation of the intra-field motion was 0.5 mm. The absolute setup error had a maximum of 16.3 mm, exceeding 5 mm in 2.2% of the imaged fields.

CONCLUSION

Compared to free breathing treatments, the primary benefit of the DIBH technique was the separation of the heart from the target rather than more accurate targeting. Despite a small gating window, occasional large errors in the chest wall position were observed for some patients, illustrating limitations of the external marker block as surrogate in a broad patient population.

Injectable Colloidal Gold for Use in Intrafractional 2D Image-Guided Radiation Therapy

In the western world, approximately 50% of all cancer patients receive radiotherapy alone or in combination with surgery or chemotherapy. Image-guided radiotherapy (IGRT) has in recent years been introduced to enhance precision of the delivery of radiation dose to tumor tissue. Fiducial markers are often inserted inside the tumor to improve IGRT precision and to enable monitoring of the tumor position during radiation therapy. In the present article, a liquid fiducial tissue marker is presented, which can be injected into tumor tissue using thin and flexible needles. The liquid fiducial has high radio-opacity, which allows for marker-based image guidance in 2D and 3D X-ray imaging during radiation therapy. This is achieved by surface-engineering gold nanoparticles to be highly compatible with a carbohydrate-based gelation matrix. The new fiducial marker is investigated in mice where they are highly biocompatible and stable after implantation. To investigate the clinical potential, a study is conducted in a canine cancer patient with spontaneous developed solid tumor in which the marker is successfully injected and used to align and image-guide radiation treatment of the canine patient. It is concluded that the new fiducial marker has highly interesting properties that warrant investigations in cancer patients.

Suitability of markerless EPID tracking for tumor position verification in gated radiotherapy

PURPOSE

To maximize the benefits of respiratory gated radiotherapy (RGRT) of lung tumors real-time verification of the tumor position is required. This work investigates the feasibility of markerless tracking of lung tumors during beam-on time in electronic portal imaging device (EPID) images of the MV therapeutic beam.

METHODS

EPID movies were acquired at ∼2 fps for seven lung cancer patients with tumor peak-to-peak motion ranges between 7.8 and 17.9 mm (mean: 13.7 mm) undergoing stereotactic body radiotherapy. The external breathing motion of the abdomen was synchronously measured. Both datasets were retrospectively analyzed in PortalTrack, an in-house developed tracking software. The authors define a three-step procedure to run the simulations: (1) gating window definition, (2) gated-beam delivery simulation, and (3) tumor tracking. First, an amplitude threshold level was set on the external signal, defining the onset of beam-on/-off signals. This information was then mapped onto a sequence of EPID images to generate stamps of beam-on/-hold periods throughout the EPID movies in PortalTrack, by obscuring the frames corresponding to beam-off times. Last, tumor motion in the superior-inferior direction was determined on portal images by the tracking algorithm during beam-on time. The residual motion inside the gating window as well as target coverage (TC) and the marginal target displacement (MTD) were used as measures to quantify tumor position variability.

RESULTS

Tumor position monitoring and estimation from beam's-eye-view images during RGRT was possible in 67% of the analyzed beams. For a reference gating window of 5 mm, deviations ranging from 2% to 86% (35% on average) were recorded between the reference and measured residual motion. TC (range: 62%-93%; mean: 77%) losses were correlated with false positives incidence rates resulting mostly from intra-/inter-beam baseline drifts, as well as sudden cycle-to-cycle fluctuations in exhale positions. Both phenomena can lead to considerable deviations (with MTD values up to a maximum of 7.8 mm) from the intended tumor position, and in turn may result in a marginal miss. The difference between tumor traces determined within the gating window against ground truth trajectory maps was 1.1 ± 0.7 mm on average (range: 0.4-2.3 mm).

CONCLUSIONS

In this retrospective analysis of motion data, it is demonstrated that the system is capable of determining tumor positions in the plane perpendicular to the beam direction without the aid of fiducial markers, and may hence be suitable as an online verification tool in RGRT. It may be possible to use the tracking information to enable on-the-fly corrections to intra-/inter-beam variations by adapting the gating window by means of a robotic couch.

Determining leaf trajectories for dynamic multileaf collimators with consideration of marker visibility: an algorithm study

The purpose of this study was to develop a leaf-setting algorithm for Dynamic Multileaf Collimator-Intensity-Modulated Radiation Therapy (DMLC-IMRT) for optimal marker visibility. Here, a leaf-setting algorithm (called a Delta algorithm) was developed with the objective of maximizing marker visibility so as to improve the tracking effectiveness of fiducial markers during treatment delivery. The initial leaf trajectories were generated using a typical leaf-setting algorithm, then the leaf trajectories were adjusted by Delta algorithm operations (analytical computations and a series of matrix calculations) to achieve the optimal solution. The performance of the Delta algorithm was evaluated with six test fields (with randomly generated intensity profiles) and 15 clinical fields from IMRT plans of three prostate cancer patients. Compared with the initial solution, the Delta algorithm kept the total delivered intensities (TDIs) constant (without increasing the beam delivery time), but improved marker visibility (the percentage ratio of marker visibility time to beam delivery time). For the artificial fields (with three markers), marker visibility increased from 68.00-72.00% for a small field (5 × 5), from 38.46-43.59% for a medium field (10 × 10), and from 28.57-37.14% for a large field (20 × 20). For the 15 clinical fields, marker visibility increased 6-30% for eight fields and > 50% for two fields but did not change for five fields. A Delta algorithm was proposed to maximize marker visibility for DMLC-IMRT without increasing beam delivery time, and this will provide theoretical fundamentals for future studies of 4D DMLC tracking radiotherapy.

Adjuvant chemotherapy and radiotherapy in the treatment of non-small cell lung cancer (NSCLC)

Lung cancer is one of the most common human malignancies and remains the leading cause of cancer related deaths worldwide. Many recent technological advances led to improved diagnostics and staging of lung cancer. With development of new treatment options such as targeted therapies there might be improvement in progression free survival of patients with advanced stage non-small cell lung cancer (NSCLC). Improvement in overall survival is still reserved for selected patients and selected treatments. One of the mostly investigated therapeutic options is adjuvant treatment. There are many open issues in selection of patients and administration of appropriate adjuvant treatment.

Reduced lung dose and improved inspiration level reproducibility in visually guided DIBH compared to audio coached EIG radiotherapy for breast cancer patients

INTRODUCTION

Patients with left-sided breast cancer with lymph node involvement have routinely been treated with enhanced inspiration gating (EIG) for a decade at our institution. In a transition from EIG to deep inspiration breath hold (DIBH) we compared the two techniques with focus on target coverage, dose to organs at risk and reproducibility of the inspiration level (IL).

MATERIAL AND METHODS

Twenty-four patients were computed tomography (CT) scanned with EIG and DIBH. For DIBH we used visual feedback and for EIG audio coaching, both during scan and treatment. Treatment plans for 50 Gy over 25 fractions were calculated. Seventeen of the patients were included in the analysis of reproducibility. They were audio coached for one minute before beam-on in DIBH at nine treatment sessions. These respiration curves were analysed with average maximum IL and standard deviation (SD) for the EIG part of the respiratory signal, and mean IL and SD for the DIBH. Comparison of dosimetric and respiration parameters were performed with the Wilcoxon signed rank-sum test.

RESULTS

In DIBH, the ipsilateral lung volume increased further compared to EIG (p < 0.0004, mean increase 11%). This lead to a 9% mean reduction (p = 0.002) of the ipsilateral lung volume receiving 20 Gy (V20 Gy). We found no other significant dosimetric differences between the two methods. The reproducibility of the IL was better with the DIBH method, observed as a significantly smaller SD in most patients (p < 0.04 for 16 of 17 patients).

CONCLUSION

The DIBH method resulted in a significantly larger lung volume and lower ipsilateral lung V20 Gy compared to EIG. The IL for visually guided DIBH was more reproducible than audio-coached EIG. Based on these findings, the DIBH technique is our new breathing adaptation standard for radiotherapy of patients with left-sided breast cancer with lymph node involvement.

Treatment of stage I and II non-small cell lung cancer: Diagnosis and management of lung cancer, 3rd ed: American College of Chest Physicians evidence-based clinical practice guidelines

BACKGROUND

The treatment of stage I and II non-small cell lung cancer (NSCLC) in patients with good or low surgical risk is primarily surgical resection. However, this area is undergoing many changes. With a greater prevalence of CT imaging, many lung cancers are being found that are small or constitute primarily ground-glass opacities. Treatment such as sublobar resection and nonsurgical approaches such as stereotactic body radiotherapy (SBRT) are being explored. With the advent of minimally invasive resections, the criteria to classify a patient as too ill to undergo an anatomic lung resection are being redefined.

METHODS

The writing panel selected topics for review based on clinical relevance to treatment of early-stage lung cancer and the amount and quality of data available for analysis and relative controversy on best approaches in stage I and II NSCLC: general surgical care vs specialist care; sublobar vs lobar surgical approaches to stage I lung cancer; video-assisted thoracic surgery vs open resection; mediastinal lymph node sampling vs lymphadenectomy at the time of surgical resection; the use of radiation therapy, with a focus on SBRT, for primary treatment of early-stage NSCLC in high-risk or medically inoperable patients as well as adjuvant radiation therapy in the sublobar and lobar resection settings; adjuvant chemotherapy for early-stage NSCLC; and the impact of ethnicity, geography, and socioeconomic status on lung cancer survival. Recommendations by the writing committee were based on an evidence-based review of the literature and in accordance with the approach described by the Guidelines Oversight Committee of the American College of Chest Physicians.

RESULTS

Surgical resection remains the primary and preferred approach to the treatment of stage I and II NSCLC. Lobectomy or greater resection remains the preferred approach to T1b and larger tumors. The use of sublobar resection for T1a tumors and the application of adjuvant radiation therapy in this group are being actively studied in large clinical trials. Every patient should have systematic mediastinal lymph node sampling at the time of curative intent surgical resection, and mediastinal lymphadenectomy can be performed without increased morbidity. Perioperative morbidity and mortality are reduced and long-term survival is improved when surgical resection is performed by a board-certified thoracic surgeon. The use of adjuvant chemotherapy for stage II NSCLC is recommended and has shown benefit. The use of adjuvant radiation or chemotherapy for stage I NSCLC is of unproven benefit. Primary radiation therapy remains the primary curative intent approach for patients who refuse surgical resection or are determined by a multidisciplinary team to be inoperable. There is growing evidence that SBRT provides greater local control than standard radiation therapy for high-risk and medically inoperable patients with NSCLC. The role of ablative therapies in the treatment of high-risk patients with stage I NSCLC is evolving. Radiofrequency ablation, the most studied of the ablative modalities, has been used effectively in medically inoperable patients with small (&lt; 3 cm) peripheral NSCLC that are clinical stage I.

Evaluation of anatomical landmark position differences between respiration-gated MRI and four-dimensional CT for radiation therapy in patients with hepatocellular carcinoma

OBJECTIVE

To measure the accuracy of position differences in anatomical landmarks in gated MRI and four-dimensional CT (4D-CT) fusion planning for radiation therapy in patients with hepatocellular carcinoma (HCC).

METHODS

From April to December 2009, gated MR and planning 4D-CT images were obtained from 53 inoperable HCC patients accrued to this study. Gated MRI and planning 4D-CT were conducted on the same day. Manual image fusions were performed by matching the vertebral bodies. Liver volumes and three specific anatomical landmarks (portal vein conjunction, superior mesenteric artery bifurcation, and other noticeable points) were contoured from each modality. The points chosen nearest the centre of the four landmark points were compared to measure the accuracy of fusion.

RESULTS

The average distance differences (±standard deviation) of four validation points were 5.1 mm (±4.6 mm), 5.6 mm (±6.2 mm), 5.4 mm (±4.5 mm) and 5.1 mm (±4.8 mm). Patients who had ascites or pulmonary disease showed larger discrepancies. MRI-CT fusion discrepancy was significantly correlated with positive radiation response (p<0.05).

CONCLUSIONS

Approximately 5-mm anatomical landmark positional differences in all directions were found between gated MRI and 4D-CT fusion planning for HCC patients; the gap was larger in patients with ascites or pulmonary disease.

ADVANCES IN KNOWLEDGE

There were discrepancies of approximately 5 mm in gated MRI-CT fusion planning for HCC patients.

The breath of life - womens' experiences of breathing adapted radiation therapy

PURPOSE

To describe and analyze how women with breast cancer experience breathing adapted radiation therapy (BART) and to explore how women manage daily radiation therapy.

METHOD

Individual interviews were conducted with 20 women treated with BART for breast cancer concerning their perception of radiation therapy. The transcribed interviews were analyzed using qualitative content analysis.

RESULTS

'The breath of life' was the overall theme, as the women experienced the breathing as a way in which to influence their treatment and thus their survival. 'Participating in one's treatment, for good or ill', was the main category with four subcategories, 'Knowing one has done something good', 'Getting an extra bonus - healthwise', 'The experience of being in control' and 'Being in a high-technology environment'. The breathing technique became the strategy by which they could manage their treatment and gave them a sense of participation which led to a feeling of being in control. The women also felt that breathing benefited their health both mentally and physically. The high-technology environment was experienced as both hopeful and frightening.

CONCLUSION

Survival or increasing the chances of survival, are of ultimate importance for a woman with breast cancer. BART requires commitment from the women, which was perceived as offering them an opportunity to participate in their own treatment, for their survival. Increasing the women's possibilities to participate in their treatment benefits their health and welfare during an otherwise turbulent time and allow the rehabilitation process to start during treatment.

Quality indicators for breast cancer: revisiting historical evidence in the context of technology changes

Radiation therapy for breast cancer has considerably changed over the years, from simple simulator-based 2-dimensional techniques to sophisticated image-guided individualized treatments, with maximally protected normal structures. This has led to a substantial improvement in the outcome of breast cancer patients in terms of disease control, survival, and quality of life. This progress is based on clinical research and paralleled by progress in delivering sophisticated radiation treatment. Clinical trials resulted in identifying patients groups who will benefit from radiation treatment. They also stimulated the development of quality assurance tools and guidelines, which are now applied in daily clinical practice. The new technical opportunities to optimize dose distributions in patients require dedicated quality assurance measures because they may be more sensitive to variations throughout the treatment. Still, a large source of variation and uncertainty in radiation therapy remains in the definition of target volumes, which is clinically significant in terms of dosimetric target coverage as well as exposure of healthy tissues. This striving for continuous improvement of patient selection and treatment will lead to further improvement of local control while at the same time improving functional and cosmetic outcome and avoiding severe late complications, including cardiac toxicity.

Helical tomotherapy in patients with breast cancer and complex treatment volumes

PURPOSE

To describe early clinical results of tomotherapy treatment in patients with breast cancer and complex treatment volumes.

METHODS AND MATERIALS

Ten patients were treated with tomotherapy between January 2009 and March 2010. Treatment planning objectives were to cover at least 95% of the planning target volume with the 95% isodose; to have a minimum dose of 90% and a maximum dose of 105%. All treatments included daily CT/megavoltage image guidance. Acute toxicity was recorded weekly.

RESULTS

Six patients were treated because constraints were not accomplished for heart, lung or contralateral breast in a previous three-dimensional conformal plan; two for preexisting cardiac or pulmonary disease, and two more for bilateral breast irradiation. Treatment volumes included the whole breast in the majority of patients, as well as the supraclavicular and the internal mammary chain nodes when indicated. Most patients were older than 50 years, and had an early breast cancer, with positive oestrogen receptors, negative HER2 expression and a poorly differentiated, infiltrating ductal carcinoma. The majority of patients had received neoadjuvant chemotherapy associated to breast-conserving surgery and adjuvant hormonotherapy. Median homogeneity index was 1.09; median coverage index was 0.81. Median V20Gy and V10Gy for ipsilateral lung was 20% and 37.1% respectively. Median V25 and V35 for heart was 15% and 4% respectively. Median dose for contralateral breast was 7 Gy. Skin acute toxicity was grade 1 in 41.7% and grade 2 in 58.3%.

CONCLUSION

Tomotherapy is a technique capable of delivering a well tolerated treatment with high homogeneity and coverage indexes and high capabilities for sparing the organs at risk in patients with anatomically complex breast cancer, bilateral breast cancer, indication for internal mammary chain node irradiation, cardiac toxicity derived from chemotherapy, or preexisting cardiac or pulmonary disease. Further studies are required to evaluate local control and late toxicity.

Evaluation of dose to cardiac structures during breast irradiation

OBJECTIVE

Adjuvant radiotherapy for breast cancer can lead to late cardiac complications. The highest radiation doses are likely to be to the anterior portion of the heart, including the left anterior descending coronary artery (LAD). The purpose of this work was to assess the radiation doses delivered to the heart and the LAD in respiration-adapted radiotherapy of patients with left-sided breast cancer.

METHODS

24 patients referred for adjuvant radiotherapy after breast-conserving surgery for left-sided lymph node positive breast cancer were evaluated. The whole heart, the arch of the LAD and the whole LAD were contoured. The radiation doses to all three cardiac structures were evaluated.

RESULTS

For 13 patients, the plans were acceptable based on the criteria set for all 3 contours. For seven patients, the volume of heart irradiated was well below the set clinical threshold whereas a high dose was still being delivered to the LAD. In 1 case, the dose to the LAD was low while 19% of the contoured heart volume received over 20 Gy. In five patients, the dose to the arch LAD was relatively low while the dose to the whole LAD was considerably higher.

CONCLUSION

This study indicates that it is necessary to assess the dose delivered to the whole heart as well as to the whole LAD when investigating the acceptability of a breast irradiation treatment. Assessing the dose to only one of these structures could lead to excessive heart irradiation and thereby increased risk of cardiac complications for breast cancer radiotherapy patients.

Dynamic gating window for compensation of baseline shift in respiratory-gated radiation therapy

PURPOSE

To analyze and evaluate the necessity and use of dynamic gating techniques for compensation of baseline shift during respiratory-gated radiation therapy of lung tumors.

METHODS

Motion tracking data from 30 lung tumors over 592 treatment fractions were analyzed for baseline shift. The finite state model (FSM) was used to identify the end-of-exhale (EOE) breathing phase throughout each treatment fraction. Using duty cycle as an evaluation metric, several methods of end-of-exhale dynamic gating were compared: An a posteriori ideal gating window, a predictive trend-line-based gating window, and a predictive weighted point-based gating window. These methods were evaluated for each of several gating window types: Superior/inferior (SI) gating, anterior/posterior beam, lateral beam, and 3D gating.

RESULTS

In the absence of dynamic gating techniques, SI gating gave a 39.6% duty cycle. The ideal SI gating window yielded a 41.5% duty cycle. The weight-based method of dynamic SI gating yielded a duty cycle of 36.2%. The trend-line-based method yielded a duty cycle of 34.0%.

CONCLUSIONS

Dynamic gating was not broadly beneficial due to a breakdown of the FSM's ability to identify the EOE phase. When the EOE phase was well defined, dynamic gating showed an improvement over static-window gating.

Cardiovascular Disease

Although important advances have been made in curing childhood cancer in the last several decades, long-term survivors face considerable morbidity and mortality because of late effects from their initial anticancer therapy. By 30 years after treatment, the cumulative mortality from treatment-related medical illness actually exceeds that of mortality from cancer recurrence. Cardiovascular disease, in particular, is a leading threat to the well-being of adult survivors of childhood cancers. Unfortunately, the mechanisms of these late cardiac effects are understudied and poorly understood. This article reviews cardiotoxicity associated with 2 major anticancer regimens used in treating childhood cancer patients: anthracycline treatment and radiation therapy. The known pathophysiology and clinical cardiac risk factors that further predispose these patients to late-onset cardiac events are discussed. Basic and translational research is urgently needed to clarify pathophysiologic mechanisms of late cardiac effects and to develop therapies to improve both long-term survival and quality of life of adults cured of pediatric cancers.

Predictive modeling of lung motion over the entire respiratory cycle using measured pressure-volume data, 4DCT images, and finite-element analysis

PURPOSE

Predicting complex patterns of respiration can benefit the management of the respiratory motion for radiation therapy of lung cancer. The purpose of the present work was to develop a patient-specific, physiologically relevant respiratory motion model which is capable of predicting lung tumor motion over a complete normal breathing cycle.

METHODS

Currently employed techniques for generating the lung geometry from four-dimensional computed tomography data tend to lose details of mesh topology due to excessive surface smoothening. Some of the existing models apply displacement boundary conditions instead of the intrapleural pressure as the actual motive force for respiration, while others ignore the nonlinearity of lung tissues or the mechanics of pleural sliding. An intermediate nonuniform rational basis spline surface representation is used to avoid multiple geometric smoothing procedures used in the computational mesh preparation. Measured chest pressure-volume relationships are used to simulate pressure loading on the surface of the model for a given lung volume, as in actual breathing. A hyperelastic model, developed from experimental observations, has been used to model the lung tissue material. Pleural sliding on the inside of the ribcage has also been considered.

RESULTS

The finite-element model has been validated using landmarks from four patient CT data sets over 34 breathing phases. The average differences of end-inspiration in position between the landmarks and those predicted by the model are observed to be 0.450 +/- 0.330 cm for Patient P1, 0.387 +/- 0.169 cm for Patient P2, 0.319 +/- 0.186 cm for Patient P3, and 0.204 +/- 0.102 cm for Patient P4 in the magnitude of error vector, respectively. The average errors of prediction at landmarks over multiple breathing phases in superior-inferior direction are less than 3 mm.

CONCLUSIONS

The prediction capability of pressure-volume curve driven nonlinear finite-element model is consistent over the entire breathing cycle. The biomechanical parameters in the model are physiologically measurable, so that the results can be extended to other patients and additional neighboring organs affected by respiratory motion.

Dynamic MLC tracking of moving targets with a single kV imager for 3D conformal and IMRT treatments

BACKGROUND

Tumor motion during radiotherapy is a major challenge for accurate dose delivery, in particular for hypofractionation and dose painting. The motion may be compensated by dynamic multileaf collimator (DMLC) tracking. Previous work has demonstrated that a single kV imager can accurately localize moving targets for DMLC tracking during rotational delivery, however this method has not been investigated for the static gantry geometry used for conformal and IMRT treatments. In this study we investigate the accuracy of single kV-imager based DMLC tracking for static-gantry delivery.

MATERIAL AND METHODS

A 5-field treatment plan with circular field shape and 200 MU per field was delivered in 20 s per field to a moving phantom with an embedded gold marker. Fluoroscopic kV images were acquired at 5 Hz perpendicular to the treatment beam axis during a 120° pre-treatment gantry rotation, during treatment delivery, and during inter-field gantry rotations. The three-dimensional marker position was estimated from the kV images and used for MLC adaptation. Experiments included 12 thoracic/abdominal tumor trajectories and five prostate trajectories selected from databases with 160 and 548 trajectories, respectively. The tracking error was determined as the mismatch between the marker position and the MLC aperture center in portal images. Simulations extended the study to all trajectories in the databases and to treatments with prolonged duration of 60 s per field.

RESULTS

In the experiments, the mean root-mean-square (rms) tracking error was 0.9 mm (perpendicular to MLC) and 1.1 mm (parallel to MLC) for thoracic/abdominal tumor trajectories and 0.6 mm (perpendicular) and 0.5 mm (parallel) for prostate trajectories. Simulations of these experiments agreed to within 0.1 mm. Simulations of all trajectories in the databases resulted in mean rms tracking errors of 0.6 mm (perpendicular) and 0.9 mm (parallel) for thorax/abdomen tumors and 0.4 mm (perpendicular) and 0.2 mm (parallel) for prostate for both 20 s and 60 s per field.

CONCLUSION

Single kV imager DMLC tracking, which is fully compatible with IMRT, was demonstrated for static fields. The mean tracking error was sub-2 mm for most tumor trajectories with respiratory motions and sub-1 mm for most prostate trajectories.

Inter- and intrafractional localisation errors in cone-beam CT guided stereotactic radiation therapy of tumours in the liver and lung

BACKGROUND

Localisation errors in cone-beam CT (CBCT) guided stereotactic body radiation therapy (SBRT) were evaluated and compared to positioning using the external coordinates of a stereotactic body frame (SBF) alone. Possible correlations to patient- or treatment-specific factors such as body mass index (BMI), planning time, treatment delivery time, and distance between tumour and spinal cord were explored to determine whether they influenced on the benefit of image-guidance.

MATERIAL AND METHODS

A total of 34 patients received SBRT (3 fractions) for tumours in the liver (15 patients) or the lung (19 patients). Immobilisation and positioning was obtained with a SBF. Pre- and post-treatment CBCT scans were registered with the bony anatomy of the planning CT to find inter- and intrafractional patient positioning errors (PPE). For lung tumour patients, matching was also performed on the tumours to find the tumour positioning errors (TPE) and baseline shifts relative to bony anatomy.

RESULTS

The mean inter- and intrafractional 3D vector PPE was 4.5 ± 2.7 mm (average ± SD) and 1.5 ± 0.6 mm, respectively, for the combined group of patients. For lung tumours, the interfractional misalignment was 5.6 ± 1.8 mm. The baseline shift was 3.9 ± 2.0 mm. Intrafractional TPE and baseline shifts were 2.1 ± 0.7 mm and 1.9 ± 0.6 mm, respectively. The magnitude of interfractional baseline shift was closely correlated with the distance between the tumour and the spinal cord. Intrafractional errors were independent of patient BMI, age or gender.

CONCLUSION

Image-guidance reduced setup errors considerably. The study demonstrated the benefit of CBCT-guidance regardless of patient specific factors such as BMI, age or gender. Protection of the spinal cord was facilitated by the correlation between the tumour position relative to the spinal cord and the magnitude of baseline shift.

Hypofractionation does not increase radiation pneumonitis risk with modern conformal radiation delivery techniques

PURPOSE

To study the interaction between radiation dose distribution and hypofractionated radiotherapy with respect to the risk of radiation pneumonitis (RP) estimated from normal tissue complication probability (NTCP) models.

MATERIAL AND METHODS

Eighteen non-small cell lung cancer patients previously treated with helical tomotherapy were selected. For each patient a 3D-conformal plan (3D-CRT) plan was produced in addition to the delivered plan. The standard fractionation schedule was set to 60 Gy in 30 fractions. Iso-efficacy comparisons with hypofractionation were performed by changing the fractionation and the physical prescription dose while keeping the equivalent tumor dose in 2 Gy fractions constant. The risk of developing RP after radiotherapy was estimated using the Mean Equivalent Lung Dose in 2-Gy fractions (MELD(2)) NTCP model with α/β=4 Gy for the residual lung. Overall treatment time was kept constant.

RESULTS

The mean risk of clinical RP after standard fractionation was 7.6% for Tomotherapy (range: 2.8-15.9%) and 9.2% for 3D-CRT (range 3.2-20.2%). Changing to 20 fractions, the Tomotherapy plans became slightly less toxic if the tumor α/β ratio, (α/β)(T), was 7 Gy (mean RP risk 7.5%, range 2.8-16%) while the 3D-CRT plans became marginally more toxic (mean RP risk 9.8%, range 3.2-21%). If (α/β)(T) was 13 Gy, the mean estimated risk of RP is 7.9% for Tomotherapy (range: 2.8-17%) and 10% for 3D-CRT (range 3.2-22%).

CONCLUSION

Modern highly conformal dose distributions are radiobiologically more forgiving with respect to hypofractionation, even for a normal tissue endpoint where α/β is lower than for the tumor in question.

Respiratory gating enhances imaging of pulmonary nodules and measurement of tracer uptake in FDG PET/CT

OBJECTIVE

The aim of this study was to evaluate prospectively the effects of respiratory gating during FDG PET/CT on the determination of lesion size and the measurement of tracer uptake in patients with pulmonary nodules in a clinical setting.

SUBJECTS AND METHODS

Eighteen patients with known pulmonary nodules (nine women, nine men; mean age, 61.4 years) underwent conventional FDG PET/CT and respiratory-gated PET acquisitions during their scheduled staging examinations. Maximum, minimum, and average standardized uptake values (SUVs) and lesion size and volume were determined with and without respiratory gating. The results were then compared using the two-tailed Student's t test and the nonparametric Wilcoxon's test to assess the effects of respiratory gating on PET acquisitions.

RESULTS

Respiratory gating reduced the measured area of lung lesions by 15.5%, the axial dimension by 10.3%, and the volume by 44.5% (p = 0.014, p = 0.007, and p = 0.025, respectively). The lesion volumes in gated studies were closer to those assessed by standard CT (difference decreased by 126.6%, p = 0.025). Respiratory gating increased the measured maximum SUV by 22.4% and average SUV by 13.3% (p < 0.001 and p = 0.002).

CONCLUSION

Our findings suggest that the use of PET respiratory gating in PET/CT results in lesion volumes closer to those assessed by CT and improved measurements of tracer uptake for lesions in the lungs.

Patient-specific finite element modeling of respiratory lung motion using 4D CT image data

Development and optimization of methods for adequately accounting for respiratory motion in radiation therapy of thoracic tumors require detailed knowledge of respiratory dynamics and its impact on corresponding dose distributions. Thus, computer aided modeling and simulation of respiratory motion have become increasingly important. In this article a biophysical approach for modeling respiratory lung motion is described: Major aspects of the process of lung ventilation are formulated as a contact problem of elasticity theory which is solved by finite element methods; lung tissue is assumed to be isotropic, homogeneous, and linearly elastic. A main focus of the article is to assess the impact of biomechanical parameters (values of elastic constants) on the modeling process and to evaluate modeling accuracy. Patient-specific models are generated based on 4D CT data of 12 lung tumor patients. Simulated motion patterns of inner lung landmarks are compared with corresponding motion patterns observed in the 4D CT data. Mean absolute differences between model-based predicted landmark motion and corresponding breathing-induced landmark displacements as observed in the CT data sets are in the order of 3 mm (end expiration to end inspiration) and 2 mm (end expiration to midrespiration). Modeling accuracy decreases with increasing tumor size both locally (landmarks close to tumor) and globally (landmarks in other parts of the lung). The impact of the values of the elastic constants appears to be small. Outcomes show that the modeling approach is an adequate strategy in predicting lung dynamics due to lung ventilation. Nevertheless, the decreased prediction quality in cases of large tumors demands further study of the influence of lung tumors on global and local lung elasticity properties.