Geometric uncertainties in voluntary deep inspiration breath hold radiotherapy for locally advanced lung cancer

Lung and Liver Stereotactic Body Radiation Therapy During Mechanically Assisted Deep Inspiration Breath-Holds: A Prospective Feasibility Trial

Purpose

Radiation therapy for tumors subject to breathing-related motion during breath-holds (BHs) has the potential to substantially reduce the irradiated volume. Mechanically assisted and noninvasive ventilation (MANIV) could ensure the target repositioning accuracy during each BH while facilitating treatment feasibility through oxygen supplementation and a perfectly replicated mechanical support. However, there is currently no clinical evidence substantiating the use of MANIV-induced BH for moving tumors. The aim of this work was, therefore, to evaluate the technique's performance under real treatment conditions.

Methods and Materials

Patients eligible for lung or liver stereotactic body radiation therapy were prospectively included in a single-arm trial. The primary endpoint corresponded to the treatment feasibility with MANIV. Secondary outcomes comprised intrafraction geometric uncertainties extracted from real-time imaging, tolerance to BH, and treatment time.

Results

Treatment was successfully delivered in 92.9% (13/14) of patients: 1 patient with a liver tumor was excluded because of a mechanically induced gastric insufflation displacing the liver cranially by more than 1 cm. In the left-right/anteroposterior/craniocaudal directions, the recalculated safety margins based on intrafraction positional data were 4.6 mm/5.1 mm/5.6 mm and 4.7 mm/7.3 mm/5.9 mm for lung and liver lesions, respectively. Compared with the free-breathing internal target volume and midposition approaches, the average reduction in the planning target volume with MANIV reached -47.2% ± 15.3%, P < .001, and -29.4% ± 19.2%, P = .007, for intrathoracic tumors and -23.3% ± 12.4%, P < .001, and -9.3% ± 15.3%, P = .073, for upper abdominal tumors, respectively. For 1 liver lesion, large caudal drifts of occasionally more than 1 cm were measured. The total slot time was 53.1 ± 10.6 minutes with a BH comfort level of 80.1% ± 10.6%.

Conclusions

MANIV enables high treatment feasibility within a nonselected population. Accurate intrafraction tumor repositioning is achieved for lung tumors. Because of occasional intra-BH caudal drifts, pretreatment assessment of BH stability for liver lesions is, however, recommended.

Robust localization of poorly visible tumor in fiducial free stereotactic body radiation therapy

BACKGROUND AND PURPOSE

Effective respiratory motion management reduces healthy tissue toxicity and ensures sufficient dose delivery to lung cancer cells in pulmonary stereotactic body radiation therapy (SBRT) with high fractional doses. An articulated robotic arm paired with an X-ray imaging system is designed for real-time motion-tracking (RTMT) dose delivery. However, small tumors (<15 mm) or tumors at challenging locations may not be visible in the X-ray images, disqualifying patients with such tumors from RTMT dose delivery unless fiducials are implanted via an invasive procedure. To track these practically invisible lung tumors in SBRT, we hereby develop a deep learning-enabled template-free tracking framework, SAFE Track.

METHODS

SAFE Track is a fully supervised framework that trains a generalizable prior for template-free target localization. Two sub-stages are incorporated in SAFE Track, including the initial pretraining on two large-scale medical image datasets (DeepLesion and Node21) followed by fine-tuning on our in-house dataset. A two-stage detector, Faster R-CNN, with a backbone of ResNet50, was selected as our detection network. 94 patients (415 fractions; 40,348 total frames) with low tumor visibility who thus had implanted fiducials were included. The cohort is categorized by the longest dimension of the tumor (<10 mm, 10-15 mm and > 15 mm). The patients were split into training (n = 66) and testing (n = 28) sets. We simulated fiducial-free tumors by removing the fiducials from the X-ray images. We classified the patients into two groups - fiducial implanted inside tumors and implanted outside tumors. To ensure the rigor of our experiment design, we only conducted fiducial removal simulation in training patients and utilized patients with fiducial implanted outside of the tumors for testing. Commercial Xsight Lung Tracking (XLT) and a Deep Match were included for comparison.

RESULTS

SAFE Track achieves promising outcomes to as accurate as 1.23±1.32 mm 3D distance in testing patients with tumor size > 15 mm where Deep Match is at 4.75±1.67 mm and XLT is at 12.23±4.58 mm 3D distance. Even for the most challenging tumor size (<10 mm), SAFE Track maintains its robustness at 1.82 plus or minus 1.67 mm 3D distance, where Deep Match is at 5.32 plus or minus 2.32 mm, and XLT is at 24.83±12.95 mm 3D distance. Moreover, SAFE Track can detect some considerably challenging cases where the tumor is almost invisible or overlapped with dense anatomies.

CONCLUSION

SAFE Track is a robust, clinically compatible, fiducial-free, and template-free tracking framework that is applicable to patients with small tumors or tumors obscured by overlapped anatomies in SBRT.

Application and dosimetric comparison of surface-guided deep inspiration breath-hold for lung stereotactic body radiotherapy

Respiratory motion management is the crucial challenge for safe and effective application of lung stereotactic body radiotherapy (SBRT). The present study implemented lung SBRT treatment in voluntary deep inspiration breath-hold (DIBH) with surface-guided radiotherapy (SGRT) system and evaluated the geometric and dosimetric benefits of DIBH to organs-at-risk (OARs), aiming to advising the choice between DIBH technology and conventional free breathing 4 dimensions (FB-4D) technology. Five patients of lung SBRT treated in DIBH with SGRT at our institution were retrospectively analyzed. CT scans were acquired in DIBH and FB-4D, treatment plans were generated for both respiratory phases. The geometric and dosimetry of tumor, ipsilateral lung, double lungs and heart were compared between the DIBH and FB-4D treatment plans. In terms of target coverage, utilizing DIBH significantly reduced the mean plan target volume (PTV) by 21.9% (p = 0.09) compared to FB-4D, the conformity index (CI) of DIBH and FB-4D were comparable, but the dose gradient index (DGI) of DIBH was higher. With DIBH expanding lung, the volumes of ipsilateral lung and double lungs were 2535.1 ± 403.0cm3 and 4864.3 ± 900.2cm3, separately, 62.2% (p = 0.009) and 73.1% (p = 0.009) more than volumes of ipsilateral lung (1460.03 ± 146.60cm3) and double lungs (2811.25 ± 603.64cm3) in FB-4D. The heart volume in DIBH was 700.0 ± 146.1cm3, 11.6% (p = 0.021) less than that in FB-4D. As for OARs protection, the mean dose, percent of volume receiving > 20Gy (V20) and percent of volume receiving > 5Gy (V5) of ipsilateral lung in DIBH were significantly lower by 33.2% (p = 0.020), 44.0% (p = 0.022) and 24.5% (p = 0.037) on average, separately. Double lungs also showed significant decrease by 31.1% (p = 0.019), 45.5% (p = 0.024) and 20.9% (p = 0.048) on average for mean dose, V20 and V5 in DIBH. Different from the lung, the mean dose and V5 of heart showed no consistency between DIBH and FB-4D, but lower maximum dose of heart was achieved in DIBH for all patients in this study. Appling lung SBRT in DIBH with SGRT was feasibly performed with high patient compliance. DIBH brought significant dosimetric benefits to lung, however, it caused more or less irradiated heart dose that depend on the patients' individual differences which were unpredictable.

The impact of motion on onboard MRI-guided pencil beam scanned proton therapy treatments

Objective.Online magnetic resonance imaging (MRI) guidance could be especially beneficial for pencil beam scanned (PBS) proton therapy of tumours affected by respiratory motion. For the first time to our knowledge, we investigate the dosimetric impact of respiratory motion on MRI-guided proton therapy compared to the scenario without magnetic field.Approach.A previously developed analytical proton dose calculation algorithm accounting for perpendicular magnetic fields was extended to enable 4D dose calculations. For two geometrical phantoms and three liver and two lung patient cases, static treatment plans were optimised with and without magnetic field (0, 0.5 and 1.5 T). Furthermore, plans were optimised using gantry angle corrections (0.5 T +5° and 1.5 T +15°) to reproduce similar beam trajectories compared to the 0 T reference plans. The effect of motion was then considered using 4D dose calculations without any motion mitigation and simulating 8-times volumetric rescanning, with motion for the patient cases provided by 4DCT(MRI) data sets. Each 4D dose calculation was performed for different starting phases and the CTV dose coverageV95%and homogeneityD5%-D95%were analysed.Main results.For the geometrical phantoms with rigid motion perpendicular to the beam and parallel to the magnetic field, a comparable dosimetric effect was observed independent of the magnetic field. Also for the five 4DCT(MRI) cases, the influence of motion was comparable for all magnetic field strengths with and without gantry angle correction. On average, the motion-induced decrease in CTVV95%from the static plan was 17.0% and 18.9% for 1.5 T and 0.5 T, respectively, and 19.9% without magnetic field.Significance.For the first time, this study investigates the combined impact of magnetic fields and respiratory motion on MR-guided proton therapy. The comparable dosimetric effects irrespective of magnetic field strength indicate that the effects of motion for future MR-guided proton therapy may not be worse than for conventional PBS proton therapy.

Dosimetric comparison of autocontouring techniques for online adaptive proton therapy

Objective.Anatomical and daily set-up uncertainties impede high precision delivery of proton therapy. With online adaptation, the daily plan is reoptimized on an image taken shortly before the treatment, reducing these uncertainties and, hence, allowing a more accurate delivery. This reoptimization requires target and organs-at-risk (OAR) contours on the daily image, which need to be delineated automatically since manual contouring is too slow. Whereas multiple methods for autocontouring exist, none of them are fully accurate, which affects the daily dose. This work aims to quantify the magnitude of this dosimetric effect for four contouring techniques.Approach.Plans reoptimized on automatic contours are compared with plans reoptimized on manual contours. The methods include rigid and deformable registration (DIR), deep-learning based segmentation and patient-specific segmentation.Main results.It was found that independently of the contouring method, the dosimetric influence of usingautomaticOARcontoursis small (<5% prescribed dose in most cases), with DIR yielding the best results. Contrarily, the dosimetric effect of using theautomatic target contourwas larger (>5% prescribed dose in most cases), indicating that manual verification of that contour remains necessary. However, when compared to non-adaptive therapy, the dose differences caused by automatically contouring the target were small and target coverage was improved, especially for DIR.Significance.The results show that manual adjustment of OARs is rarely necessary and that several autocontouring techniques are directly usable. Contrarily, manual adjustment of the target is important. This allows prioritizing tasks during time-critical online adaptive proton therapy and therefore supports its further clinical implementation.

Radiotherapy for mediastinal lymphoma in breath hold using surface monitoring and nasal high flow oxygen: Clinical experiences and breath hold stability

PURPOSE

In this study we describe the clinical introduction and evaluation of radiotherapy in mediastinal lymphoma in breath hold using surface monitoring combined with nasal high flow therapy (NHFT) to prolong breath hold duration.

MATERIALS AND METHODS

11 Patients with mediastinal lymphoma were evaluated. 6 Patients received NHFT, 5 patients were treated in breath hold without NHFT. Breath hold stability as measured by a surface scanning system was evaluated, as well as internal movement based on cone beam computed tomography (CBCT) before and after treatment. Based on internal movement, margins were determined. In a parallel planning study we compared free breathing plans with breath hold plans using the determined margins.

RESULTS

Average inter breath hold stability was 0.6 mm for NHFT treatments, and 0.5 mm for non-NHFT treatments (p > 0.1). Intra breath hold stability was 0.8 vs. 0.6 mm (p > 0.1) on average. Using NHFT, average breath hold duration increased from 34 s to 60 s (p < 0.01). Residual CTV motion derived from CBCTs before and after each fraction was 2.0 mm for NHFT vs 2.2 mm for non-NHFT (p > 0.1). Combined with inter-fraction motion, a uniform mediastinal margin of 5 mm appears to be sufficient. In breath hold, mean lung dose is reduced by 2.6 Gy (p < 0.001), while mean heart dose is reduced by 2.0 Gy (p < 0.001).

CONCLUSION

Treatment of mediastinal lymphoma in breath hold is feasible and safe. The addition of NHFT approximately increases breath hold durations with a factor two while stability is maintained. By reducing breathing motion, margins can be decreased to 5 mm. A considerable dose reduction in heart, lungs, esophagus, and breasts can be achieved with this method.

Limitations of phase-sorting based pencil beam scanned 4D proton dose calculations under irregular motion

Objective.4D dose calculation (4DDC) for pencil beam scanned (PBS) proton therapy is typically based on phase-sorting of individual pencil beams onto phases of a single breathing cycle 4DCT. Understanding the dosimetric limitations and uncertainties of this approach is essential, especially for the realistic treatment scenario with irregular free breathing motion.Approach.For three liver and three lung cancer patient CTs, the deformable multi-cycle motion from 4DMRIs was used to generate six synthetic 4DCT(MRI)s, providing irregular motion (11/15 cycles for liver/lung; tumor amplitudes ∼4-18 mm). 4DDCs for two-field plans were performed, with the temporal resolution of the pencil beam delivery (4-200 ms) or with 8 phases per breathing cycle (500-1000 ms). For the phase-sorting approach, the tumor center motion was used to determine the phase assignment of each spot. The dose was calculated either using the full free breathing motion or individually repeating each single cycle. Additionally, the use of an irregular surrogate signal prior to 4DDC on a repeated cycle was simulated. The CTV volume with absolute dose differences >5% (V_dosediff>5%) and differences in CTVV_95%andD_5%-D_95%compared to the free breathing scenario were evaluated.Main results.Compared to 4DDC considering the full free breathing motion with finer spot-wise temporal resolution, 4DDC based on a repeated single 4DCT resulted inV_dosediff>5%of on average 34%, which resulted in an overestimation ofV_95%up to 24%. However, surrogate based phase-sorting prior to 4DDC on a single cycle 4DCT, reduced the averageV_dosediff>5%to 16% (overestimationV_95%up to 19%). The 4DDC results were greatly influenced by the choice of reference cycle (V_dosediff>5%up to 55%) and differences due to temporal resolution were much smaller (V_dosediff>5%up to 10%).Significance.It is important to properly consider motion irregularity in 4D dosimetric evaluations of PBS proton treatments, as 4DDC based on a single 4DCT can lead to an underestimation of motion effects.

Clinical experience of volumetric-modulated flattening filter free stereotactic body radiation therapy of lesions in the lung with deep inspiration breath-hold

This clinical study aimed to evaluate lung cancer patients' ability to perform deep inspiration breath-hold (DIBH) during CT simulation and throughout the treatment course of stereotactic body radiation therapy (SBRT). In addition, target sizes, organ at risk (OAR) sizes, and doses to the respective volumes in filter-free volumetric-modulated arc therapy plans performed under free-breathing (FB) and DIBH conditions were evaluated. Twenty-one patients with peripheral lesions were included, of which 13 were eligible for SBRT. All patients underwent training for breath-hold during CT, and if they complied with the requirements, two CT scans were obtained: CT scan in DIBH and a four-dimensional CT scan in FB. The treatment plans in FB and DIBH were generated, and the dose parameters and volume sizes were compared. The endpoints for evaluation were patient compliance, target dose coverage, and doses to the OARs. This clinical study showed high patient DIBH compliance during both CT simulation and treatment for patients with lung cancer. A significant reduction in target volumes was achieved with SBRT in DIBH, in addition to significantly decreased doses to the heart, chest wall, and lungs. DIBH in SBRT of lung lesions is feasible, and a routine to manage intra-fractional deviation should be established upon implementation.

Ultra-fast pencil beam scanning proton therapy for locally advanced non-small-cell lung cancers: field delivery within a single breath-hold

PURPOSE

The use of motion mitigation techniques such as breath-hold can reduce the dosimetric uncertainty of lung cancer proton therapy. We studied the feasibility of pencil beam scanning (PBS) proton therapy field delivery within a single breath-hold at PSI's Gantry 2.

METHODS

In PBS proton therapy, the delivery time for a field is determined by the beam-on time and the dead time between proton spots (the time required to change the energy and/or lateral position). We studied ways to reduce beam-on and lateral scanning time, without sacrificing dosimetric plan quality, aiming at a single field delivery time of 15 seconds at maximum. We tested this approach on 10 lung cases with varying target volumes. To reduce the beam-on time, we increased the beam current at the isocenter by developing new beam optics for PSI's PROSCAN beamline and Gantry 2. To reduce the dead time between the spots, we used spot-reduced plan optimization.

RESULTS

We found that it is possible to achieve conventional fractionated (2 Gy(RBE)/fraction) and hypofractionated (6 Gy(RBE)/fraction) field delivery times within a single breath-hold (<15 sec) for a variety non-small-cell lung cancer cases.

CONCLUSION

In summary, the combination of spot reduction and improved beam line transmission is a promising approach for the treatment of mobile tumours within clinically achievable breath-hold durations.

Combined proton-photon therapy for non-small cell lung cancer

PURPOSE

Advanced non-small cell lung cancer (NSCLC) is still a challenging indication for conventional photon radiotherapy. Proton therapy has the potential to improve outcomes, but proton treatment slots remain a limited resource despite an increasing number of proton therapy facilities. This work investigates the potential benefits of optimally combined proton-photon therapy delivered using a fixed horizontal proton beam line in combination with a photon Linac, which could increase accessibility to proton therapy for such a patient cohort.

MATERIALS AND METHODS

A treatment planning study has been conducted on a patient cohort of seven advanced NSCLC patients. Each patient had a planning CT and multiple repeated CTs from three different days and for different breath-holds on each day. Treatment plans for combined proton-photon therapy (CPPT) were calculated for individual patients by optimizing the combined cumulative dose on the initial planning CT only (non-adapted) as well as on each daily CT respectively (adapted). The impact of inter-fractional changes and/or breath-hold variability was then assessed on the repeat breath-hold CTs. Results were compared to plans for IMRT or IMPT alone, as well as against combined treatments assuming a proton gantry. Plan quality was assessed in terms of dosimetric, robustness and NTCP metrics.

RESULTS

Combined treatment plans improved plan quality compared to IMRT treatments, especially in regard to reductions of low and medium doses to organs at risk (OARs), which translated into lower NTCP estimates for three side effects. For most patients, combined treatments achieved results close to IMPT-only plans. Inter-fractional changes impact mainly the target coverage of combined and IMPT treatments, while OARs doses were less affected by these changes. With plan adaptation however, target coverage of combined treatments remained high even when taking variability between breath-holds into account.

CONCLUSIONS

Optimally combined proton-photon plans improve treatment plan quality compared to IMRT only, potentially reducing the risk of toxicity while also allowing to potentially increase accessibility to proton therapy for NSCLC patients. This article is protected by copyright. All rights reserved.

Intrafraction target shift comparison using two breath-hold systems in lung stereotactic body radiotherapy

Background and purpose

In lung Stereotactic Body Radiotherapy (SBRT) respiratory management is used to reduce target motion due to respiration. This study aimed (1) to estimate intrafraction shifts through a Cone Beam Computed Tomography (CBCT) acquired during the first treatment arc when deep inspiration breath-hold (DIBH) was performed using spirometry-based (SB) or surface-guidance (SG) systems and (2) to analyze the obtained results depending on lesion localization.

Material and methods

A sample of 157 patients with 243 lesions was analyzed. A total of 860 and 410 fractions were treated using SB and SG. Averaged intrafraction shifts were estimated by the offsets obtained when registering a CBCT acquired during the first treatment arc with the planning CT. Offsets were recorded in superior-inferior (SI), left-right (LR) and anterior-posterior (AP). Significance tests were applied to account for differences in average offsets and variances between DIBH systems. Systematic and random errors were computed.

Results

Average offset moduli were 2.4 ± 2.2 mm and 3.5 ± 2.6 mm for SB and SG treatments (p < 0.001). When comparing SB and SG offset distributions in each direction no differences were found in average values (p > 0.3). However, variances were statistically smaller for SB than for SG (p < 0.001). The number of vector moduli offsets greater than 5 mm was 2.1 times higher for SG. Compared to other locations, lower lobe lesions moduli were at least 2.3 times higher.

Conclusions

Both systems were accuracy-equivalent but not precision-equivalent systems. Furthermore, the SB system was more precise than the SG one. Despite DIBH, patients with lower lobe lesions had larger offsets than superior lobe ones, mainly in SI.

Synthetic 4DCT(MRI) lung phantom generation for 4D radiotherapy and image guidance investigations

Purpose

Respiratory motion is one of the major challenges in radiotherapy. In this work, a comprehensive and clinically plausible set of 4D numerical phantoms, together with their corresponding “ground truths,” have been developed and validated for 4D radiotherapy applications.

Methods

The phantoms are based on CTs providing density information and motion from multi‐breathing‐cycle 4D Magnetic Resonance imagings (MRIs). Deformable image registration (DIR) has been utilized to extract motion fields from 4DMRIs and to establish inter‐subject correspondence by registering binary lung masks between Computer Tomography (CT) and MRI. The established correspondence is then used to warp the CT according to the 4DMRI motion. The resulting synthetic 4DCTs are called 4DCT(MRI)s. Validation of the 4DCT(MRI) workflow was conducted by directly comparing conventional 4DCTs to derived synthetic 4D images using the motion of the 4DCTs themselves (referred to as 4DCT(CT)s). Digitally reconstructed radiographs (DRRs) as well as 4D pencil beam scanned (PBS) proton dose calculations were used for validation.

Results

Based on the CT image appearance of 13 lung cancer patients and deformable motion of five volunteer 4DMRIs, synthetic 4DCT(MRI)s with a total of 871 different breathing cycles have been generated. The 4DCT(MRI)s exhibit an average superior–inferior tumor motion amplitude of 7 ± 5 mm (min: 0.5 mm, max: 22.7 mm). The relative change of the DRR image intensities of the conventional 4DCTs and the corresponding synthetic 4DCT(CT)s inside the body is smaller than 5% for at least 81% of the pixels for all studied cases. Comparison of 4D dose distributions calculated on 4DCTs and the synthetic 4DCT(CT)s using the same motion achieved similar dose distributions with an average 2%/2 mm gamma pass rate of 90.8% (min: 77.8%, max: 97.2%).

Conclusion

We developed a series of numerical 4D lung phantoms based on real imaging and motion data, which give realistic representations of both anatomy and motion scenarios and the accessible “ground truth” deformation vector fields of each 4DCT(MRI). The open‐source code and motion data allow foreseen users to generate further 4D data by themselves. These numeric 4D phantoms can be used for the development of new 4D treatment strategies, 4D dose calculations, DIR algorithm validations, as well as simulations of motion mitigation and different online image guidance techniques for both proton and photon radiation therapy.

A Planning Comparison of IMRT vs. Pencil Beam Scanning for Deep Inspiration Breath Hold Lung Cancers

Deep inspiration breath hold (DIBH) has dosimetric advantages for lung cancer patients treated with external beam therapy, but is difficult for many patients to perform. Proton therapy permits sparing of the downstream organs at risk (OAR). We compared conventionally fractionated proton (p) and photon(x) plans on both free breathing (FB) and DIBH planning CTs to determine the effect of DIBH with proton therapy. We evaluated 24 plans from 6 lung cancer patients treated with photon DIBH on a prospective protocol. All patients were re-planned using pencil beam scanning (PBS) proton therapy. New plans were generated for FB datasets with both modalities. All plans were renormalized to 60 Gy. We evaluated dosimetric parameters for heart, lung and esophagus. We also compared FB_p to DIBH_x parameters to quantify how FB_p plans compare to DIBH_x plans. Significant differences were found for lung metrics V20 and mean lung dose between FB and DIBH plans regardless of treatment modality. Furthermore, lung metrics for FB_p were comparable or superior to DIBH_x, suggesting that FB protons may be a viable alternative for those patients that cannot perform DIBH with IMRT. The heart dose metrics were significantly different for the 5 out of 6 patients where the PTV overlapped the heart as DIBH moved heart out of the high dose volume. Heart dose metrics were further reduced by proton therapy. DIBH offers similar relative advantages for lung sparing for PBS as it does for IMRT but the magnitude of the DIBH related gains in OAR sparing were smaller for PBS than IMRT. FB_p plans offer similar or better lung and heart sparing compared to DIBH_x plans. For IMRT patients who have difficulty performing DIBH, FB protons may offer an alternative.

Treatment planning and 4D robust evaluation strategy for proton therapy of lung tumors with large motion amplitude

Purpose

Intensity‐modulated proton therapy (IMPT) for lung tumors with a large tumor movement is challenging due to loss of robustness in the target coverage. Often an upper cut‐off at 5‐mm tumor movement is used for proton patient selection. In this study, we propose (1) a robust and easily implementable treatment planning strategy for lung tumors with a movement larger than 5 mm, and (2) a four‐dimensional computed tomography (4DCT) robust evaluation strategy for evaluating the dose distribution on the breathing phases.

Materials and methods

We created a treatment planning strategy based on the internal target volume (ITV) concept (aim 1). The ITV was created as a union of the clinical target volumes (CTVs) on the eight 4DCT phases. The ITV expanded by 2 mm was the target during robust optimization on the average CT (avgCT). The clinical plan acceptability was judged based on a robust evaluation, computing the voxel‐wise min and max (VWmin/max) doses over 28 error scenarios (range and setup errors) on the avgCT. The plans were created in RayStation (RaySearch Laboratories, Stockholm, Sweden) using a Monte Carlo dose engine, commissioned for our Mevion S250i Hyperscan system (Mevion Medical Systems, Littleton, MA, USA). We developed a new 4D robust evaluation approach (4DRobAvg; aim 2). The 28 scenario doses were computed on each individual 4DCT phase. For each scenario, the dose distributions on the individual phases were deformed to the reference phase and combined to a weighted sum, resulting in 28 weighted sum scenario dose distributions. From these 28 scenario doses, VWmin/max doses were computed. This new 4D robust evaluation was compared to two simpler 4D evaluation strategies: re‐computing the nominal plan on each individual 4DCT phase (4DNom) and computing the robust VWmin/max doses on each individual phase (4DRobInd). The treatment planning and dose evaluation strategies were evaluated for 16 lung cancer patients with tumor movement of 4–26 mm.

Results

The ratio of the ITV and CTV volumes increased linearly with the tumor amplitude, with an average ratio of 1.4. Despite large ITV volumes, a clinically acceptable plan fulfilling all target and organ at risk (OAR) constraints was feasible for all patients. The 4DNom and 4DRobInd evaluation strategies were found to under‐ or overestimate the dosimetric effect of the tumor movement, respectively. 4DRobInd showed target underdosage for five patients, not observed in the robust evaluation on the avgCT or in 4DRobAvg. The accuracy of dose deformation used in 4DRobAvg was quantified and found acceptable, with differences for the dose‐volume parameters below 1 Gy in most cases.

Conclusion

The proposed ITV‐based planning strategy on the avgCT was found to be a clinically feasible approach with adequate tumor coverage and no OAR overdosage even for large tumor movement. The new proposed 4D robust evaluation, 4DRobAvg, was shown to give an easily interpretable understanding of the effect of respiratory motion dose distribution, and to give an accurate estimate of the dose delivered in the different breathing phases.

Investigation of interfractional variation in lung tumor position under expiratory-phase breath hold using cone-beam computed tomography in stereotactic body radiation therapy

PURPOSE

We investigated the interfractional variation in the tumor position during lung stereotactic body radiotherapy (SBRT) under expiratory-phase breath hold (BH) using cone-beam computed tomography (CBCT).

METHODS

A total of 79 patients with lung cancer were treated with lung SBRT, wherein the Abches system under expiratory-phase BH was used to study interfractional variation. The tumors were located in the upper lobe in 31 cases, in the middle lobe in 11 cases, and in the lower lobe in 37 cases. Planning CTs were scanned under expiratory-phase BH with the Abches system. The 3-degrees-of-freedom (DOF) tumor-based setup using CBCT images under expiratory-phase BH was performed after a 6-DOF bony vertebrae-based setup using an ExacTrac X-ray system. Interfractional variation in the lung tumor position was defined as the difference in the position of the lung tumor relative to the bone anatomy in the left-right (LR), antero-posterior (AP), and craniocaudal (CC) directions represented as absolute values.

RESULTS

The interfractional variation in the lung tumor position was very similar in all the lung regions, and its mean ± standard deviation values in all patients were 1.0 ± 1.1, 1.6 ± 1.9, and 1.6 ± 1.9 mm in the LR, AP, and CC directions, respectively. Further, 99.1%, 92.4%, and 92.7% of all the fractions for the interfractional tumor positional variation in the LR, AP, and CC directions were less than 5 mm, respectively.

CONCLUSION

The interfractional variation in the tumor position was small for lung cancer patients treated with the Abches system under expiratory-phase BH.

An approach for estimating dosimetric uncertainties in deformable dose accumulation in pencil beam scanning proton therapy for lung cancer

Deformable image registration (DIR) is an important component for dose accumulation and associated clinical outcome evaluation in radiotherapy. However, the resulting deformation vector field (DVF) is subject to unavoidable discrepancies when different algorithms are applied, leading to dosimetric uncertainties of the accumulated dose. We propose here an approach for proton therapy to estimate dosimetric uncertainties as a consequence of modeled or estimated DVF uncertainties. A patient-specific DVF uncertainty model was built on the first treatment fraction, by correlating the magnitude differences of five DIR results at each voxel to the magnitude of any single reference DIR. In the following fractions, only the reference DIR needs to be applied, and DVF geometric uncertainties were estimated by this model. The associated dosimetric uncertainties were then derived by considering the estimated geometric DVF uncertainty, the dose gradient of fractional recalculated dose distribution and the direction factor from the applied reference DIR of this fraction. This estimated dose uncertainty was respectively compared to the reference dose uncertainty when different DIRs were applied individually for each dose warping. This approach was validated on seven NSCLC patients, each with nine repeated CTs. The proposed model-based method is able to achieve dose uncertainty distribution on a conservative voxel-to-voxel comparison within ±5% of the prescribed dose to the 'reference' dosimetric uncertainty, for 77% of the voxels in the body and 66%-98% of voxels in investigated structures. We propose a method to estimate DIR induced uncertainties in dose accumulation for proton therapy of lung tumor treatments.

Visually guided inspiration breath-hold facilitated with nasal high flow therapy in locally advanced lung cancer

BACKGROUND AND PURPOSE

Reducing breathing motion in radiotherapy (RT) is an attractive strategy to reduce margins and better spare normal tissues. The objective of this prospective study (NCT03729661) was to investigate the feasibility of irradiation of non-small cell lung cancer (NSCLC) with visually guided moderate deep inspiration breath-hold (IBH) using nasal high-flow therapy (NHFT).

MATERIAL AND METHODS

Locally advanced NSCLC patients undergoing photon RT were given NHFT with heated humidified air (flow: 40 L/min with 80% oxygen) through a nasal cannula. IBH was monitored by optical surface tracking (OST) with visual feedback. At a training session, patients had to hold their breath as long as possible, without and with NHFT. For the daily cone beam CT (CBCT) and RT treatment in IBH, patients were instructed to keep their BH as long as it felt comfortable. OST was used to analyze stability and reproducibility of the BH, and CBCT to analyze daily tumor position. Subjective tolerance was measured with a questionnaire at 3 time points.

RESULTS

Of 10 included patients, 9 were treated with RT. Seven (78%) completed the treatment with NHFT as planned. At the training session, the mean BH length without NHFT was 39 s (range 15-86 s), and with NHFT 78 s (range 29-223 s) (p = .005). NHFT prolonged the BH duration by a mean factor of 2.1 (range 1.1-3.9s). The mean overall stability and reproducibility were within 1 mm. Subjective tolerance was very good with the majority of patients having no or minor discomfort caused by the devices. The mean inter-fraction tumor position variability was 1.8 mm (-1.1-8.1 mm;SD 2.4 mm).

CONCLUSION

NHFT for RT treatment of NSCLC in BH is feasible, well tolerated and significantly increases the breath-hold duration. Visually guided BH with OST is stable and reproducible. We therefore consider this an attractive patient-friendly approach to treat lung cancer patients with RT in BH.

Enhanced Deep-Inspiration Breath Hold Superior to High-Frequency Percussive Ventilation for Respiratory Motion Mitigation: A Physiology-Driven, MRI-Guided Assessment Toward Optimized Lung Cancer Treatment With Proton Therapy

Background: To safely treat lung tumors using particle radiation therapy (PRT), motion-mitigation strategies are of critical importance to ensure precise irradiation. Therefore, we compared applicability, effectiveness, reproducibility, and subjects' acceptance of enhanced deep-inspiration breath hold (eDIBH) with high-frequency percussive ventilation (HFPV) by MRI assessment within 1 month.

Methods: Twenty-one healthy subjects (12 males/9 females; age: 49.5 ± 5.8 years; BMI: 24.7 ± 3.3 kg/m⁻²) performed two 1.5 T MRI scans in four visits at weekly intervals under eDIBH and HFPV conditions, accompanied by daily, home-based breath-hold training and spirometric assessments over a 3-week period. eDIBH consisted of 8-min 100% O₂ breathing (3 min resting ventilation, 5 min controlled hyperventilation) prior to breath hold. HFPV was set at 200–250 pulses min⁻¹ and 0.8–1.2 bar. Subjects' acceptance and preference were evaluated by questionnaire. To quantify inter- and intrafractional changes, a lung distance metric representing lung topography was computed for 10 reference points: a motion-invariant spinal cord and nine lung structure contours (LSCs: apex, carina, diaphragm, and six vessels as tumor surrogates distributed equally across the lung). To parameterize individual LSC localizability, measures of their spatial variabilities were introduced and lung volumes calculated by automated MRI analysis.

Results: eDIBH increased breath-hold duration by > 100% up to 173 ± 73 s at visit 1, and to 217 ± 67 s after 3 weeks of home-based training at visit 4 (p < 0.001). Measures of vital capacity and lung volume remained constant over the 3-week period. Two vessels in the lower lung segment and the diaphragm yielded a two- to threefold improved positional stability with eDIBH, whereby absolute distance variability was significantly smaller for five LSCs; ≥70% of subjects showed significantly better intrafractional lung motion mitigation under reproducible conditions with eDIBH compared with HFPV with smaller ranges most apparent in the anterior-posterior and cranial-caudal directions. Approximately 80% of subjects preferred eDIBH over HFPV, with “less discomfort” named as most frequent reason.

Conclusions: Both, eDIBH, and HFPV were well-tolerated. eDIBH duration was long enough to allow for potential PRT. Variability in lung volume was smaller and position of lung structures more precise with eDIBH. Subjects preferred eDIBH over HFPV. Thus, eDIBH is a very promising tool for lung tumor therapy with PRT, and further investigation of its applicability in patients is warranted.

Dosimetric influence of deformable image registration uncertainties on propagated structures for online daily adaptive proton therapy of lung cancer patients

PURPOSE

A major burden of introducing an online daily adaptive proton therapy (DAPT) workflow is the time and resources needed to correct the daily propagated contours. In this study, we evaluated the dosimetric impact of neglecting the online correction of the propagated contours in a DAPT workflow.

MATERIAL AND METHODS

For five NSCLC patients with nine repeated deep-inspiration breath-hold CTs, proton therapy plans were optimised on the planning CT to deliver 60 Gy-RBE in 30 fractions. All repeated CTs were registered with six different clinically used deformable image registration (DIR) algorithms to the corresponding planning CT. Structures were propagated rigidly and with each DIR algorithm and reference structures were contoured on each repeated CT. DAPT plans were optimised with the uncorrected, propagated structures (propagated DAPT doses) and on the reference structures (ideal DAPT doses), non-adapted doses were recalculated on all repeated CTs.

RESULTS

Due to anatomical changes occurring during the therapy, the clinical target volume (CTV) coverage of the non-adapted doses reduces on average by 9.7% (V95) compared to an ideal DAPT doses. For the propagated DAPT doses, the CTV coverage was always restored (average differences in the CTV V95 < 1% compared to the ideal DAPT doses). Hotspots were always reduced with any DAPT approach.

CONCLUSION

For the patients presented here, a benefit of online DAPT was shown, even if the daily optimisation is based on propagated structures with some residual uncertainties. However, a careful (offline) structure review is necessary and corrections can be included in an offline adaption.

Geometric Reproducibility of Fiducial Markers and Efficacy of a Patient-Specific Margin Design Using Deep Inspiration Breath Hold for Stereotactic Body Radiation Therapy for Pancreatic Cancer

Purpose

In patients undergoing stereotactic body radiation therapy (SBRT) for pancreatic adenocarcinoma, the reproducibility of tumor positioning between deep-inspiration breath holds is unclear. We characterized this variation with fiducials at simulation and treatment and investigated whether a patient-specific breath-hold (PSBH) margin would help account for intrafraction variation at treatment.

Methods and Materials

We analyzed 20 consecutive patients with pancreatic cancer who underwent SBRT with deep-inspiration breath holds. At simulation, 3 additional breath-hold scans were acquired immediately after the contrast-enhanced planning computed tomography (CT) scan and used to quantify the mean and maximum variations in the simulation fiducial position (Sim_Var_avg and Sim_Var_max), as well as to design the internal target volume (ITV) incorporating a PSBH margin.

Results

At treatment, a mean of 5 breath-hold cone beam CT (CBCT) scans were acquired per fraction for each patient to quantify the mean and maximum variations in the treatment fiducial position (Tx_Var_avg and Tx_Var_max). Various planning target volume (PTV) margins on the gross tumor volume (GTV) versus ITV were evaluated using CBCT scans, with the goal of >95% of fiducials being covered at treatment. The Sim_Var_avg and Sim_Var_max were 0.9 ± 0.5 mm and 1.5 ± 0.8 mm in the left-right (LR) direction, 0.9 ± 0.4 mm and 1.4 ± 0.4 mm in the anteroposterior (AP) direction, and 1.5 ± 0.9 mm and 2.1 ± 1.0 mm in the superoinferior (SI) direction, respectively. The Tx_Var_avg and Tx_Var_max were 1.2 ± 0.4 mm and 2.0 ± 0.7 mm in the LR direction, 1.1 ± 0.4 mm and 1.8 ± 0.6 mm in the AP direction, and 1.9 ± 1.0 mm and 3.1 ± 1.4 mm in the SI direction, respectively. The ITV was increased by 21.0% ± 8.6% compared with the GTV alone. The PTV margin necessary to encompass >95% of the fiducial locations was 2 mm versus 4 mm in both LR and AP and 4 mm versus 6 mm in SI for the ITV and the GTV, respectively.

Conclusions

The interbreath-hold variation is not insignificant, especially in the SI direction. Acquiring multiple breath-hold CT scans at simulation can help quantify the reproducibility of the interbreath hold and design a PSBH margin for treatment.

Comparison of virtual non-contrast dual-energy CT and a true non-contrast CT for contouring in radiotherapy of 3D printed lung tumour models in motion: a phantom study

OBJECTIVES

This work aims to investigate whether virtual non-contrast (VNC) dual-energy CT(DECT) of contrasted lung tumours can be used as an alternative for true non-contrast (TNC) images in radiotherapy. Two DECT techniques and a TNC CT were compared and influences on gross tumour volume (GTV) volume and CT number from motion artefacts in three-dimensional printed lung tumour models (LTM) in amotion phantom were examined.

METHODS

Two spherical LTMs (diameter 3.0 cm) with different inner shapes were created in a three-dimensional printer. The inner shapes contained water or iodine (concentration 5 mg ml^-1) and were scanned with a dual-source DECT (ds-DECT), single-source sequential DECT (ss-DECT) and TNC CT in a respiratory motion phantom (15 breaths/min, amplitude 1.5 cm). CT number and volume of LTMs were measured. Therefore, two GTVs were contoured.

RESULTS

Deviations in GTV volume (outer shape) of LTMs in motion for contrast-enhanced ss-DECT and ds-DECT VNC images compared to TNC images are not significant (p > 0.05). Relative GTV volume and CT number deviations (inner shapes) of LTMs in motion were 6.6 ± 0.6% and 104.4 ± 71.2 HU between ss-DECT and TNC CT and -8.4 ± 10.6% and 25.5 ± 58.5 HU between ds-DECT and TNC, respectively.

CONCLUSION

ss-DECT VNC images could not sufficiently subtract iodine from water in LTMs inmotion, whereas ds-DECT VNC images might be a valid alternative to a TNC CT.

ADVANCES IN KNOWLEDGE

ds-DECT provides a contrasted image for contouring and a non-contrasted image for radiotherapy treatment planning for LTM in motion.

Technological quality requirements for stereotactic radiotherapy : Expert review group consensus from the DGMP Working Group for Physics and Technology in Stereotactic Radiotherapy

This review details and discusses the technological quality requirements to ensure the desired quality for stereotactic radiotherapy using photon external beam radiotherapy as defined by the DEGRO Working Group Radiosurgery and Stereotactic Radiotherapy and the DGMP Working Group for Physics and Technology in Stereotactic Radiotherapy. The covered aspects of this review are 1) imaging for target volume definition, 2) patient positioning and target volume localization, 3) motion management, 4) collimation of the irradiation and beam directions, 5) dose calculation, 6) treatment unit accuracy, and 7) dedicated quality assurance measures. For each part, an expert review for current state-of-the-art techniques and their particular technological quality requirement to reach the necessary accuracy for stereotactic radiotherapy divided into intracranial stereotactic radiosurgery in one single fraction (SRS), intracranial fractionated stereotactic radiotherapy (FSRT), and extracranial stereotactic body radiotherapy (SBRT) is presented. All recommendations and suggestions for all mentioned aspects of stereotactic radiotherapy are formulated and related uncertainties and potential sources of error discussed. Additionally, further research and development needs in terms of insufficient data and unsolved problems for stereotactic radiotherapy are identified, which will serve as a basis for the future assignments of the DGMP Working Group for Physics and Technology in Stereotactic Radiotherapy. The review was group peer-reviewed, and consensus was obtained through multiple working group meetings.

Evaluation of interbreath-hold lung tumor position reproducibility with vector volume histogram using the breath-hold technique

Tumor geometric reproducibility for lung stereotactic body radiotherapy (SBRT) is an important issue in the breath-hold (BH) technique. We investigated the inter-BH reproducibility of the tumor position in expiratory BH using our proposed vector volume histogram (VVH) method. Subjects comprising 14 patients with lung cancer who were treated with lung SBRT under expiratory BH conditions were monitored by the Abches system. Multiple computed tomography (CT) scans were performed to evaluate the inter-BH reproducibility of the tumor position at the expiratory BH in the simulation session. Gross tumor volume was delineated by a physician. Deformable image registration was used to deform the images from the 3 expiratory BH-CTs to the treatment planning expiratory BH-CT. To evaluate the inter-BH reproducibility of the tumor positions, we measured the largest motion extent within the organ of 3 dimensions (left-right, LR; anterior-posterior, AP; cranio-caudal, CC) and a 3D vector using the VVH method. The average and standard deviations of the inter-BH reproducibility of the tumor position in the LR, AP, and CC directions, and the 3D vector were 1.7 ± 0.5, 2.0 ± 0.7, 2.1 ± 0.7, and 2.7 ± 0.7 mm, respectively. Ten patients exhibited inter-BH displacements of the lung tumor >3 mm in the 3D vector. No displacement >5 mm was observed in any direction for all patients. Our study indicated that the inter-BH variation of the tumor position was small for lung cancer patients, using the Abches system and the VVH method.

Current status of proton therapy techniques for lung cancer

Proton beams have been used for cancer treatment for more than 28 years, and several technological advancements have been made to achieve improved clinical outcomes by delivering more accurate and conformal doses to the target cancer cells while minimizing the dose to normal tissues. The state-of-the-art intensity modulated proton therapy is now prevailing as a major treatment technique in proton facilities worldwide, but still faces many challenges in being applied to the lung. Thus, in this article, the current status of proton therapy technique is reviewed and issues regarding the relevant uncertainty in proton therapy in the lung are summarized.

Deep inspiration breath hold in locally advanced lung cancer radiotherapy: validation of intrafractional geometric uncertainties in the INHALE trial

OBJECTIVES

Patients with locally advanced non-small cell lung cancer (NSCLC) were included in a prospective trial for radiotherapy in deep inspiration breath hold (DIBH). We evaluated DIBH compliance and target position reproducibility.

METHODS

Voluntary, visually guided DIBHs were performed with optical tracking. Patients underwent three consecutive DIBH CT scans for radiotherapy planning. We evaluated the intrafractional uncertainties in the position of the peripheral tumour, lymph nodes and differential motion between them, enabling PTV margins calculation. Patients who underwent all DIBH imaging and had tumour position reproducibility <8 mm were up-front DIBH compliant. Patients who performed DIBHs throughout the treatment course were overall DIBH compliant. Clinical parameters and DIBH-related uncertainties were validated against our earlier pilot study.

RESULTS

69 of 88 included patients received definitive radiotherapy. 60/69 patients (87%) were up-front DIBH compliant. DIBH plan was not superior in seven patients and three lost DIBH ability during the treatment, leaving 50/69 patients (72%) overall DIBH compliant.The systematic and random errors between consecutive DIBHs were small but differed from the pilot study findings. This led to slightly different PTV margins between the two studies.

CONCLUSIONS

DIBH compliance and reproducibility was high. Still, this validation study highlighted the necessity of designing PTV margins in larger, representative patient cohorts.

ADVANCES IN KNOWLEDGE

We demonstrated high DIBH compliance in locally advanced NSCLC patients. DIBH does not eliminate but mitigates the target position uncertainty, which needs to be accounted for in treatment margins. Margin design should be based on data from larger representative patient groups.

Advanced dose calculation algorithms in lung cancer radiotherapy: Implications for SBRT and locally advanced disease in deep inspiration breath hold

PURPOSE

Evaluating performance of modern dose calculation algorithms in SBRT and locally advanced lung cancer radiotherapy in free breathing (FB) and deep inspiration breath hold (DIBH).

METHODS

For 17 patients with early stage and 17 with locally advanced lung cancer, a plan in FB and in DIBH were generated with Anisotropic Analytical Algorithm (AAA). Plans for early stage were 3D-conformal SBRT, 45 Gy in 3 fractions, prescribed to 95% isodose covering 95% of PTV and aiming for 140% dose centrally in the tumour. Locally advanced plans were volumetric modulated arc therapy, 66 Gy in 33 fractions, prescribed to mean PTV dose. Calculation grid size was 1 mm for SBRT and 2.5 mm for locally advanced plans. All plans were recalculated with AcurosXB with same MU as in AAA, for comparison on target coverage and dose to risk organs.

RESULTS

Lung volume increased in DIBH, resulting in decreased lung density (6% for early and 13% for locally-advanced group). In SBRT, AAA overestimated mean and near-minimum PTV dose (p-values < 0.01) compared to AcurosXB, with largest impact in DIBH (differences of up to 11 Gy). These clinically relevant differences may be a combination of small targets and large dose gradients within the PTV. In locally advanced group, AAA overestimated mean GTV, CTV and PTV doses by median less than 0.8 Gy and near-minimum doses by median 0.4-2.7 Gy. No clinically meaningful difference was observed for lung and heart dose metrics between the algorithms, for both FB and DIBH.

CONCLUSIONS

AAA overestimated target coverage compared to AcurosXB, especially in DIBH for SBRT.

Interfractional diaphragm changes during breath-holding in stereotactic body radiotherapy for liver cancer

Aim and background

IGRT based on bone matching may produce a large target positioning error in terms of the reproducibility of expiration breath-holding on SBRT for liver cancer. We evaluated the intrafractional and interfractional errors using the diaphragm position at the end of expiration by utilising Abches and analysed the factor of the interfractional error.

Materials and methods

Intrafractional and interfractional errors were measured using a couple of frontal kV images, planning computed tomography (pCT) and daily cone-beam computed tomography (CBCT). Moreover, max-min diaphragm position within daily CBCT image sets with respect to pCT and the maximum value of diaphragm position difference between CBCT and pCT were calculated.

Results

The mean ± SD (standard deviation) of the intra-fraction diaphragm position variation in the frontal kV images was 1.0 ± 0.7 mm in the C-C direction. The inter-fractional diaphragm changes were 0.4 ± 4.6 mm in the C-C direction, 1.4 ± 2.2 mm in the A-P direction, and -0.6 ± 1.8 mm in the L-R direction. There were no significant differences between the maximum value of the max-min diaphragm position within daily CBCT image sets with respect to pCT and the maximum value of diaphragm position difference between CBCT and pCT.

Conclusions

Residual intrafractional variability of diaphragm position is minimal, but large interfractional diaphragm changes were observed. There was a small effect in the patient condition difference between pCT and CBCT. The impact of the difference in daily breath-holds on the interfractional diaphragm position was large or the difference in daily breath-holding heavily influenced the interfractional diaphragm change.

Robust optimization of VMAT for lung cancer: Dosimetric implications of motion compensation techniques

In inverse planning of lung radiotherapy, techniques are required to ensure dose coverage of target disease in the presence of tumor motion as a result of respiration. A range of published techniques for mitigating motion effects were compared for dose stability across 5 breath cycles of ±2 cm. Techniques included planning target volume (PTV) expansions, internal target volumes with (OITV) and without tissue override (ITV), average dataset scans (ADS), and mini-max robust optimization. Volumetric arc therapy plans were created on a thorax phantom and verified with chamber and film measurements. Dose stability was compared by DVH analysis in calculations across all geometries. The lung override technique resulted in a substantial lack of dose coverage (-10%) to the tumor in the presence of large motion. PTV, ITV and ADS techniques resulted in substantial (up to 25%) maximum dose increases where solid tissue travelled into low density optimized regions. The results highlight the need for care in optimization of highly heterogeneous where density variations may occur with motion. Robust optimization was shown to provide greater stability in both maximum (<3%) and minimum dose variations (<2%) over all other techniques.

Impact of beam angle choice on pencil beam scanning breath-hold proton therapy for lung lesions

INTRODUCTION

The breath-hold technique inter alia has been suggested to mitigate the detrimental effect of motion on pencil beam scanned (PBS) proton therapy dose distributions. The aim of this study was to evaluate the robustness of incident proton beam angles to day-to-day anatomical variations in breath-hold.

MATERIALS AND METHODS

Single field PBS plans at five degrees increments in the transversal plane were made and water-equivalent path lengths (WEPLs) were derived on the planning breath-hold CT (BHCT) for 30 patients diagnosed with locally-advanced non-small cell lung cancer (NSCLC), early stage NSCLC or lung metastasis. Our treatment planning system was subsequently used to recalculate the plans and derive WEPL on a BHCT scan acquired at the end of the treatment. Changes to the V_95%, D₉₅ and mean target dose were evaluated.

RESULTS

The difference in WEPL as a function of the beam angle was highly patient specific, with a median of 3.3 mm (range: 0.0-41.1 mm). Slightly larger WEPL differences were located around the lateral or lateral anterior/posterior beam angles. Linear models revealed that changes in dose were associated to the changes in WEPL and the tumor baseline shift (p < 0.05).

CONCLUSIONS

WEPL changes and tumor baseline shift can serve as reasonable surrogates for dosimetric uncertainty of the target coverage and are well-suited for routine evaluation of plan robustness. The two lateral beam angles are not recommended to use for PBS proton therapy of lung cancer patients treated in breath-hold, due to the poor robustness for several of the patients evaluated.

Target position uncertainty during visually guided deep-inspiration breath-hold radiotherapy in locally advanced lung cancer

PURPOSE

The purpose of this study was to estimate the uncertainty in voluntary deep-inspiration breath-hold (DIBH) radiotherapy for locally advanced non-small cell lung cancer (NSCLC) patients.

METHODS

Perpendicular fluoroscopic movies were acquired in free breathing (FB) and DIBH during a course of visually guided DIBH radiotherapy of nine patients with NSCLC. Patients had liquid markers injected in mediastinal lymph nodes and primary tumours. Excursion, systematic- and random errors, and inter-breath-hold position uncertainty were investigated using an image based tracking algorithm.

RESULTS

A mean reduction of 2-6mm in marker excursion in DIBH versus FB was seen in the anterior-posterior (AP), left-right (LR) and cranio-caudal (CC) directions. Lymph node motion during DIBH originated from cardiac motion. The systematic- (standard deviation (SD) of all the mean marker positions) and random errors (root-mean-square of the intra-BH SD) during DIBH were 0.5 and 0.3mm (AP), 0.5 and 0.3mm (LR), 0.8 and 0.4mm (CC), respectively. The mean inter-breath-hold shifts were -0.3mm (AP), -0.2mm (LR), and -0.2mm (CC).

CONCLUSION

Intra- and inter-breath-hold uncertainty of tumours and lymph nodes were small in visually guided breath-hold radiotherapy of NSCLC. Target motion could be substantially reduced, but not eliminated, using visually guided DIBH.

Robustness of 4D-optimized scanned carbon ion beam therapy against interfractional changes in lung cancer

BACKGROUND AND PURPOSE

Moving targets could be conformally treated with actively scanned carbon ion beams using 4D-optimization. As this heavily exploits 4D-CTs, an important question is whether the conformity also upholds in the context of interfractional changes, i.e. variable positioning, anatomy and breathing patterns.

MATERIALS AND METHODS

In 4 lung cancer patients, 6 weekly 4D-CTs were available. 4D-CTs and their phases were non-rigidly registered to propagate contours and 4D-doses. On the first 4D-CT, a 4D-optimized plan delivering a uniform dose to each motion phase (total dose 9.4Gy(RBE)) was simulated, as well as an ITV plan for comparison. On the five following 4D-CTs, 4D-dose was forward calculated and evaluated for target coverage and conformity. Variable uniform (3-7mm) and range margins (2mm/%) were investigated.

RESULTS

For all patients, target coverage (V95>95% accumulated over 5 fractions) could be achieved, but with variable margin size weakly depending on motion amplitude and range changes. The same margins were also necessary for ITV plans, which lead to lower conformity and higher integral doses.

CONCLUSION

4D-optimization appears feasible also under interfractional changes and maintains a dosimetric advantage over less conformal ITV irradiations. Further studies are needed to identify patients benefiting most from the technically more complex 4D-optimization.

Deep inspiration breath-hold radiotherapy for lung cancer: impact on image quality and registration uncertainty in cone beam CT image guidance

OBJECTIVE

We investigated the impact of deep inspiration breath-hold (DIBH) and tumour baseline shifts on image quality and registration uncertainty in image-guided DIBH radiotherapy (RT) for locally advanced lung cancer.

METHODS

Patients treated with daily cone beam CT (CBCT)-guided free-breathing (FB) RT had an additional CBCT in DIBH at three fractions. These CBCT scans were offline rigidly registered (on tumour) to FB and DIBH CT scans acquired at planning. All registrations were repeated to evaluate the intraobserver uncertainty. CBCT scans were scored on degree of streak artefacts and visualization of tumour and anatomical structures. We examined the impact of tumour baseline shift between consecutive DIBHs on CBCT image quality.

RESULTS

CBCT scans from 15 patients were analysed. Intraobserver image registration uncertainty was approximately 2 mm in both FB and DIBH, except for the craniocaudal direction in FB, where it was >3 mm. On the 31st fraction, the intraobserver uncertainty increased compared with the second fraction. This increase was more pronounced in FB. Image quality scores improved in DIBH compared with FB for all parameters in all patients. Simulated tumour baseline shifts ≤2 mm did not affect the CBCT image quality considerably.

CONCLUSION

DIBH CBCT improved image quality and reduced registration uncertainty in the craniocaudal direction in image-guided RT of locally advanced lung cancer. Baseline shifts ≤2 mm in DIBH during CBCT acquisition did not affect image quality. Advances in knowledge: DIBH RT has dosimetric advantages over FB; this work demonstrates an additional benefit of DIBH in terms of registration accuracy because of improved image quality.