Free Breathing Plans Research Articles

TH-A-137-08: On the Validity of Target Density Overrides for RapidArc Lung SBRT Treatment Planning

Purpose: It is common practice to calculate lung SBRT plans on free breathing CT scans. Recently, literature has suggested using time average scans or ITV density overridden scans to more accurately calculate dose in lung. We evaluate each of these techniques. Methods: A free breathing (FB) and 4D-CT were acquired for a Quasar Motion Phantom with lung insert and 3 cm spherical target (Modus Medical Devices Inc, Canada). Target motion was set to ± 1cm parallel to CT table travel at 16 cycles per minute. An ITV was defined on the 4D-CT with a PTV defined as a 5 mm expansion. RapidArc plans giving 1000 cGy to 96–97% of the PTV in two arcs were created on four image sets: the time average, FB, FB with the ITV set to tissue density, and FB with the PTV set to tissue density. Plans were delivered on a Varian TrueBeam STx (Varian Medical Systems, Palo Alto, CA) and measured on EBT3 film placed in the phantom target. Reference dose distributions that accounted for film motion were created for each plan by shifting the dose in 2 mm steps over the range of motion and convolving these shifted doses. The film was analyzed in with a gamma metric of 1mm/1%. Results: The passing gamma percentages were 77.9%, 83.6%, 89.1%, and 92.7% for the FB, time average, ITV, and PTV plans, respectively. Profiles along the direction of travel showed hot spots of 5-10% above the predicted dose just outside the target for the FB and average plans. The ITV plan showed similar hot spots ∼2% above predicted dose. The PTV profile was in good agreement with predicted dose. Conclusion: Though this study is a greatly simplified case, it shows that planning to a tissue density target in lung may produce the most accurate dose predictions.

MO‐A‐137‐07: Assessment of Metrics to Identify Which Left‐Sided Breast Cancer Patients Will Benefit From a Moderate Deep Inspiration Breath‐Hold Technique

Purpose: To derive and examine the robustness of metrics for prediction of dosimetric benefits for left‐sided breast cancer patients from a moderate deep inspiration breath‐hold (mDIBH) technique. Methods: Radiotherapy treatment plans using a field‐in‐field tangent approach were retrospectively generated on both free‐breathing (FB) and mDIBH CT scans acquired for fourteen consecutive left‐sided breast cancer patients. Breast PTV‐eval, heart and left anterior descending coronary artery (LAD) were defined as per RTOG and the heart atlas (Feng et al. IJROBP 79:10‐18, 2011). The same PTV coverage was maintained in both FB and mDIBH plans. Volumetric metrics, irradiated heart and lung volume, and distance metrics from representative images were defined. Correlations with dosimetric parameters and robustness were examined. Results for single planner retrospective plans were compared with results from treated plans prepared by a group of dosimetrists for both the cohort of 14 mDIBH patients as well as a cohort of 19 previously treated FB left‐sided breast cancer patients. Results: FB cardiac dosimetric parameters for our cohort appeared to follow exponential distributions, with a significant reduction of sigma shown for mDIBH. The extent of dosimetric benefit is patient specific. Linear correlations were found between cardiac and pulmonary dosimetric parameters and volumetric metrics using irradiated volumes, as well as for distance metrics if volumetric corrections were applied. Linear correlations were preserved (R > 0.9) regardless of whether determined from a single planner or a group of dosimetrists. Fitted parameters were slightly higher for data derived from the group of planners, however, the difference was not found to be significant via signed rank test. Conclusion: Linear correlations between cardiac and pulmonary dosimetric parameters and several metrics were identified and determined to be robust for left‐sided breast cancer patients planned at our institution, which facilitates reliable selection of patients who will benefit from a mDIBH technique.

The deep inspiration breath hold technique using Abches reduces cardiac dose in patients undergoing left-sided breast irradiation

PurposeWe explored whether the deep inspiration breath hold (DIBH) technique using Abches during left-sided breast irradiation was effective for minimizing the amount of radiation to the heart and lung compared to free breathing (FB).Materials and MethodsBetween February and July 2012, a total of 25 patients with left-sided breast cancer underwent two computed tomography scans each with the DIBH using Abches and using FB after breast-conserving surgery. The scans were retrospectively replanned using standardized criteria for the purpose of this study. The DIBH plans for each patient were compared with FB plans using dosimetric parameters.ResultsAll patients were successfully treated with the DIBH technique using Abches. Significant differences were found between the DIBH and FB plans for mean heart dose (2.52 vs. 4.53 Gy), heart V30 (16.48 vs. 45.13 cm3), V20 (21.35 vs. 54.55 cm3), mean left anterior descending coronary artery (LAD) dose (16.01 vs. 26.26 Gy, all p < 0.001), and maximal dose to 0.2 cm3 of the LAD (41.65 vs. 47.27 Gy, p = 0.017). The mean left lung dose (7.53 vs. 8.03 Gy, p = 0.073) and lung V20 (14.63% vs. 15.72%, p = 0.060) of DIBH using Abches were not different significantly compared with FB.ConclusionWe report that the use of a DIBH technique using Abches in breathing adapted radiotherapy for left-sided breast cancer is easily feasible in daily practice and significantly reduces the radiation doses to the heart and LAD, therefore potentially reducing cardiac risk.

Improved heart, lung and target dose with deep inspiration breath hold in a large clinical series of breast cancer patients

Background and purposeThis study aims at evaluating the effect of deep-inspiration breath hold (DIBH) on target coverage and dose to organs at risk in a large series of breast cancer patients. Materials and methodsClinical dose plans for 319 breast cancer patients were evaluated: 144 left-sided patients treated with DIBH and 175 free-breathing (FB) patients (83 left-sided and 92 right-sided). All patients received whole breast irradiation with tangential fields, based on a forward-planned intensity-modulated radiation therapy (IMRT) technique. Dose to heart, ipsi-lateral lung and ipsi-lateral breast were assessed and median values compared between patient groups. ResultsComparing group median values, DIBH plans show large reductions of dose to the heart compared with left-sided FB plans; V20Gy (relative volume receiving ⩾20Gy) for the heart is reduced from 7.8% to 2.3% (−70%, p<0.0001), V40Gy from 3.4% to 0.3% (−91%, p<0.0001) and mean dose from 5.2 to 2.7Gy (−48%, p<0.0001). Lung dose also shows a small reduction in V20Gy (p<0.04), while median target coverage is slightly improved (p=0.0002). ConclusionsIn a large series of clinical patients we find that implementation of DIBH in daily clinical practice results in reduced irradiation of heart and lung, without compromising target coverage.

Poster - Thur Eve - 67: Clinical results of deep inspiration breath hold radiation treatment for the left breast patients.

Adjuvant radiotherapy for left breast cancers increases local tumor control, but also increases the risk of radiation-induced cardiac disease. Deep Inspiration Breath Hold (DIBH) can minimize dose to the heart for left breast patients where the heart is within the tangential field. In this study, we evaluated the dosimetric benefit of DIBH technique comparing to free breathing (FB) radiotherapy for left breast cancer patients. Five patients with left breast cancer treated with DIBH technique were selected randomly. The CT scans of breath hold (BH) and FB were taken for every DIBH patient. Standard clinical DIBH intensity-modulated radiotherapy (IMRT) plans were generated with BH scan dataset using the Varian Eclipse TP system. The prescription dose is 4250 cGy in 16 fractions. The BH plan was copied to the FB scan dataset and shifted accordingly to have the same coverage for the breast tissue, and the dose was re-calculated. Dose-volume histograms (DVH) of the heart and lung; mean dose and maximum dose of the heart were calculated and compared from the BH and FB plans for every patient. The lung volume is increased during BH and hence the heart is moved out of the field, resulting in the lower heart maximum dose. The mean dose is almost less than 1 Gy for all BH plans. The average mean heart dose is 0.8 Gy for BH plan compared to 1.6 Gy for FB plan. Patients benefit significantly from DIBH technique due to the very low heart dose.

Poster - Thur Eve - 26: Interfraction reproducibility of heart position during breast irradiation using Active Breathing Control.

The moderate deep-inspiration breath hold (mDIBH) technique using the Active Breathing Coordinator (ABC) from Elekta is used in our clinic to lower the heart dose during left breast irradiations. The purpose of this work was to investigate the interfraction reproducibility of the heart to chest distance during these treatments and to evaluate the dosimetric effect of any changes in the heart position. Daily CBCT images were available for 5 patients who had been treated with ABC tangents and a cavity boost. On these images, one-dimensional measurements of the distance between the heart and the chest wall were taken at two anatomical locations corresponding roughly with the location where the radiation field most likely intercepts the heart. The average change in this distance was interpreted as a shift of the heart position. To assess the effect of this shift on the delivered heart dose, the heart contours in the clinical plans of the corresponding patients were shifted towards the treatment field using standard Pinnacle tools. Although the ABC device allows good reproducibility of the volume of air held, this does not warrant reproducibility of heart position for all patients during treatment. The largest average heart shift extracted from CBCT images in this study was 6.2mm. The heart dose reconstructed using this shift for the corresponding patient also showed the largest effect. However, even in the presence of a systematic heart shift of this magnitude, the ABC plan still showed superior heart dose reduction compared to the free-breathing plan.

SU-E-T-605: RapidArc Combined with DIBH Technique for Thoracic Esophageal Carcinoma: The Potential Value of Target Immobilization and Reduced Lung Density in Dose Escalation

To compare the dosimetric benefits of Rapidarc (RA) combined with deep inspiration breath-hold (DIBH) with those of other standard techniques, including free breathing (FB) during fixed-field intensity modulated radiation therapy (IMRT) and dual arc RA, in the treatment of patients with thoracic esophageal carcinoma (EC). Ten patients with EC underwent computed tomography (CT) scans under 2 respiration conditions: free-breathing (FB) and DIBH. These scans were used to generate 3-dimensional conformal treatment plans. For breath-hold scans, the patients were brought to reproducible respiration levels using active breathing control (ABC) maneuvers. Planning target volumes (PTVs) for FB plans included a 0.5 cm margin for setup plus a 1 cm margin equal to the extent of tumor motion for respiration. PTVs for DIBH plans included a 0.5 cm margin for setup error and a 0.5 cm margin for residual uncertainty in tumor position. Using a dose level of 60 Gy to the PTV, three treatment plans were generated: IMRT-FB, RA-FB and RA-ABC, and the target and normal tissue volumes were compared, as were the dosimetry parameters. On average, the DIBH technique resulted in increased lung volumes compared with FB techniques. There was no significant differences in gross tumor volume between the two breathing states (p > 0.05); but PTV and heart volume were larger for FB than for DIBH (p < 0.05). The overall CI and HI for the RA-ABC plan was slightly inferior to those of the IMRT- FB and RA-FB plans (p < 0.05 each). With DIBH, the heart was partly out of the beam portals and the average mean heart dose was reduced. Compared with conventional FB, RA combined with DIBH significantly reduced cardiac and pulmonary doses without compromising the target coverage and may reduce treatment toxicity, enabling dose escalation in future prospective studies of patients with EC.

Dosimetric comparison of treatment plans based on free breathing, maximum, and average intensity projection CTs for lung cancer SBRT

To determine whether there is a CT dataset may be more favorable for planning and dose calculation by comparing dosimetric characteristics between treatment plans calculated using free breathing (FB), maximum and average intensity projection (MIP and AIP, respectively) CTs for lung cancer patients receiving stereotactic body radiation therapy (SBRT). Twenty lung cancer SBRT patients, treated on a linac with 2.5 mm width multileaf-collimator (MLC), were analyzed retrospectively. Both FB helical and four-dimensional CT scans were acquired for each patient. Internal target volume (ITV) was delineated based on MIP CTs and modified based on both ten-phase datasets and FB CTs. Planning target volume (PTV) was then determined by adding additional setup margin to ITV. The PTVs and beams in the optimized treatment plan based on FB CTs were copied to MIP and AIP CTs, with the same isocenters, MLC patterns and monitor units. Mean effective depth (MED) of beams, and some dosimetric parameters for both PTVs and most important organ at risk (OAR), lung minus PTV, were compared between any two datasets using two-tail paired t test. The MEDs in FB and AIP plans were similar but significantly smaller (Ps < 0.001) than that in MIP plans. Minimum dose, mean dose, dose covering at least 90% and 95% of PTVs in MIP plans were slightly higher than two other plans (Ps < 0.008). The absolute volume of lung minus PTV receiving greater than 5, 10, and 20 Gy in MIP plans were significantly smaller than those in both FB and AIP plans (Ps < 0.008). Conformity index for FB plans showed a small but statistically significantly higher. Dosimetric characteristics of AIP plans are similar to those of FB plans. Slightly better target volume coverage and significantly lower low-dose region (≤30 Gy) in lung was observed in MIP plans. The decrease in low-dose region in lung was mainly caused by the change of lung volume contoured on two datasets rather than the differences of dose distribution between AIP and MIP plans. Compare with AIP datasets, FB datasets were more prone to significant image artifacts and MIP datasets may overestimate or underestimate the target volume when the target is closer to the denser tissue, so AIP seems favorable for planning and dose calculation for lung SBRT.

Radiation during deep inspiration allows loco-regional treatment of left breast and axillary-, supraclavicular- and internal mammary lymph nodes without compromising target coverage or dose restrictions to organs at risk

Background and purpose. Loco-regional radiotherapy of left-sided breast cancer represents a treatment planning challenge when the internal mammary chain (IMC) lymph nodes are included in the target volume. This treatment planning study evaluates the reduction in cardiopulmonary doses when radiation is given during deep inspiration breath-hold (DIBH). This was achieved without compromising dose coverage to the planning target volume (PTV). Patients and methods. Seventeen patients with early breast cancer, referred for adjuvant radiotherapy, were included. For each patient two computed tomography (CT)-scans were acquired; the first during free breathing (FB) and the second during DIBH. The scans were monitored by the Varian RPMTM respiratory gating system. Audio-visual guidance was used. The treatment planning of the two CT studies was performed focusing on good coverage (V95% > 98%) of the PTV. Doses to the heart, left anterior descending (LAD) coronary artery, lungs and contralateral breast were assessed. Results. With equal PTV coverage, average mean heart dose was reduced from 6.2 Gy to 3.1 Gy in DIBH plans as compared to FB. Average volume receiving 25 Gy or more (V25Gy) was reduced from 6.7% to 1.2%, and the number of patients with V25Gy > 5% was reduced from 8 to 1 utilizing DIBH. The average mean dose to the LAD coronary artery was reduced from 25.0 Gy to 10.9 Gy. The average ipsilateral lung volume receiving 20 Gy or more (V20Gy) was reduced from 44.5% to 32.7% with DIBH. In 11 of the DIBH plans V20Gy was lower than 35%, in accordance with national guidelines, while none of the FB plans fulfilled this recommendation. Conclusion. Respiratory gated radiotherapy during DIBH is a suitable technique for loco-regional breast irradiation even when IMC lymph nodes are included in the PTV. Cardiopulmonary doses are considerably decreased for all dose levels without compromising the dose coverage to PTV.

In vivo MR acoustic radiation force imaging in the porcine liver

High intensity focused ultrasound (HIFU) in the abdomen can be sensitive to acoustic aberrations that can exist in the beam path of a single sonication. Having an accurate method to quickly visualize the transducer focus without damaging tissue could assist with executing the treatment plan accurately and predicting these changes and obstacles. By identifying these obstacles, MR acoustic radiation force imaging (MR-ARFI) provides a reliable method for visualizing the transducer focus quickly without damaging tissue and allows accurate execution of the treatment plan. MR-ARFI was used to view the HIFU focus, using a gated spin echo flyback readout-segmented echo-planar imaging sequence. HIFU spots in a phantom and in the livers of five live pigs under general anesthesia were created with a 550 kHz extracorporeal phased array transducer initially localized with a phase-dithered MR-tracking sequence to locate microcoils embedded in the transducer. MR-ARFI spots were visualized, observing the change of focal displacement and ease of steering. Finally, MR-ARFI was implemented as the principle liver HIFU calibration system, and MR-ARFI measurements of the focal location relative to the thermal ablation location in breath-hold and breathing experiments were performed. Measuring focal displacement with MR-ARFI was achieved in the phantom and in vivo liver. In one in vivo experiment, where MR-ARFI images were acquired repeatedly at the same location with different powers, the displacement had a linear relationship with power [y = 0.04x + 0.83 μm (R(2) = 0.96)]. In another experiment, the displacement images depicted the electronic steering of the focus inside the liver. With the new calibration system, the target focal location before thermal ablation was successfully verified. The entire calibration protocol delivered 20.2 J of energy to the animal (compared to greater than 800 J for a test thermal ablation). ARFI displacement maps were compared with thermal ablations during seven breath-hold ablations. The error was 0.83 ± 0.38 mm in the S/I direction and 0.99 ± 0.45 mm in the L/R direction. For six spots in breathing ablations, the mean error in the nonrespiration direction was 1.02 ± 0.89 mm. MR-ARFI has the potential to improve free-breathing plan execution accuracy compared to current calibration and acoustic beam adjustment practices. Gating the acquisition allows for visualization of the focal spot over the course of respiratory motion, while also being insensitive to motion effects that can complicate a thermal test spot. That MR-ARFI measures a mechanical property at the focus also makes it insensitive to high perfusion, of particular importance to highly perfused organs such as the liver.

SU‐E‐T‐889: Target Definition in TomoTherapy Lung SBRT Treatment Plans

Purpose: An unresolved question in lung stereotactic body radiotherapy (SBRT) treatment planning is whether a free breathing computed tomography (CT) image set, an image set with the electron density of the internal target volume (ITV) set to water, or an averaged 4DCT scan provides the most realistic representation of the physical situation. Methods: TomoTherapy Hi Art treatment plans created on free breathing scans for three patients receiving lung SBRT at our institution were compared to plans created on 1) a scan with the ITV electron density set to water, 2) a scan with the planning target volume (PTV) electron density set to water, 3) a phase averaged 4DCT, and 4) a time averaged 4DCT. The comparison plans were made by creating TomoTherapy delivery quality assurance (DQA) phantoms from the additional image sets, then recalculating the free breathing plan on these phantoms. All doses were exported to MIMVista for analysis. Results: The maximum isodose line covering the entire PTV and ITV showed little change among the different plans for each patient. The maximum point dose to the ITV and the PTV was similar for the free breathing and average scans. The maximum point doses showed a slight increase for both the ITV and PTV override plans for all patients. The PTV override plan pushed the maximum point dose out of the ITV into the PTV. Conclusions: Plans calculated on the scans with density overridden target volumes show a marked change in dose distribution and target coverage from plans calculated on free breathing and average scans. The different treatment planning techniques are clearly not equivalent. The use of a poor model could lead to over or under‐dosing of the target. Further study is needed to determine which plan most accurately models the true physical situation.

Dose evaluation and risk estimation for secondary cancer in contralateral breast and a study of correlation between thorax shape and dose to organs at risk following tangentially breast irradiation during deep inspiration breath-hold and free breathing

Purpose: To assess the impact of using breathing adapted radiotherapy on contralateral breast (CB) dose, to relate the thorax shape with the dose to the organs at risk (OARs) and to predict the risk for induced malignancies in CB using linear and non-linear models, following tangential irradiation of breast. Material and methods. Sixteen patients with stage I-II breast cancer treatment planned with tangential fields using deep inspiration breath hold (DIBH) and free breathing (FB) techniques were included in this analysis. The dose results mainly based on DVH analysis were compared. Four parameters were defined to describe thoracic shape. Excess relative risk (ERR) for cancer induction in CB, employing linear and non-linear models was calculated. Results. Average CB volumes exposed to a dose of 1 Gy is 1.3 times higher in DIBH plans than in FB plans. No significant difference in average V3Gy and V5Gy for DIBH and FB plans is observed. The average mean CB dose for DIBH and FB plans is 0.33 and 0.28 Gy, respectively. No correlation between thorax shape parameters and mean OARs dose is observed. The estimated average mean ERR with linear model is lower in FB plans (0.12) than for the DIBH plans (0.14). The estimated ERR with non-linear model is 0.14 for DIBH plans and 0.15 for FB plans. Conclusion. No significant difference in CB dose between DIBH and FB plans is observed. The four thorax shape parameters defined in this study can not be related to the dose at OARs using DIBH and FB radiation techniques. The ERR estimates for secondary CB cancer are nearly the same for FB and DIBH planning when using a linear and non-linear risk prediction models.

Analysis of cardiac and pulmonary complication probabilities after radiation therapy for patients with early-stage breast cancer

The purpose of this study was to evaluate the radiobiological implications of clinical use of respiratory-gated techniques for postoperative radiation therapy of early-stage left-sided breast cancer after breast-conserving surgery. Radiation therapy treatment plans of 80 patients with early-stage breast cancer (stage I-II), receiving whole breast irradiation after breast-conserving therapy, were analyzed. The control group consisting of 47 patients received standard radiation therapy, and the respiratory-gated group consisting of 33 patients received deep inspiration-gated radiation therapy. Normal tissue complication probabilities (NTCP) for cardiac mortality and for clinical radiation-induced pneumonitis were calculated for all patients included in present study, using relative seriality model. NTCP data were analyzed for 113 radiation therapy plans, which included free breathing plans for the respiratory-gated groups. Pneumonitis probability was 0.6% (range 0.0-2.8%) and 0.3% (0.0-1.2%) for control and respiratory-gated group, respectively. Cardiac mortality was 1.3% (0.0-5.0%) and 0.2% (0.0-2.8%) for control and respiratory-gated group, respectively. Using respiratory-gated radiation therapy, NTCP was reduced in comparison with the control group by 83% (P<0.00001) and by 55% (P=0.01270) for cardiac mortality and for clinical radiation-induced pneumonitis, respectively. Use of respiratory-gated radiation therapy, for postoperative treatment of early-stage breast cancer, significantly reduces excessive cardiac mortality probability and pulmonary complication probability, as compared to standard radiation therapy techniques. This is especially important from heart complication probability point of view, as cardiac mortality remains one of the important issues of postoperative breast irradiation in patients with early stage breast cancer.

Fast free-breathing planning in cardiac MR imaging

Fast free-breathing planning in cardiac MR imaging

Application of active breathing control in 3-dimensional conformal radiation therapy for hepatocellular carcinoma: The feasibility and benefit

Background and purpose To investigate the feasibility and effectiveness of utilizing active breathing coordinator (ABC) in 3DCRT for HCC. Materials and methods A dosimetric comparison between the free-breathing (FB) plan and ABC plan in HCC 3DCRT was performed. Set-up errors and reproducibility of diaphragm position using ABC were measured, and patients’ acceptance was also recorded. Results From April 2005 to February 2007, 28 HCC were irradiated with ABC and they tolerated ABC well. The mean dose to normal liver was reduced from 16.9 Gy in FB plan to 14.3 Gy in ABC plan. PTV for ABC and FB plans were 529 cm 3 and 781 cm 3, respectively, and V 23 were reduced from 45% to 30%. The predicted incidences of radiation-induced liver disease by Lyman model were 1% and 2.5%, respectively, in favor of ABC plan. The systematic and random errors for the ABC and FB plans were 1.2 mm vs. 4.7 mm, 1.6 mm vs. 3.5 mm, and 1.8 mm vs. 2.7 mm, respectively, in cranio-caudal, anterior-posterior, and left-right directions. The average intrafraction reproducibility of diaphragm position in cranio-caudal direction was 1.6 mm, and the interfraction, 6.7 mm. Conclusions The utilization of ABC in HCC 3DCRT is feasible, and can reduce liver irradiation.

Dosimetric Advantages of Four-Dimensional Adaptive Image-Guided Radiotherapy for Lung Tumors Using Online Cone-Beam Computed Tomography

This study compares multiple planning techniques designed to improve accuracy while allowing reduced planning target volume (PTV) margins though image-guided radiotherapy (IGRT) with four-dimensional (4D) cone-beam computed tomography (CBCT). Free-breathing planning and 4D-CBCT scans were obtained in 8 patients with lung tumors. Four plans were generated for each patient: 3D-conformal, 4D-union, 4D-offline adaptive with a single correction (offline ART), and 4D-online adaptive with daily correction (online ART). For the 4D-union plan, the union of gross tumor volumes from all phases of the 4D-CBCT was created with a 5-mm expansion applied for setup uncertainty. For offline and online ART, the gross tumor volume was delineated at the mean position of tumor motion from the 4D-CBCT. The PTV margins were calculated from the random components of tumor motion and setup uncertainty. Adaptive IGRT techniques provided better PTV coverage with less irradiated normal tissues. Compared with 3D plans, mean relative decreases in PTV volumes were 15%, 39%, and 44% using 4D-union, offline ART, and online ART planning techniques, respectively. This resulted in mean lung volume receiving > or = 20Gy (V20) relative decreases of 21%, 23%, and 31% and mean lung dose relative decreases of 16%, 26%, and 31% for the 4D-union, 4D-offline ART, and 4D-online ART, respectively. Adaptive IGRT using CBCT is feasible for the treatment of patients with lung tumors and significantly decreases PTV volume and dose to normal tissues, allowing for the possibility of dose escalation. All analyzed 4D planning strategies resulted in improvements over 3D plans, with 4D-online ART appearing optimal.

4D Proton treatment planning strategy for mobile lung tumors

To investigate strategies for designing compensator-based 3D proton treatment plans for mobile lung tumors using four-dimensional computed tomography (4DCT) images. Four-dimensional CT sets for 10 lung cancer patients were used in this study. The internal gross tumor volume (IGTV) was obtained by combining the tumor volumes at different phases of the respiratory cycle. For each patient, we evaluated four planning strategies based on the following dose calculations: (1) the average (AVE) CT; (2) the free-breathing (FB) CT; (3) the maximum intensity projection (MIP) CT; and (4) the AVE CT in which the CT voxel values inside the IGTV were replaced by a constant density (AVE_RIGTV). For each strategy, the resulting cumulative dose distribution in a respiratory cycle was determined using a deformable image registration method. There were dosimetric differences between the apparent dose distribution, calculated on a single CT dataset, and the motion-corrected 4D dose distribution, calculated by combining dose distributions delivered to each phase of the 4DCT. The AVE_RIGTV plan using a 1-cm smearing parameter had the best overall target coverage and critical structure sparing. The MIP plan approach resulted in an unnecessarily large treatment volume. The AVE and FB plans using 1-cm smearing did not provide adequate 4D target coverage in all patients. By using a larger smearing value, adequate 4D target coverage could be achieved; however, critical organ doses were increased. The AVE_RIGTV approach is an effective strategy for designing proton treatment plans for mobile lung tumors.

Dosimetric comparison of stereotactic body radiotherapy in different respiration conditions: A modeling study

Purpose To evaluate the dosimetric consequences for irradiated lung tissue for different respiration conditions for hypofractionated stereotactic body radiotherapy (SBRT). Methods and materials Thirteen patients with lung lesion undergoing SBRT treatment in shallow breathing with abdominal compression (SB + AP) underwent additional multislice CT studies in free breathing (FB), deep inspiration and expiration breath hold (DIBH, DEBH). For each patient 6 different treatment plans were designed for the various respiration conditions applying standard (7/7/10 mm), reduced (5/5/5 mm) and individual margins. The FB plan with standard margins was used as a reference. The percentage of volume of the ipsilateral lung receiving total doses ⩾12, ⩾15 and ⩾18 Gy, mean lung dose ( D mean), NTCP corrected for fractionation effects and the total monitor units (MU) were evaluated. Results With DIBH it was possible to reduce all lung dose parameters by about 20%. Applying reduced margins in DIBH, this reduction was even increased to about 40%. The standard technique (SB + AP) with individual margins showed similar results as DIBH with standard margins. DEBH showed some improvement over FB only when reduced margins were applied. Only for 5/13 patients NTCP values >1% were obtained. For these patients a significant NTCP reduction was achieved with DIBH techniques. Conclusions In SBRT shallow breathing with abdominal compression produces acceptable results concerning lung DVHs. DIBH, especially with reduced margins, showed the best lung sparing. For the clinical implementation of such a technique some form of gating is advisable. However, there are some practical limitations due to high fractional doses.

SU-FF-J-15: Evaluation of Lung Tumor Dose Coverage in Free-Breathing Treatment Plans Using Respiration-Correlated CT

Purpose: We quantify the effect of respiration on 3D conformal radiotherapy of lungtumors in terms of dosimetric end‐points using respiration‐correlated CT scans (RCCT). Method and Materials: Six patients received RCCT scans (GE Advantage4D CT). Repeat slices were obtained at each couch position and retrospectively sorted according to respiratory phase using an external respiratory monitor. We generated simulated free‐breathing (FB) CT sets by assembling slices at different respiratory phases from the RCCT study. FB treatment plans similar to the clinical plan were created based on a 1.5 cm expansion of the gross tumor volume (GTV) in one patient and 1.0 cm in two other patients. Superposition‐convolution dose calculations (ADAC/Pinnacle system) were used. We applied the FB plan to the RCCT data sets at various respiratory phases and evaluated dose coverage of the GTV and clinical target volume (CTV=GTV+6mm). Results: Initial results are presented for an extreme situation: the patient with the largest tumor motion (2.3 cm, inferior/superior direction) and the start phase of the simulated FB scan chosen to have the tumor at its most superior position. The D99% values are: 1) FB plan: GTV 5833 cGy, CTV 5706 cGy. 2) FB plan applied to end‐inspiration: GTV 5828 cGy, CTV 5433 cGy. The GTV and CTV are only modestly underdosed at end‐inspiration (tumor most inferior), in this worst case scenario (FB plan based on a tumor at a superior position). Dosimetric indices at end‐expiration show less change. Preliminary studies on two other patients with smaller tumor excursions show similarly small dosimetric changes. Conclusion:Tumor coverage by free‐breathing based plans with standard margins changes only slightly with breathing phase in an extreme case. This study examines only respiratory motion and not interfractional tumor motion or setup error which also affect tumordose coverage.

TU‐D‐J‐6C‐05: Investigation of the Actually Delivered Patient Dose in Lung IMRT Treatment Based On Deformable Registration of 4D CT Data and Monte Carlo Simulations

Purpose: To accurately calculate the difference between prescribed and delivered dose caused by respiratory motion and artifacts in free breathing helical CT (fbCT) for lung IMRT treatments, and to estimate the impact of this difference in clinical outcome. Method and Materials: Ten cases were studied. For all patients, fbCT and 4DCT reconstructed at 10 phases of the respiratory cycle were available. Patients were selected to have a wide range of tumor motion, size, and position. CORVUS was used to make two IMRT plans calculated on the fbCT for GTV (volume determined on fbCT) and ITV (volume determined by 4DCT) targets respectively. The prescribed dose was 60Gy. The fluence maps and beam setup parameters of the IMRT plans were used by the Monte Carlo dose calculation engine MCSIM for absolute dose calculation on free breathing and 4DCT data. CT deformable registration between the breathing phases was performed and the composite dose over the whole breathing cycle was calculated for the GTV, CTV and lungs. EUD and TCP/NTCP values were determined for the composite dose over the whole breathing cycle based on both plans. Results: For a patient with 4 cm peak to peak tumor movement, a planned CTV EUD of 58 Gy was reduced to 54Gy and 33Gy respectively for the composite 4D dose distribution of ITV and GTV plans. 3‐year control estimates for this patient, assuming that 3‐year local control after 60Gy in the GTV is 30%, were reduced to 29% and 23% for the ITV and the free breathing plans respectively. Conclusion: With the advent of 4DCT, CT data deformable registration and Monte Carlo dose calculations, it is feasible to accurately calculate in retrospect the actually delivered dose and compare it with what historically was assumed to be delivered.