A motion phantom study on helical tomotherapy: the dosimetric impacts of delivery technique and motion

The top 100 most cited articles in helical tomotherapy: a scoping review

Objective

The purpose of this scoping review was to explore the top 100 most cited articles in helical tomotherapy (HT) through bibliometric analysis and visualization tools, help researchers comprehensively understand the research hotspots of HT, and provide clear and intuitive network visualization.

Methods

The Web of Science Core Collection and the search strategy of "Title (TI)=(tomotherapy)" were used to search for articles related to HT as of 27 May 2023. The top 100 most cited articles were obtained by sorting "citations: highest first". From these top 100 most cited articles, the following information was extracted: journals, years and months, countries, authors, types of tumor treated, and topics. The VOSviewer software was introduced for visualizing all the articles related to HT.

Results

The top 100 most cited articles in HT were published between 1999 and 2019. The citation counts of these articles ranges from 326 to 45, with a total of 8,422 citations at the time of searching. The index of citations per year (CPY) ranges from 22.32 to 2.45. These articles originated from 17 countries, with most publications from the United States (n=50), followed by Canada (n=12), Italy (n=10), Germany (n=7) and Belgium (n=5). The International Journal of Radiation Oncology, Biology, Physics published the highest number of articles (n=31), followed by Radiotherapy and Oncology (n=20), Medical Physics (n=13) and Strahlentherapie und Onkologie (n=12). In terms of specific tumor types, head and neck cancer (n=15) is the most common disease, followed by cancers with complex target structures (n=14), breast cancer (n=12), prostate cancer (n=10) and lung cancer (n=8). The most common research topics also include dosimetric comparison (n = 44), quality assurance (n = 12) and Megavoltage CT (n = 8).

Conclusion

This scoping review provides a comprehensive list of the 100 most cited articles in HT. This analysis offers valuable insights into the current research directions of HT that can be utilized by researchers, clinicians, and policy-makers.

Dose delivery accuracy on helical tomotherapy for 4-dimensional tumor motion - a phantom study

Background

The advances in image guidance and capability of highly conformal dose deliveries made possible the use of helical tomotherapy (HT) for lung cancer treatment. To determine the effect of respiratory motion on the delivered dose in HT, film dosimetry using a dynamic phantom was performed. This was a phantom study to determine the effect of motion on the delivered dose in HT.

Materials and methods

4D computed tomography (4DCT) was acquired for various target motions of CIRS dynamic phantom (CIRS Inc., Norfolk, USA) with 2.5cm diameter spherical target of volume 8.2 cc moving in the COS⁴ motion pattern. AveIP images and treatment plans were generated in the HT planning system. Target excursions during treatment delivery were changed in the superior-inferior, anteroposterior and lateral directions. The breathing cycle time was varied from 4 to 5 sec. and also the delivery interruptions were introduced. A film was exposed for each delivery and gamma analysis was performed.

Results

The gamma pass rate (GPR) with 3%, 2 mm criteria for the target motion in the S-I direction showed a significant reduction from 97.5% to 54.4% as the motion increased from 3 mm to 8 mm (p = 0.03). For the target motion in S-I = 8 mm, L-R = A-P = 3 mm, the percentage decrease in the GPR was 74% (p = 0.001) for three interruptions.

Conclusion

The ITV based approach in HT is ideal for a shallow breathing situation when the tumor excursions were confined to 5 mm in the S-I and 3 mm in L-R and A-P directions.

Pulmonary function in stereotactic body radiotherapy with helical tomotherapy for primary and metastatic lung lesions

AIMS

This retrospective study reports outcomes after stereotactic body radiation therapy (SBRT) as delivered by helical tomotherapy (HT) for lung lesions. It promotes a dose escalation program.

METHODS

Histological and/or radiological findings and/or case histories identified 41 primary and 15 metastatic lesions. Thirty patients received 40 Gy in 5 fractions (BED 72 Gy_10Gy) and 26 50 Gy in 5 fractions (BED 100Gy_10Gy). Primary end point was lung toxicity. Secondary end points were respiratory function, local control and local progression-free survival.

RESULTS

Acute toxicity developed in 18/56 patients and late toxicity in 8/54. Median FEV-1 variations versus baseline were - 0.5% (range - 16 to + 43%) at 6 months and - 4.00% (range - 42 to + 18%) at 24 months. Median DLCO variations versus baseline were - 1% (range - 38 to + 36%) at 6 months and - 12.2% (range - 48 to + 11%) at 24 months. At 6 months, a significant positive correlation emerged between FEV-1 change and KPS (p = 0.047). At 24 months, a significant negative correlation emerged between FEV-1 change and the ipsilateral lung V5 (p = 0.006). A low baseline DLCO correlated with more marked DLCO worsening at 6 months (p = 0.012). At 24 months, DLCO worsening correlated significantly with the median contralateral lung dose (p = 0.003). At the last checkup, 23 patients were in complete remission, 16 were in partial remission, 5 had stable disease, and 7 were in relapse. Median follow-up was 12 months (range 5-56).

CONCLUSIONS

In patients with lung disease, SBRT, as delivered by HT, was well tolerated and provided good local control.

Dose deviations induced by respiratory motion for radiotherapy of lung tumors: Impact of CT reconstruction, plan complexity, and fraction size

A thorax phantom was used to assess radiotherapy dose deviations induced by respiratory motion of the target volume. Both intensity modulated and static, non‐modulated treatment plans were planned on CT scans of the phantom. The plans were optimized using various CT reconstructions, to investigate whether they had an impact on robustness to target motion during delivery. During irradiation, the target was programmed to simulate respiration‐induced motion of a lung tumor, using both patient‐specific and sinusoidal motion patterns in three dimensions. Dose was measured in the center of the target using an ion chamber. Differences between reference measurements with a stationary target and dynamic measurements were assessed. Possible correlations between plan complexity metrics and measured dose deviations were investigated. The maximum observed motion‐induced dose differences were 7.8% and 4.5% for single 2 Gy and 15 Gy fractions, respectively. The measurements performed with the largest target motion amplitude in the superior–inferior direction yielded the largest dosimetric deviations. For 2 Gy fractionation schemes, the summed dose deviation after 33 fractions is likely to be less than 2%. Measured motion‐induced dose deviations were significantly larger for one CT reconstruction compared to all the others. Static, non‐modulated plans showed superior robustness to target motion during delivery. Moderate correlations between the modulation complexity score applied to VMAT (MCSv) and measured dose deviations were found for 15 Gy SBRT treatment plans. Correlations between other plan complexity metrics and measured dose deviations were not found.

Evaluation of the target dose coverage of stereotactic body radiotherapy for lung cancer using helical tomotherapy: A dynamic phantom study

Aim

To evaluate the target dose coverage for lung stereotactic body radiotherapy (SBRT) using helical tomotherapy (HT) with the internal tumor volume (ITV) margin settings adjusted according to the degree of tumor motion.

Background

Lung SBRT with HT may cause a dosimetric error when the target motion is large.

Materials and methods

Two lung SBRT plans were created using a tomotherapy planning station. Using these original plans, five plans with different ITV margins (4.0-20.0 mm for superior-inferior [SI] dimension) were generated. To evaluate the effects of respiratory motion on HT, an original dynamic motion phantom was developed. The respiratory wave of a healthy volunteer was used for dynamic motion as the typical tumor respiratory motion. Five patterns of motion amplitude that corresponded to five ITV margin sizes and three breathing cycles of 7, 14, and 28 breaths per minute were used. We evaluated the target dose change between a static delivery and a dynamic delivery with each motion pattern.

Results

The target dose difference increased as the tumor size decreased and as the tumor motion increased. Although a target dose difference of <5 % was observed at ≤10 mm of tumor motion for each condition, a maximum difference of -9.94 % ± 7.10 % was observed in cases of small tumors with 20 mm of tumor motion under slow respiration.

Conclusions

Minimizing respiratory movement is recommended as much as possible for lung SBRT with HT, especially for cases involving small tumors.

Investigating the impact of tumour motion on TomoTherapy stereotactic ablative body radiotherapy (SABR) deliveries on 3-dimensional and 4-dimensional computed tomography

TomoTherapy can provide highly accurate SABR deliveries, but currently it does not have any effective motion management techniques. Shallow breathing has been identified as one possible motion management solution on TomoTherapy, which has been made possible with the BreatheWell audiovisual biofeedback (AVB) device. Since both the shallow breathing technique and the clinical use of the BreatheWell device are novel, their implementation requires comprehensive verification and validation work. As the first stage of the validation, this paper investigates the impact of target motion on a TomoTherapy SABR delivery is assessed on both 3D CT and 4D CT using a 4D respiratory phantom. A dosimetric study on a 4D respiratory phantom was conducted, with the phantom's insert designed to move at four different amplitudes in the superior-inferior direction. SABR plans on 3D and 4D CT scans were created and measured. Critical plan statistics and measurement results were compared. It is found that for TomoTherapy SABR deliveries, by reducing the targets respiratory motion, target coverage, organ-at-risk (OAR) sparing, and delivery accuracy were improved.

Evaluation of TomoTherapy dose calculations with intrafractional motion and motion compensation

PURPOSE

Anatomical motion, both cyclical and aperiodic, can impact the dose delivered during external beam radiation. In this work, we evaluate the use of a research version of the clinical TomoTherapy^® dose calculator to calculate dose with intrafraction rigid motion. We also evaluate the feasibility of a method of motion compensation for helical tomotherapy using the jaws and MLC.

METHODS

Treatment plans were created using the TomoTherapy treatment planning system. Dose was recalculated for several simple rigid motion traces including a 4 mm step motion applied either longitudinally or transversely, and a sinusoidal motion. The calculated dose volumes were compared to dose measurements that were performed by translating the phantom with the same motion traces used in the calculations. Measurements were made using film and ion chambers. Finally, the delivery plans were modified to compensate for the motion by sweeping the jaws for longitudinal motion and shifting the MLC leaves for transverse motion, and the calculations and measurements were repeated.

RESULTS

A transverse step motion shifted the dose that was delivered after the step occurred, but otherwise did not impact the dose distribution. Film measurements agreed with dose calculations to within 2%/2 mm for 99% of dose points within the 50% isodose line. A shift in the MLC leaf delivery pattern successfully compensated for the step motion to within the 3 mm accuracy allowed by the finite leaf widths. A longitudinal step motion impacted the dose in the interior of the target volume to a degree that was dependent on the planning field width and step size. Film measurements agreed with dose calculations to within 2%/2 mm for 98% of dose points within the 50% isodose line. Shifts in the jaw position successfully compensated for the longitudinal step motion. Sinusoidal (breathing-like) motion was also studied, with similar results.

CONCLUSIONS

A research version of the clinical TomoTherapy dose calculator has been shown to accurately calculate the dose from treatment plans delivered in the presence of arbitrary rigid motion. Modifications to the delivery plan using jaw and MLC leaf shifts that follow the motion can successfully compensate for the target motion.

Recent advances in radiation oncology

Radiotherapy (RT) is very much a technology-driven treatment modality in the management of cancer. RT techniques have changed significantly over the past few decades, thanks to improvements in engineering and computing. We aim to highlight the recent developments in radiation oncology, focusing on the technological and biological advances. We will present state-of-the-art treatment techniques, employing photon beams, such as intensity-modulated RT, volumetric-modulated arc therapy, stereotactic body RT and adaptive RT, which make possible a highly tailored dose distribution with maximum normal tissue sparing. We will analyse all the steps involved in the treatment: imaging, delineation of the tumour and organs at risk, treatment planning and finally image-guidance for accurate tumour localisation before and during treatment delivery. Particular attention will be given to the crucial role that imaging plays throughout the entire process. In the case of adaptive RT, the precise identification of target volumes as well as the monitoring of tumour response/modification during the course of treatment is mainly based on multimodality imaging that integrates morphological, functional and metabolic information. Moreover, real-time imaging of the tumour is essential in breathing adaptive techniques to compensate for tumour motion due to respiration. Brief reference will be made to the recent spread of particle beam therapy, in particular to the use of protons, but also to the yet limited experience of using heavy particles such as carbon ions. Finally, we will analyse the latest biological advances in tumour targeting. Indeed, the effectiveness of RT has been improved not only by technological developments but also through the integration of radiobiological knowledge to produce more efficient and personalised treatment strategies.

The radiation techniques of tomotherapy & intensity-modulated radiation therapy applied to lung cancer

Radiotherapy (RT) plays an important role in the management of lung cancer. Development of radiation techniques is a possible way to improve the effect of RT by reducing toxicities through better sparing the surrounding normal tissues. This article will review the application of two forms of intensity-modulated radiation therapy (IMRT), fixed-field IMRT and helical tomotherapy (HT) in lung cancer, including dosimetric and clinical studies. The advantages and potential disadvantages of these two techniques are also discussed.

Comparison of Effects Between Central and Peripheral Stage I Lung Cancer Using Image-Guided Stereotactic Body Radiotherapy via Helical Tomotherapy

Lung cancer is a common malignant tumor with high morbidity and mortality. Here we compared the effects and outcome between central and peripheral stage I lung cancer using image-guided stereotactic body radiotherapy. From June 2011 to July 2013, a total of 33 patients with stage I lung cancer were enrolled. A total of 50 Gy in 10 fractions or 60 Gy in 10 fractions was delivered in the central arm (n = 18), while 50 Gy in 5 fractions in the peripheral arm (n = 15). Statistical analyses were performed using logistic regression analysis and Kaplan-Meier method. The mean follow-up time was 38.1 months. Three-month, 1-, 2-, and 3-year overall response rates were 66.7%, 83.3%, 61.1%, and 72.2% and 66.7%, 80%, 80%, and 80% in the central and peripheral arms, respectively. Three-year local control rates (94.4% vs 93.3%, P = .854), regional control rates (94.4% vs 86.7%, P = .412), and distant control rates (64.2% vs 61.7%, P = .509) had no differences between the central and the peripheral arms. Grade 2 radiation pneumonitis was observed in 6 of 18 patients in the central arm and in 1 of 15 patients in the peripheral arm (P = .92). Grade 2 radiation esophagitis was 5.7% in the central arm, while none occurred in the peripheral arm (P = .008). Five (15.1%) of all patients felt slight fatigue during radiotherapy. Other major complications were not observed. In conclusion, helical image-guided stereotactic body radiotherapy for central stage I lung cancer is safe and effective compared to peripheral stage I lung cancer.

Safety and efficacy of intensity-modulated stereotactic body radiotherapy using helical tomotherapy for lung cancer and lung metastasis

Stereotactic body radiotherapy (SBRT) proved to be an effective treatment with acceptable toxicity for lung tumors. However, the use of helical intensity-modulated (IM) SBRT is controversial. We investigated the outcome of lung tumor patients treated by IMSBRT using helical tomotherapy with a Japanese standard fractionation schedule of 48 Gy in 4 fractions (n = 37) or modified protocols of 50-60 Gy in 5-8 fractions (n = 35). Median patient's age was 76 years and median follow-up period for living patients was 20 months (range, 6-46). The median PTV was 6.9 cc in the 4-fraction group and 14 cc in the 5- to 8-fraction group (P = 0.001). Grade 2 radiation pneumonitis was seen in 2 of 37 patients in the 4-fraction group and in 2 of 35 patients in the 5- to 8-fraction group (log-rank P = 0.92). Other major complications were not observed. The LC rates at 2 years were 87% in the 4-fraction group and 83% in the 5- to 8-fraction group. Helical IMSBRT for lung tumors is safe and effective. Patients with a high risk of developing severe complications may also be safely treated using 5-8 fractions. The results of the current study warrant further studies of helical IMSBRT.

Outcome and toxicity of stereotactic body radiotherapy with helical tomotherapy for inoperable lung tumor: analysis of Grade 5 radiation pneumonitis

To analyze outcomes and toxicities of stereotactic body radiotherapy with helical tomotherapy (HT-SBRT) for inoperable lung tumors, the medical records of 30 patients with 31 lung tumors treated with HT-SBRT were reviewed. The 3-year local control, cause-specific survival and overall survival rates (LC, CCS and OS, respectively) were analyzed using the Kaplan-Meier method. Toxicities were graded using Common Terminology Criteria for Adverse Events ver. 4. To investigate the factors associated with Grade 5 radiation pneumonitis (G5 RP), several parameters were analyzed: (i) patient-specific factors (age, gross tumor volume and PTV, and the interstitial pulmonary shadow on pretreatment CT); and (ii) dosimetry-specific factors (conformity index, homogeneity index, mean lung dose, and V5, V10, V15, V20 and V25 of the total lungs). The median duration of observation for all patients was 36.5 months (range, 4-67 months). The 3-year LC, CCS and OS were 82, 84 and 77%, respectively. Regarding Grade 3 or higher toxicities, two patients (6.7%) developed G5 RP. GTV was significantly associated with G5 RP (P = 0.025), and there were non-significant but slight associations with developing G5 RP for V5 (P = 0.067) and PTV (P = 0.096). HT-SBRT led to standard values of LC, CCS and OS, but also caused a markedly higher incidence of G5 RP. It is essential to optimize patient selection so as to avoid severe radiation pneumonitis in HT-SBRT.

A margin-based analysis of the dosimetric impact of motion on step-and-shoot IMRT lung plans

Purpose

Intrafraction motion during step-and-shoot (SNS) IMRT is known to affect the target dosimetry by a combination of dose blurring and interplay effects. These effects are typically managed by adding a margin around the target. A quantitative analysis was performed, assessing the relationship between target motion, margin size, and target dosimetry with the goal of introducing new margin recipes.

Methods

A computational algorithm was used to calculate 1,174 motion-encoded dose distributions and DVHs within the patient’s CT dataset. Sinusoidal motion tracks were used simulating intrafraction motion for nine lung tumor patients, each with multiple margin sizes.

Results

D_95% decreased by less than 3% when the maximum target displacement beyond the margin experienced motion less than 5 mm in the superior-inferior direction and 15 mm in the anterior-posterior direction. For target displacements greater than this, D_95% decreased rapidly.

Conclusions

Targets moving in excess of 5 mm outside the margin can cause significant changes to the target. D_95% decreased by up to 20% with target motion 10 mm outside the margin, with underdosing primarily limited to the target periphery. Multi-fractionated treatments were found to exacerbate target under-coverage. Margins several millimeters smaller than the maximum target displacement provided acceptable motion protection, while also allowing for reduced normal tissue morbidity.

Fast, simple, and informative patient-specific dose verification method for intensity modulated total marrow irradiation with helical tomotherapy

Background

Patient-specific dose verification for treatment planning in helical tomotherapy is routinely performed using a homogeneous virtual water cylindrical phantom of 30 cm diameter and 18 cm length (Cheese phantom). Because of this small length, treatment with total marrow irradiation (TMI) requires multiple deliveries of the dose verification procedures to cover a wide range of the target volumes, which significantly prolongs the dose verification process. We propose a fast, simple, and informative patient-specific dose verification method which reduce dose verification time for TMI with helical tomotherapy.

Methods

We constructed a two-step solid water slab phantom (length 110 cm, height 8 cm, and two-step width of 30 cm and 15 cm), termed the Whole Body Phantom (WB phantom). Three ionization chambers and three EDR-2 films can be inserted to cover extended field TMI treatment delivery. Three TMI treatment plans were conducted with a TomoTherapy HiArt Planning Station and verified using the WB phantom with ion chambers and films. Three regions simulating the head and neck, thorax, and pelvis were covered in a single treatment delivery. The results were compared to those with the cheese phantom supplied by Accuray, Inc. following three treatment deliveries to cover the body from head to pelvis.

Results

Use of the WB phantom provided point doses or dose distributions from head and neck to femur in a single treatment delivery of TMI. Patient-specific dose verification with the WB phantom was 62% faster than with the cheese phantom. The average pass rate in gamma analysis with the criteria of a 3-mm distance-to-agreement and 3% dose differences was 94% ± 2% for the three TMI treatment plans. The differences in pass rates between the WB and cheese phantoms at the upper thorax to abdomen regions were within 2%. The calculated dose agreed with the measured dose within 3% for all points in all five cases in both the WB and cheese phantoms.

Conclusions

Our dose verification method with the WB phantom provides simple and rapid quality assurance without limiting dose verification information in total marrow irradiation with helical tomotherapy.

Motion as a perturbation: measurement-guided dose estimates to moving patient voxels during modulated arc deliveries

PURPOSE

To present a framework for measurement-guided VMAT dose reconstruction to moving patient voxels from a known motion kernel and the static phantom data, and to validate this perturbation-based approach with the proof-of-principle experiments.

METHODS

As described previously, the VMAT 3D dose to a static patient can be estimated by applying a phantom measurement-guided perturbation to the treatment planning system (TPS)-calculated dose grid. The fraction dose to any voxel in the presence of motion, assuming the motion kernel is known, can be derived in a similar fashion by applying a measurement-guided motion perturbation. The dose to the diodes in a helical phantom is recorded at 50 ms intervals and is transformed into a series of time-resolved high-density volumetric dose grids. A moving voxel is propagated through this 4D dose space and the fraction dose to that voxel in the phantom is accumulated. The ratio of this motion-perturbed, reconstructed dose to the TPS dose in the phantom serves as a perturbation factor, applied to the TPS fraction dose to the similarly situated voxel in the patient. This approach was validated by the ion chamber and film measurements on four phantoms of different shape and structure: homogeneous and inhomogeneous cylinders, a homogeneous cube, and an anthropomorphic thoracic phantom. A 2D motion stage was used to simulate the motion. The stage position was synchronized with the beam start time with the respiratory gating simulator. The motion patterns were designed such that the motion speed was in the upper range of the expected tumor motion (1-1.4 cm∕s) and the range exceeded the normally observed limits (up to 5.7 cm). The conformal arc plans for X or Y motion (in the IEC 61217 coordinate system) consisted of manually created narrow (3 cm) rectangular strips moving in-phase (tracking) or phase-shifted by 90° (crossing) with respect to the phantom motion. The XY motion was tested with the computer-derived VMAT MLC sequences. For all phantoms and plans, time-resolved (10 Hz) ion chamber dose was collected. In addition, coronal (XY) films were exposed in the cube phantom to a VMAT beam with two different starting phases, and compared to the reconstructed motion-perturbed dose planes.

RESULTS

For the X or Y motions with the moving strip and geometrical phantoms, the maximum difference between perturbation-reconstructed and ion chamber doses did not exceed 1.9%, and the average for any motion pattern∕starting phase did not exceed 1.3%. For the VMAT plans on the cubic and thoracic phantoms, one point exhibited a 3.5% error, while the remaining five were all within 1.1%. Across all the measurements (N = 22), the average disagreement was 0.5 ± 1.3% (1 SD). The films exhibited γ(3%∕3 mm) passing rates ≥90%.

CONCLUSIONS

The dose to an arbitrary moving voxel in a patient can be estimated with acceptable accuracy for a VMAT delivery, by performing a single QA measurement with a cylindrical phantom and applying two consecutive perturbations to the TPS-calculated patient dose. The first one accounts for the differences between the planned and delivered static doses, while the second one corrects for the motion.

A comparison of the dosimetric effects of intrafraction motion on step-and-shoot, compensator, and helical tomotherapy-based IMRT

Intrafraction motion during intensity‐modulated radiation therapy can cause differences between the planned and delivered patient dose. The magnitude of these differences is dependent on a number of variables, including the treatment modality. This study was designed to compare the relative susceptibility of plans generated with three different treatment modalities to intrafraction motion. The dosimetric effects of motion were calculated using computational algorithms for seven lung tumor patients. Three delivery techniques — MLC‐based step‐and‐shoot (SNS), beam attenuating compensators, and helical tomotherapy (HT) — were investigated. In total 840 motion‐encoded dose‐volume histograms (DVHs) were calculated for various combinations of CTV margins and sinusoidal CTV motion including CTV offsets. DVH‐based metrics (e.g., D95% and D05%) were used to score plan degradations. For all three modalities, dosimetric degradations were typically smaller than 3% if the CTV displacement was smaller than the CTV margin. For larger displacements, technique and direction‐specific sensitivities existed. While the HT plans show similar D95% degradations for motion in the SI and AP directions, SNS and compensator plans showed larger D95% degradations for motion in the SI direction than for motion in the AP direction. When averaged over all motion/margin combinations, compensator plans resulted in 0.9% and 0.6% smaller D95% reductions compared to SNS and HT plans, respectively. These differences were statistically significant. No statistically significant differences in D95% degradations were found between SNS and HT for data averaged over all margin and motion track combinations. For CTV motion that is larger than the CTV margin, the dosimetric impact on the CTV varies with treatment technique and the motion direction. For the cases presented here, the effect of motion on CTV dosimetry was statistically smaller for compensator deliveries than SNS and HT, likely due to the absence of the interplay effect which is present for the more dynamic treatment deliveries. The differences between modalities were, however, small and might not be clinically significant. As expected, margins that envelop the CTV motion provide dosimetric protection against motion for all three modalities.

PACS numbers: 87.53.Jw, 87.55.dk, 87.55.de

Intensity-modulated radiation therapy using static ports of tomotherapy (TomoDirect): comparison with the TomoHelical mode

Purpose

With the new mode of Tomotherapy, irradiation can be delivered using static ports of the TomoDirect mode. The purpose of this study was to evaluate the characteristics of TomoDirect plans compared to conventional TomoHelical plans.

Methods

TomoDirect and TomoHelical plans were compared in 46 patients with a prostate, thoracic wall or lung tumor. The mean target dose was used as the prescription dose. The minimum coverage dose of 95% of the target (D95%), conformity index (CI), uniformity index (UI), dose distribution in organs at risk and treatment time were evaluated. For TomoDirect, 2 to 5 static ports were used depending on the tumor location.

Results

For the prostate target volume, TomoDirect plans could not reduce the rectal dose and required a longer treatment time than TomoHelical. For the thoracic wall target volume, the V5Gy of the lung or liver was lower in TomoDirect than in TomoHelical (p = 0.02). For the lung target volume, TomoDirect yielded higher CI (p = 0.009) but smaller V5Gy of the lung (p = 0.005) than TomoHelical. Treatment time did not differ significantly between the thoracic wall and lung plans.

Conclusion

Prostate cancers should be treated with the TomoHelical mode. Considering the risk of low-dose radiation to the lung, the TomoDirect mode could be an option for thoracic wall and lung tumors.

A study of longitudinal tumor motion in helical tomotherapy using a cylindrical phantom

Tumor motion during radiation treatment on a helical tomotherapy unit may create problems due to interplay with motion of the multileaf collimator, gantry rotation, and patient couch translation through the gantry. This study evaluated this interplay effect for typical clinical parameters using a cylindrical phantom consisting of 1386 diode detectors placed on a respiratory motion platform. All combinations of radiation field widths (1, 2.5, and 5 cm) and gantry rotation periods (16, 30, and 60 s) were considered for sinusoidal motions with a period of 4 s and amplitudes of 5, 6, 7, 8, 9, and 10 mm, as well as real patient breathing pattern. Gamma comparisons with 2% dose difference and 2 mm distance to agreement and dose profiles were used for evaluation. The required motion margins were determined for each set of parameters. The required margin size increased with decreasing field width and increasing tumor motion amplitude, but was not affected by rotation period. The plans with the smallest field width of 1 cm have required motion margins approximately equal to the amplitude of motion (± 25%), while those with the largest field width of 5 cm had required motion margins approximately equal to 20% of the motion amplitude (± 20%). For tumor motion amplitudes below 6 mm and field widths above 1 cm, the required additional motion margins were very small, at a maximum of 2.5 mm for sinusoidal breathing patterns and 1.2 mm for the real patient breathing pattern.

Helical tomotherapy for SIB and hypo-fractionated treatments in lung carcinomas: a 4D Monte Carlo treatment planning study

PURPOSE

To evaluate the impact of intra-fraction motion induced by regular breathing on treatment quality for helical tomotherapy treatments.

MATERIAL AND METHODS

Four patients treated by simultaneous-integrated boost (SIB) and three by hypo-fractionated stereotactic treatments (hypo-fractionated, 18 Gy/fraction) were included. All patients were coached to ensure regular breathing. For the SIB group, the tumor volume was delineated using CT information only (CTV(CT)) and the boost region was based on PET information (GTV(PET), no CTV extension). In the hypo-fractionated group, a GTV based on CT information was contoured. In both groups, ITVs were defined according to 4D data. The PTV included the ITV plus a setup error margin. The treatment was planned using the tomotherapy TPS on 3D CT images. In order to verify the impact of intra-fraction motion and interplay effects, dose calculations were performed using a previously validated Monte Carlo model of tomotherapy (TomoPen): first on the planning 3D CT ("planned dose") and second, on the 10 phases of the 4D scan. For the latter, two dose distributions, termed "interplay simulated" or "no interplay" were computed with and without beamlet-phase correlation over the 10 phases and combined using deformable dose registration.

RESULTS

In all cases, DVHs of "interplay simulated" dose distributions complied within 1% of the original clinical objectives used for planning, defined according to ICRU (report 83) and RTOG (trials 0236 and 0618) recommendations, for SIB and hypo-fractionated groups, respectively. For one patient in the hypo-fractionated group, D(mean) to the CTV(CT) was 2.6% and 2.5% higher than "planned" for "interplay simulated" and "no interplay", respectively.

CONCLUSION

For the patients included in this study, assuming regular breathing, the results showed that interplay of breathing and tomotherapy delivery motions did not affect significantly plan delivery accuracy. Hence, accounting for intra-fraction motion through the definition of an ITV volume was sufficient to ensure tumor coverage.

Dosimetric selection for helical tomotherapy based stereotactic ablative radiotherapy for early-stage non-small cell lung cancer or lung metastases

BACKGROUND

No selection criteria for helical tomotherapy (HT) based stereotactic ablative radiotherapy (SABR) to treat early stage non-small cell lung cancer (NSCLC) or solitary lung metastases has been established. In this study, we investigate the dosimetric selection criteria for HT based SABR delivering 70 Gy in 10 fractions to avoid severe toxicity in the treatment of centrally located lesions when adequate target dose coverage is desired.

MATERIALS AND METHODS

78 HT-SABR plans for solitary lung lesions were created to prescribe 70 Gy in 10 fractions to the planning target volume (PTV). The PTV was set to have ≥95% PTV receiving 70 Gy in each case. The cases for which dose constraints for ≥1 OAR could not be met without compromising the target dose coverage were compared with cases for which all target and OAR dose constraints were met.

RESULTS

There were 23 central lesions for which OAR dose constraints could not be met without compromising PTV dose coverage. Comparing to cases for which optimal HT-based SABR plans were generated, they were associated with larger tumor size (5.72±1.96 cm vs. 3.74±1.49 cm, p<0.0001), higher lung dose, increased number of immediately adjacent OARs ( 3.45±1.34 vs. 1.66±0.81, p<0.0001), and shorter distance to the closest OARs (GTV: 0.26±0.22 cm vs. 0.88±0.54 cm, p<0.0001; PTV 0.19±0.18 cm vs. 0.48±0.36 cm, p = 0.0001).

CONCLUSION

Delivery of 70 Gy in 10 fractions with HT to meet all the given OAR and PTV dose constraints are most likely when the following parameters are met: lung lesions ≤3.78 cm (11.98 cc), ≤2 immediately adjacent OARs which are ≥0.45 cm from the gross lesion and ≥0.21 cm from the PTV.

Feasibility study of multi-pass respiratory-gated helical tomotherapy of a moving target via binary MLC closure

Gated radiotherapy of lung lesions is particularly complex for helical tomotherapy, due to the simultaneous motions of its three subsystems (gantry, couch and collimator). We propose a new way to implement gating for helical tomotherapy, namely multi-pass respiratory gating. In this method, gating is achieved by delivering only the beam projections that occur within a respiratory gating window, while blocking the rest of the beam projections by fully closing all collimator leaves. Due to the continuous couch motion, the planned beam projections must be delivered over multiple passes of radiation deliveries. After each pass, the patient couch is reset to its starting position, and the treatment recommences at a different phase of tumour motion to 'fill in' the previously blocked beam projections. The gating process may be repeated until the plan dose is delivered (full gating), or halted after a certain number of passes, with the entire remaining dose delivered in a final pass without gating (partial gating). The feasibility of the full gating approach was first tested for sinusoidal target motion, through experimental measurements with film and computer simulation. The optimal gating parameters for full and partial gating methods were then determined for various fractionation schemes through computer simulation, using a patient respiratory waveform. For sinusoidal motion, the PTV dose deviations of -29 to 5% observed without gating were reduced to range from -1 to 3% for a single fraction, with a 4 pass full gating. For a patient waveform, partial gating required fewer passes than full gating for all fractionation schemes. For a single fraction, the maximum allowed residual motion was only 4 mm, requiring large numbers of passes for both full (12) and partial (7 + 1) gating methods. The number of required passes decreased significantly for 3 and 30 fractions, allowing residual motion up to 7 mm. Overall, the multi-pass gating technique was shown to be a promising way to reduce the impact of lung tumour motion during helical tomotherapy.

Helical tomotherapy for solitary lung tumor: feasibility study and dosimetric evaluation of treatment plans

The purpose of this study is to evaluate the feasibility of and treatment plans for helical tomotherapy (HT) for solitary lung tumors. Nine patients with stage IA non-small-cell lung cancer (NSCLC) and three patients with solitary lung metastasis were treated with HT. All patients were immobilized with the BodyFIX system, and the target volume shown on computed tomography in the 3 respiration phases were superimposed onto the 3-dimensional radiation treatment planning system to represent the internal target motion. All patients were treated with 54-60 Gy in 12 fractions over 8 to 13 days. The median follow-up time was 14 months (range: 3-16 months). The overall response rate was 92%. The local control rate at 1 year was 100% with no difference between NSCLC and metastasis. Of 12 patients, one patient experienced Grade 2 radiation pneumonitis (RP), and the other patient with severe interstitial pneumonia and emphysema experienced Grade 5 RP. The mean lung dose (MLD) and the dose-volume histogram (DVH) for the normal lung volumes were converted into normalized total dose with an alpha/beta ratio of 3 Gy. The DVH of the normal lung volumes demonstrated that the mean volumes of V(10), V(30), V(50), and V(70) were 40.4 +/- 9.4%, 21.3 +/- 6.4%, and 12.8 +/- 4.6%, and 9.3 +/- 4.2% in all patients, 28.8%, 18.7%, 12.3%, and 10.5% in the patient with Grade 2 RP, and 29.3%, 17.9%, 7.7%, and 5.4% in the patient with Grade 5 RP. The mean MLD of all patients was 13.5 +/- 3.9 Gy, and those values of patients with Grade 2 and 5 RP were 12.9 and 21.8 Gy, respectively. Our study found that only the MLD was significantly correlated with RP >Grade 2 (p = 0.030) using the Student's t-test. Our study found the conformity index value of 1.48 +/- 0.15 and the homogeneity index value of 1.066 +/- 0.023, which suggested the feasibility of using HT to treat lung tumors with hypofractionation. In conclusion, HT is a feasible non-invasive technique for treating solitary lung tumors and achieves high accuracy in terms of dose conformity and homogeneity. The decision of the indications for HT might be required caution in cases in which a severe pulmonary toxicity is predicted from the high MLD, especially in cases involving a severe pulmonary comorbidity.

Radiosurgery in the treatment of spinal metastases: tumor control, survival, and quality of life after helical tomotherapy

OBJECTIVE

The effectiveness and limitations of spinal radiosurgery using a helical TomoTherapy system for the treatment of spinal metastases are reviewed in this article.

METHODS

This is a retrospective review of patients who underwent stereotactic radiosurgery for spinal metastases between July 2004 and December 2007. Radiographic follow-up consisted of magnetic resonance imaging to assess tumor growth control as well as pre- and posttreatment x-rays, which were used to measure changes in segmental angulation and deformity. Clinical performance was assessed using the Karnofsky Performance Scale, Oswestry Disability Index, and visual analog scale.

RESULTS

Forty patients were treated for 110 metastatic tumors (range, 1-6 tumors per patient). The mean age at the time of radiosurgical treatment was 67 years (age range, 35-81 years). Twenty-three patients (57.5%) had undergone previous surgical resection. Pain was the most common presenting symptom, seen in 32 patients (80%). The mean Oswestry Disability Index score at presentation was 43 (range, 20-90), and the mean visual analog scale score was 6.2 (range, 0-10). The mean radiosurgical dose to the tumor was 17.3 Gy (range, 10-24 Gy). At a mean follow-up duration of 12.7 months (range, 4-32 months), decreased or stable tumor volume was seen in 90 (82%) of the tumors treated. There was improvement in pain in 34 patients (85%). The mean postradiosurgical Oswestry Disability Index score was 25 (range, 10-90), whereas the postradiosurgical visual analog scale score was 3.2 (range, 0-9). Progression of kyphosis was the most common radiographic sequela, experienced by 73% of patients alive at 12 months, with a mean change in angulation of 7.3 +/- 4.5 degrees.

CONCLUSION

Radiosurgery is effective as either primary or adjunctive treatment of metastatic tumors of the spine.

Accelerated treatment using intensity-modulated radiation therapy plus concurrent capecitabine for unresectable hepatocellular carcinoma

BACKGROUND

: Patients with unresectable hepatocellular carcinoma (HCC) have limited treatment options. In this study, the authors investigated the feasibility, toxicity, and efficacy associated with intensity-modulated radiation therapy (IMRT) and concurrent, chronomodulated capecitabine in the treatment of unresectable HCC.

METHODS

: Twenty patients underwent treatment planning for HCC confined to the liver with helical tomotherapy-based IMRT. Fifty-five percent of patients had Child-Pugh Class A disease, and 45% of patients had Class B disease. Ninety-five percent of patients were prescribed 50 gray (Gy) of radiotherapy to the planning target volume delivered in 20 fractions with concurrent, chronomodulated capecitabine. Transcatheter arterial chemoembolization preceded radiotherapy in 11 patients, and 9 patients received IMRT alone because of portal vein thrombosis, esophageal varices, or tumor size.

RESULTS

: The mean greatest tumor dimension was 9 cm (range, 1.3-17.4 cm), the mean dose to normal liver was 22.6 Gy (range, 10-29.2 Gy), and the average volume of liver that received >30 Gy (V30) was 27.2% (range, 12%-43%). Eighteen patients (90%) completed the prescribed treatment of 50 Gy. There was no increase from baseline in acute or late toxicity greater than 2 grades. Partial response or disease stability was achieved at 3 months to 6 months after treatment in 15 of 16 patients (94%). The median survival (+/-standard deviation) for patients who had Child-Pugh Class A and B disease was 22.5 +/- 5.1 months and 8 +/- 3.3 months, respectively.

CONCLUSIONS

: In this initial experience with accelerated IMRT plus capecitabine for patients who had large HCC lesions, the results demonstrated acceptable toxicity with promising local control. The relatively low acute and late toxicity observed with this program suggested that dose intensification can be incorporated into the treatment regimen if needed. Cancer 2009. (c) 2009 American Cancer Society.

Surface dose in the treatment of breast cancer with helical tomotherapy

PURPOSE

Investigation of the effects of breathing motion- and misregistration-induced errors on the superficial dose in the treatment of breast cancer using helical tomotherapy (HT).

MATERIAL AND METHODS

Surface dose measurements were performed with thermoluminescence dosimetry (TLD). Two treatment plans with different planning target volume (PTV) definitions of the left breast were used: PTVskin had its ventral border exactly on skin level, while PTVair included also a 10-mm extension ventral to the PTVskin. With a thoracic static phantom, misregistration errors in an HT were simulated. A dynamic phantom was used to simulate a breathing patient during HT. Surface doses of breast cancer patients were measured both for an HT (179 points) and a conventional three-dimensional conformal treatment (70 points).

RESULTS

In the static phantom misregistration setup, dose deviations of -31.9% for PTVskin to +35.4% for PTVair could be observed. The dynamic phantom measurements resulted in surface dose deviations from those in a static position between 0.8% and 3.8% without a significant difference for the PTV definitions. The measured surface doses on patients averaged (mean +/- standard deviation) 1.65 +/- 0.13 Gy for the HT and 1.42 +/- 0.11 Gy for the three-dimensional conformal treatment.

CONCLUSION

HT enables a homogeneous and reproducible surface dose with small dose deviations in the treatment of breast cancer. HT is a feasible method to treat breast cancer under free shallow breathing of the patient using a treatment plan with a ventral PTV border on the skin level.

The implication of non-cyclic intrafractional longitudinal motion in SBRT by TomoTherapy

To determine the dosimetric impact of non-cyclic longitudinal intrafractional motion, TomoTherapy plans with different field sizes were interrupted during a phantom delivery, and a displacement between -5 mm and 5 mm was induced prior to the delivery of the completion procedure. The planar dose was measured by film and a cylindrical phantom, and under-dosed or over-dosed volume was observed for either positive or negative displacement. For a 2.5 cm field, there was a 4% deviation for every mm of motion and for a 1 cm field, the deviation was 8% per mm. The dimension of the under/over-dosed area was independent of the motion but dependent on the field size. The results have significant implication in small-field high-dose treatments (i.e. stereotactic body radiation therapy (SBRT)) that deliver doses in only a few fractions. Our studies demonstrate that a small longitudinal motion may cause a dose error that is difficult to compensate; however, dividing a SBRT fraction into smaller passes is helpful to reduce such adverse effects.

Adjuvant radiotherapy for gastric cancer: A dosimetric comparison of 3-dimensional conformal radiotherapy, tomotherapy and conventional intensity modulated radiotherapy treatment plans

Some patients with gastric cancer benefit from post-operative chemo-radiotherapy, but adequately irradiating the planning target volume (PTV) whilst avoiding organs at risk (OAR) can be difficult. We evaluate 3-dimensional conformal radiotherapy (CRT), conventional intensity-modulated radiotherapy (IMRT) and helical tomotherapy (TT). TT, 2 and 5-field (F) CRT and IMRT treatment plans with the same PTV coverage were generated for 5 patients and compared. Median values are reported. The volume of left/right kidney receiving at least 20Gy (V20) was 57/51% and 51/60% for 2 and 5F-CRT, and 28/14% for TT and 27/19% for IMRT. The volume of liver receiving at least 30Gy (V30) was 45% and 62% for 2 and 5F-CRT, and 37% for TT and 35% for IMRT. With TT, 98% of the PTV received 95-105% of the prescribed dose, compared with 45%, 34% and 28% for 2F-CRT, 5F-CRT and IMRT respectively. Using conventional metrics, conventional IMRT can achieve comparable PTV coverage and OAR sparing to TT, but at the expense of PTV dose heterogeneity. Both irradiate large volumes of normal tissue to low doses. Additional studies are needed to demonstrate the clinical impact of these technologies.

Experimental measurements and Monte Carlo simulations for dosimetric evaluations of intrafraction motion for gated and ungated intensity modulated arc therapy deliveries

Respiratory gated radiation therapy allows for a smaller margin expansion for the planning target volume (PTV) to account for respiratory induced motion and is emerging as a common method to treat lung and liver tumors. We investigated the dosimetric effect of free motion and gated delivery for intensity modulated arc therapy (IMAT) with experimental measurements and Monte Carlo simulations. The impact of PTV margin and duty cycle for gated delivery is studied with Monte Carlo simulations. A motion phantom is used for this study. Two sets of contours were drawn on the mid-inspiration CT scan of this motion phantom. For each set of contours, an IMAT plan to be delivered with constant dose rate was created. The plans were generated on a CT scan of the phantom in the static condition with 3 mm PTV margin and applied to the motion phantom under four conditions: static, full superior-inferior (SI) motion (A = 1 cm, T = 4 s) and gating conditions (25% and 50% duty cycles) with full SI motion. A 6 by 15 cm piece of radiographic film was placed in the sagittal plane of the phantom and then irradiated under all measurement conditions. Film calibration was performed with a step-wedge method to convert optical density to dose. Gated IMAT delivery was first validated in 2D by comparing static film with that from gating and full motion. A previously verified simulation tool for IMRT that takes the log files from the multileaf collimator (MLC) controller and the gating system were adapted to simulate the delivered IMAT treatment for full 3D dosimetric analysis. The IMAT simulations were validated against the 2D film measurements. The resultant IMAT simulations were evaluated with dose criteria, dose-volume histograms and 3D gamma analysis. We validated gated IMAT deliveries when we compared the static film with the one from gating using 25% duty cycle using 2D gamma analysis. Within experimental and setup uncertainties, film measurements agreed with their corresponding simulated plans using 2D gamma analysis. Finally, when planning with margins designed for gating with 25% duty cycle and applying 50% or no gating during treatment, the dose differences in D(min,) D(99%) and D(95%) of the clinical target volume can be up to 27 cGy, 20 cGy and 18 cGy, respectively, for a plan with 200 cGy prescription dose. We have experimentally delivered gated IMAT with constant dose rate to a motion phantom and assessed their accuracies with film dosimetry and Monte Carlo simulations. Film dosimetry demonstrated that 25% gating and static plans are within 2%, 2 mm. The Monte Carlo simulation method was employed to generate dose delivered in 3D to a motion phantom, and the dosimetric results were reported. Since our film measurements agreed well with Monte Carlo simulations, we can reliably use this simulation tool to further study the dosimetric effects of target motion and effectiveness of gating for IMAT deliveries.

On the impact of longitudinal breathing motion randomness for tomotherapy delivery

The purpose of this study is to explain the unplanned longitudinal dose modulations that appear in helical tomotherapy (HT) dose distributions in the presence of irregular patient breathing. This explanation is developed by the use of longitudinal (1D) simulations of mock and surrogate data and tested with a fully 4D HT delivered plan. The 1D simulations use a typical mock breathing function which allows more flexibility to adjust various parameters. These simplified simulations are then made more realistic by using 100 surrogate waveforms all similarly scaled to produce longitudinal breathing displacements. The results include the observation that, with many waveforms used simultaneously, a voxel-by-voxel probability of a dose error from breathing is found to be proportional to the realistically random breathing amplitude relative to the beam width if the PTV is larger than the beam width and the breathing displacement amplitude. The 4D experimental test confirms that regular breathing will not result in these modulations because of the insensitivity to leaf motion for low-frequency dynamics such as breathing. These modulations mostly result from a varying average of the breathing displacements along the beam edge gradients. Regular breathing has no displacement variation over many breathing cycles. Some low-frequency interference is also possible in real situations. In the absence of more sophisticated motion management, methods that reduce the breathing amplitude or make the breathing very regular are indicated. However, for typical breathing patterns and magnitudes, motion management techniques may not be required with HT because typical breathing occurs mostly between fundamental HT treatment temporal and spatial scales. A movement beyond only discussing margins is encouraged for intensity modulated radiotherapy such that patient and machine motion interference will be minimized and beneficial averaging maximized. These results are found for homogeneous and longitudinal on-axis delivery for unplanned longitudinal dose modulations.

Treatment planning comparison between conformal radiotherapy and helical tomotherapy in the case of locally advanced-stage NSCLC

BACKGROUND AND PURPOSE

To investigate the impact of Helical Tomotherapy (HT) upon the dose distribution when compared to our routinely delivered 3D conformal radiotherapy (CRT) in the case of patients affected by stage III non-small-cell lung cancer (NSCLC).

MATERIAL AND METHODS

Thirteen stage III inoperable NSCLC patients were scheduled to receive 61.2-70.2Gy, 1.8Gy/fraction. Two treatment techniques (HT and CRT) were considered, and in the case of CRT the dose calculation was performed using both the pencil beam (PB) and Anisotropic Analytical Algorithm (AAA) available on the Varian Eclipse planning system. Dose volume constraints for PTV coverage and OAR sparing were assessed for the HT inverse planning with the highest priority upon PTV coverage and spinal cord sparing. The three plans were compared in terms of dose-volume histograms (DVHs) and normal tissue complication probability (NTCP). A statistical analysis was performed using non-parametric Wilcoxon matched pairs tests.

RESULTS

In CRT the use of a less accurate algorithm (PB) decreased the monitor unit number by 2.4%. HT significantly improved dose homogeneity within PTV compared with CRT_AAA. For lung parenchyma V20-V40 were lower with HT, corresponding to a decrease of 7% in the risk of radiation pneumonitis. The volume of the heart and esophagus irradiated to >45-60Gy were reduced using HT plans. For eight PTs with an esophagus-PTV overlap >5%, HT significantly reduced both late and acute esophageal complication probability.

CONCLUSIONS

Our findings obtained in stage III NSCLC patients underline that HT guarantees an important sparing of lungs and esophagus, thus HT has the potential to improve therapeutic ratio, when compared with CRT, by means of dose escalation and/or combined treatment strategy. In CRT of locally advanced lung cancers, the use of a more advanced algorithm would give significantly better modeling of target dose and coverage.

Comparative analysis of 60Co intensity-modulated radiation therapy

In this study, we perform a scientific comparative analysis of using (60)Co beams in intensity-modulated radiation therapy (IMRT). In particular, we evaluate the treatment plan quality obtained with (i) 6 MV, 18 MV and (60)Co IMRT; (ii) different numbers of static multileaf collimator (MLC) delivered (60)Co beams and (iii) a helical tomotherapy (60)Co beam geometry. We employ a convex fluence map optimization (FMO) model, which allows for the comparison of plan quality between different beam energies and configurations for a given case. A total of 25 clinical patient cases that each contain volumetric CT studies, primary and secondary delineated targets, and contoured structures were studied: 5 head-and-neck (H&N), 5 prostate, 5 central nervous system (CNS), 5 breast and 5 lung cases. The DICOM plan data were anonymized and exported to the University of Florida optimized radiation therapy (UFORT) treatment planning system. The FMO problem was solved for each case for 5-71 equidistant beams as well as a helical geometry for H&N, prostate, CNS and lung cases, and for 3-7 equidistant beams in the upper hemisphere for breast cases, all with 6 MV, 18 MV and (60)Co dose models. In all cases, 95% of the target volumes received at least the prescribed dose with clinical sparing criteria for critical organs being met for all structures that were not wholly or partially contained within the target volume. Improvements in critical organ sparing were found with an increasing number of equidistant (60)Co beams, yet were marginal above 9 beams for H&N, prostate, CNS and lung. Breast cases produced similar plans for 3-7 beams. A helical (60)Co beam geometry achieved similar plan quality as static plans with 11 equidistant (60)Co beams. Furthermore, 18 MV plans were initially found not to provide the same target coverage as 6 MV and (60)Co plans; however, adjusting the trade-offs in the optimization model allowed equivalent target coverage for 18 MV. For plans with comparable target coverage, critical structure sparing was best achieved with 6 MV beams followed closely by (60)Co beams, with 18 MV beams requiring significantly increased dose to critical structures. In this paper, we report in detail on a representative set of results from these experiments. The results of the investigation demonstrate the potential for IMRT radiotherapy employing commercially available (60)Co sources and a double-focused MLC. Increasing the number of equidistant beams beyond 9 was not observed to significantly improve target coverage or critical organ sparing and static plans were found to produce comparable plans to those obtained using a helical tomotherapy treatment delivery when optimized using the same well-tuned convex FMO model. While previous studies have shown that 18 MV plans are equivalent to 6 MV for prostate IMRT, we found that the 18 MV beams actually required more fluence to provide similar quality target coverage.

Dose as a function of lung volume and planned treatment volume in helical tomotherapy intensity-modulated radiation therapy-based stereotactic body radiation therapy for small lung tumors

PURPOSE

To evaluate the limitations of Hi-Art Helical Tomotherapy (Middleton, WI) stereotactic body radiotherapy (SBRT) for lung lesions, and to provide an initial report on patients treated with this method. Stereotactic body radiotherapy was shown to be an effective, well-tolerated treatment for early-stage, non-small-cell lung carcinoma (NSCLC). The Radiation Therapy Oncology Group (RTOG) 0236 protocol is currently evaluating three-dimensional conformal SBRT that delivers 60 Gy in three fractions.

METHODS AND MATERIALS

Inverse treatment planning for hypothetical lung gross tumor volumes (GTV) and planned treatment volume (PTV) expansions were performed. We tested the hypothesis that the maximum acceptable dose (MAD) to be delivered to the lesion by SBRT could be predicted by PTV and lung volume. Dose constraints on normal tissue were as designated by the RTOG protocol. Inverse planning was performed to find the maximum tolerated SBRT dose up to 60 Gy.

RESULTS

Regression analysis of the data obtained indicated a linear relationship between MAD, PTV, and lung volume. This generated two equations which may be useful predictive tools. Seven patients with Stage I and II NSCLC treated at the University of Virginia with this method tolerated the treatment extremely well, and suffered no greater than grade I toxicity, with no evidence of disease recurrence in follow-up from 2-20 months.

CONCLUSIONS

Helical tomotherapy SBRT for lung lesions is well-tolerated. In addition, the likely MAD for patients considered for this type of treatment can be predicted by PTV and lung volume.

A delivery transfer function (DTF) analysis for helical tomotherapy

The previous theoretical work of a delivery transfer function (DTF) in radiotherapy is expanded to include the unique intensity modulation method of helical tomotherapy. In addition to the collimation of each beamlet, and the Gaussian scatter convolution spreading of the dose that other radiotherapy units have, helical tomotherapy uses 51 small arcs of varying lengths to adjust the intensity. The blurring from these arcs is not taken into account during treatment planning. A theoretical DTF is constructed, and a calculation is performed which includes this unique source motion in relation to the other DTF components. Various typical delivery parameters are used to generate resolution maps for a constant intensity projection. Near the isocenter, the transverse (to a given beam direction) blurring is small but at larger radii (>6 cm), the source blurring dominates over leaf size. For most clinical situations, this inherent source motion blurring is expected to be negligible.