Stereotactic Image-Guided Intensity Modulated Radiotherapy Using the HI-ART II Helical Tomotherapy System

Helical Therapy is Safe for Lung Stereotactic Body Radiation Therapy Despite Limitations in Achieving Sharp Dose Gradients

PURPOSE

We observed that many of our helical therapy lung stereotactic body radiation therapy plans did not meet the Radiation Therapy Oncology Group (RTOG) recommended R50% (volume of 50% of the prescription dose/planning target volume), which characterizes the steepness of dose fall off. We hypothesized that despite not meeting R50%, helical therapy lung stereotactic body radiation therapy plans would confer similar local control and minimal side effects as previously reported using nonhelical treatment platforms.

MATERIALS AND METHODS

We report a retrospective review of all consecutive patients treated off-protocol with stereotactic body radiation therapy for peripheral lung lesions from 2008 to 2013 utilizing helical therapy. Seventy-four patients (81 lesions and 79 plans) were treated with doses ranging from 48 to 60 Gy in 3 to 5 fractions prescribed to the edge of the planning target volume.

RESULTS

Forty-eight (61%) plans had major deviation from R50%. Only 1 (<1%) plan had a major deviation from the R100%. All plans had > 95% planning target volume coverage by prescription dose, 7(8.6%) plans with 121% to 133% maximum dose, and lung V20 Gy <10% in 70 (89%) plans. With a median follow-up of 4.7 years (95% confidence interval: 4.1-5.3), local control for all patients at 1, 2, and 5 years was 94.6%, 83.4%, and 74%, respectively. For patients with primary stage I-II lung cancer (n = 46), the 1, 2, and 5-year local control: 97.2%, 94.2%, and 86.9%; RC: 97.6%, 82.5%, and 69.5%; and DM: 3%, 16%, and 33.4%, respectively. Patients treated for lung metastases (n = 26) had worse local control at 1, 2, and 5 years: 94.4%, 69.3%, and 55.5%, respectively. Side effects were rare with 2 (3%) patients reporting chest wall pain and 6 (8%) patients experiencing radiation pneumonitis, including 1 patient who had grade 5 radiation pneumonitis.

CONCLUSIONS

Helical therapy delivers a safe and effective lung stereotactic body radiation therapy plan, despite not being able to meet RTOG's recommended R50 conformality constraint.

Geometric validation of MV topograms for patient localization on TomoTherapy

Our goal was to geometrically validate the use of mega-voltage orthogonal scout images (MV topograms) as a fast and low-dose alternative to mega-voltage computed tomography (MVCT) for daily patient localization on the TomoTherapy system. To achieve this, anthropomorphic head and pelvis phantoms were imaged on a 16-slice kilo-voltage computed tomography (kVCT) scanner to synthesize kilo-voltage digitally reconstructed topograms (kV-DRT) in the Tomotherapy detector geometry. MV topograms were generated for couch speeds of 1-4 cm s(-1) in 1 cm s(-1) increments with static gantry angles in the anterior-posterior and left-lateral directions. Phantoms were rigidly translated in the anterior-posterior (AP), superior-inferior (SI), and lateral (LAT) directions to simulate potential setup errors. Image quality improvement was demonstrated by estimating the noise level in the unenhanced and enhanced MV topograms using a principle component analysis-based noise level estimation algorithm. Average noise levels for the head phantom were reduced by 2.53 HU (AP) and 0.18 HU (LAT). The pelvis phantom exhibited average noise level reduction of 1.98 HU (AP) and 0.48 HU (LAT). Mattes Mutual Information rigid registration was used to register enhanced MV topograms with corresponding kV-DRT. Registration results were compared to the known rigid displacements, which assessed the MV topogram localization's sensitivity to daily positioning errors. Reduced noise levels in the MV topograms enhanced the registration results so that registration errors were <1 mm. The unenhanced head MV topograms had discrepancies < 2.1 mm and the pelvis topograms had discrepancies < 2.7 mm. Result were found to be consistent regardless of couch speed. In total, 64.7% of the head phantom MV topograms and 60.0% of the pelvis phantom MV topograms exactly measured the phantom offsets. These consistencies demonstrated the potential for daily patient positioning using MV topogram pairs in the context bony-anatomy based procedures such as total marrow irradiation, total body irradiation, and cranial spinal irradiation.

Feasibility of non-coplanar tomotherapy for lung cancer stereotactic body radiation therapy

To quantify the dosimetric gains from non-coplanar helical tomotherapy (HT) arcs for stereotactic body radiation therapy (SBRT) of lung cancer, we created oblique helical arcs by rotating patient's CT images. Ten, 20 and 30 degrees of yaws were introduced in the treatment planning for a patient with a hypothetical lung tumor at the upper, middle and lower portion of the right lung, and the upper and middle left lung. The planning target volume (PTV) was 43 cm(3). 60 Gy was prescribed to the PTV. Dose to organs at risk (OARs), which included the lungs, heart, spinal cord and chest wall, was optimized using a 2.5 cm jaw, 0.287 pitch and modulation factor of 2.5. Composite plans were generated by dose summation of the resultant plans. These plans were evaluated for its conformity index (R(x)) and percentile volume of lung receiving radiation dose of x Gy (V(x)). Conformity index was defined by the ratio of x percent isodose volume and PTV. The results show that combination of non-coplanar arcs reduced R(50) by 4.5%, R(20) by 26% and R(10) by 30% on average. Non-coplanar arcs did not affect V(20) but reduced V(10) and V(5) by 10% and 24% respectively. Composite of the non-coplanar arcs also reduced maximum dose to the spinal cord by 20-39%. Volume of chest wall receiving higher than 30 Gy was reduced by 48% on average. Heart dose reduction was dependent on the location of the PTV and the choice of non-coplanar orientations. Therefore we conclude that non-coplanar HT arcs significantly improve critical organ sparing in lung SBRT without changing the PTV dose coverage.

A technique for stereotactic radiosurgery treatment planning with helical tomotherapy

The purpose of this study was to develop an efficient and effective planning technique for stereotactic radiosurgery using helical tomotherapy. Planning CTs and contours of 20 patients, previously treated in our clinic for brain metastases with linac-based radiosurgery using circular collimators, were used to develop a robust TomoTherapy planning technique. Plan calculation times as well as delivery times were recorded for all patients to allow for an efficiency evaluation. In addition, conformation and homogeneity indices were calculated as metrics to compare plan quality with that which is achieved with conventional radiosurgery delivery systems. A robust and efficient planning technique was identified to produce plans of radiosurgical quality using the TomoTherapy treatment planning system. Dose calculation did not exceed a few hours and resulting delivery times were less than 1 hour, which allows the process to fit into a single day radiosurgery workflow. Plan conformity compared favorably with published results for gamma knife radiosurgery. In addition, plan homogeneity was similar to linac-based approaches. The TomoTherapy planning software can be used to create plans of acceptable quality for stereotactic radiosurgery in a time that is appropriate for a radiosurgery workflow that requires that planning and delivery occur within 1 treatment day.

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.

Planning and delivery of whole brain radiation therapy with simultaneous integrated boost to brain metastases and synchronous limited-field thoracic radiotherapy using helical tomotherapy: a preliminary experience

Lung cancer is the commonest source of brain metastases, which has been traditionally treated with Whole Brain Radiation Therapy (WBRT) with or without focal boost. We herein report our preliminary experience of the planning and delivery of WBRT with Simultaneous Integrated Boost (SIB) to brain metastases along with synchronous limited-field thoracic radiotherapy using Helical TomoTherapy in four patients with lung cancer. All plans were iteratively optimized for maximal target volume coverage and organ-at-risk (OAR) sparing. Standardized dose metrics were used for plan evaluation. All treated regions were imaged with a megavoltage computed tomography (CT) prior to treatment and co-registered with planning CT for image-guidance. Helical TomoTherapy was able to achieve highly conformal and homogeneous dose distributions with excellent OAR sparing both in the brain and the chest. The mean (standard deviation) Dose Homogeneity Index (DHI) and Conformity Index (CI) was 0.06 (0.01) & 0.79 (0.07); 0.04 (0.02) & 0.57 (0.22); and 0.03 (0.02) & 0.77 (0.06) for whole brain, brain metastases, and chest, respectively. The mean monitor units (MU) per fraction and time taken for delivery were 8595 and 9898 MU and 9.8 and 11.3 minutes for the brain and chest plans, respectively. Although the dosimetric equivalence of SIB to a single fraction radiosurgery might still be questionable, our preliminary experience of WBRT with SIB to individual brain metastases using Helical TomoTherapy has been encouraging. In addition, it allows synchronous irradiation of other involved primary or metastatic sites for palliative effect.

Does intensity-modulated stereotactic radiotherapy achieve superior target conformity than conventional stereotactic radiotherapy in different intracranial tumours?

AIMS

To compare the dosimetric outcome of various conventional stereotactic radiotherapy (SRT) techniques with intensity-modulated stereotactic radiotherapy (IMSRT) in brain tumours of varying shape, size, location and proximity to organs at risk (OARs).

MATERIALS AND METHODS

Fused computed tomography and magnetic resonance imaging datasets of four patients with different brain tumours previously treated with non-coplanar static conformal fields (SCF) were re-planned on the BrainScan treatment planning system using non-coplanar conformal arcs (CA), dynamic conformal arcs (DCA) and IMSRT with coplanar (IMSRT_CP) or non-coplanar (IMSRT_NCP) beam arrangement. Beam shaping and intensity modulation were carried out using a BrainLab micromultileaf collimator. The primary objective for each plan was to encompass >or=99% of the planning target volume (PTV) by >95% of the prescribed dose while minimising the dose to OARs.

RESULTS

The mean PTV coverage in SCF, CA, DCA, IMSRT_NCP and IMSRT_CP was 99.2, 99.5, 99.4, 99.2 and 99.2%, respectively. The highest dose within the target was <107% of the prescribed dose in all plans. Conformity was found to vary depending on the shape and location of the target. The best mean conformity index, ranging from 0.74 (CA) to 0.84 (IMSRT_NCP) was observed in spherical tumours. Among the three conventional SRT techniques, DCA and SCF appeared comparable (mean conformity index 0.72 and 0.71, respectively) and more conformal than CA (mean conformity index 0.67). In all cases, IMSRT showed better target conformity than conventional SRT techniques with a mean conformity index of 0.83 for non-coplanar and 0.81 for coplanar beam arrangement. The maximum improvement in conformity index was observed for IMSRT_NCP in complex, concave and irregularly shaped targets. The volume of normal brain and other OARs irradiated to high (>or=80%) and low (>or=30%) dose varied depending on the tumour shape, size, and location, but was essentially comparable in all three conventional SRT techniques. IMSRT (both coplanar as well as non-coplanar) reduced the volume of normal brain being irradiated to moderate to high doses compared with conventional SRT techniques, more so for large and irregular targets.

CONCLUSIONS

DCA and SCF are preferred conventional SRT techniques in terms of target conformity and reduction of doses to OARs. The use of IMSRT_NCP further improves conformity and reduces doses to OARs in a range of brain tumours commonly considered for stereotactic irradiation.