Pelvic organ motion and dosimetric implications during horizontal patient rotation for prostate radiation therapy.

PURPOSE: Gantry-free radiation therapy systems utilizing patient rotation would be simpler and more cost effective than the conventional gantry-based systems. Such a system could enable the expansion of radiation therapy to meet global demand and reduce capital costs. Recent advances in adaptive radiation therapy could potentially be applied to correct for gravitational deformation during horizontal patient rotation. This study aims to quantify the pelvic organ motion and the dosimetric implications of horizontal rotation for prostate intensity-modulated radiation therapy (IMRT) treatments. METHODS: Eight human participants who previously received prostate radiation therapy were imaged in a clinical magnetic resonance imaging (MRI) scanner using a bespoke patient rotation system (PRS). The patients were imaged every 45 degrees during a full roll rotation (0-360 degrees). Whole pelvic bone, prostate, rectum, and bladder motion were compared to the supine position using dice similarity coefficient (DSC) and mean absolute surface distance (MASD). Prostate centroid motion was compared in the left-right (LR), superior-inferior (SI), and anterior-posterior (AP) direction prior to and following pelvic bone-guided rigid registration. Seven-field prostate IMRT treatment plans were generated for each patient rotation angles under three adaption scenarios: No plan adaption, rigid planning target volume (PTV)-guided alignment to the prostate, and plan re-optimization. Prostate, rectum, and bladder doses were compared for each adaption scenario. RESULTS: Pelvic bone motion within the PRS of up to 53 mm relative to the supine position was observed for some participants. Internal organ motion was greatest at the 180-degree PRS couch angle (prone), with prostate centroid motion range < 2 mm LR, 0 mm to 14 mm SI, and -11 mm to 4 mm AP. Rotation with no adaption of the treatment plan resulted in an underdose to the PTV -- in some instances up to 75% (D95%: 78 ± 0.3 Gy at supine to 20 ± 15.0 Gy at the 225-degree PRS couch angle). Bladder dose was reduced during the rotation by up to 98% (V60 Gy: 15.0 ± 9.4% supine to 0.3 ± 0.5% at the 225-degree PRS couch angle). In some instances, the rectum dose increased during rotation (V60Gy: 20.0 ± 4.5% supine to 25.0 ± 15.0% at the 135-degree PRS couch angle). Rigid PTV-guided alignment resulted in PTV coverage which, though statistically lower (P < 0.05 for all D95% values), was within 1 Gy of the supine plans. Plan re-optimization resulted in a statistically equivalent PTV coverage compared to the supine plans (P > 0.05 for all D95% metrics and all within ±0.4 Gy). For both rigid PTV-guided alignment and plan re-optimization, rectum dose volume metrics were reduced compared to the supine position between the 90- and 225-degree PRS couch angles (P < 0.05). Bladder dose volume metrics were not impacted by rotation. CONCLUSION: Pelvic bone and internal organ motion are present during patient rotation. Rigid PTV-guided alignment to the prostate will be a requirement if prostate IMRT is to be safely delivered using patient rotation. Plan re-optimization for each PRS couch angle to account for anatomical deformations further improves the PTV coverage.

Anatomical deformation due to horizontal rotation: towards gantry-free radiation therapy.

Gantry-free radiation therapy systems may be simpler and more cost effective, particularly for MRI-guided photon or hadron therapy. This study aims to understand and quantify anatomical deformations caused by horizontal rotation with scan sequences sufficiently short to facilitate integration into an MRI-guided workflow. Rigid and non-rigid pelvic deformations due to horizontal rotation were quantified for a cohort of 8 healthy volunteers using a bespoke patient rotation system and a clinical MRI scanner. For each volunteer a reference scan was acquired at 0° followed by sequential faster scans in 45° increments through to 360°. All fast scans were registered to the 0° image via a three-step process: first, images were aligned using MR visible couch markers. Second, the scans were pre-processed then rigidly registered to the 0° image. Third, the rigidly registered scans were non-rigidly registered to the 0° image to assess soft tissue deformation. The residual differences after rigid and non-rigid registration were determined from the transformation matrix and the deformation vector field, respectively. The rigid registration yielded mean rotations of ⩽2.5° in all cases. The average 3D translational magnitudes range was 5.8 ± 2.9 mm-30.0 ± 11.0 mm. Translations were most significant in the left-right (LR) direction. Smaller translations were observed in the anterior-posterior (AP) and superior-inferior (SI) directions. The maximum deformation magnitudes range was: 10.0 ± 0.9 mm-28.0 ± 2.8 mm and average deformation magnitudes range: 2.3 ± 0.6 mm-7.5 ± 1.0 mm. Average non-rigid deformation magnitude was correlated with BMI (correlation coefficient 0.84, p  = 0.01). Rigid pelvic deformations were most significant in the LR direction but could be accounted for with on-line adjustments. Non-rigid deformations can be significant and will need to be accounted for in order to facilitate the delivery of gantry-free therapy with an automated patient rotation system.

A prospective study of weekly intensity modulated radiation therapy plan adaptation for head and neck cancer: improved target coverage and organ at risk sparing.

This prospective study of weekly CT scanning and plan adaption during H&N IMRT reports on the frequency of plan adaptations based on dosimetric differences between original and re-optimised IMRT plans. The volumetric and geometric change occurring in target volumes and salivary glands is also described. Ten H&N cancer patients underwent weekly planning CT imaging and re-optimisation of the IMRT plan if PTV or OAR coverage was unacceptable. Comparisons of PTV and parotid gland dosimetry between the original and adaptive plans were made. Parotid and submandibular gland volume changes and shift were calculated. Eight of ten patients required one or more plan adaptations, with 41% of adaptations occurring by fraction ten. Salivary glands reduced in volume, with a medial shift of the lateral border of the parotid gland and a superior shift of the submandibular gland. Change in PTV coverage did not correlate with weight loss or nutritional score. Inadequate PTV coverage, requiring plan adaptation, occurs early in the course of IMRT. A weekly Adaptive RT (ART) protocol results in significant improvement of PTV coverage. Implementation of a clinical ART protocol should include imaging and dose calculation within the first ten fractions.

A simple model for transit dosimetry based on a water equivalent EPID.

PURPOSE: The aim of this study was to demonstrate a new model for implementing a transit dosimetry system as a means of in vivo dose verification with a water equivalent electronic portal imaging device (WE-EPID) and a conventional treatment planning system (TPS). METHOD AND MATERIALS: A standard amorphous silicon (a-Si) EPID was modified to a WE-EPID configuration by replacing the metal-plate/phosphor screen situated above the photodiode detector with a 3 cm thick water equivalent plastic x ray converter material. A clinical TPS was used to calculate dose to the WE-EPID in its conventional EPID position behind the phantom/patient. The "extended phantom" concept was used to facilitate dose calculation at the EPID position, which is outside the CT field of view (FOV). The CT images were manipulated from 512 × 512 into 1024 × 1024 and padded pixels were assigned the density of air before importing to the TPS. The virtual WE-EPID was added as an RT structure of water density at the EPID plane. The accuracy of TPS dose calculations at the EPID plane in transit geometry was first evaluated for different field sizes and thickness of object in the beam by comparison with the dose measured using a 2D ion chamber array (ICA) and the WE-EPID. Following basic dose response tests, clinical fields including direct single fields (open and wedged) and modulated fields (integrated or control point by control point doses for VMAT) were measured for 6 MV photons with varying of solid water thickness or an anthropomorphic phantom present in beam. The EPID images were corrected for dark signal and pixel sensitivity and converted to dose using a single dose calibration factor. The 2D dose evaluation was conducted using 3%/3 and 2%/2 mm gamma-index criteria. RESULTS: The measured dose-response with the ICA and WE-EPID for all basic dose-response tests agreed with TPS dose calculations to within 1.5%. The maximum difference in dose profiles for the largest measured field size of 25 × 25 cm2 was 2.5%. Gamma evaluation showed at least 94% (3%/3 mm criteria) and 90% (2%/2 mm) agreement in both integrated and control-point doses for all clinical fields acquired by the WE-EPID and ICA when compared with TPS-calculated portal dose images. CONCLUSION: A new approach to transit dose verification has been demonstrated utilizing a water equivalent EPID and a commercial TPS. The accuracy of dose calculations at the EPID plane using a commercial TPS beam model was experimentally confirmed. The model proposed in this study provides an accurate method to directly verify doses delivered during treatment without the additional uncertainties inherent in modelling the complex dose-response of standard EPIDs.

Defining and assessing an anisotropic delineation margin for modern radiotherapy.

PURPOSE: Uncertainty in target volume delineation for modern radiotherapy impacts dosimetry and patient outcomes. Delineation uncertainty is generally overlooked in practice as a source of error, potentially since historically, other uncertainties have been the main focus. This work defined and assessed an anisotropic delineation margin in both polar and spherical coordinate systems in order to account for the spatially varying nature of this uncertainty using a whole breast radiotherapy cohort as a proof of concept. METHODS: A cohort of 21 whole breast radiotherapy patient datasets with clinical target volumes delineated by eight independent observers was utilized. Patients were divided into categories based on target volume and laterality. An anisotropic delineation margin for each category was determined by multiplying the average standard deviation in observer contours in each category by a factor of two. Standard deviation was determined in both polar and spherical coordinates at angular increments. This anisotropic approach was compared to a conventional clinical approach, where the delineation margin was applied in the cardinal directions only. The assessment of the delineation margin was undertaken by comparing the encompassment of the observer volumes by the target volume with added margin. The extra, presumed healthy tissue included in the margin and the malignant tissue missed by the margin were determined. RESULTS: The proposed delineation margin is effective at accounting for inter-observer variation, producing >95% coverage of all CTVs for polar, spherical, and Cartesian margins in 82%, 79%, and 92% of cases, respectively. Additionally, <1% malignant tissue was missed for 65%, 70%, and 91% of cases and <37% healthy tissue was included in 95%, 89%, and 97% of cases. A conventional delineation margin approach is most appropriate for small and gold standard target volumes. However, for large target volumes, an anisotropic margin is necessary, producing significantly greater coverage of CTVs, including significantly less presumed healthy tissue and missing significantly less malignant tissue. CONCLUSIONS: All delineation margin methods that account for target volume and laterality proved to be adequate, with appropriate encompassment of interobserver variation and minimal inclusion of extra excess healthy tissue and exclusion of possible malignant tissue. The anisotropic approach was found to be superior to a conventional approach for target volumes >1400 cm3 only with significantly greater encompassment of interobserver variation, less missed malignant tissue and less included healthy tissue. This methodology has been validated for a whole breast radiotherapy cohort as a proof of concept, however could be applied to other anatomical sites.

In vivo skin dose measurement using MOSkin detectors in tangential breast radiotherapy.

The purpose of this study is to measure patient skin dose in tangential breast radiotherapy. Treatment planning dose calculation algorithm such as Pencil Beam Convolution (PBC) and in vivo dosimetry techniques such as radiochromic film can be used to accurately monitor radiation doses at tissue depths, but they are inaccurate for skin dose measurement. A MOSFET-based (MOSkin) detector was used to measure skin dose in this study. Tangential breast radiotherapies ("bolus" and "no bolus") were simulated on an anthropomorphic phantom and the skin doses were measured. Skin doses were also measured in 13 patients undergoing each of the techniques. In the patient study, the EBT2 measurements and PBC calculation tended to over-estimate the skin dose compared with the MOSkin detector (p<0.05) in the "no bolus radiotherapy". No significant differences were observed in the "bolus radiotherapy" (p>0.05). The results from patients were similar to that of the phantom study. This shows that the EBT2 measurement and PBC calculation, while able to predict accurate doses at tissue depths, are inaccurate in predicting doses at build-up regions. The clinical application of the MOSkin detectors showed that the average total skin doses received by patients were 1662±129cGy (medial) and 1893±199cGy (lateral) during "no bolus radiotherapy". The average total skin doses were 4030±72cGy (medial) and 4004±91cGy (lateral) for "bolus radiotherapy". In some cases, patient skin doses were shown to exceed the dose toxicity level for skin erythema. Hence, a suitable device for in vivo dosimetry is necessary to accurately determine skin dose.

Dose enhancement in radiotherapy of small lung tumors using inline magnetic fields: A Monte Carlo based planning study.

PURPOSE: To report on significant dose enhancement effects caused by magnetic fields aligned parallel to 6 MV photon beam radiotherapy of small lung tumors. Findings are applicable to future inline MRI-guided radiotherapy systems. METHODS: A total of eight clinical lung tumor cases were recalculated using Monte Carlo methods, and external magnetic fields of 0.5, 1.0, and 3 T were included to observe the impact on dose to the planning target volume (PTV) and gross tumor volume (GTV). Three plans were 6 MV 3D-CRT plans while 6 were 6 MV IMRT. The GTV's ranged from 0.8 to 16 cm(3), while the PTV's ranged from 1 to 59 cm(3). In addition, the dose changes in a 30 cm diameter cylindrical water phantom were investigated for small beams. The central 20 cm of this phantom contained either water or lung density insert. RESULTS: For single beams, an inline magnetic field of 1 T has a small impact in lung dose distributions by reducing the lateral scatter of secondary electrons, resulting in a small dose increase along the beam. Superposition of multiple small beams leads to significant dose enhancements. Clinically, this process occurs in the lung tissue typically surrounding the GTV, resulting in increases to the D98% (PTV). Two isolated tumors with very small PTVs (3 and 6 cm(3)) showed increases in D98% of 23% and 22%. Larger PTVs of 13, 26, and 59 cm(3) had increases of 9%, 6%, and 4%, describing a natural fall-off in enhancement with increasing PTV size. However, three PTVs bounded to the lung wall showed no significant increase, due to lack of dose enhancement in the denser PTV volume. In general, at 0.5 T, the GTV mean dose enhancement is around 60% lower than that at 1 T, while at 3 T, it is 5%-60% higher than 1 T. CONCLUSIONS: Monte Carlo methods have described significant and predictable dose enhancement effects in small lung tumor plans for 6 MV radiotherapy when an external inline magnetic field is included. Results of this study indicate that future clinical inline MRI-guided radiotherapy systems will be able to deliver a dosimetrically superior treatment to small (PTV < 15 cm(3)), isolated lung tumors over non-MRI-Linac systems. This increased efficacy coincides with the reimbursement in the United States of lung CT screening and the likely rapid growth in the number of patients with small lung tumors to be treated with radiotherapy.

Proton beam deflection in MRI fields: Implications for MRI-guided proton therapy.

PURPOSE: This paper investigates, via magnetic modeling and Monte Carlo simulation, the ability to deliver proton beams to the treatment zone inside a split-bore MRI-guided proton therapy system. METHODS: Field maps from a split-bore 1 T MRI-Linac system are used as input to geant4 Monte Carlo simulations which model the trajectory of proton beams during their paths to the isocenter of the treatment area. Both inline (along the MRI bore) and perpendicular (through the split-bore gap) orientations are simulated. Monoenergetic parallel and diverging beams of energy 90, 195, and 300 MeV starting from 1.5 and 5 m above isocenter are modeled. A phase space file detailing a 2D calibration pattern is used to set the particle starting positions, and their spatial location as they cross isocenter is recorded. No beam scattering, collimation, or modulation of the proton beams is modeled. RESULTS: In the inline orientation, the radial symmetry of the solenoidal style fringe field acts to rotate the protons around the beam's central axis. For protons starting at 1.5 m from isocenter, this rotation is 19° (90 MeV) and 9.8° (300 MeV). A minor focusing toward the beam's central axis is also seen, but only significant, i.e., 2 mm shift at 150 mm off-axis, for 90 MeV protons. For the perpendicular orientation, the main MRI field and near fringe field act as the strongest to deflect the protons in a consistent direction. When starting from 1.5 m above isocenter shifts of 135 mm (90 MeV) and 65 mm (300 MeV) were observed. Further to this, off-axis protons are slightly deflected toward or away from the central axis in the direction perpendicular to the main deflection direction. This leads to a distortion of the phase space pattern, not just a shift. This distortion increases from zero at the central axis to 10 mm (90 MeV) and 5 mm (300 MeV) for a proton 150 mm off-axis. In both orientations, there is a small but subtle difference in the deflection and distortion pattern between protons fired parallel to the beam axis and those fired from a point source. This is indicative of the 3D spatially variant nature of the MRI fringe field. CONCLUSIONS: For the first time, accurate magnetic and Monte Carlo modeling have been used to assess the transport of generic proton beams toward a 1 T split-bore MRI. Significant rotation is observed in the inline orientation, while more complex deflection and distortion are seen in the perpendicular orientation. The results of this study suggest that due to the complexity and energy-dependent nature of the magnetic deflection and distortion, the pencil beam scanning method will be the only choice for delivering a therapeutic proton beam inside a potential MRI-guided proton therapy system in either the inline or perpendicular orientation. Further to this, significant correction strategies will be required to account for the MRI fringe fields.

Electron contamination modeling and reduction in a 1 T open bore inline MRI-linac system.

PURPOSE: A potential side effect of inline MRI-linac systems is electron contamination focusing causing a high skin dose. In this work, the authors reexamine this prediction for an open bore 1 T MRI system being constructed for the Australian MRI-Linac Program. The efficiency of an electron contamination deflector (ECD) in purging electron contamination from the linac head is modeled, as well as the impact of a helium gas region between the deflector and phantom surface for lowering the amount of air-generated contamination. METHODS: Magnetic modeling of the 1 T MRI was used to generate 3D magnetic field maps both with and without the presence of an ECD located immediately below the MLC's. Forty-seven different ECD designs were modeled and for each the magnetic field map was imported into Geant4 Monte Carlo simulations including the linac head, ECD, and a 30 × 30 × 30 cm(3) water phantom located at isocenter. For the first generation system, the x-ray source to isocenter distance (SID) will be 160 cm, resulting in an 81.2 cm long air gap from the base of the ECD to the phantom surface. The first 71.2 cm was modeled as air or helium gas, with the latter encased between two windows of 50 µm thick high density polyethlyene. 2D skin doses (at 70 µm depth) were calculated across the phantom surface at 1 × 1 mm(2) resolution for 6 MV beams of field size of 5 × 5, 10 × 10, and 20 × 20 cm(2). RESULTS: The skin dose was predicted to be of similar magnitude as the generic systems modeled in previous work, 230% to 1400% of D(max) for 5 × 5 to 20 × 20 cm(2), respectively. Inclusion of the ECD introduced a nonuniformity to the MRI imaging field that ranged from ∼20 to ∼140 ppm while the net force acting on the ECD ranged from ∼151 N to ∼1773 N. Various ECD designs were 100% efficient at purging the electron contamination into the ECD magnet banks; however, a small percentage were scattered back into the beam and continued to the phantom surface. Replacing a large portion of the extended air-column between the ECD and phantom surface with helium gas is a key element as it significantly minimized the air-generated contamination. When using an optimal ECD and helium gas region, the 70 µm skin dose is predicted to increase moderately inside a small hot spot over that of the case with no magnetic field present for the jaw defined square beams examined here. These increases include from 12% to 40% of [Formula: see text] for 5 × 5 cm(2), 18% to 55% of D(max) for 10 × 10 cm(2), and from 23% to 65% of D(max) for 20 × 20 cm(2). CONCLUSIONS: Coupling an efficient ECD and helium gas region below the MLCs in the 160 cm isocenter MRI-linac system is predicted to ameliorate the impact electron contamination focusing has on skin dose increases. An ECD is practical as its impact on the MRI imaging distortion is correctable, and the mechanical forces acting on it manageable from an engineering point of view.

BrachyView: proof-of-principle of a novel in-body gamma camera for low dose-rate prostate brachytherapy.

PURPOSE: The conformity of the achieved dose distribution to the treatment plan strongly correlates with the accuracy of seed implantation in a prostate brachytherapy treatment procedure. Incorrect seed placement leads to both short and long term complications, including urethral and rectal toxicity. The authors present BrachyView, a novel concept of a fast intraoperative treatment planning system, to provide real-time seed placement information based on in-body gamma camera data. BrachyView combines the high spatial resolution of a pixellated silicon detector (Medipix2) with the volumetric information acquired by a transrectal ultrasound (TRUS). The two systems will be embedded in the same probe so as to provide anatomically correct seed positions for intraoperative planning and postimplant dosimetry. Dosimetric calculations are based on the TG-43 method using the real position of the seeds. The purpose of this paper is to demonstrate the feasibility of BrachyView using the Medipix2 pixel detector and a pinhole collimator to reconstruct the real-time 3D position of low dose-rate brachytherapy seeds in a phantom. METHODS: BrachyView incorporates three Medipix2 detectors coupled to a multipinhole collimator. Three-dimensionally triangulated seed positions from multiple planar images are used to determine the seed placement in a PMMA prostate phantom in real time. MATLAB codes were used to test the reconstruction method and to optimize the device geometry. RESULTS: The results presented in this paper show a 3D position reconstruction accuracy of the seed in the range of 0.5-3 mm for a 10-60 mm seed-to-detector distance interval (Z direction), respectively. The BrachyView system also demonstrates a spatial resolution of 0.25 mm in the XY plane for sources at 10 mm distance from Medipix2 detector plane, comparable to the theoretical value calculated for an equivalent gamma camera arrangement. The authors successfully demonstrated the capability of BrachyView for real-time imaging (using a 3 s data acquisition time) of different brachytherapy seed configurations (with an activity of 0.05 U) throughout a 60 × 60 × 60 mm(3) Perspex prostate phantom. CONCLUSIONS: The newly developed miniature gamma camera component of BrachyView, with its high spatial resolution and real time capability, allows accurate 3D localization of seeds in a prostate phantom. Combination of the gamma camera with TRUS in a single probe will complete the BrachyView system.

Electron contamination modeling and skin dose in 6 MV longitudinal field MRIgRT: Impact of the MRI and MRI fringe field.

PURPOSE: In recent times, longitudinal field MRI-linac systems have been proposed for 6 MV MRI-guided radiotherapy (MRIgRT). The magnetic field is parallel with the beam axis and so will alter the transport properties of any electron contamination particles. The purpose of this work is to provide a first investigation into the potential effects of the MR and fringe magnetic fields on the electron contamination as it is transported toward a phantom, in turn, providing an estimate of the expected patient skin dose changes in such a modality. METHODS: Geant4 Monte Carlo simulations of a water phantom exposed to a 6 MV x-ray beam were performed. Longitudinal magnetic fields of strengths between 0 and 3 T were applied to a 30 × 30 × 20 cm(3) phantom. Surrounding the phantom there is a region where the magnetic field is at full MRI strength, consistent with clinical MRI systems. Beyond this the fringe magnetic field entering the collimation system is also modeled. The MRI-coil thickness, fringe field properties, and isocentric distance are varied and investigated. Beam field sizes of 5 × 5, 10 × 10, 15 × 15 and 20 × 20 cm(2) were simulated. Central axis dose, 2D virtual entry skin dose films, and 70 µm skin depth doses were calculated using high resolution scoring voxels. RESULTS: In the presence of a longitudinal magnetic field, electron contamination from the linear accelerator is encouraged to travel almost directly toward the patient surface with minimal lateral spread. This results in a concentration of electron contamination within the x-ray beam outline. This concentration is particularly encouraged if the fringe field encompasses the collimation system. Skin dose increases of up to 1000% were observed for certain configurations and increases above Dmax were common. In nonmagnetically shielded cases, electron contamination generated from the jaw faces and air column is trapped and propagated almost directly to the phantom entry region, giving rise to intense dose hot spots inside the x-ray treatment field. These range up to 1000% or more of Dmax at the CAX, depending on field size, isocenter, and coil thickness. In the case of a fully magnetically shielded collimation system and the lowest MRI field of 0.25 T, the entry skin dose is expected to increase to at least 40%, 50%, 65%, and 80% of Dmax for 5 × 5, 10 × 10, 15 × 15, and 20 × 20 cm(2), respectively. CONCLUSIONS: Electron contamination from the linac head and air column may cause considerable skin dose increases or hot spots at the beam central axis on the entry side of a phantom or patient in longitudinal field 6 MV MRIgRT. This depends heavily on the properties of the magnetic fringe field entering the linac beam collimation system. The skin dose increase is also related to the MRI-coil thickness, the fringe field, and the isocenter distance of the linac. The results of this work indicate that the properties of the MRI fringe field, electron contamination production, and transport must be considered carefully during the design stage of a longitudinal MRI-linac system.

Monte Carlo characterization of skin doses in 6 MV transverse field MRI-linac systems: effect of field size, surface orientation, magnetic field strength, and exit bolus.

PURPOSE: The main focus of this work is to continue investigations into the Monte Carlo predicted skin doses seen in MRI-guided radiotherapy. In particular, the authors aim to characterize the 70 microm skin doses over a larger range of magnetic field strength and x-ray field size than in the current literature. The effect of surface orientation on both the entry and exit sides is also studied. Finally, the use of exit bolus is also investigated for minimizing the negative effects of the electron return effect (ERE) on the exit skin dose. METHODS: High resolution GEANT4 Monte Carlo simulations of a water phantom exposed to a 6 MV x-ray beam (Varian 2100C) have been performed. Transverse magnetic fields of strengths between 0 and 3 T have been applied to a 30 x 30 x 20 cm3 phantom. This phantom is also altered to have variable entry and exit surfaces with respect to the beam central axis and they range from -75 degrees to +75 degrees. The exit bolus simulated is a 1 cm thick (water equivalent) slab located on the beam exit side. RESULTS: On the entry side, significant skin doses at the beam central axis are reported for large positive surface angles and strong magnetic fields. However, over the entry surface angle range of -30 degrees to -60 degrees, the entry skin dose is comparable to or less than the zero magnetic field skin dose, regardless of magnetic field strength and field size. On the exit side, moderate to high central axis skin dose increases are expected except at large positive surface angles. For exit bolus of 1 cm thickness, the central axis exit skin dose becomes an almost consistent value regardless of magnetic field strength or exit surface angle. This is due to the almost complete absorption of the ERE electrons by the bolus. CONCLUSIONS: There is an ideal entry angle range of -30 degrees to -60 degrees where entry skin dose is comparable to or less than the zero magnetic field skin dose. Other than this, the entry skin dose increases are significant, especially at higher magnetic fields. On the exit side there is mostly moderate to high skin dose increases for 0.2-3 T with the only exception being large positive angles. Exit bolus of 1 cm thickness will have a significant impact on lowering such exit skin dose increases that occur as a result of the ERE.

Rectal dose reduction with IMRT for prostate radiotherapy.

Dose escalation in radiation therapy has led to increased control rates with some clinical trial evidence that rectal toxicity may be reduced when using intensity-modulated radiotherapy (IMRT) over 3D conformal radiotherapy (3DCRT) for dose-escalated prostate radiotherapy. However, IMRT for prostate patients is not yet standard in many Australian radiation oncology centres. This study investigates dosimetric changes that can be observed between IMRT and 3DCRT in prostate radiotherapy. Fifteen patients were selected for analysis. Two target definitions were investigated--prostate-only and prostate plus seminal vesicles (p + SVs). A five-field 3DCRT and seven-field IMRT plan were created for each patient and target definition. The planning target volume coverage was matched for both plans. Doses to the rectum, bladder and femoral heads were compared using dose volume histograms. The rectal normal tissue complication probabilities (NTCPs) were calculated and compared for the 3DCRT and IMRT plans. The delivery efficiency was investigated. The IMRT plans resulted in reductions in the V25, V50, V60, V70 and V75 Gy values for both the prostate-only and p + SVs targets. Rectal NTCP was reduced with IMRT for three different sets of model parameters. The reductions in rectal dose and NTCP were much larger for the p + SVs target. Delivery of IMRT plans was less efficient than for 3DCRT plans. IMRT resulted in superior plans based on dosimetric and biological endpoints. The dosimetric gains with IMRT were greater for the more complex p + SVs target. The gains made came at the cost of decreased delivery efficiency.

High resolution entry and exit Monte Carlo dose calculations from a linear accelerator 6 MV beam under the influence of transverse magnetic fields.

A current concern with 6 MV transverse field MRI-linac hybrid systems is the predicted increases in skin dose (both the entry and exit sides) caused by the effects of the magnetic field on secondary electrons. In this work high resolution GEANT4 Monte Carlo simulations have been performed at the beam central axis in the entry and exit regions of a water phantom to predict surface (0 microm depth) and skin (70 microm depth) doses when placed in such a hybrid system. A 30 x 30 x 20 cm3 water phantom with 10 microm thick voxels has been simulated by being irradiated perpendicularly with a 6 MV photon beam (Varian 2100C) of sizes of 5 x 5, 10 x 10, 15 x 15, and 20 x 20 cm2. Uniform transverse magnetic fields of 0.2, 0.75, 1.5, and 3 T with varying thickness above the phantom have been investigated. Simulations with and without lepton contamination have been performed. In the entry region the high resolution scoring has yielded unexpected surface and skin doses. There is a small amount of nonpurged air-generated lepton contamination that originates immediately above the phantom surface and delivers its dose over very short longitudinal distances in the entry region. At 0.2 T the surface and skin doses are not accurately predicted using lepton-contamination-free simulations and extrapolated lower resolution scoring. Lepton-free simulations are up to 7% of Dmax lower than simulations with leptons. However, compared to 0 T, entry skin dose is reduced at 0.2 and 0.75 T but increases to 28%-31% of Dmax at 3 T. For skin doses at the central axis in the exit region, high resolution scoring shows relative increases of 38%-106%, depending on the magnetic field strength and field size. These values are also up to 20% higher than lower resolution results. The shape of the exit dose profiles varies unpredictably and so extrapolation of low resolution data is insufficient. In order to achieve accurate Monte Carlo skin dosimetry in a transverse field MRI-linac system, the authors recommend using high resolution scoring. In systems of 0.2 T the inclusion of air-generated lepton contamination is also recommended.

A comparison of proton therapy and IMRT treatment plans for prostate radiotherapy.

Proton therapy (PT) is becoming a more widely available treatment option on the world stage and there is some interest in investment in this treatment option in Australia. The benefit of PT has been shown for a number of tumour sites, particularly for paediatric patients. The workload from these patients may not completely fill the maximum yearly workload of a machine. This work aims to ascertain if prostate cancer would be a suitable candidate to fill the rest of the workload at an Australian PT facility. Passive and intensity modulated proton therapy (IMPT) plans were generated for a prostate patient. These were compared to 7 field sliding window and step and shoot IMRT plans. All plans used a prescription dose of 78 CGE. IMRT and IMPT plans used inverse planning for optimisation. Homogeneity in the PTV was best for the IMPT plan. IMPT also gave the best rectal sparing. The bladder and femoral heads were exposed to less dose in both proton plans. Proton plans exposed normal tissue outside the PTV to less than 50% of the dose given by the IMRT plans. PT, particularly IMPT, is a suitable treatment option for the prostate cancer patient presented here.

Multicentre quality assurance of intensity-modulated radiation therapy plans: a precursor to clinical trials.

A multicentre planning study comparing intensity-modulated radiation therapy (IMRT) plans for the treatment of a head and neck cancer has been carried out. Three Australian radiotherapy centres, each with a different planning system, were supplied a fully contoured CT dataset and requested to generate an IMRT plan in accordance with the requirements of an IMRT-based radiation therapy oncology group clinical trial. Plan analysis was carried out using software developed specifically for reviewing multicentre clinical trial data. Two out of the three plans failed to meet the prescription requirements with one misinterpreting the prescription and the third failed to meet one of the constraints. Only one plan achieved all of the dose objectives for the critical structures and normal tissues. Although each centre used very similar planning parameters and beam arrangements the resulting plans were quite different. The subjective interpretation and application of the prescription and planning objectives emphasize one of the many difficulties in carrying out multicentre IMRT planning studies. The treatment prescription protocol in a clinical trial must be both lucid and unequivocally stated to avoid misinterpretation. Australian radiotherapy centres must show that they can produce a quality IMRT plan and that they can adhere to protocols for IMRT planning before using it in a clinical trial.

Measurement of radiotherapy x-ray skin dose on a chest wall phantom.

Sufficient skin dose needs to be delivered by a radiotherapy chest wall treatment regimen to ensure the probability of a near surface tumor recurrence is minimized. To simulate a chest wall treatment a hemicylindrical solid water phantom of 7.5 cm radius was irradiated with 6 MV x-rays using 20x20 cm2 and 10x20 cm2 fields at 100 cm source surface distance (SSD) to the base of the phantom. A surface dose profile was obtained from 0 to 180 degrees, in 10 degrees increments around the circumference of the phantom. Dosimetry results obtained from radiochromic film (effective depth of 0.17 mm) were used in the investigation, the superficial doses were found to be 28% (of Dmax) at the 0 degrees beam entry position and 58% at the 90 degrees oblique beam position. Superficial dose results were also obtained using extra thin thermoluminescent dosimeters (TLD) (effective depth 0.14 mm) of 30% at 0 degrees, 57% at 90 degrees, and a metal oxide semiconductor field effect transistor (MOSFET) detector (effective depth 0.5 mm) of 43% at 0 degrees, 62% at 90 degrees. Because the differences in measured superficial doses were significant and beyond those related to experimental error, these differences are assumed to be mostly attributable to the effective depth of measurement of each detector. We numerically simulated a bolus on/bolus off technique and found we could increase the coverage to the skin. Using an alternate "bolus on," "bolus off" regimen, the skin would receive 36.8 Gy at 0 degrees incidence and 46.4 Gy at 90 degrees incidence for a prescribed midpoint dose of 50 Gy. From this work it is evident that, as the circumference of the phantom is traversed the SSD increases and hence there is an inverse square fluence fall-off, this is more than offset by the increase in skin dose due to surface curvature to a plateau at about 90 degrees. Beyond this angle it is assumed that beam attenuation through the phantom and inverse square fall-off is causing the surface dose to reduce.

Assessment of large single-fraction, low-energy X-ray dose with radiochromic film.

PURPOSE: To investigate the accuracy of in vivo dosimetry using radiochromic film for large single-fraction, low-energy irradiations. METHODS AND MATERIALS: Gafchromic MD-55-2 radiochromic film and LiF thermoluminescent dosimeters (TLDs) were placed in vivo on 25 patients to ascertain their effectiveness for assessment of dose. All patients received 10 Gy single fractions at energies ranging from 100 kVp (half-value layer [HVL] = 3.5 mm Al) up to 250 kVp (HVL = 2.3 mm Cu). Effects of small air gaps were also investigated using LiF TLDs and radiochromic film. RESULTS: Radiochromic film adequately measured applied dose for 25 patients in vivo with a standard deviation of 5.5% from prescribed dose. LiF TLDs recorded a standard deviation of 4.1% from measured to applied dose. Small air gaps which can be created under the film or TLDs during in vivo dosimetry were shown to have a measurable but minimal effect on results for gaps less than 5 mm. CONCLUSIONS: Gafchromic film has adequately measured applied dose in vivo at low energy for large 10 Gy single-fraction irradiation.

Dosimetry of blood irradiation with radiochromic film.

It has been shown that radiochromic film is an ideal dosimeter for assessment and verification of delivered dose to irradiated blood products. Using a parallel opposing two-field technique on a medical linear accelerator, blood is irradiated to diminish the risk of transfusion-associated graft vs. host disease (TA-GVHD). The blood products are irradiated in a Perspex blood box to an applied dose of 29.5-31.7 Gy. Verification of applied dose has been performed with thimble ionization chambers and radiochromic film. Radiochromic film results have matched absorbed dose measurements from ionization chambers at all sites within the 'active' treatment volume within +/-6% for a 95% confidence limit. Using a sample of 100 in-vitro measurements, radiochromic film has measured the average applied dose to blood products to be 30.95+/-2.6 Gy for two standard deviations. Like currently available 'irradiated' film labels, the radiochromic film also serves as a visible reminder that the blood products have been irradiated.