The effect of the nasopharyngeal air cavity on x-ray interface doses

Accuracy Evaluation of Collapsed Cone Convolution Superposition Algorithms for the Nasopharynx Interface in the Early Stage of Nasopharyngeal Carcinoma

This study combined the use of radiation dosimeteric measurements and a custom-made anthropomorphic phantom in order to evaluate the accuracy of therapeutic dose calculations at the nasopharyngeal air-tissue interface. The doses at the nasopharyngeal air-tissue interface obtained utilizing the Pinnacle and TomoTherapy TPS, which are based on collapsed cone convolution superposition (CCCS) algorithms, were evaluated and measured under single $10\times 10\text{c}{\text{m}}^2$ , $2\times 2\text{c}{\text{m}}^2$ , two parallel opposed $2\times 2\text{c}{\text{m}}^2$ and clinical fields for early stage of nasopharyngeal carcinoma by using EBT3, GR-200F, and TLD 100. At the air-tissue interface under a $10\times 10\text{c}{\text{m}}^2$ field, the TPS dose calculation values were in good agreement with the dosimeter measurement with all differences within 3.5%. When measured the single field $2\times 2\text{c}{\text{m}}^2$ , the differences between the average dose were measured at the distal interface for EBT3, GR-200F, and TLD-100 and the calculation values were -15.8%, -16.4%, and -4.9%, respectively. When using the clinical techniques such as IMRT, VMAT, and tomotherapy, the measurement results at the interface for all three techniques did not imply under dose. Small-field sizes will lead to dose overestimation at the nasopharyngeal air-tissue interface due to electronic disequilibrium when using CCCS algorithms. However, under clinical applications of multiangle irradiation, the dose errors caused by this effect were not significant.

Dynamic IMRT Treatments of Sinus Region Tumors: Comparison of Monte Carlo Calculations with Treatment Planning System Calculations and Ion Chamber Measurements

Results are presented comparing Monte Carlo (MC) calculations for dynamic IMRT treatments of tumors in the sinus region with Eclipse treatment planning system dose calculations, and ion chamber measurements. The EGS4nrc MC code, BEAMnrc, was commissioned to simulate a Varian 21Ex Linac for both open and IMRT fields. The accuracy of the simulation for IMRT plans was evaluated using a head phantom by comparing MC, Eclipse, TLD results, and ion chamber in solid water phantom measurements. The MC code was then used to simulate dose distributions for five patients who were treated using dynamic IMRT for tumors in the sinus region. The results were compared with absolute and relative dose distributions calculated using Eclipse (pencil beam, modified-Batho inhomogeneity correction). Absolute dose differences were also compared with ion chamber results. Comparison of the doses calculated on the head phantom using MC, compared with Eclipse, ion chamber, and TLD measurements showed differences of −3.9%, −1.4%, and −2.0%, respectively (MC is colder). Relative dose distributions for the patient plans calculated using MC agreed well with those calculated using Eclipse with respect to targets and critical organs, indicating the modified-Batho correction is adequate. Average agreement for mean absolute target doses between MC and Eclipse was −3.0 ± 2.3% (1 s.d.). Agreement between ion chamber and Eclipse for these patients was −2.2 ± 1.9%, compared with 0.2 ± 2.0% for all head and neck IMRT patients. When Eclipse doses were corrected based on ion chamber results, agreement between MC and Eclipse was −0.7 ± 2.0%, indicating a small systematic uncertainty in the doses calculated using the treatment planning system for this subset of patients.

Dosimetric effect by shallow air cavities in high energy electron beams

This study evaluates the dosimetric impact caused by an air cavity located at 2 mm depth from the top surface in a PMMA phantom irradiated by electron beams produced by a Siemens Primus linear accelerator. A systematic evaluation of the effect related to the cavity area and thickness as well as to the electron beam energy was performed by using Monte Carlo simulations (EGSnrc code), Pencil Beam algorithm and Gafchromic EBT2 films. A home-PMMA phantom with the same geometry as the simulated one was specifically constructed for the measurements. Our results indicate that the presence of the cavity causes an increase (up to 70%) of the dose maximum value as well as a shift forward of the position of the depth-dose curve, compared to the homogeneous one. Pronounced dose discontinuities in the regions close to the lateral cavity edges are observed. The shape and magnitude of these discontinuities change with the dimension of the cavity. It is also found that the cavity effect is more pronounced (6%) for the 12 MeV electron beam and the presence of cavities with large thickness and small area introduces more significant variations (up to 70%) on the depth-dose curves. Overall, the Gafchromic EBT2 film measurements were found in agreement within 3% with Monte Carlo calculations and predict well the fine details of the dosimetric change near the cavity interface. The Pencil Beam calculations underestimate the dose up to 40% compared to Monte Carlo simulations; in particular for the largest cavity thickness (2.8 cm).

Dependences of mucosal dose on photon beams in head-and-neck intensity-modulated radiation therapy: a Monte Carlo study

Dependences of mucosal dose in the oral or nasal cavity on the beam energy, beam angle, multibeam configuration, and mucosal thickness were studied for small photon fields using Monte Carlo simulations (EGSnrc-based code), which were validated by measurements. Cylindrical mucosa phantoms (mucosal thickness = 1, 2, and 3 mm) with and without the bone and air inhomogeneities were irradiated by the 6- and 18-MV photon beams (field size = 1 × 1 cm(2)) with gantry angles equal to 0°, 90°, and 180°, and multibeam configurations using 2, 4, and 8 photon beams in different orientations around the phantom. Doses along the central beam axis in the mucosal tissue were calculated. The mucosal surface doses were found to decrease slightly (1% for the 6-MV photon beam and 3% for the 18-MV beam) with an increase of mucosal thickness from 1-3 mm, when the beam angle is 0°. The variation of mucosal surface dose with its thickness became insignificant when the beam angle was changed to 180°, but the dose at the bone-mucosa interface was found to increase (28% for the 6-MV photon beam and 20% for the 18-MV beam) with the mucosal thickness. For different multibeam configurations, the dependence of mucosal dose on its thickness became insignificant when the number of photon beams around the mucosal tissue was increased. The mucosal dose with bone was varied with the beam energy, beam angle, multibeam configuration and mucosal thickness for a small segmental photon field. These dosimetric variations are important to consider improving the treatment strategy, so the mucosal complications in head-and-neck intensity-modulated radiation therapy can be minimized.

Monte Carlo study on mucosal dose in oral and naval cavity using photon beams with small field

We study how mucosal dose in the oral or nasal cavity depends on the irradiated small segmental photon fields varying with beam energy, beam angle and mucosa thickness. Dose ratio (mucosal dose with bone underneath to dose at the same point without bone) reflecting the dose enhancement due to the bone backscatter was determined by Monte Carlo simulation (EGSnrc-based code), validated by measurements. Phase space files based on the 6 and 18 MV photon beams with small field size of 1 × 1 cm2, produced by a Varian 21 EX linear accelerator, were generated using the BEAMnrc Monte Carlo code. Mucosa phantoms (mucosa thickness = 1, 2 and 3 mm) with and without a bone under the mucosa were irradiated by photon beams with gantry angles varying from 0 to 30°. Doses along the central beam axis in the mucosa and the dose ratio were calculated with different mucosa thicknesses. For the 6 MV photon beams, the dose at the mucosa-bone interface increased by 44.9–41.7%, when the mucosa thickness increased from 1 to 3 mm for the beam angle ranging from 0 to 30°. These values were lower than those (58.8–53.6%) for the 18 MV photon beams with the same beam angle range. For both the 6 and 18 MV photon beams, depth doses in the mucosa were found to increase with an increase of the beam angle. Moreover, the dose gradient in the mucosa was greater for the 18 MV photon beams compared to the 6 MV. For the dose ratio, it was found that the dose enhancement due to the bone backscatter increased with a decrease of mucosa thickness, and was more significant at both the air-mucosa and mucosa-bone interface. Mucosal dose with bone was investigated by Monte Carlo simulations with different experimental configurations, and was found vary with the beam energy, beam angle and mucosa thickness for a small segmental photon field. The dosimetric information in this study should be considered when searching for an optimized treatment strategy to minimize the mucosal complications in the head-and-neck intensity-modulated radiation therapy.

Dosimetry of a small air cavity for clinical electron beams: A Monte Carlo study

This study investigated dosimetric changes in a water phantom when a small air cavity was presented at the central axis of a clinical electron beam. We used 6-, 9-, and 16-MeV electron beams with a 10 x 10 cm(2) applicator and cutout produced by a Varian 21 EX linear accelerator. Percentage depth doses (PDDs) for different depths (0.5-7 cm), thicknesses (2-10 mm), and widths (1-5 cm) of air cavities were calculated using Monte Carlo simulations (EGSnrc code) validated by film measurements. By comparing PDDs of phantoms with and without the air cavity, it was found that when the depth or thickness of cavity was changed, the PDD curve below the cavity was shifted with a distance equal to the thickness of the cavity. However, when the width of the air cavity was changed, both the PDD curve and its slope within and below the cavity were changed. A larger width of the air cavity resulted in a shallower PDD curve within the cavity. The slope of the PDD curve below the cavity tended towards a value as the width of the air cavity was increased to 3-5 cm for the 6-, 9-, and 16-MeV electron beams. The dependence of the depth dose on the width of the air cavity is a result of the contribution of the electron side scattering in the water surrounding the cavity. The change in depth dose resulting from the presence of an air cavity can cause discrepancies between the calculated and actual dose during radiotherapy, unless the effects of the air cavity are properly characterized during treatment planning. From the dosimetry data in this study, neglecting an air cavity of 1-cm thickness in the build-up region of a 6-MeV electron beam resulted in a delivered dose 10-12% larger than the original prescription. Delivered doses 3% and 6% higher than the prescribed dose were observed when doses were prescribed at R(80) for a 16-MeV electron beam. These results were obtained by neglecting air cavities with thicknesses equal to 2 and 4 mm, respectively, at a depth of 5 cm.

High-Dose-Rate Intracavitary Brachytherapy Boost for Early T Stage Nasopharyngeal Carcinoma{PRIVATE}

PURPOSE

To investigate any possible therapeutic gain from dose escalation with brachytherapy for early T stage nasopharyngeal carcinoma (NPC).

METHODS AND MATERIALS

One hundred forty-five patients with T1-2b N0-3 NPC were boosted with high-dose-rate intracavitary brachytherapy after completion of two-dimensional external radiotherapy (ERT) during the period from 1999 to 2003. To compare the efficacy of brachytherapy boost, another 142 patients with T1-2b N0-3 disease who were treated with ERT alone during 1994 to 1999 were evaluated. All patients were treated with ERT to a total dose of 66 Gy in 6.5 weeks. The brachytherapy boost group was given 10-12 Gy in 2 weekly fractions.

RESULTS

Dose escalation beyond 66 Gy with brachytherapy boost was shown to improve local control and survival. The 5-year actuarial local failure-free survival, regional failure-free survival, distant metastasis-free survival, progression-free survival, cancer-specific survival, and overall survival rates for the brachytherapy group and the control group were 95.8% and 88.3% (p = 0.020), 96% and 94.6% (p = 0.40), 95% and 83.2% (p = 0.0045), 89.2% and 74.8% (p = 0.0021), 94.5% and 83.4% (p = 0.0058), and 91.1% and 79.6% (p = 0.0062), respectively. The 5-year major-complication-free survival rate was 89.5% for the brachytherapy group and 85.6% for the control group (p = 0.23).

CONCLUSIONS

For patients who are treated with two-dimensional treatment techniques, dose escalation with brachytherapy boost improves local control and overall survival of patients with T1-T2a and possibly non-bulky T2b disease.

Gamma Knife 3-D dose distribution near the area of tissue inhomogeneities by normoxic gel dosimetry

The accuracy of the Leksell GammaPlan, the dose planning system of the Gamma Knife Model-B, was evaluated near tissue inhomogeneities, using the gel dosimetry method. The lack of electronic equilibrium around the small-diameter gamma beams can cause dose calculation errors in the neighborhood of an air-tissue interface. An experiment was designed to investigate the effects of inhomogeneity near the paranosal sinuses cavities. The homogeneous phantom was a spherical glass balloon of 16 cm diameter, filled with MAGIC gel; i.e., the normoxic polymer gel. Two hollow PVC balls of 2 cm radius, filled with N2 gas, represented the air cavities inside the inhomogeneous phantom. For dose calibration purposes, 100 ml gel-containing vials were irradiated at predefined doses, and then scanned in a MR unit. Linearity was observed between the delivered dose and the reciprocal of the T2 relaxation time constant of the gel. Dose distributions are the results of a single shot of irradiation, obtained by collimating all 201 cobalt sources to a known target in the phantom. Both phantoms were irradiated at the same dose level at the same coordinates. Stereotactic frames and fiducial markers were attached to the phantoms prior to MR scanning. The dose distribution predicted by the Gamma Knife planning system was compared with that of the gel dosimetry. As expected, for the homogeneous phantom the isodose diameters measured by the gel dosimetry and the GammaPlan differed by 5% at most. However, with the inhomogeneous phantom, the dose maps in the axial, coronal and sagittal planes were spatially different. The diameters of the 50% isodose curves differed 43% in the X axis and 32% in the Y axis for the Z =90 mm axial plane; by 44% in the X axis and 24% in the Z axis for the Y=90 mm coronal plane; and by 32% in the Z axis and 42% in the Y axis for the X=92 mm sagittal plane. The lack of ability of the GammaPlan to predict the rapid dose fall off, due to the air cavities behind or near the lesion led to an overestimation of the dose that was actually delivered. Clinically, this can result in underdosing of lesions near tissue inhomogeneities in patients under treatment.

Accuracy of inhomogeneity correction algorithm in intensity-modulated radiotherapy of head-and-neck tumors

We examined the degree of calculated-to-measured dose difference for nasopharyngeal target volume in intensity-modulated radiotherapy (IMRT) based on the observed/expected ratio using patient anatomy with humanoid head-and-neck phantom. The plans were designed with a clinical treatment planning system that uses a measurement-based pencil beam dose-calculation algorithm. Two kinds of IMRT plans, which give a direct indication of the error introduced in routine treatment planning, were categorized and evaluated. The experimental results show that when the beams pass through the oral cavity in anthropomorphic head-and-neck phantom, the average dose difference becomes significant, revealing about 10% dose difference to prescribed dose at isocenter. To investigate both the physical reasons of the dose discrepancy and the inhomogeneity effect, we performed the 10 cases of IMRT quality assurance (QA) with plastic and humanoid phantoms. Our result suggests that the transient electronic disequilibrium with the increased lateral electron range may cause the inaccuracy of dose calculation algorithm, and the effectiveness of the inhomogeneity corrections used in IMRT plans should be evaluated to ensure meaningful quality assurance and delivery.

Relative dose distribution in gamma knife treatment near tissue inhomogeneties

The primary goal in this study was to investigate 3-D dose distribution, near the areas of tissue inhomogeneities, in Gamma Knife Radiosurgery with the gel dosimetry. The spherical glass balloon of a diameter of 16 cm filled with the gel forms the homogeneous phantom; and an identical balloon with two corks placed on each side to represent the air cavities forms the inhomogeneous phantom. Dose calibration is performed by irradiating vials at known doses and then utilizing the R2- dose calibration curve. Stereotactic frames and fudicial markers were attached to the phantoms for MR scanning and image processing. Dose distributions from a single shot, using all 201 Cobalt sources, delivered to a known point with identical coordinates, are calculated both in homogeneous and inhomogeneous gel phantoms. In the aspect of dosimetrical quality control, the Gamma Knife planning system predicted dose distribution is compared with the experimental results. In the homogeneous phantom, the gel dosimetry calculated dose distribution is in good agreement with the GammaPlan predicted dose distribution. However, with the inhomogeneous phantom, the dose distribution is spatially different and significant differences in dose levels are observed.

Dose build-up behind air cavities for Co-60, 4, 6 and 8 MV. Measurements and Monte Carlo simulations

It has been shown in several studies that the build-up in photon beams behind air cavities (such as in the head and neck) increases with energy. In this study this effect is investigated over a broad range of energies that have been used for treating head and neck tumours. The study addresses the question of whether an energy lower than 6 MV is desirable and is based on measurements and Monte Carlo (MC) simulations. In a PMMA phantom containing an air cavity (3 x 16 x 3 cm3 at 3 cm depth) an ionization chamber (Capintec PS-033) was used to measure the dose build-up behind the cavity for 4, 6 and 8 MV beam qualities for different field sizes (from 3 x 6 cm2 to 8 x 8 cm2). MC simulations were made using the EGSnrc code for the same geometry and energies as well as for Co-60. Measurements and MC simulations agree well when the fixed-separation plane-parallel chamber measurements have been corrected for the expected over-response in the build-up region. This work demonstrates that the build-up effect of 6 MV is 'closer' to the build-up effect of 8 MV than to that of 4 MV. This suggests that if the build-up effect is of concern when the target volume is in the vicinity of air cavities, 4 MV should be preferred over both 6 MV and 8 MV. This work also shows that the build-up effect for Co-60 is significantly smaller than that of 4 MV. Moreover, the build-up effect increases as the field size decreases. With the increasing use of IMRT (and radiosurgery), small fields are used more frequently making these issues even more relevant. This should be taken into consideration when choosing the accelerator energies for a radiotherapy department.

Application of the quality index methodology for dosimetric verification of build-up effect beyond air–tissue interface in treatment planning system algorithms

We have designed a simple benchmark test for the user of a treatment planning system to check the calculation algorithm's ability to model the build up effect beyond an air/tissue interface. The expected result is expressed as an inhomogeneity correction factor CF derived from measurements and from Monte Carlo calculations for a full range of photon beam qualities. The linear regression lines obtained from plotting CF as a function of beam quality index form the basis for a quantitative check of the algorithm performance.

Does 4MV perform better compared to 6MV in the presence of air cavities in the head and neck region?

BACKGROUND AND PURPOSE

The underdose near air cavities in the head and neck region at photon energies of 4 MV and 6 MV was studied in search for clinical advantages of the 4 MV over 6 MV treatments.

MATERIALS AND METHODS

The on-axis and off-axis dose distributions were measured with a parallel-plate ionization chamber and films in polystyrene phantoms containing an air cavity of appropriate size based on the results of computed tomography scans.

RESULTS

Although most results are similar for both energies, the 4 MV photon beams give a somewhat smaller underdose effect and a faster re-build up than the 6 MV. For both energies a significant underdose effect was observed at the edge of the field in the larynx phantom. This proved to be true for small and large fields, for smaller and larger cavities, for one-beam as well as parallel-opposed beams.

CONCLUSION

For most clinically relevant situations there is no remarkable benefit in the use of either of the two energies.

Monte Carlo dosimetric evaluation of high energy vs low energy photon beams in low density tissues

BACKGROUND AND PURPOSE

Low megavoltage photon beams are often the treatment choice in radiotherapy when low density heterogeneities are involved, because higher energies show some undesirable dosimetric effects. This work is aimed at investigating the effects of different energy selection for low density tissues.

PATIENTS AND METHODS

BEAMnrc was used to simulate simple treatment set-ups in a simple and a CT reconstructed lung phantom and an air-channel phantom. The dose distribution of 6, 15 and 20 MV photon beams was studied using single, AP/PA and three-field arrangements.

RESULTS

Our results showed no significant changes in the penumbra width in lung when a pair of opposed fields were used. The underdosage at the anterior/posterior tumor edge caused by the dose build-up at the lung-tumor interface reached 7% for a 5 x 5 cm AP/PA set-up. Shrinkage of the 90% isodose volume was noticed for the same set-up, which could be rectified by adding a lateral field. For the CT reconstructed phantom, the AP/PA set-up offered better tumor coverage when lower energies were used but for the three field set-up, higher energies resulted to better sparing of the lung tissue. For the air-channel set-up, adding an opposed field reduced the penumbra width. Using higher energies resulted in a 7% cold spot around the air-tissue interface for a 5 x 5 cm field.

CONCLUSIONS

The choice of energy for treatment in the low density areas is not a straightforward decision but depends on a number of parameters such as the beam set-up and the dosimetric criteria. Updated calculation algorithms should be used in order to be confident for the choice of energy of treatment.

Evaluation of underdosage in the external photon beam radiotherapy of glottic carcinoma: Monte Carlo study

PURPOSE

Underdosage in the human larynx may be the true factor behind the decrease in local control rates.

PATIENTS AND METHODS

To evaluate underdosage with Monte Carlo a CT-based geometrical model of the patient's neck (mathematical neck) was created. Dose was calculated for a pair of 6 Me V parallel-opposed photon beams modulated with 15 degree steel wedges.

RESULTS

At least 5% of volume of 3.5 cm(3) hypothetical tumor near the air wall of the larynx receives less than 86% of the maximum tumor dose. The same volume received less than 91% of the maximum tumor dose when the mathematical neck had no air cavities.

CONCLUSIONS

We conclude the significant underdosage at the air-tissue interface in the larynx occurs in traditional radiotherapy treatments, especially in the glottic part of the larynx.

Monte Carlo simulation of the Leksell Gamma Knife®: II. Effects of heterogeneous versus homogeneous media for stereotactic radiosurgery

The absence of electronic equilibrium in the vicinity of bone-tissue or air-tissue heterogeneity in the head can misrepresent deposited dose with treatment planning algorithms that assume all treatment volume as homogeneous media. In this paper, Monte Carlo simulation (PENELOPE) and measurements with a specially designed heterogeneous phantom were applied to investigate the effect of air-tissue and bone-tissue heterogeneity on dose perturbation with the Leksell Gamma Knife. The dose fall-off near the air-tissue interface caused by secondary electron disequilibrium leads to overestimation of dose by the vendor supplied treatment planning software (GammaPlan) at up to 4 mm from an interface. The dose delivered to the target area away from an air-tissue interface may be underestimated by up to 7% by GammaPlan due to overestimation of attenuation of photon beams passing through air cavities. While the underdosing near the air-tissue interface cannot be eliminated with any plug pattern, the overdosage due to under-attenuation of the photon beams in air cavities can be eliminated by plugging the sources whose beams intersect the air cavity. Little perturbation was observed next to bone-tissue interfaces. Monte Carlo results were confirmed by measurements. This study shows that the employed Monte Carlo treatment planning is more accurate for precise dosimetry of stereotactic radiosurgery with the Leksell Gamma Knife for targets in the vicinity of air-filled cavities.

Importance of accurate dose calculations outside segment edges in intensity modulated radiotherapy treatment planning

BACKGROUND AND PURPOSE

To assess the effect of differences in the calculation of the dose outside segment edges on the overall dose distribution and the optimisation process of intensity modulated radiation therapy (IMRT) treatment plans.

PATIENTS AND METHODS

Accuracy of dose calculations of two treatment planning systems (TPS1 and TPS2) was assessed, to ensure that they are both suitable for IMRT treatment planning according to published guidelines. Successively, 10 treatment plans for patients with prostate and head and neck tumours were calculated in both systems. The calculations were compared in selected points as well as in combination with volumetric parameters concerning the planning target volume (PTV) and organs at risk.

RESULTS

For both planning systems, the calculations agree within 2.0% or 3 mm with the measurements in the high-dose region for single and multiple segment dose distributions. The accuracy of the dose calculation is within the tolerances proposed by recent recommendations. Below 35% of the prescribed dose, TPS1 overestimates and TPS2 underestimates the measured dose values, TPS2 being closer to the experimental data. The differences between TPS1 and TPS2 in the calculation of the dose outside segments explain the differences (up to 50% of the local value) found in point dose comparisons. For the prostate plans, the discrepancies between the TPS do not translate into differences in PTV coverage, normal tissue complication probability (NTCP) values and results of the plan optimisation process. The dose-volume histograms (DVH) of the rectal wall differ below 60 Gy, thus affecting the plan optimisation if a cost function would operate in this dose region. For the head and neck cases, the two systems give different evaluations of the DVH points for the PTV (up to 22% differences in target coverage) and the parotid mean dose (1.0-3.0 Gy). Also the results of the optimisation are influenced by the choice of the dose calculation algorithm.

CONCLUSIONS

In IMRT, the accuracy of the dose calculation outside segment edges is important for the determination of the dose to both organs at risks and target volumes and for a correct outcome of the optimisation process. This aspect should therefore be of major concern in the commissioning of a TPS intended for use in IMRT. Fulfilment of the accuracy criteria valid for conformal radiotherapy is not sufficient. Three-dimensional evaluation of the dose distribution is needed in order to assess the impact of dose calculation accuracy outside the segment edges on the total dose delivered to patients treated with IMRT.

Experimental verification of convolution/superposition photon dose calculations for radiotherapy treatment planning

This work describes an experimental verification of the two-photon dose calculation engines available on the Helax-TMS (version 6.1) commercial radiotherapy treatment planning system. The performance of the pencil beam convolution and the collapsed cone superposition algorithms was examined for 4, 6, 15 MV beams, under a range of clinically relevant irradiation geometries. Comparisons against measurements were carried out in terms of absolute dose, thus assessment of the accuracy of monitor unit (MU) calculations was also carried out. Results show that both algorithms agree with measurement to acceptable tolerance levels in most cases in homogeneous water-equivalent media irradiated under full scatter conditions. The collapsed cone algorithm slightly overestimates the penumbra width and this is mainly due to discretization effects of the fluence matrix. The accuracy of this algorithm strongly depends on the resolution of the patient density matrix. It is recommended that the density matrix voxel size used for dose calculations is less than 5 x 5 x 5 mm3. The dose in media irradiated under missing tissue geometry, or in the presence of low or high-density heterogeneities, is modelled best with the collapsed cone algorithm. This is of particular clinical interest in treatment planning of the breast and of the thorax. For these treatment sites, a retrospective study of treatment plans indicated in certain cases significant overestimation of the dose to the planning target volume when using the pencil beam convolution model.

The value of radiochromic film dosimetry around air cavities: experimental results and Monte Carlo simulations

In this study we investigate radiochromic film dosimetry around air cavities with particular focus on the perturbation of the dose distribution by the film when the film is parallel to the beam axis. We considered a layered polystyrene phantom containing an air cavity as a model for the air-soft tissue geometry that may occur after surgical resection of a paranasal sinus tumour. A radiochromic film type MD-55 was positioned within the phantom so that it intersected the cavity. Two phantom set-ups were examined. In the first case, the air cavity is at the centre of the phantom, thus the film is lying along the central beam axis. In the second case, the cavity and film are located 2 cm offset from the phantom centre and the central beam axis. In order to examine the influence of the film on the dose distribution and to interpret the film-measured results, Monte Carlo simulations were performed. The film was modelled rigorously to incorporate the composition and structure of the film. Two field configurations, a 1 x 10 cm2 field and a 10 x 10 cm2 field, were examined. The dose behind the air cavity is reduced by 6 to 7% for both field configurations when a film that intersects the cavity contains the central beam axis. This is due to the attenuation exerted by the film when photons cross the cavity. Offsetting the beam to the cavity and the film by 2 cm removes the dose reduction behind the air cavity completely. Another result was that the rebuild-up behind the cavity for the 10 x 10 cm2 field, albeit less significant than for the 1 x 10 cm2 field, could only be measured by the film that was placed offset with respect to the central beam axis. Although radiochromic film is approximately soft-tissue equivalent and energy independent as compared to radiographic films, care should be taken in the case of inhomogeneous phantoms when the film intersects air cavities and contains the beam central axis. Errors in dose measurement can be expected distal to the air cavity due to attenuation in the film itself. This attenuation would not occur in the absence of the film. Both experiments and Monte Carlo computations support this conclusion.

Underdosage of the upper-airway mucosa for small fields as used in intensity-modulated radiation therapy: a comparison between radiochromic film measurements, Monte Carlo simulations, and collapsed cone convolution calculations

Head-and-neck tumors are often situated at an air-tissue interface what may result in an underdosage of part of the tumor in radiotherapy treatments using megavoltage photons, especially for small fields. In addition to effects of transient electronic disequilibrium, for these small fields, an increased lateral electron range in air will result in an important extra reduction of the central axis dose beyond the cavity. Therefore dose calculation algorithms need to model electron transport accurately. We simulated the trachea by a 2 cm diameter cylindrical air cavity with the rim situated 2 cm beneath the phantom surface. A 6 MV photon beam from an Elekta SLiplus linear accelerator, equipped with the standard multileaf collimator (MLC), was assessed. A 10 x 2 cm2 and a 10 x 1 cm2 field, both widthwise collimated by the MLC, were applied with their long side parallel to the cylinder axis. Central axis dose rebuild-up was studied. Radiochromic film measurements were performed in an in-house manufactured polystyrene phantom with the films oriented either along or perpendicular to the beam axis. Monte Carlo simulations were performed with BEAM and EGSnrc. Calculations were also performed using the pencil beam (PB) algorithm and the collapsed cone convolution (CCC) algorithm of Helax-TMS (MDS Nordion, Kanata, Cahada) version 6.0.2 and using the CCC algorithm of Pinnacle (ADAC Laboratories, Milpitas, CA, USA) version 4.2. A very good agreement between the film measurements and the Monte Carlo simulations was found. The CCC algorithms were not able to predict the interface dose accurately when lateral electronic disequilibrium occurs, but were shown to be a considerable improvement compared to the PB algorithm. The CCC algorithms overestimate the dose in the rebuild-up region. The interface dose was overestimated by a maximum of 31% or 54%, depending on the implementation of the CCC algorithm. At a depth of 1 mm, the maximum dose overestimation was 14% or 24%.

Evaluating the effectiveness of a longitudinal magnetic field in reducing underdosing of the regions around upper respiratory cavities irradiated with photon beams--a Monte Carlo study

The problem of underdosing lesions adjacent to upper respiratory cavities and a proposal to correct it are presented in this work. The EGS4 Monte Carlo code was used to simulate a 6 MV x-ray beam passing through a block of tissues with air cavities 2, 4, and 6 cm wide. The geometry used approximates the tracheal geometry used by previous researchers who investigated the underdosing phenomenon. A uniform longitudinal magnetic field of 0.5 T strength is used to reduce secondary electron outscatter caused by the presence of an air gap, and thus improving the dose at the distal surface of air cavities. We introduce the term "percent dose reduction" (PDR), which is defined as the difference between the dose after the air cavity and the dose at the same depth in soft-tissue phantom normalized to the dose in the tissue phantom, to quantify the reduction in dose after an air gap. We also introduce the term dose improvement ratio (DIR), which is defined as the dose ratio with magnetic field to the dose, at the same point, without magnetic field, to quantify the improvement in dose when the magnetic field is applied. For 2 x 2 x 20 cm3 and 4 x 4 x 20 cm3 air cavities irradiated by 2 x 2 cm2 beams, we found PDRs of 38% and 52%, respectively. This means that for these cavities, there is a 38% and a 52% reduction in dose at the cavity edge compared to the same dose in tissue at the same depth for each cavity. The dose improved by 30% (DIR= 1.3) and 87% (DIR= 1.87), respectively, when applying the magnetic field. The worst effect on dose at the distal side came from larger cavities irradiated with small fields. In these situations, the improvement in dose due to the presence of magnetic field was the largest. This article deals with "ideal" head and neck geometries with a uniform magnetic field. In a paper to follow we will use a CT-based phantom to study the effect in realistic geometries with the presence of a magnetic field from a Helmholtz coil pair.

Monte Carlo evaluation of tissue inhomogeneity effects in the treatment of the head and neck

PURPOSE

To use Monte Carlo dose calculation to assess the degree to which tissue inhomogeneities in the head and neck affect static field conformal, computed tomography (CT)-based 6-MV photon treatment plans.

METHODS AND MATERIALS

We retrospectively studied the three-dimensional treatment plans that had been used for the treatment of 5 patients with tumors in the nasopharyngeal or paranasal sinus regions. Two patients had large surgical cavities. The plans were designed with a clinical treatment planning system that uses a measurement-based pencil-beam dose-calculation algorithm with an equivalent path-length inhomogeneity correction. Each plan employs conformally-shaped 6-MV photon beams. Patient anatomy and electron densities were obtained from the treatment planning CT images. For each plan, the dose distribution was recalculated with the Monte Carlo method, utilizing the same beam geometry and CT images. The Monte Carlo method accurately accounts for the perturbation effects of local tissue heterogeneities. The Monte Carlo calculated dose distributions were compared with those from the clinical treatment planning system.

RESULTS

The degree to which tissue inhomogeneity affects the dose distributions of individual fields varies with the specific anatomic geometry, especially the size and location of air cavities in relation to the beam orientation and field size. Most of the beam apertures completely enclose the air cavities within or adjacent to the gross tumor volume (GTV). Equivalent squares (including blocking) ranged from approximately 5 to 9.5 cm. A common feature observed for individual fields is that the Monte Carlo calculated doses to tissue directly behind and within an air cavity are lower. However, after combining the fields employed in each treatment plan, the overall dose distribution shows only small differences between the two methods. For all 5 patients, the Monte Carlo calculated treatment plans showed a slightly lower dose received by the 95% of target volume (D(95)) than the plans calculated with the pencil-beam algorithm. The average difference in the target volume encompassed by the prescription isodose line was less than 2.2%. The difference between the dose-volume histograms (DVHs) of the GTV was generally small. For the brainstem and chiasm, the DVHs of the two plans were similar. For the spinal cord, differences in the details of the DHV and the dose to 1 cc (D(1cc)) of the structure were observed, with Monte Carlo calculation generally predicting increased dose indices to the spinal cord. However, these changes are not expected to be clinically significant.

CONCLUSION

For 6-MV photons, the effects of both normal tissue inhomogeneities and surgical air cavities on the target coverage were adequately accounted for by conventional pencil beam methods for all of the cases studied. Although differences in details of the DVHs of the normal structures were observed, depending on whether Monte Carlo or pencil-beam algorithm was used for calculation, these differences are not expected to be clinically significant. In general, the pencil-beam calculation corrected for primary attenuation by the equivalent pathlength is a sufficiently accurate method for head-and-neck treatment planning using 6-MV photons.

Dose perturbation by air cavities in megavoltage photon beams: implications for cavity surface doses

Gas-filled cavities in the body are known to perturb megavoltage photon beams in radiation therapy. The bowel represents one such cavity where cavity dimensions can vary throughout a treatment course. This has implications for doses to the surface of the bowel. Doses to the first 0.1 mm and 1.0 mm of cavity surfaces were calculated for 6 MV and 18 MV photon beams for a range of cavity sizes. It is found that relatively minor variations in cavity surface doses result over the typical range of rectal cavity size (2-3 cm). However, the variations in surface dose should be considered whenever bowel reactions must be considered, or when the target cells of interest line the bowel itself.

Reducing loss in lateral charged-particle equilibrium due to air cavities present in x-ray irradiated media by using longitudinal magnetic fields

The underdosing of lesions distal to air cavities, such as those found in upper respiratory passages, occurs due to the loss in lateral charged-particle equilibrium (CPE). The degree of underdosing worsens for smaller field sizes, resulting in more frequent recurrence of the cancer treated. Higher photon energies further aggravate the outcome by producing longer second build-up regions beyond the cavity. Besides underdosing, the larger lateral spread of secondary electron fluence in the air cavity produces diffuse dose distributions at the tissue-air interface for shaped or intensity modulated fields. These disequilibrium effects create undesirable deviations from the intended treatment. The clinical concern is further intensified by the failure of traditional treatment planning systems to even account for such defects. In this work, the use of longitudinal magnetic fields on the order of 0.5 T is proposed for alleviating lateral electronic disequilibrium due to the presence of air cavities in the irradiated volume. The magnetic field enforces lateral CPE by restricting the lateral range of electrons in the air cavity. The problem is studied in a simple water-air-water slab geometry using EGS4 Monte Carlo simulations for 6 MV photons. Electronic disequilibrium is evaluated for beams of various sizes, shapes and intensity distributions constructed by linear superposition of the dose distributions for 0.5 x 0.5 cm2 beamlets. Comparison is also made with 60Co irradiation. The results indicate that the lateral confinement of secondary electrons in the air cavity by sub-MRI strength longitudinal fields is effective in reducing deterioration of dose distributions near tissue-air interfaces. This can potentially reduce recurrence rates of cancers such as the larynx carcinoma.

A systematic evaluation of air cavity dose perturbation in megavoltage x-ray beams

The EGS4 Monte Carlo radiation transport code was used to systematically study the dose perturbation near planar and cylindrical air cavities in a water medium irradiated by megavoltage x-ray beams. The variables of the problem included x-ray energy, cavity shape and dimension, and depth of the cavity in water. The Monte Carlo code was initially validated against published measurements and its results were found to agree within 2% with the published measurements. The study results indicate that the dose perturbation is strongly dependent on x-ray energy, field size, depth, and size of cavity in water. For example, the Monte Carlo calculations show dose reductions of 42% and 18% at 0.05 and 2 mm, respectively, beyond the air-water interface distal to the radiation source for a 3 cm thick air slab irradiated by a single 5x5 cm2 15 MV beam. The dose reductions are smaller for a parallel-opposed pair of 5x5 cm2 15 MV x-ray beams, being 21% and 11% for the same depths. The combined set of Monte Carlo calculations showed that the dose reduction near an air cavity is greater for: (a) Smaller x-ray field size, (b) higher x-ray energy, (c) larger air-cavity size, and (d) smaller depth in water where the air cavity is situated. A potential clinical application of these results to the treatment of prostate cancer is discussed.

Diamond detector measurements near simulated air channels for narrow photon beams

BACKGROUND AND PURPOSE

To evaluate the combined effect of increased photon transmission, reduced photon scatter, increased secondary electron range and loss of electronic equilibrium for narrow 6-MV beams in and around a simulated air channel.

MATERIALS AND METHODS

A measuring method was developed in-house for relative dose measurements near simulated air-like/soft-tissue interfaces in an automated water phantom. A Styrofoam cylinder (density 0.03 g/cm3) of 2-cm diameter was submersed in the water phantom and irradiated with small rectangular radiation fields. The field length was fixed at 10 cm and the field widths ranged from 1 to 4 cm. The axis of the foam cylinder and the long side of the field were parallel. A water layer of 2 cm was realised upstream of the cylinder. Relative depth dose and profiles behind the foam cavity were assessed using a diamond detector with a sensitive crystal thickness of 0.21 mm located at 1 mm from the top of the encapsulation.

RESULTS

The dose at central axis 1.1 mm behind the cavity was found to be 92 and 74% for a field size of 10 x 2 and 10 x 1 cm2, respectively. The highly convex dose profile of the 10 x 1-cm2 field, characterising the homogeneous case, is flattened.

CONCLUSIONS

The diamond detector is an excellent choice as a detector in small photon fields with high-dose gradients as they occur near air channels, provided the orientation of the detector is appropriate. Doses near air channels are subject to significant local variations as a function of small changes of field width, and local underdosing may occur in particular cases.

The roles of multileaf collimators and micro-multileaf collimators in conformal and conventional nasopharyngeal carcinoma radiotherapy treatments

The purpose of this work is to study the efficacy and limitations of using standard multileaf collimators (MLCs) and micro-multileaf collimators (mMLCs) in the treatment of nasopharyngeal carcinoma (NPC) by conventional and conformal radiotherapy techniques. The penumbra characteristics of MLC, mMLC, and customized block collimated beams are measured with respect to leaf edge angle, beam energy, treatment depth, and field size and compared with those generated by a commercial three-dimensional planning computer system. Upon verification of the planning system, it is used to evaluate the treatment plans generated with these beam shapers for conventional and conformal NPC treatments. The effective penumbra of a MLC beam is strongly influenced by its edge angle, leaf width, and treatment depth. The suitability of standard MLCs in conventional NPC treatments is determined mainly by the edge angle to be used. For conformal NPC treatments involving six or more fields, dose volume histograms comparable to those of customized beam blocks are obtained with a standard MLC. The mMLC does not have the same restrictions as those on standard MLC but is limited to phase II treatment by its small usable field size. Both standard MLCs and mMLCs can be used to replace customized divergent beam blocks in both conventional and conformal NPC treatments. However, a MLC, due to its larger effective penumbra, may be unsuitable for use in cases when the tumor volumes extend very close to the critical normal structures. A mMLC, on the other hand, is limited by its small maximum field size and can only be used for collimating the facial portals in the second phase treatment.