The effect of slice thickness on target and organs at risk volumes, dosimetric coverage and radiobiological impact in IMRT planning.

PURPOSE: Dose-volume histogram (DVH) has become an important tool for evaluation of radiation outcome as reflected from many clinical protocols. While dosimetric accuracy in treatment planning system (TPS) is well quantified, the variability in volume estimation is uncertain due to reconstruction algorithm that is investigated in this study. In addition, the impact of dose distribution and tumor control probability (TCP) were also investigated with CT slice thickness for IMRT planning. MATERIALS AND METHODS: A water phantom containing various objects with accurately known volume ranging from 1 to 100 cm(3) was scanned with 1, 2, 3, 5, and 10 mm slice thickness. The CT data sets were sent to Eclipse TPS for contour delineation and volume estimation. The data were compared with known volume for the estimation of error in the volume of each structure. IMRT Plans were generated on phantom containing four objects with different slice thickness (1-5 mm) to calculate TCP. ICRU-83-recommended dose points such as D 2%, D 50%, D 98%, as well as homogeneity and conformity index were also calculated. RESULTS: The variability of volumes with CT slice thickness was significant especially for small volume structures. A maximum error of 92% was noticed for 1 cm(3) volume of object with 10 mm slice thickness, whereas it was ~19% for 1 mm slice thickness. For 2 and 3 cm(3) objects, the maximum error of 99% was noticed with 10 mm slice thickness and ~60% with 5 mm. The differences are smaller for larger volumes with a cutoff at about 20 cm(3). The calculated volume of the objects is a function of reconstruction algorithm and slice thickness. The PTV mean dose and TCP decreased with increasing slice thickness. Maximum variation of ~5% was noticed in mean dose and ~2% in TCP with change in slice thickness from 1 to 5 mm. The relative decrease in target volume receiving 95% of the prescribed dose is ~5% with change in slice thickness from 1 to 5 mm. The homogeneity index increases up to 163% and conformity index decreases by 4% between 1 and 5 mm slice thickness, producing highly inhomogeneous and least conformal treatment plan. CONCLUSIONS: Estimation of a volume is dependent on CT slice thickness and the contouring algorithm in a TPS. During commissioning of TPS and for all clinical protocols, evaluation of volume should be included to provide the limit of accuracy in DVH from TPS, especially for small objects. A smaller slice thickness provides superior dosimetry with improved TCP. Thus, the smallest possible slice thickness should be used for IMRT planning, especially when smaller structures are present.

Beam angle selection for intensity-modulated radiotherapy (IMRT) treatment of unresectable pancreatic cancer: are noncoplanar beam angles necessary?

BACKGROUND AND PURPOSE: External beam radiation therapy with concurrent chemotherapy (CRT) is widely used for the treatment of unresectable pancreatic cancer. Noncoplanar (NCP) 3D conformal radiotherapy (3DCRT) and coplanar (CP) IMRT have been reported to lower the radiation dose to organs at risk (OARs). The purpose of this article is to examine the utility of noncoplanar beam angles in IMRT for the management of pancreatic cancer. MATERIALS AND METHODS: Sixteen patients who were treated with CRT for unresectable adenocarcinoma of the pancreatic head or neck were re-planned using CP and NCP beams in 3DCRT and IMRT with the Varian Eclipse treatment planning system. RESULTS: Compared to CP IMRT, NCP IMRT had similar target coverage with slightly increased maximum point dose, 5,799 versus 5,775 cGy (p = 0.008). NCP IMRT resulted in lower mean kidney dose, 787 versus 1,210 cGy (p < 0.0001) and higher mean liver dose, 1,208 versus 1,061 cGy (p < 0.0001). Also, NCP IMRT resulted in similar mean stomach dose, 1,257 versus 1,248 cGy (p = 0.86) but slightly higher mean small bowel dose, 981 versus 866 cGy (p < 0.0001). CONCLUSIONS: The NCP IMRT was able to significantly decrease bilateral kidney dose, but did not improve other dose-volume criteria. The use of NCP beam angles is preferred only in patients with risk factors for treatment-related kidney dysfunction (AU)

Beam angle selection for intensity-modulated radiotherapy (IMRT) treatment of unresectable pancreatic cancer: are noncoplanar beam angles necessary?

BACKGROUND AND PURPOSE: External beam radiation therapy with concurrent chemotherapy (CRT) is widely used for the treatment of unresectable pancreatic cancer. Noncoplanar (NCP) 3D conformal radiotherapy (3DCRT) and coplanar (CP) IMRT have been reported to lower the radiation dose to organs at risk (OARs). The purpose of this article is to examine the utility of noncoplanar beam angles in IMRT for the management of pancreatic cancer. MATERIALS AND METHODS: Sixteen patients who were treated with CRT for unresectable adenocarcinoma of the pancreatic head or neck were re-planned using CP and NCP beams in 3DCRT and IMRT with the Varian Eclipse treatment planning system. RESULTS: Compared to CP IMRT, NCP IMRT had similar target coverage with slightly increased maximum point dose, 5,799 versus 5,775 cGy (p = 0.008). NCP IMRT resulted in lower mean kidney dose, 787 versus 1,210 cGy (p < 0.0001) and higher mean liver dose, 1,208 versus 1,061 cGy (p < 0.0001). Also, NCP IMRT resulted in similar mean stomach dose, 1,257 versus 1,248 cGy (p = 0.86) but slightly higher mean small bowel dose, 981 versus 866 cGy (p < 0.0001). CONCLUSIONS: The NCP IMRT was able to significantly decrease bilateral kidney dose, but did not improve other dose-volume criteria. The use of NCP beam angles is preferred only in patients with risk factors for treatment-related kidney dysfunction.

Determination of the depth dose distribution of proton beam using PRESAGE™ dosimeter.

PRESAGE™ dosimeter dosimeter has been proved useful for 3D dosimetry in conventional photon therapy and IMRT [1-5]. Our objective is to examine the use of PRESAGE™ dosimeter for verification of depth dose distribution in proton beam therapy. Three PRESAGE™ samples were irradiated with a 79 MeV un-modulated proton beam. Percent depth dose profile measured from the PRESAGE™ dosimeter is compared with data obtained in a water phantom using a parallel plate Advanced Markus chamber. The Bragg-peak position determined from the PRESAGE™ is within 2 mm compared to measurements in water. PRESAGE™ shows a highly linear response to proton dose. However, PRESAGE™ also reveals an underdosage around the Bragg peak position due to LET effects. Depth scaling factor and quenching correction factor need further investigation. Our initial result shows that PRESAGE™ has promising dosimetric characteristics that could be suitable for proton beam dosimetry.

Dosimetric evaluation and clinical application of virtual mini-multileaf collimator.

One of the major concerns with multileaf collimators (MLC) is the jagged field edge that produces a larger penumbra compared with that produced by a Cerrobend block. The dosimetric undulation of the MLC can be minimized by replacing an existing MLC with a mini-MLC, an expensive replacement, or by software implementation, which essentially converts a regular MLC into a virtual mini-MLC. In this study, the dosimetry in the penumbra region of a virtual mini-MLC replacing the Cerrobend block is investigated for clinical applications. HD270, a software program implemented by Siemens (Concord, CA), combines the use of an MLC and a table translation perpendicular to the leaf plane to produce a smooth field edge, thus reducing isodose undulation. Three different step resolutions are available: 5 mm, 3 mm, and 2 mm. Using film dosimetry, the penumbra regions are studied at two different depths for clinical blocks and corresponding MLC setup, as well as HD270 with different resolutions for both 6-MV and 15-MV x-ray beams. The dose delivery time for HD270 on auto-sequencing mode is compared with the use of Cerrobend blocks. The clinical applications of HD270 in head-and-neck (head and neck) and prostate treatments are investigated. For single-field irradiation, the 80-20% penumbra widths for both the 45 degrees block and the circular block are reduced with HD270 compared with MLC for both 6 and 15 MV at different depths. At 2-mm resolution, the scalloping isodose lines (IDLs) with MLC completely disappear, although the penumbra is still larger than the Cerrobend block. On the other hand, the difference in dose undulations between 2-mm and 3-mm resolution is small. In the head and neck irradiation, the 80-20% widths with HD270 are 1 to 2 mm less than MLC, but they are still 2 mm wider than with a Cerrobend block. The 50% IDL is reduced by 2 mm with HD270 compared with MLC, which provides safety near spinal cord. Dose-volume histogram (DVH) calculations for the different shielding techniques indicate that the HD270 improves the spinal cord dose distribution significantly compared with MLC. A similar improvement in dose undulation is observed for the prostate case. In the dose region, >60% of the prescribed dose, there is approximately 10% less irradiated volume for the rectum when HD270 (3 mm resolution) is employed compared with MLC. The treatment time was compared with that from the Cerrobend block, and it was found that even at 3-mm resolution, there is a 20% reduction in treatment time in a head and neck treatment; with a 2-mm resolution, there is a 15% increase in time. The isodose undulation due to MLC can be significantly reduced with the HD270. Clinical application with HD270 for head and neck and prostate irradiation provides a smaller penumbra region compared with MLC, although it still gives a larger one compared with the Cerrobend block. In the clinical cases presented in this study, the 3-mm resolution is the most effective in improving the penumbra and delivery time. The HD270 implementation is a versatile and cost-effective solution for reducing MLC undulation.

Comparison of beam characteristics in intensity modulated radiation therapy (IMRT) and those under normal treatment condition.

In the step-and-shoot delivery of an IMRT plan with a Siemens Primus accelerator, radiation is turned off by desynchronizing the injector while the field parameters are being changed. When the machine is ready again a trigger pulse is sent to the injector to start the beam instantaneously. The objective of this study is to investigate the beam characteristics of the machine operating in the IMRT mode and to study the effect of the Initial Pulse Forming Network (IPEN) on the dark current. The central axis (CAX) output for a 10 x 10 cm2 field over the range 1-100 MU was measured with an ion chamber in a polystyrene phantom for both 6 and 15 MV x rays. Beam profiles were also measured over the range of 2-40 MU with the machine operating in the IMRT mode and compared with those in the normal mode. By adjusting the IPFN value, dark current radiation (DCR) was measured using ion chamber measurements. For both the normal and IMRT modes, dose versus MU is nonlinear in the range 1-5 MUs. Above 5 MU, dose varies linearly with MU for both 6 and 15 MV x rays. For stability of dose profiles, the 2 MU-IM group exhibit 20% variation from one subfield to another. The variation is about 5% for the 8 MU-IM group and <5% for 10 MU and higher. The results are similar in the normal treatment mode. With the IPFN at >80% of the PFN value, a spurious radiation associated with dark current at approximately 0.7% of the dose at isocenter for a 10 x 10 cm2 field is detected during the "PAUSE" state of the accelerator for 15 MV x rays. When the IPFN is lowered to <80% of the PFN value, no DCR is detected. For 6 MV x rays, no measurable DCR was detected regardless of the IPFN setting.

Role of multileaf collimator in replacing shielding blocks in radiation therapy.

To facilitate the use of multileaf collimator (MLC) in field shaping, we tested the hypothesis that the changes in the penumbra due to MLC replacing a Cerrobend block can be related to a change in the margin of the block. We also investigated if it is possible to estimate the effect of MLC replacing a block in terms of a change in the block margin. Calculations were performed for a single field as well as a multiple field setup. For the single field setup, blocks with equal areas were drawn at the four corners of a 16 x 20 cm(2) field at angles of 20 degrees, 40 degrees, 60 degrees, and 80 degrees with the horizontal axis. The blocks were then replaced with MLC leaves. For 6 MV x-rays, dose profiles in the penumbra regions of the blocks at 5- and 10-cm depths were compared with those obtained with the corresponding MLC setup. For multiple fields, the same sets of blocks were set up on the anterio-posterior (AP-PA) pair of a four-field setup. The margins of the blocks were increased (i.e., block shaved) in 1 mm steps to a maximum of 6 mm. The similarity between MLC and the change in the block margin was examined by comparing the dose-volume histogram (DVH) of the normal tissues in the penumbral regions for the different setups. To correlate the effect of MLC with a change in the block margin, difference dose-volume histograms (DDVH) of the normal tissues relative to the original block were compared for the MLC setup with those for the changes in the block margin. The correlation obtained was used to predict the effect on the penumbra region of the MLC setup for the lateral fields of a patient irradiated with a four-field setup. The calculations were carried out with 15 MV x-rays. For the single field setup, dose undulation is largest for the 50% isodose line (IDL) as reflected in the largest increase in the 50% to 20% isodose region compared with the 90% to 10% and the 80% to 20% regions. The increase in the penumbral width is largest for the 20 degrees block when replaced by the MLC and is smaller as the angle increases. The increase in the penumbral width also increases with depth. The effect of replacing a Cerrobend block with an MLC is similar to an increase in the block margin. For 15 MV x-rays, the increase varies inversely with the angle of the blocks, from > 6 mm increase in block margin for the 20 degrees block to about 1 mm for the 80 degrees block. In the clinical example, replacing the blocks in the lateral fields of a four-field irradiation with MLC is similar to changing the margin of the blocks. For the posterior block, MLC is similar to a 1- to 2-mm increase in the margin of the block, whereas for the anterior block the effect is similar to 1 mm for the straight portion of the block to about 6 mm in the superior portion of the block. Characterization of an MLC setup replacing a Cerrobend block is necessary for adequate coverage of target volume. The effect of MLC replacing a Cerrobend block is similar to a change in the block margin. It is possible to estimate with reasonable accuracy the effect of MLC replacing a Cerrobend block.

Characteristics of bremsstrahlung in electron beams.

Clinical electron beams contain an admixture of bremsstrahlung produced in structures in the accelerator head, in field-defining cerrobend or lead cutouts, and in the irradiated patient or water phantom. Accurate knowledge of these components is important for dose calculations and treatment planning. In this study, the bremsstrahlung components are separated for electron beams (energy 6-22 MeV, diameter 0-5 cm) using measurements in water and calculations. The results show that bremsstrahlung from the accelerator head dominates and increases with field size for electron beams generated by accelerators equipped with scattering foils. The bremsstrahlung from the field-defining cerrobend accounts for 10% to 30% of the total bremsstrahlung and decreases with increasing beam radius. The bremsstrahlung is softer than the x-ray beams of corresponding nominal energy since the latter are hardened by the flattening filter. For the 6, 12, and 22 MeV electron beams, the effective attenuation coefficients in water for the bremsstrahlung are 0.058, 0.050, and 0.043 cm(-1). The depths of maximum dose at 100 cm SSD are 0.8, 1.7, and 3.0 cm. The position of the virtual source of the bremsstrahlung shifts downstream from the nominal source position by 20, 13, 5.6 cm, respectively. The lateral bremsstrahlung dose distribution is more forward-peaked for higher electron energy. The bremsstrahlung components could be described for any machine by a set of simple measurements and can be modeled by an analytical expression.

Gold microspheres: a selective technique for producing biologically effective dose enhancement.

PURPOSE: To investigate dose enhancement and radiosensitization associated with electrons produced and scattered from gold particles suspended in cells in vitro and with tumour cells growing in vivo irradiated with low-energy photons. MATERIALS AND METHODS: CHO-K1, EMT-6 and DU-145 cells were irradiated with kilovoltage X-ray and Cs-137 beams in slowly stirred suspensions in the presence of various concentrations of gold particles ( 1.5-3.0 microm); cell survival was measured by clonogenic assay. Gold particles were injected directly into EMT-6 tumours growing in scid mice prior to their irradiation. Tumour cell killing was assayed by an in vivo-in vitro technique. RESULTS: Dose enhancement was confirmed by both Fricke dosimetry and cell killing for 100, 140, 200 and 240 kVp X-rays, but not for Cs-137 gamma-rays. For the chemical dosimeter, a dose enhancement (DMF) of 1.42 was measured for 1% gold particle solutions irradiated with 200 kVp X-rays. When rodent and human cells were irradiated in the presence of 1% gold particles, DMF values at the 10% survival level ranged from 1.36 to 1.54, with an overall average value of 1.43. Preliminary attempts to deliver these gold particles to tumour cells in vivo by intra-tumour injection resulted in modest radiosensitization but extremely heterogeneous distribution. CONCLUSIONS: An increased biologically effective dose can be produced by gold microspheres suspended in cell culture or distributed in tumour tissue exposed to kilovoltage photon beams. With the increasing use of interstitial brachytherapy with isotopes that produce low-energy photons, high-Z particles might find a role for significantly improving the therapeutic ratio.

Optimum beam angles for the conformal treatment of lung cancer: a CT simulation study.

Treatment of lung cancer is often performed with cone-down oblique beams to spare spinal cord and normal structures. However, there is no optimum technique to determine oblique beam angles when a CT simulation is not available. Impact of oblique beam angle was investigated in this study. Fifteen patients with centrally located lung tumors were immobilized and scanned using a CT simulator. The target volumes, left and right lungs, and spinal cord were delineated on each slice. Patients were simulated starting with anterior-posterior treatment beams and subsequently an oblique opposed pair beam from 0 degrees up to 60 degrees at an interval of 5 degrees to optimize the projection of target-to-cord distance and minimize the lung volume in the treatment fields. Analysis was performed with a dose volume histogram (DVH) in each beam orientation. The distance between the target volume and spinal cord was linearly related to the angle of the beam. A larger angle facilitated further sparing of the spinal cord; however, progressively more lung volume was exposed. The 50% DVH data for lung volume was used as an indicator of lung volume. Although, the minimum lung volume was irradiated with an angle of 30 degrees, the additional lung treated increased by only 8 +/- 7% of the total lung volume for 30-60 degrees beam angles and cord distance increased by 18.5 mm. A 30 degrees oblique parallel-opposed beam for the cone-down treatment of lung provided minimum lung volume in the irradiated field; however, the spinal cord distance increased linearly with beam angle. A CT simulator is ideally suited for simulation of lung cancer to maximize the clearance from the spinal cord and minimize the additional lung volume irradiated. Int. J. Cancer (Radiat. Oncol. Invest.) 90, 359-365 (2000).

Condensed chromatin and cell inactivation by single-hit kinetics.

Mammalian cells are extremely sensitive to gamma rays at mitosis, the time at which their chromatin is maximally condensed. The radiation-induced killing of mitotic cells is well described by single-hit inactivation kinetics. To investigate if radiation hypersensitivity by single-hit inactivation correlated with chromatin condensation, Chinese hamster ovary (CHO) K1 (wild-type) and xrs-5 (radiosensitive mutant) cells were synchronized by mitotic shake-off procedures and the densities of their chromatin cross sections and their radiosensitivities were measured immediately and 2 h into G1 phase. The chromatin of G1-phase CHO K1 cells was dispersed uniformly throughout their nuclei, and its average density was at least three times less than in the chromosomes of mitotic CHO K1 cells. The alpha-inactivation co-efficient of mitotic CHO K1 cells was approximately 2.0 Gy(-1) and decreased approximately 10-fold when cells entered G1 phase. The density of chromatin in CHO xrs-5 cell chromosomes at mitosis was greater than in CHO K1 cell chromosomes, and the radiosensitivity of mitotic CHO xrs-5 cells was the greatest with alpha = 5.1 Gy(-1). In G1 phase, CHO xrs-5 cells were slightly more resistant to radiation than when in mitosis, but a significant proportion of their chromatin was found to remain in condensed form adjacent to the nuclear membrane. These studies indicate that in addition to their known defects in DNA repair and V(D)J recombination, CHO xrs-5 cells may also be defective in some process associated with the condensation and/or dispersion of chromatin at mitosis. Their radiation hypersensitivity could result, in part, from their DNA remaining in compacted form during interphase. The condensation status of DNA in other mammalian cells could define their intrinsic radiosensitivity by single-hit inactivation, the mechanism of cell killing which dominates at the dose fraction size (1.8-2.0 Gy) most commonly used in radiotherapy.

Treatment plan evaluation using dose-volume histogram (DVH) and spatial dose-volume histogram (zDVH).

OBJECTIVE: The dose-volume histogram (DVH) has been accepted as a tool for treatment-plan evaluation. However, DVH lacks spatial information. A new concept, the z-dependent dose-volume histogram (zDVH), is presented as a supplement to the DVH in three-dimensional (3D) treatment planning to provide the spatial variation, as well as the size and magnitude of the different dose regions within a region of interest. MATERIALS AND METHODS: Three-dimensional dose calculations were carried out with various plans for three disease sites: lung, breast, and prostate. DVHs were calculated for the entire volume. A zDVH is defined as a differential dose-volume histogram with respect to a computed tomographic (CT) slice position. In this study, zDVHs were calculated for each CT slice in the treatment field. DVHs and zDVHs were compared. RESULTS: In the irradiation of lung, DVH calculation indicated that the treatment plan satisfied the dose-volume constraint placed on the lung and zDVH of the lung revealed that a sizable fraction of the lung centered about the central axis (CAX) received a significant dose, a situation that warranted a modification of the treatment plan due to the removal of one lung. In the irradiation of breast with tangential fields, the DVH showed that about 7% of the breast volume received at least 110% of the prescribed dose (PD) and about 11% of the breast received less than 98% PD. However, the zDVHs of the breast volume in each of seven planes showed the existence of high-dose regions of 34% and 15%, respectively, of the volume in the two caudal-most planes and cold spots of about 40% in the two cephalic planes. In the treatment planning of prostate, DVHs showed that about 15% of the bladder and 40% of the rectum received 102% PD, whereas about 30% of the bladder and 50% of the rectum received the full dose. Taking into account the hollow structure of both the bladder and the rectum, the dose-surface histograms (DSH) showed larger hot-spot volume, about 37% of the bladder wall and 43% of the rectal wall. The zDVHs of the bladder revealed that the hot-spot region was superior to the central axis. The zDVHs of the rectum showed that the high-dose region was an 8-cm segment mostly superior to the central axis. The serial array-like of the rectum warrants a closer attention with regard to the complication probability of the organ. CONCLUSIONS: Although DVH provides an averaged dose-volume information, zDVH provides differential dose-volume information with respect to the CT slice position. zDVH is a 2D analog of a 3D DVH and, in some situations, more superior. It provides additional information on plan evaluation that otherwise could not be appreciated. The zDVH may be used along with DVH for plan evaluation and for the correlation of radiation outcome.

Monte Carlo modelling of a virtual wedge.

Compared with a set of physical photon wedges, a non physical wedge (virtual or dynamic wedge), realized by a moving collimator jaw, offers an alternative that allows creation of a wedged field with any arbitrary wedge angle instead of the traditional four physical wedges (15 degrees, 30 degrees, 45 degrees and 60 degrees). It is commonly assumed that non-physical wedges do not alter the photon spectrum compared with physical wedges that introduce beam hardening and loss of dose uniformity in the unwedged direction. In this study, we investigated the influence of a virtual wedge on the photon spectra of a 6-10 MV Siemens MD2 accelerator with the Monte Carlo code EGS4/BEAM. Good agreement was obtained between calculated and measured lateral dose profiles at the depth of maximum dose and at 10 cm depth for 20 x 20 cm2 fields for 6 and 10 MV photon beams. By comparing Monte Carlo models of a physical wedge and the virtual wedge that was studied in this work, it is confirmed that the latter has an insignificant effect on the beam quality, whereas the former can introduce significant beam hardening.

Ionization chamber shift correction and surface dose measurements in electron beams.

Cylindrical ionization chambers produce perturbations (gradient and fluence) in the medium, and hence the point of measurement is not accurately defined in electron beam dosimetry. The gradient perturbation is often corrected by a shift method depending on the type of ion chamber. The shift is in the range of 0.33-0.85 times the inner radius (r) of the ion chamber, upstream from the centre of the chamber, depending upon the dosimetry protocol. This variation in shift causes the surface dose to be uncertain due to the high dose gradient. An investigation was conducted to estimate the effective point of measurement of cylindrical ion chambers in electron beams. Ionization measurements were taken with the ion chamber in air and in a phantom at source to chamber distances of <100 cm and >100 cm respectively. The data in air and in the phantom were fitted with the inverse square and electron depth dose functions, respectively. The intersection of the two functions provides an accurate estimate of the ion chamber shift and the surface dose. Our results show that the shift correction for an ion chamber is energy dependent. The measured shifts vary from 0.9r to 0.5r between 6 MeV and 20 MeV beams respectively. The surface dose measured with the ion chambers and mathematically determined values are in agreement to within 3%. The method presented in this report is unambiguous, fast and reliable for the estimation of surface dose and the shift needed in electron beam dosimetry.

Monte Carlo dosimetry study of a 6 MV stereotactic radiosurgery unit.

Small-field and stereotactic radiosurgery (SRS) dosimetry with radiation detectors, used for clinical practice, have often been questioned due to the lack of lateral electron equilibrium and uncertainty in beam energy. A dosimetry study was performed for a dedicated 6 MV SRS unit, capable of generating circular radiation fields with diameters of 1.25-5 cm at isocentre using the BEAM/EGS4 Monte Carlo code. With this code the accelerator was modelled for radiation fields with a diameter as small as 0.5 cm. The radiation fields and dosimetric characteristics (photon spectra, depth doses, lateral dose profiles and cone factors) in a water phantom were evaluated. The cone factor (St) for a specific cone c at depth d is defined as St(d, c) = D(d, c)/D(d, c(ref)), where c(ref) is the reference cone. To verify the Monte Carlo calculations, measurements were performed with detectors commonly used in SRS such as small-volume ion chambers, a diamond detector, TLDs and films. Results show that beam energies vary with cone diameter. For a 6 MV beam, the mean energies in water at the point of maximum dose for a 0.5 cm cone and a 5 cm cone are 2.05 MeV and 1.65 MeV respectively. The values of St obtained by the simulations are in good agreement with the results of the measurements for most detectors. When the lateral resolution of the detectors is taken into account, the results agree within a few per cent for most fields and detectors. The calculations showed a variation of St with depth in the water. Based on calculated electron spectra in water, the validity of the assumption that measured dose ratios are equal to measured detector readings was verified.

Beam characteristics of a retrofitted double-focused multileaf collimator.

Multileaf collimators (MLCs) are generally believed to be convenient and cost-effective tools for intensity modulation and conformal therapy. They are becoming a standard feature on new accelerators; however, the older units can be retrofitted with modern MLCs. Before such a unit can be clinically used, the beam characteristics must be verified. In this study the beam characteristics of a Siemens double-focused MLC retrofitted to an MD2 linear accelerator are presented. The head leakage along with inter- and intra-leaf radiation transmission were measured using film. The collimator (Sc), phantom (Sp), total (Scp) scatter factors, central axis depth dose, beam profiles for off-axis ratios, penumbra, and surface dose were evaluated for square, rectangular, and irregularly shaped fields. The maximum head leakage was estimated to be < 0.05% in any plane at a distance of 1 m and maximum transmission through the MLC leaves was estimated to be < 1.4% and < 1.1% for the 10 MV and 6 MV beams, respectively. The maximum differences between pre- and post-MLC installation data for the Sc and Scp were < or = 0.7% and < or = 1.4%, respectively. Similarly, the percent depth dose data for all fields and both beam energies were within 1.5% of the original data. The beam profiles measured at various depths were also in agreement with those of the pre-MLC installation data. The measured beam penumbra (20%-80%) showed a range of 7.8 mm-11.0 mm for the 6 MV and 8.4 mm-11.1 mm for the 10 MV beams from smallest to largest fields. These ranges differ by less than a millimeter from those of the old data. The surface dose measurements were slightly lower than the conventional jaw values suggesting that MLC does not produce significant electron contamination. It is concluded that the retrofitted MLC maintains the integrity of the original beam and may provide a cost-effective conformal therapy.

Ion recombination and polarity effect of ionization chambers in kilovoltage x-ray exposure measurements.

Exposure measurements with ionization chambers are dependent on the correction factors related to the beam energy (ke), temperature and pressure (ktp), ionization recombination (Pion), and polarity (kpol) effects. In this work, six different chambers commonly used in diagnostic radiology were investigated for the Pion and kpol at various exposure rates by changing the tube voltage, beam current, exposure time, and distance. A special triaxial connector was used to connect chambers to an electrometer capable of measuring positive and negative polarity and 150 V and 300 V electrode potentials to measure the kpol and Pion, respectively. A mammography unit (24-35 kVp) and a diagnostic x-ray unit (60-125 kVp) were used. Results indicate that the magnitude of the Pion is linearly dependent on kVp for large volume (> 150 cm3) chambers and independent for small volume (< or = 150 cm3) chambers. In general, Pion is higher at higher exposures (increasing kVp, mAs, and decreasing distance); however, kpol is independent of exposure rate and kVp, but strongly depends on the sensitive volume of an ion chamber. Pion and kpol vary between 1-48% and 1-16%, respectively, among various chambers and exposure conditions. Chambers with larger volumes have higher values of Pion and kpol. The desired accuracy of +/- 5% in exposure measurements might not be feasible unless both the polarity and recombination effects are known and accounted accurately.

Radiation fields backscattered from material interfaces: I. Biological effectiveness.

Confluent cultures of CHO-K1 and CHO-xrs5 cells were irradiated attached to 6 microm Mylar with 137Cs gamma rays and 200 kVp X rays adjacent to scattering materials consisting of polystyrene, glass, aluminum, copper, tin and lead. The absorbed dose in cell nuclei was estimated from measurements of backscattered dose made with a parallel-plate ion chamber with a 5-microm Mylar window and a gas volume whose thickness was equivalent to approximately 2.6 microm of cells or tissue. Cell inactivation after various doses was measured by clonogenic assays after trypsinization and enumeration. Survival curves constructed from data pooled from at least two independent experiments were best fitted to a linear-quadratic (LQ) or a linear equation for CHO-K1 and CHO-xrs5 cells, respectively. An average distance of 9.3+/-1.9 microm from the scattering surfaces to the midline of nuclei for both the cell lines was estimated from electron micrographs of fixed cell sections. The major differences in biological effect observed when the cells were irradiated adjacent to these materials could be largely explained by the differences in the physical dose. Further analyses using the LQ equation suggested additional biological effects with implications for the mechanisms involved. CHO-K1 cells showed a small but consistent increase in the low-dose (alpha-inactivation coefficient) mechanism for both radiations scattered from high-Z material. An increased value of the alpha coefficient suggests an increase in RBE which could be associated with a higher proportion of low-energy and track-end electrons in these fields. The radiation fields which produced maximum single-hit killing in CHO-K1 cells also produced less killing by the quadratic (beta-inactivation coefficient) mechanism. In contrast, when similarly irradiated, CHO-xrs5 cells exhibited significantly lower alpha coefficients of inactivation. The mechanistic basis for this opposite effect of backscattered radiations in these cell lines is as yet unknown.

Lung and heart dose volume analyses with CT simulator in radiation treatment of breast cancer.

PURPOSE: Radiation pneumonitis and cardiac effects are directly related to the irradiated lung and heart volumes in the treatment fields. The central lung distance (CLD) from a tangential breast radiograph is shown to be a significant indicator of ipsilateral irradiated lung volume. Retrospective analysis of the pattern of dose volume of lung and heart with actual volume data from a CT simulator in the treatment of breast cancer is presented with respect to CLD. METHODS AND MATERIALS: The heart and lung volumes in the tangential treatment fields were analyzed in 108 consecutive cases (52 left and 56 right breast) referred for CT simulation. All patients in this study were immobilized and placed on an inclined breast board in actual treatment setup. Both arms were stretched over head to avoid collision with the scanner aperture. Radiopaque marks were placed on the medial and lateral borders of the tangential fields. All patients were scanned in spiral mode with slice width and thickness of 3 mm each, respectively. The lung and heart structures as well as irradiated areas were delineated on each slice and respective volumes were accurately measured. The treatment beam parameters were recorded and the digitally reconstructed radiographs (DRRs) were generated for the measurement of the CLD and analysis. RESULTS: Using CT data the mean volume and standard deviation of left and right lungs were 1307.7+/-297.7 cm3 and 1529.6+/-298.5 cm3, respectively. The magnitude of irradiated volume in left and right lung is nearly equal for the same CLD that produces different percent irradiated volumes (PIV). The left and right PIV lungs are 8.3+/-4.7% and 6.6+/-3.7%, respectively. The PIV data have shown to correlate with CLD with second- and third-degree polynomials; however, in this study a simple straight line regression is used to provide better confidence than the higher order polynomials. The regression lines for the left and right breasts are very different based on actual CT data. The slopes of regression lines for the left and right lung are 0.6%/mm and 0.5%/mm, respectively which is statistically different with thep value of 0.01. A maximum heart PIV of >3.0% is observed in 80% of the patients. The heart PIV is inversely correlated with gantry angle and weakly correlated with CLD. CONCLUSIONS: The CT-simulator provides accurate volumetric information of the heart and lungs in the treatment fields. The lung PIV is directly correlated to the CLD (0.6%/mm and 0.5%/mm for the left and right lungs). Left and right lungs have different volumes and hence, different regression lines are recommended. An additional 12% lung volume could be irradiated in the supraclavicular field. Heart volume is not correlated with the CLD. The heart PIV is associated to the beam angle. Heart volume may not be accurately visualized in a tangential radiograph; however, this can be easily seen in a DRR with contour delineation and can be minimized with proper beam parameters iteratively with a virtual simulator. Lung and heart PIV along with dose volume histograms (DVH) are essential in reducing pulmonary and cardiac complications.