Chee Wai Cheng

Accelerator beam data commissioning equipment and procedures: Report of the TG-106 of the Therapy Physics Committee of the AAPM

For commissioning a linear accelerator for clinical use, medical physicists are faced with many challenges including the need for precision, a variety of testing methods, data validation, the lack of standards, and time constraints. Since commissioning beam data are treated as a reference and ultimately used by treatment planning systems, it is vitally important that the collected data are of the highest quality to avoid dosimetric and patient treatment errors that may subsequently lead to a poor radiation outcome. Beam data commissioning should be performed with appropriate knowledge and proper tools and should be independent of the person collecting the data. To achieve this goal, Task Group 106 (TG-106) of the Therapy Physics Committee of the American Association of Physicists in Medicine was formed to review the practical aspects as well as the physics of linear accelerator commissioning. The report provides guidelines and recommendations on the proper selection of phantoms and detectors, setting up of a phantom for data acquisition (both scanning and no-scanning data), procedures for acquiring specific photon and electron beam parameters and methods to reduce measurement errors (<1%), beam data processing and detector size convolution for accurate profiles. The TG-106 also provides a brief discussion on the emerging trend in Monte Carlo simulation techniques in photon and electron beam commissioning. The procedures described in this report should assist a qualified medical physicist in either measuring a complete set of beam data, or in verifying a subset of data before initial use or for periodic quality assurance measurements. By combining practical experience with theoretical discussion, this document sets a new standard for beam data commissioning.

Patterns of dose variability in radiation prescription of breast cancer

PURPOSEnComparison of radiation outcome of various treatment protocols is difficult due to the variability of dose prescription. A retrospective analysis of the pattern and intercomparison of dose prescriptions is presented for the treatment of breast cancer.nnnMATERIALS AND METHODSnTo represent the clinical practice for breast irradiation with tangential fields, commonly used prescription points were chosen that lie on the perpendicular bisector of the chest wall separation (s) that represents the breast apex height (h). These points are located at 1.5 cm from the posterior beam edge, at the chest wall-lung interface (2-3 cm), at distances of h/3 and h/2, and at the isocenter. One hundred consecutive patients treated with intact breast following excisional biopsy were used in this study. For analysis, treatment planning was carried out without lung correction with a 6 MV beam for all patients, even though some of the patients were treated with high energy beams for dose uniformity. Dose distributions were optimized with wedges and beam weights to provide a symmetrical dose distribution on the central axis plane. The statistical analyses of the different parameters, s, h, maximum dose, and doses at various prescription points were carried out.nnnRESULTSnThe maximum dose (hot spot) in breast varied from +5% to +27% above the prescribed dose among the patient population. The hot spot was directly related to s, and appeared to be independent of h and the ratio h/s. Among 55%, 40%, and 5% of the patients, the magnitude of the hot spot was 5-10%, 10-15%, and >15%, respectively. Except for the magnitude of the hot spot, the doses at various prescription points were independent of the breast size. For a prescription point at h/3 or at the lung-chest wall interface, the dose variation within +/- 1% is observed for 90% of the patient population. On the other hand, the average dose variation is about +/- 3% among other protocols with dose prescription point varying up to the h/2 point. With the prescription point at the isocenter, an average and maximum variation of 4-5% and 11% were observed, respectively. The maximum dose inhomogeneity for some patients was significantly higher, i.e. up to +27% even without the lung correction.nnnCONCLUSIONSnA wide variation in prescription dose is observed among the different treatment protocols commonly used in breast treatment. For a total dose of 46-50 Gy delivered at 2 Gy/fraction to the breast, the prescribed dose may vary between 50 and 55 Gy and the hot spot dose per fraction may range between 2.3 and 2.5 Gy depending on the protocol and breast size. Thus dose normalization at hot spot and the isocenter should be discouraged unless the total dose to the breast is modified. A uniform definition of dose prescription for breast treatment is greatly required for intercomparison of clinical data.

The effect of the number of computed tomographic slices on dose distributions and evaluation of treatment planning systems for radiation therapy of intact breast

PURPOSEnThis study was undertaken to answer the following questions in breast irradiation: (a) How many calculation planes are sufficient for three-dimensional (3-D) treatment planning? (b) Is pseudo-3-D planning system sufficiently accurate for 3-D treatment planning of a breast?nnnMETHODS AND MATERIALSnWe carried out dose calculations and differential dose-volume analysis on three representative patients covering the range of breast size encountered in a clinic. The breast volumes were reconstructed from computed tomography (CT) scans using three slices, five slices and the full CT scan respectively. An established 3-D dose algorithm and two pseudo-3-D commercial systems were used in the calculations. Comparison of isodose distributions were made between the central axis plane, a cephalic and a caudal plane 6 cm above or below the central axis respectively.nnnRESULTSnWhen comparing isodose distributions generated with conventional two-dimensional treatment planning with 3-D dose calculations, the former underestimated the size and magnitude of the hot spots in the medial and the lateral subcutaneous (SC) regions. When comparing the three-slice with the full CT model, while the three-slice model was found to be adequate for the small and the medium size patients, the full CT model provided a more accurate representation of dose distributions for the large patient. Comparison of a true 3-D algorithm with pseudo-3-D algorithms showed that while the latter systems were adequate for the small and the medium patients, significant differences were noted between the true 3-D and the pseudo-3-D algorithms for the large patient.nnnCONCLUSIONnFor patients whose breast contours vary slowly within the tangential fields, a three-slice CT scan as well as a pseudo-3-D approach appears to be adequate for clinical decision. However, for patients with large variation of contours within the tangential fields, a full scale CT scan with a true 3-D dose algorithm is more accurate than either the three-slice or the five-slice model.

TREATMENT PLAN EVALUATION USING DOSE-VOLUME HISTOGRAM (DVH) AND SPATIAL DOSE-VOLUME HISTOGRAM (zDVH)

OBJECTIVEnThe 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.nnnMATERIALS AND METHODSnThree-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.nnnRESULTSnIn 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.nnnCONCLUSIONSnAlthough 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.

Dosimetry of high energy photon and electron beams with CEA films

With all the advantages of film dosimetry in the megavoltage energy range, the use of film as a dosimeter is still limited due to the various difficulties associated with films such as energy dependence, film orientation, and sensitometric nonlinearity. Recently, therapy verification and localization films (CEA TVS and TLF films) from a Swedish manufacturer have become available in vacuum-sealed water-proof packaging in the US. The packaging renders the CEA films useful in a water phantom and ideal for photon and electron dosimetry. A systematic study has been carried out to investigate the potential of dosimetric application of the new films for high energy photon and electron beams. For the TVS films, the characteristic curve is generally energy independent but appears to be dependent on the source of the radiation, i.e., whether it is gamma rays or bremsstrahlung x rays. Compared to Kodak Readypack XV films, the CEA TVS film is linear in optical density over a much larger range of radiation dose. The inter- and intra-variation of the TVS films is less than 2%. For electrons, the characteristic curve is linear over a similar density range as photons but exhibit a slight energy dependence. TVS film is slightly directional dependent on the incident radiation for both photons and electrons. The perpendicular orientation results in higher optical density than the parallel orientation. The differences are within +/- 2% except in the buildup region for photons and in the exponential fall-off region of the electron beams where differences up to 4% are noted. For the CEA TLF film which is about three times faster than the TVS film, the characteristic curve is reasonably linear over the dose range of 0-15 cGy and energy independent within the experimental uncertainty (+/- 5%). Percent depth dose and isodose measurements with the TVS films are in good agreement with ion chamber results.

Determination of zero-field size percent depth doses and tissue maximum ratios for stereotactic radiosurgery and IMRT dosimetry : Comparison between experimental measurements and Monte Carlo simulation

In this study, zero-field percent depth dose (PDD) and tissue maximum ratio (TMR) for 6MV x rays have been determined by extrapolation from dosimetric measurements over the field size range 1×1-10×10cm2. The key to small field dosimetry is the selection of a proper dosimeter for the measurements, as well as the alignment of the detector with the central axis (CAX) of beam. The measured PDD results are compared with those obtained from Monte Carlo (MC) simulation to examine the consistency and integrity of the measured data from which the zero-field PDD is extrapolated. Of the six most commonly used dosimeters in the clinic, the stereotactic diode field detector (SFD), the PTW Pinpoint, and the Exradin A14 are the most consistent and produce results within 2% of each other over the entire field size range 1×1-40×40cm2. Although the diamond detector has the smallest sensitive volume, it is the least stable and tends to disagree with all other dosimeters by more than 10%. The zero-field PDD data extrapolated from larger field measurements obtained with the SFD are in good agreement with the MC results. The extrapolated and MC data agree within 2.5% over the clinical depth range (dmax-30cm), when the MC data for the zero field are derived from a 1×1cm2 field simulation using a miniphantom (1×1×48cm3). The agreement between the measured PDD and the MC data based on a full phantom (48×48×48cm3) simulation is fairly good within 1% at shallow depths to approximately 5% at 30cm. Our results seem to indicate that zero-field TMR can be accurately calculated from PDD measurements with a proper choice of detector and a careful alignment of detector axis with the CAX.

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 10×10 cm 2 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 currentradiation (DCR) was measured using ion chambermeasurements. 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 80% of the PFN value, a spurious radiation associated with dark current at approximately 0.7% of the dose at isocenter for a 10×10 cm 2 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.

Dosimetric comparison of treatment planning systems in irradiation of breast with tangential fields

PURPOSEnThe objectives of this study are: (1) to investigate the dosimetric differences of the different treatment planning systems (TPS) in breast irradiation with tangential fields, and (2) to study the effect of beam characteristics on dose distributions in tangential breast irradiation with 6 MV linear accelerators from different manufacturers.nnnMETHODS AND MATERIALSnNine commercial and two university-based TPS are evaluated in this study. The computed tomographic scan of three representative patients, labeled as small, medium and large based on their respective chest wall separations in the central axis plane (CAX) were used. For each patient, the tangential fields were set up in each TPS. The CAX distribution was optimized separately with lung correction, for each TPS based on the same set of optimization conditions. The isodose distributions in two other off-axis planes, one 6 cm cephalic and the other 6 cm caudal to the CAX plane were also computed. To investigate the effect of beam characteristics on dose distributions, a three-dimensional TPS was used to calculate the isodose distributions for three different linear accelerators, the Varian Clinac 6/100, the Siemens MD2 and the Philips SL/7 for the three patients. In addition, dose distributions obtained with 6 MV X-rays from two different accelerators, the Varian Clinac 6/100 and the Varian 2100C, were compared.nnnRESULTSnFor all TPS, the dose distributions in all three planes agreed qualitatively to within +/- 5% for the small and the medium patients. For the large patient, all TPS agreed to within +/- 4% on the CAX plane. The isodose distributions in the caudal plane differed by +/- 5% among all TPS. In the cephalic plane in which the patient separation is much larger than that in the CAX plane, six TPS correctly calculated the dose distribution showing a cold spot in the center of the breast contour. The other five TPS showed that the center of the breast received adequate dose. Isodose distributions for 6 MV X-rays from three different accelerators differed by about +/- 3% for the small patient and more than +/- 5% for the large patient. For two different 6 MV machines of the same manufacturer, the isodose distribution agreed to within +/- 2% for all three planes for the large patient.nnnCONCLUSIONnThe differences observed among the various TPS in this study were within +/- 5% for both the small and the medium patients while doses at the hot spot exhibit a larger variation. The large discrepancy observed in the off-axis plane for the large patient is largely due to the inability of most TPS to incorporate the collimator angles in the dose calculation. Only six systems involved agreed to within +/- 5% for all three patients in all calculation planes. The difference in dose distributions obtained with three accelerators from different manufacturers is probably due to the difference in beam profiles. On the other hand, the 6 MV X-rays from two different models of linear accelerators from the same manufacturer have similar beam characteristics and the dose distributions are within +/- 2% of each other throughout the breast volume. In general, multi-institutional breast treatment data can be compared within a +/- 5% accuracy.

Determination of zero-field size percent depth doses and tissue maximum ratios for stereotactic radiosurgery and IMRT dosimetry

In this study, zero-field percent depth dose (PDD) and tissue maximum ratio (TMR) for 6MV x rays have been determined by extrapolation from dosimetric measurements over the field size range 1×1-10×10cm2. The key to small field dosimetry is the selection of a proper dosimeter for the measurements, as well as the alignment of the detector with the central axis (CAX) of beam. The measured PDD results are compared with those obtained from Monte Carlo (MC) simulation to examine the consistency and integrity of the measured data from which the zero-field PDD is extrapolated. Of the six most commonly used dosimeters in the clinic, the stereotactic diode field detector (SFD), the PTW Pinpoint, and the Exradin A14 are the most consistent and produce results within 2% of each other over the entire field size range 1×1-40×40cm2. Although the diamond detector has the smallest sensitive volume, it is the least stable and tends to disagree with all other dosimeters by more than 10%. The zero-field PDD data extrapolated from larger field measurements obtained with the SFD are in good agreement with the MC results. The extrapolated and MC data agree within 2.5% over the clinical depth range (dmax-30cm), when the MC data for the zero field are derived from a 1×1cm2 field simulation using a miniphantom (1×1×48cm3). The agreement between the measured PDD and the MC data based on a full phantom (48×48×48cm3) simulation is fairly good within 1% at shallow depths to approximately 5% at 30cm. Our results seem to indicate that zero-field TMR can be accurately calculated from PDD measurements with a proper choice of detector and a careful alignment of detector axis with the CAX.

Transmission and dose perturbations with high-Z materials in clinical electron beams

High density and atomic number (Z) materials used in various prostheses, eye shielding, and beam modifiers produce dose enhancements on the backscatter side in electron beams and is well documented. However, on the transmission side the dose perturbation is given very little clinical importance, which is investigated in this study. A simple and accurate method for dose perturbation at metallic interfaces with soft tissues and transmission through these materials is required for all clinical electron beams. Measurements were taken with thin-window parallel plate ion chambers for various high-Z materials (Al, Ti, Cu, and Pb) on a Varian and a Siemens accelerator in the energy range of 5-20 MeV. The dose enhancement on both sides of the metallic sheet is due to increased electron fluence that is dependent on the beam energy and Z. On the transmission side, the magnitude of dose enhancement depends on the thickness of the high-Z material. With increasing thickness, dose perturbation reduces to the electron transmission. The thickness of material to reduce 100% (range of dose perturbation), 50% and 10% transmission is linear with the beam energy. The slope (mm/MeV) of the transmission curve varies exponentially with Z. A nonlinear regression expression (t=E[alpha+beta exp(-0.1Z)]) is derived to calculate the thickness at a given transmission, namely 100%, 50%, and 10% for electron energy, E, which is simple, accurate and well suited for a quick estimation in clinical use. Caution should be given to clinicians for the selection of thickness of high-Z materials when used to shield critical structures as small thickness increases dose significantly at interfaces.