Cephas Mubata

Portal Imaging Protocol for Radical Dose-Escalated Radiotherapy Treatment of Prostate Cancer

PURPOSE The use of escalated radiation doses to improve local control in conformal radiotherapy of prostatic cancer is becoming the focus of many centers. There are, however, increased side effects associated with increased radiotherapy doses that are believed to be dependent on the volume of normal tissue irradiated. For this reason, accurate patient positioning, CT planning with 3D reconstruction of volumes of interest, clear definition of treatment margins and verification of treatment fields are necessary components of the quality control for these procedures. In this study electronic portal images are used to (a) evaluate the magnitude and effect of the setup errors encountered in patient positioning techniques, and (b) verify the multileaf collimator (MLC) field patterns for each of the treatment fields. METHODS AND MATERIALS The Phase I volume, with a planning target volume (PTV) composed of the gross tumour volume (GTV) plus a 1.5 cm margin is treated conformally with a three-field plan (usually an anterior field and two lateral or oblique fields). A Phase II, with no margin around the GTV, is treated using two lateral and four oblique fields. Portal images are acquired and compared to digitally reconstructed radiographs (DRR) and/or simulator films during Phase I to assess the systematic (CT planning or simulator to treatment error) and the daily random errors. The match results from these images are used to correct for the systematic errors, if necessary, and to monitor the time trends and effectiveness of patient imobilization systems used during the Phase I treatment course. For the Phase II, portal images of an anterior and lateral field (larger than the treatment fields) matched to DRRs (or simulator images) are used to verify the isocenter position 1 week before start of Phase II. The Portal images are acquired for all the treatment fields on the first day to verify the MLC field patterns and archived for records. The final distribution of the setup errors was used to calculate modified dose-volume histograms (DVHs). This procedure was carried out on 36 prostate cancer patients, 12 with vacuum-molded (VacFix) bags for immobilization and 24 with no immobilization. RESULTS The systematic errors can be visualized and corrected for before the doses are increased above the conventional levels. The requirement for correction of these errors (e.g., 2.5 mm AP shift) was demonstrated, using DVHs, in the observed 10% increase in rectal volume receiving at least 60 Gy. The random (daily) errors observed showed the need for patient fixation devices when treating with reduced margins. The percentage of fields with displacements of < or = 5.0 mm increased from 82 to 96% with the use of VacFix bags. The rotation of the pelvis is also minimized when the bags are used, with over 95% of the fields with rotations of < or = 2.0 degrees compared to 85% without. Currently, a combination of VacFix and thermoplastic casts is being investigated. CONCLUSION The systematic errors can easily be identified and corrected for in the early stages of the Phase I treatment course. The time trends observed during the course of Phase I in conjunction with the isocenter verification at the start of Phase II give good prediction of the accuracy of the setup during Phase II, where visibility of identifiable structures is reduced in the small fields. The acquisition and inspection of the portal images for the small Phase I fields has been found to be an effective way of keeping a record of the MLC field patterns used. Incorporation of the distribution of the setup errors into the planning system also gives a clearer picture of how the prescribed dose was delivered. This information can be useful in dose-escalation studies in determining the relationship between the local control or morbidity rates and prescribed dose.

Monte Carlo calculation of output factors for circular, rectangular, and square fields of electron accelerators (6-20 MeV)

Monte Carlo (MC) techniques can be used to build a simulation model of an electron accelerator to calculate output factors for electron fields. This can be useful during commissioning of electron beams from a linac and in clinical practice where irregular fields are also encountered. The Monte Carlo code BEAM/EGS4 was used to model electron beams (6-20 MeV) from a Varian 2100C linear accelerator. After optimization of the Monte Carlo simulation model, agreement within 1% to 2% was obtained between calculated and measured (with a Si diode) lateral and depth dose distributions or within 1 mm in the penumbral regions. Output factors for square, rectangular, and circular fields were measured using two different plane-parallel ion chambers (Markus and NACP) and compared to MC simulations. The agreement was usually within 1% to 2%. This study was not primarily concerned with minimizing the simulation time required to obtain output factors but some considerations with respect to this are presented. It would be particularly useful if the MC model could also be used to calculate output factors for other, similar linacs. To see if this was possible, the primary electron energies in the MC model were retuned to model a recently commissioned similar linac. Good agreement between calculated and measured output factors was obtained for most field sizes for this second accelerator.

IMRT clinical implementation: Prostate and pelvic node irradiation using Helios and a 120-leaf multileaf collimator

Dynamic intensity modulated radiation therapy (IMRT) to treat prostate and pelvic nodes using the Varian 120‐leaf Millennium multileaf collimator (MLC) has been implemented in our clinic. This paper describes the procedures that have been undertaken to achieve this, including some of the commissioning aspects of Helios, verification of the dynamic dose delivery, and quality assurance (QA) of the dose delivered to the patient. Commissioning of Helios included measurements of transmission through the 120‐leaf MLC, which were found to be 1.7% for 6 mV and 1.8% for 10 MV. The rounded leaf edge effect, known as the dosimetric separation, was also determined using two independent methods. Values of 1.05 and 1.65 mm were obtained for 6 and 10 MV beams. Five test patients were planned for prostate and pelvic node irradiation to 70 and 50 Gy, respectively. Dose and fluence verification were carried out on specially designed phantoms and dose points in the prostate were measured to be within 2.0% (mean 0.9%, s.d. 0.6%) of the calculated dose and in the nodes within 3.0% (mean 1.6%, s.d. 1.1%). Following the results of this commissioning and implementation study, we have started to treat men with a target volume including the prostate and pelvic nodes using Helios optimized dynamic IMRT delivery in a dose escalation protocol. PACS number(s): 87.53.–j, 87.90.+y

Comparison of a multi-leaf collimator with conformal blocks for the delivery of stereotactically guided conformal radiotherapy.

Stereotactically-guided conformal radiotherapy is a practical technique for irradiating irregular lesions in the brain. The shaping of the conformal fields may be achieved using lead alloy blocks, a conventional multi-leaf collimator (MLC) or a mini/micro-MLC. Although the former gives more precise shaping, it is labour intensive. The latter methods are more practical as both mould room and treatment room times are reduced, but the shaping is limited by the finite leaf-width. This study compares treatment plans, in terms of normal tissue doses and tumour coverage, for fields shaped using conformal blocks and a conventional MLC in two series of geometrical shapes and nine patient tumours. For the range of tumour sizes considered (volumes 14-264 cm3, minimum dimension 30 mm, maximum 102 mm), the MLC treats, on average, 14% (range 3-34%) and 17% (range 0-36%) more normal brain tissue than conformal blocks to >50% and >80% of the prescription dose, respectively. The large variability is due to strong dependence on tumour shape and the presence of partial leaf-widths in the MLC fit. It is therefore important to consider both of these effects when deciding whether the MLC is appropriate for a particular target volume.

A quality assurance procedure for the Varian multi-leaf collimator

A quick, simple set of tests has been devised to assess and record the quality assurance aspects of the Varian multi-leaf collimator (MLC) when used for clinical treatments on a regular basis. Pre-treatment, daily and weekly checks are performed by the radiographers while more detailed quality assurance is carried out at monthly and quarterly intervals by physicists.

Considerations for modelling MLCs with Monte Carlo techniques

In order to avoid systematic errors when sometimes a large number of small radiation fields are superimposed in intensity modulated photon radiation therapy (IMRT), accurate knowledge of the influence of the beam defining multileaf collimator (MLC) leaves on the radiation field is essential. Measuring the two-dimensional dose distributions of all radiation fields that might contribute to an DVIRT treatment is impractical. An alternative is Monte Carlo (MC) simulations of the individual field arrangements. This requires an exact model of the MLC leaves. The current BEAM distribution provides two modules; one with a flat leaf end surface and one that models the curved shape of a cylindrical leaf end. The former is the standard MLC module in BEAM, the latter, MLCQ, is an extension of the module MLCP, developed by us and described in [1]. In this work, lateral dose profiles for two different accelerators equipped with MLCs are calculated. The differences in the penumbral regions are shown in a homogeneous phantom and in patient geometry.

Speeding up Monte Carlo simulation of electron cut-outs in treatment planning

Treatment planning using Monte Carlo technique is becoming increasingly popular. This is especially the case for electron beams, since electrons are directly ionising, so a fewer number of histories are required to attain a given statistical uncertainty compared to photons. On most linear accelerators, there are a finite number of energies and applicators. The only patient dependent beam shaping device then is the electron cut-out at the end of the applicator or on the skin. It is possible to simulate and collect the beam characteristics in terms of phase space data or beam models at the top of the cerrobend or lead cut-outs, (see Figure 1). Simulation of These cut-outs can be time consuming if variance reduction techniques are not used. These can take the form of sampling from particles inside the block and discarding the ones int the block material, raising the cut-off energies, and range reduction techniques.

A Monte Carlo study of backscatter in the monitor ion chamber for clinical photon and electron beams

For monitor unit calculations based on dose-to-fluence formalisms [1] the contribution of backscattered radiation from the jaws into the monitor ion chamber of a linear accelerator has to be known. In principle, Monte Carlo (MC) simulations of the complete accelerator geometry for Monte Carlo treatment planning can take the backscatter to the monitor chamber into account, provided an accurate model for the energy deposited in it is available. Output factors for different fields are influenced by forward scatter towards the measuring point and by backscatter towards the monitor chamber. When MC simulations are used to calculate output factors, only the first effect is automatically taken into account. In order to be able to compare MC calculated output factors to measured ones, the backscatter contribution should be used as a correction to the MC output factors. The effect is also of importance when modelling a dynamic wedge.