A contemporary review of stereotactic radiotherapy: Inherent dosimetric complexities and the potential for detriment

Figures & data

Figure 1. An indication (based on a PubMed search of published literature) of the increasing implementation of both intra- and extra-cranial stereotactic radiotherapy. Note the logarithmic scale. ‘Stereotactic’ includes both intra- and extra-cranial.

Figure 2. An overview of some studies on SBRT for the treatment of primary lung cancer, showing the number of patients, N, dose, number of fractions and local control rate. Median follow-up varied between 11 and 43 months [42,44,56–64].

Table I. The Radiation Therapy Oncology Group (RTOG, United States) definitions of the various grades of pulmonary toxicity.

Toxicity grade	Definition
1	Mild dry cough or dyspnoea on exertion (DOE) not requiring clinical intervention
2	Cough requiring narcotic antitussives (cough suppressant) or dyspnoea not at rest
3	Severe cough not responsive to narcotics, or dyspnoea at rest; intermittent oxygen or steroids may be required
4	Continuous oxygen or assisted ventilation
5	Fatal

Figure 3. Various recent studies reporting observations of toxicity at or above Grade 3 (refer to Table I for definition) post-SBRT. The dose in Gy is given, along with the number of fractions. N refers to the total patient population, and n refers to the number of patients exhibiting toxicity (these numbers are given explicitly in the figure) [57,70–75].

Figure 4. Various recent studies reporting observations of toxicity at or above Grade 3 post-SBRT for liver lesions. The dose in Gy is given, along with the number of fractions. N refers to the total patient population, and n refers to the number of patients exhibiting toxicity (the latter is shown explicitly in the figure) [78–81].

Figure 5. A sketch of a tumour and isodose curves illustrating different concepts of dose prescription.

Figure 6. The main figure shows a comparison of different detector types as applied to small field measurement, based on data from McNiven et al. [158]. Shown are the relative central axis dose factors for stereotactic fields as a function of field diameter (or diameter of equivalent circle) for a 2 mm plane-parallel ionisation chamber, silicon electron diode, Kodak EDR2 radiographic film, micro-MOSFET, Type 31006 PTW PinPoint cylindrical ionisation chamber and GafChromic Type HS Radiochromic film. Measurements are in each case normalised to the standard 10 × 10 cm² reference field. Beneath the main figure is a sub-plot of the same data presented relative to the 2 mm plane-parallel ionisation chamber.

Figure 7. (a) The agreement between nominal beam diameters and those measured (FWHM) with VPL radiosensitive gel. The y = x line indicates perfect agreement. (b) A comparison of beam diameters as measured with various dosimeters, expressed as a ratio with the measured data plotted in (a). The PinPoint detector suggests larger penumbra, while the diamond and silicon diode (DOSI) detectors give lower estimates of the penumbra. Based on data from Pappas et al. [186].

Figure 8. This shows the measured output factors for 5 mm, 7.5 mm and 10 mm beams from a CyberKnife unit. The majority of dosimeters give lower output factors than those measured with gel (VIPAR). For the Gafchromic film [192,193], diode [193–196] and PinPoint chamber [192,194,195] measurements the error bars represent the standard deviation in different published values while the error bars for the gel [192] and TLD measurements [196] correspond to the uncertainty of the published measurements.

Table II. A qualitative overview of the advantages and disadvantages of the different dosimeters as applied to small field dosimetry.

Dosimeter	Advantages	Disadvantages	General comments
Ionisation chamber	High precision; wide variety of chambers available; real time measurement; no dose rate dependence; standard dosimeter that is well documented and investigated.	Zero-dimensional (0D) or one-dimensional (1D, by profile scanning); geometry restricts applications; typically inappropriate for stereotactic fields because of volume averaging effects.	Standard clinical tool for calibration; acquisition of data for planning, QA.
Radiosensitive film	Two-dimensional (2D) dose information; integrative; good spatial resolution.	Single-use; energy and (to a lesser extent) dose dependence; geometry may restrict applications.	Standard clinical tool for measurements in solid phantoms, qualitative dosimetry, QA.
Semi-conductor	Small size; real-time measurement; 1D or arrays for (low-resolution) 2D information	Degradation (finite life and changing radiation sensitivity); energy, temperature, dose rate and directional dependence; requires recalibration.	Diodes increasingly common (MOSFET less so); can be used for water tank measurements and in vivo.
TLD	Small size; various materials and forms available; large number can be used; small size and lack of cables etc allows greater flexibility in measurement geometry; standard LiF good for stereotactic spectral qualities.	Precision typically lower than ionisation chambers; read-out is delayed and relationship between dose and light emission is very complex; sensitivity may change with history	Measurements in phantom (slab or anthropomorphic) and in vivo; personal dosimetry; may be used for inter-clinic comparisons.
Diamond detector	Small size; real time measurement; no directional dependence.	Dose-rate dependence; requires pre-irradiation for stability (if not used for >1 hr).	Have been used for measurement of penumbra and stereotactic fields; CVD diamond used as TLDs.
Radiosensitive gel	Water equivalent; simultaneously acts as phantom and dosimeter; three-dimensional dose information; high spatial resolution; large number of types available; integrative; multiple readout methods; high flexibility in geometry.	Requires lab for manufacture or as-needed purchase from commercial vendors; each batch requires calibration; single-use; MRI readout may not be feasible at some clinics; optical readout of polymer-type gels nontrivial.	Limited clinical application; have been used for stereotactic fields; commercially available; published ‘recipes’ allow in-house manufacture. Polyurethane-based (e.g. Presage) solid dosimeters show promise.

Figure 9. A breakdown of 37 relevant scientific papers (employing general Monte Carlo codes) indicates that the Electron Gamma Shower codes are the primary codes employed in the study of stereotactic fields, with EGS4 being employed most frequently in the early 2000s and EGSnrc being employed in the late 2000s.

Table III. A summary of different Monte Carlo codes optimised for radiotherapy applications.

Code	Ref.	Description
MMC	[238,239]	Macro Monte Carlo (MMC) implemented in commercial TPS (‘eMC’, Varian) for electron beams. Data pre-calculated with EGSnrc for electrons in small spherical voxels. Transport through CT phantom based on latter.
PEREGRINE	[240]	Simulates radiation through collimators using pre-calculated source input. Source obtained using BEAM modelling of linac head. Transport through beam shapers involves several approximations [241].
MCDOSE	[242,243]	Based on EGS4. Modifications (for efficiency) are mostly to electron transport algorithms. Uses source model for accelerator head; input into patient-specific beam modifiers (tongue-and-groove effect in mini-multileaf collimators ignored). Shown to match EGS4 well in various geometries [243].
VMC, XMVC, VMC++	[244–246]	Voxel Monte Carlo (VMC) originally intended for electron beams; photons incorporated later (XVMC). Kawrakow and Fippel later developed VMC++, with modifications such as relativistic spin and ‘exact’ multiple scattering [247]. VMC-based codes being incorporated into commercial TPS packages by several vendors, including Elekta, BrainLAB, Nucletron and Varian.
MCV	[248]	Monte Carlo Vista (MCV) implemented as part of commercial TPS (Pinnacle, Philips Radiation Oncology Systems). Accelerator head modelled with BEAM, stored and used as input for patient-specific geometries. Patient-specific calculations may be performed using DOSXYZnrc (an EGSnrc usercode), VMC++ or MCVRTP – a code developed by Philips (based on EGS4).
RTMCNP	[249]	Radiotherapy-oriented MC code, functions as a pre-processor for MCNP4. Only implemented for research applications so far. Source library is used, and RTMCNP converts patient CT into lattice geometry with set number of defined media. Patch used to modify standard transport algorithms.

Figure 10. A comparison of the simulation times of various treatment planning optimised and full Monte Carlo radiation transport codes. The data is based on that compiled by Chetty et al. [236]. The times presented are relative to EGS4 (which were performed using the PRESTA algorithm and in a Cartesian geometry, DOSXYZ). The simulations were undertaken for the simple geometry specified by Rogers and Mohan [237], under which conditions the codes generally agreed within 1%. Note the logarithmic scale.

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