Note RADIATION QUALITY OF A TOMOTHERAPY PHOTON FAN BEAM

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1 Note RADIATION QUALITY OF A TOMOTHERAPY PHOTON FAN BEAM Abstract Tomotherapy, a novel radiotherapy technique, uses narrow fan beams for cancer patient treatment. Photon energy spectra for a rectangular 10 1 cm 2 photon beam were analyzed in central axis and penumbra regions at depths of 3 to 10 cm in a water phantom. A6MVbeam of a Varian 2100C/D Linear Accelerator was modeled using BEAM99 Monte Carlo calculations to simulate energy transport in a water phantom. Arrays of 4 2mm 2 scoring regions were arranged to cover the central axis and penumbra areas. Radiation quality factors were calculated based on dose-mean linear energy transfer. Although there appears to be a trend towards higher quality factor values in the penumbra area, this change is fairly small, at most 3% in penumbra region. We conclude that change in radiation quality is not likely to be an issue in a tomotherapeutic approach when 6 MV x rays are used. Health Phys. 87(2): ; 2004 Key words: radiation therapy; Monte Carlo; photons; medical radiation INTRODUCTION MODERN RADIOTHERAPY techniques involve multiple beam incidence angles and use of multi-leaf collimators (MLC) to deliver highly conformal and intensity modulated radiotherapy. The use of many small fields to create one intensity modulated field in intensity modulated radiation therapy (IMRT) magnifies the effect of any collimation device. One concern related to the complexity of IMRT modalities is that higher contribution from scattered photons can lead to softer photon spectra. If this is the case, dose and dose fractionation become insufficient predictors of biological outcome. While corrections for photon spectra have been noted for brachytherapy sources and superficial units (Bentzen 1992; Zellmer et al. 1994; Antipas et al. 2001), it has been presumed that * London Regional Cancer Centre, 790 Commissioners Road East, London, Ontario N6A 4L6, Canada; present address: Fraser Valley Cancer Centre, th Avenue, Surrey, BC, V3V 122, Canada. For correspondence or reprints contact: T. Kron at the above address, or at tomas.kron@lrcc.on.ca. (Manuscript received 10 October 2003; revised manuscript received 5 December 2003, accepted 13 May 2004) /04/0 Copyright 2004 Health Physics Society V. Moiseenko,* M. Mulligan,* and T. Kron* 166 for linac-based external therapy this is not an issue. Current guidelines for tolerance doses in normal tissue are based on pre-imrt technologies. It is conceivable that new technologies will lead not only to complex, highly non-uniform dose distributions, not representative of old technologies, but will also warrant a correction for radiation quality. This mostly relates to normal tissues and predicted complication probability, which is often a dose limiting factor in treatment planning. A recent report (Liu and Verhaegen 2002) indicated substantial changes in dose-mean lineal energy for conventional linac based IMRT photon fields. Depending on location and field settings, the quality factor, as reported by the authors, varied by as much as 20%. It appears that dose-mean lineal energy, Y D, changes by more than 5% only for regions with dose below 50% of maximum reported dose. Even though the corrections in the regions of biologically significant doses are not expected to be substantial, they are systematic deviations and one expects planning to be as accurate as 3% (Fraass et al. 1998). Therefore, any systematic and predictable corrections of the same order of magnitude should be determined and quantified. Helical tomotherapy (Mackie et al. 1993; Mackie et al. 1999) takes the IMRT field segmentation to an extreme. A narrow fan beam rotates around the patient as the patient moves through the ring gantry. The delivery pattern created by the binary MLC is altered as a function of gantry angle, a process which allows very flexible dose delivery. Because the penumbra region may pass through the same volume a number of times, changes in radiation quality may cause a larger effect for tomotherapy. In this report we present results of Monte Carlo simulations for tomotherapy photon beams in central axis and penumbra regions. MATERIALS AND METHODS A fan-beam field was simulated with dimensions 10 1cm 2 on the surface of a water phantom at 100 cm source to surface distance. A 6MV Varian 2100C/D Linear Accelerator was modeled using BEAM99 from

2 Radiation quality of a tomotherapy photon fan beam V. MOISEENKO ET AL. 167 target to MLC leaves to generate phase-space files. At the time of commencement of this project no modules suitable for simulating tomotherapy were available. The difference in photon energy spectrum between 6 MV Varian 2100C/D and tomotherapy units was assumed to be insignificant. The use of Varian 2100C/D data was therefore considered adequate for the purposes of this study. The generated phase-space files were subsequently used in the water phantom. Scoring regions of 4 2mm 2 were arranged in arrays of 6 to cover the central axis and penumbra areas, as shown in Fig. 1. These arrays were located at depths 3, 5 and 10 cm. 750 million particles were simulated in the 10 1cm 2 field contained in the water phantom and the resulting phasespace file was used with BEAMDP to generate the electron spectrum in a single scoring region. Electron energy spectra were calculated for each scoring region. These electron spectra were further converted into dose distributions in linear energy transfer (LET) and dosemean LET, L D, was calculated for each scoring region. To convert electron energy spectra to LET distributions, Fig. 1. Schematic view of the fan beam and scoring regions. The arrays were located at depths 3, 5, and 10 cm. LET-energy data for monoenergetic electrons in liquid water were used (Watt 1996). For a particular tomotherapy plan, the effect of radiation quality will be enhanced or decreased depending on the choice of pitch and slice width. Modeling of a single fan beam is the most generic case. A specific plan can be constructed as a superposition of multiple fan beams. Therefore, analysis of a specific plan may actually mask or exaggerate effects of change in radiation quality. We report the most basic data, which can be further used and applied for specific plans. Extensive dose-response data are now available for cell survival and induction of chromosome aberrations including effects of dose fractionation, dose rate and radiation quality (Kellerer and Rossi 1978; Lloyd and Edwards 1983). The latter can be related to LET or microdosimetric quantities, such as dose-mean lineal energy. Radiation quality estimates quantified as dosemean LET as well as quality factors, as they were used in radiation protection (Drexler et al. 1990), would serve as a useful guide to evaluate effects of changes in photon spectra of tomotherapy radiation fields. Dose-response for cell survival for low-let radiation can be adequately described with a linear-quadratic equation: ln S D G D 2, (1) where S is cell survival, D is dose, and are linear and quadratic coefficients, and G is a Lea-Catcheside factor accounting for dose fractionation/protraction (Sachs et al. 1997). Radiation quality is assumed to affect the linear term only and if limiting (low dose, i.e., when contribution of the quadratic term is negligible) relative biological effectiveness (RBE) for particular radiation is established relative to reference radiation, then we can simply write: r RBE m, (2) where r is a linear coefficient for the reference radiation, RBE m is limiting RBE for the considered radiation. One can see that effect of radiation quality will depend on dose. Importance of radiation quality in radiation therapy can be estimated by comparing typical dose per fraction with / ratio, i.e., dose at which linear and quadratic components contribute to cell kill equally. For tumor cells, typical / values are of the order of 10 Gy; for late effects in normal tissues approximately 3 Gy (Wigg 2001). In radiotherapy, the dose per fraction is typically 2 Gy, and most of normal tissue volume receives only a fraction of this dose. Therefore, in radiation therapy situation, full effect of radiation quality will be seen. Detailed description of RBE effects on biological indices

3 168 Health Physics August 2004, Volume 87, Number 2 is beyond the scope of this paper and can be found elsewhere (Antipas et al. 2001). In this report, radiation quality is quantified in terms of dose-mean LET and radiation quality factors calculated from dose-mean LET. To calculate quality factor values two expressions were used. The first expression, Q L D, where L D is dose-mean LET in kev m 1, was recommended in ICRU 16 (ICRU 1970). This expression is applicable to LET up to 100 kev m 1, and therefore adequate for photons. The second expression, Q L D, numerically gives results resembling those derived from ICRU 40 recommendation (Kellerer and Hahn 1988; ICRU 1984). ICRU 40 relates quality factor to the lineal energy and has a strong microdosimetric foundation. Most of the results presented in this report were calculated using equation Q L D. Results obtained with ICRU 16 formalism are discussed for comparison. Detailed reviews of quality factors for photons can be found elsewhere (Drexler et al. 1990). As a reference, an open cm 2 photon field was used with a scoring region of 4 4mm 2 located at 5 cm depth in a water phantom. primary particle in region 18 are orders of magnitude lower, which results in a less smooth curve. Fig. 3 (upper panel) shows dose drop-off for scoring regions 7 to 12 at depths 3, 5, and 10 cm normalized to the center of the field dose at 5 cm depth. Most of the dose-mean LET values fell within the range of 0.34 to 0.37 kev m 1. The lower panel of Fig. 3 shows dose-mean LET and quality factor values calculated for these scoring regions using equation Q L D. The dashed line shows quality factor for the reference, cm 2, open field at 5 cm depth. The other scoring region arrays showed the same trend towards slightly higher radiation quality factors for the scoring regions in the penumbra area. For all except seven scoring regions, relative to the reference photon field, the quality factor values range between and Notably, none of the inner-most scoring RESULTS AND DISCUSSION Fig. 2 shows electron energy spectra for the reference field and scoring region 18 at 5 cm depth. The reference spectrum in a large field basically reflects the initial photon spectrum and the energy distribution between photons and electrons in Compton effect. Compared to this, the electron spectrum in the penumbra is significantly skewed to lower energies due to increasing contribution of scattered photons. Both spectra are normalized to 100%; however, the counts per incident Fig. 2. Electron energy spectra for reference field and scoring region 18 at 5 cm depth. P(E) is proportion of electrons with the energy E or larger. Fig. 3. Upper panel: dose fall-off relative to dose in scoring region 7 at depth 5 cm; lower panel: dose-mean LET (right axis) and quality factor (left axis) values calculated for scoring regions Quality factor values were calculated from Q L D. Dashed line in the lower panel is the quality factor for the reference radiation field.

4 Radiation quality of a tomotherapy photon fan beam V. MOISEENKO ET AL. 169 regions (1 3, 7 9, and 13 15) showed changes in radiation quality larger than 1.5% relative to the reference field. Fig. 4 shows dose-mean LET and quality factors for the most peripheral scoring regions 6, 12, and 18 at depths 3, 5, and 10 cm relative to reference field quality factor (dashed line), using Q L D. It is notable that for scoring region 18, the quality factor value at 10 cm depth is lower than at 3 and 5 cm. This is because beam divergence leads to a larger contribution from the primary beam. Therefore, the scatter component at this depth is lower compared to shallower depths for the same scoring region. The change in radiation quality demonstrated in Fig. 3 (lower panel) for tomotherapy fan beams is small but reproducible. Although this confirms that the photon spectrum is softer in the penumbra region, the change in beam quality is small. The effect on biological outcome is even smaller when equation Q L D is used because of the smaller dependence of quality factor dose-mean LET. When this formalism was applied, quality factor values did not exceed that for reference field at 5 cm depth by more than 1.5%. Because the corrections apply to dose regions which receive only a fraction of the maximum dose (see upper panel, Fig. 3), any corrections for radiation quality in radiotherapy using tomotherapy fan beams appear unwarranted. However, if even thinner fan beams are used (e.g., to maintain high spatial accuracy in sup/inf directions), and additional intensity modulation within the fan beam is applied, the effect could be larger. There is a discrepancy between our findings and the results reported by Liu and Verhaegen (2002). In this study we modeled only a fully open fan beam, rather than Fig. 4. Dose-mean LET (right axis) and quality factor (left axis) values for scoring regions 6, 12, and 18 relative to reference field, Q L D. a full IMRT treatment plan. However, this cannot explain the difference because our maximum change in radiation quality applicable to penumbra was one order of magnitude below the 20% reported by Liu and Verhaegen (2002). Further investigation of radiation quality of beams used in conjunction with MLC in IMRT modalities is required in which MLC shape and tongue and groove design are explicitly taken into consideration. CONCLUSION Small, but reproducible change in radiation quality was observed for penumbra regions for tomotherapy fan beam; Corrections for radiation quality in radiotherapy using tomotherapy fan beams appear unwarranted. REFERENCES Antipas V, Dale RG, Coles IP. A theoretical investigation into the role of tumour radiosensitivity, clonogen repopulation, tumour shrinkage and radionuclide RBE in permanent brachytherapy implants of 125-I and 103-Pd. Phys Med Biol 46: ; Bentzen SM. Steepness of the clinical dose-control curve and variation in the in vitro radiosensitivity of head and neck squamous cell carcinoma. Int J Radiat Biol 61: ; Drexler G, Veit R, Zankl M. The quality factor for photons. Radiat Prot Dosim 32:83 89; Fraass B, Doppke K, Hunt M, Kutcher G, Starkschall G, Stern R, Van Dyk J. American association of physicists in medicine radiation therapy committee task group 53: quality assurance for clinical radiotherapy treatment planning. Med Phys 25: ; International Commission on Radiation Units and Measurements. Linear energy transfer. Bethesda, MD: ICRU Publications; Report No 16; International Commission on Radiation Units and Measurements. The quality factor in radiation protection. Bethesda, MD: ICRU Publications; Report No 40; Kellerer AM, Hahn K. Options for a reformulation of the quality factor. Institut fur medizinische Stahlenkunde; IMSK 88/119; Kellerer AM, Rossi HH. A generalized formulation of dual radiation action. Rad Res 75: ; Liu HH, Verhaegen F. An investigation of energy spectrum and lineal energy variations in mega-voltage photon beams used in radiotherapy. Radiat Prot Dosim 99: ; Lloyd DC, Edwards AA. Chromosome aberrations in human lymphocytes: effect of radiation quality, dose, and dose rate. In: Radiation-induced chromosome damage in man. New York: Alan R. Liss; 1983: Mackie TR, Holmes TW, Swerdloff S, Reckwerdt P, Deasy JO, Yang J, Paliwal B, Kinsella T. Tomotherapy: a new concept for the delivery of conformal radiotherapy. Med Phys 20: ; Mackie TR, Balog J, Ruchala K, Shepard DM, Aldridge KS, Fitchard EE, Reckwerdt P, Olivera GH, McNutt T, Mehta M. Tomotherapy. Semin Radiat Oncol 9: ; Sachs RK, Hahnfeld P, Brenner DJ. The link between low-let dose-response relations and the underlying kinetics of

5 170 Health Physics August 2004, Volume 87, Number 2 damage production/repair/misrepair. Int J Rad Biol 72: 351:374; Watt DE. Quantities for dosimetry of ionizing radiations in liquid water. Gateshead: Athenaeum Press Ltd; Taylor & Francis; Wigg DR. Applied Radiobiology and Bioeffect Planning. Madison, WI: Medical Physics Publishing; Zellmer DL, Shadley JD, Gillin MT. Comparison of measured biological response and predictions from microdosimetric data applicable to brachytherapy. Radiat Prot Dosim 52: ; ff