British Journal of Radiology (2006) 79, 331-335
© 2006 British Institute of Radiology
doi: 10.1259/bjr/16208090
Peripheral dose from uniform dynamic multileaf collimation fields: implications for sliding window intensity-modulated radiotherapy
D S Sharma, MSc, DipRP1,
Animesh, MSc, DipRP2,
S S Deshpande, MSc, DipRP1,
R D Phurailatpam, MSc, DipRP3,
D D Deshpande, PhD1,
S K Shrivastava, MD4 and
K A Dinshaw, FRCR4
Departments of 1 Medical Physics and 4 Radiation Oncology, Tata Memorial Hospital, Dr. Ernest Borges Marg, Parel, Mumbai, 400 012, India, 2 Kirloskar Theratronics, Mumbai and 3 Clinical Research Centre, ACTREC, New Mumbai, India
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Abstract
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The increase in the number of monitor units in sliding window intensity-modulated radiotherapy, compared with conventional techniques for the same target dose, may lead to an increase in peripheral dose (PD). PD from a linear accelerator was measured for 6 MV X-ray using 0.6 cm3 ionization chamber inserted at 5 cm depth into a 35 cm x 35 cm x 105 cm plastic water phantom. Measurements were made for field sizes of 6 cm x 6 cm, 10 cm x 10 cm and 14 cm x 14 cm, shaped in both static and dynamic multileaf collimation (DMLC) mode, employing strip fields of fixed width 0.5 cm, 1.0 cm, 1.5 cm, and 2.0 cm, respectively. The effect of collimator rotation and depth of measurement on peripheral dose was investigated for 10 cm x 10 cm field. Dynamic fields require 2 to 14 times the number of monitor units than does a static open field for the same dose at the isocentre, depending on strip field width and field size. Peripheral dose resulting from dynamic fields manifests two distinct regions showing a crest and trough within 30 cm from the field edge and a steady exponential fall beyond 30 cm. All dynamic fields were found to deliver a higher PD compared with the corresponding static open fields, being highest for smallest strip field width and largest field size; also, the percentage increase observed was highest at the largest out-of-field distance. For 6 cm x 6 cm field, dynamic fields with 0.5 cm and 2 cm strip field width deliver PDs 8 and 2 times higher than that of the static open field. The corresponding factors for 14 cm x 14 cm field were 15 and 6, respectively. The factors by which PD for DMLC fields increase, relative to jaws-shaped static fields for out-of-field distance beyond 30 cm, are almost the same as the corresponding increases in the number of monitor units. Reductions of 20% and 40% in PD were observed when the measurements were done at a depth of 10 cm and 15 cm, respectively. When the multileaf collimator executes in-plane (collimator 90°) motion, peripheral dose decreases by as much as a factor of 3 compared with cross-plane data. The knowledge of PD from DMLC field is necessary to estimate the increase in whole-body dose and the likelihood of radiation induced secondary malignancy.
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Introduction
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Absorbed dose outside the primary radiation field (peripheral dose; PD) is of clinical interest in estimating out-of-field organ dose and subsequent long-term radiation sequelae. Potential effects of PD such as cataract formation, gonadal dysfunction and infertility, and damage to fetus, and their threshold doses, have been summarized by Fraass and van de Geijn [1]. The potential damage to the fetus from PD sometimes creates clinical dilemmas during decision making in radiation therapy of pregnant patients [2–4].
Prior investigators have confirmed that increase in the number of monitor units (MU) in tomotherapy, compared with a conventional technique for the same tumour dose, results in a higher whole-body dose [5–7]. For a dose of 70 Gy to a head and neck tumour, whole-body dose has been shown to increase from between 0.2 Sv and 0.25 Sv for conventional therapy to nearly 2 Sv for tomotherapy; this in turn may increase the risk of radiation-induced secondary malignancies by a factor of eight [8]. The increase in MU is a function of complexity of intensity-modulation, its delivery technique and collimator design of the treatment machine. Therefore, PD needs to be measured separately for the technique employed on the treatment machine. This study was designed to measure PD from 6 MV X-rays employing static and dynamic multileaf collimation (DMLC), which is the basis for dynamic intensity-modulated radiotherapy (IMRT). Dependence of PD on out-of-field distance, strip field width, field size, direction of multileaf collimator (MLC) motion and depth of measurement were investigated.
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Methods and materials
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Peripheral dose was measured for 6 MV X-ray using a linear accelerator (LA) (Clinac 2100 C/D; Varian Associates, Palo Alto, CA) equipped with an MLC consisting of 26 leaf pairs, each leaf projecting to a 10 mm leaf width at the isocentre distance. All measurements were carried out using a 0.6 cm3 Farmer type ionization chamber (PTW, Friedberg, Germany) inserted at 5 cm depth into a 35 cm x 35 cm x 105 cm plastic water phantom (Nuclear Associates, USA) under isocentric conditions. Field sizes of 6 cm x 6 cm, 10 cm x 10 cm and 14 cm x 14 cm defined by conventional jaws were simulated in DMLC field mode, wherein strip fields of constant width 0.5 cm, 1 cm, 1.5 cm and 2 cm created by the opposing MLCs were moved with constant speed from one bank to the other in a rectilinear fashion perpendicular to the radial axis of the LA. Thus for every open field, six sets of PDs were measured – two from static open fields shaped by jaws and MLCs and four from DMLC. When the collimator is set at 0° (IEC 61217) [9], and MLCs are used either in static or dynamic mode, the upper (Y) jaws define the superior and inferior borders and the lower (X) jaws are positioned 0.8 cm distal to the most retracted leaf position on each side of the field. The detail performance characteristic of this MLC in dynamic dose delivery has been described elsewhere [10–12].
The number of MU required to deliver 1 Gy at 5 cm depth on the central axis was found by matching meter readings from the different DMLC fields to that from the corresponding, static jaw-shaped open field. These MUs were used for the subsequent PD measurement along a longitudinal axis to an out-of-field distance up to 60 cm. The factor by which planned MU increases for different DMLC fields over that of the corresponding static fields for the same dose is defined to be MU multiplication factor.
The same measurements were repeated for 10 cm x 10 cm field size with collimator at 90°, wherein the MLC executes in-plane motion. Variation in PD with depth was investigated for 10 cm and 15 cm depth under isocentric set up using 10 cm x 10 cm field size having 1 cm strip field width.
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Results
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The MU multiplication factor for different DMLC fields was found to increase with decreasing strip field width and increasing field size (Table 1
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Table 1. Monitor unit(MU) multiplication factor (ratio of MU from dynamic multileaf collimation (DMLC) fields to that from correspondingopen field) for 6 MV X-ray for different DMLC fields having effective field sizes of 6 cmx6 cm, 10 cmx10 cm and 14 cmx14 cm
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The measured PD data for 6 MV X-ray at 5 cm depth and for 0° collimator angle are shown in Figure 1 as a function of out-of-field distance. Each family of six curves represents the PD data for jaws and MLC shaped static field and four DMLC fields having different strip field widths. The dose at the depth of maximum dose (Dmax) on the central axis of every jaws-shaped static field was normalized to 100%. All the PD data were expressed as a percentage of the respective Dmax. PD data of all DMLC fields were found to manifest two distinct regions.
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Figure 1. Peripheral dose in phantom from 6 MV X-ray for 0° collimator and field sizes (a) 6 cm x 6 cm; (b) 14 cm x 14 cm at 5 cm depth, normalized to 100% on the central axis at depth of maximum dose of static open field. MLC, multilead collimator.
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PD < 30 cm
The region extending up to 30 cm out-of-field distance, shows crests and troughs of varying amplitude; this is prominent for 6 cm x 6 cm (Figure 1a
) and is found to grow progressively smaller for larger fields (Figure 1b).
PD > 30 cm
In this region PD curves from DMLC fields of different strip field width are seen running parallel and maintaining constant slope. The factor by which PD for different DMLC fields increase over jaws-shaped static field is observed to be almost the same as that of the corresponding increase in MU. Although the absolute value of PD is comparatively small, the percentage increase in PD is higher for this region, showing maximum increase at 60 cm out-of-field distance.
As expected, PD of all DMLC fields was found to be higher than the corresponding static open fields. For the same field size the smaller strip field width is seen delivering higher PD compared with larger strip field width. Also for the same strip field width, PD consistently increases with increasing field sizes. For 6 cm x 6 cm field, DMLC with 0.5 cm and 2 cm strip field width deliver a maximum of 8 and 2 times higher PD than that of static open fields (Figure 1a). The corresponding factor for 14 cm x 14 cm field is 15 and 6, respectively (Figure 1b). DMLC field with 0.5 cm strip field width delivers 1.5 and 3 times higher PD than that of 2 cm strip field width at 1 cm and 60 cm out-of-field distance, respectively.
As an exception, jaws-shaped static fields may deliver more PD than certain DMLC fields with larger strip field width. This exceptional phenomenon is observed for out-of-field distances extending up to 15 cm for 6 cm x 6 cm and up to 7 cm for 14 cm x 14 cm. Also, PD data of MLC shaped static field is found to be less than that of jaw-shaped static fields for out-of-field distances up to 30 cm, beyond which MLC shaped static fields show a slight increase in PD over jaws-shaped fields.
PD data for static and DMLC fields measured with 90° collimation shows a reduction in value compared with data measured with 0° collimation. The magnitude of reduction is seen to depend on out-of-field distance and strip field width. For 10 cm x 10 cm field, maximum reduction in PD data for jaws and MLC shaped static field are found to be by a factor of 2 and 2.5, respectively (Figure 2). For DMLC fields, a maximum reduction factor of 3 is observed at 30 cm out-of-field distance for the smallest strip field width of 0.5 cm (Figure 3). Similar characteristics were observed for larger strip field widths, albeit with progressively lower PD. MLC shaped static and DMLC fields show no change in PD at 20 cm from the field edge for these two collimator orientations.
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Figure 2. Composite peripheral dose(PD) distribution of 6 MV X-ray measured at 5 cm depth with 0° and 90° collimator angle from 10 cm x 10 cm static field shaped by jaws and multileaf collimator (MLC), normalized to 100% on the central axis at depth of maximum dose of static open field.
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Figure 3. Composite peripheral dose(PD) distribution of 6 MV X-ray measured at 5 cm depth with 0° and 90° collimator angles from 10 cm x 10 cm field simulated in dynamic multileaf collimation (DMLC) mode using 0.5 cm sweeping gap width, normalized to 100% on the central axis at depth of maximum dose of static open field.
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PD measured under isocentric condition at 5 cm, 10 cm and 15 cm depth using 10 cm x 10 cm fields and 1 cm strip field width are shown in Figure 4. A maximum reduction of 20% and 40% at 60 cm from the field edge is observed for 10 cm and 15 cm depth, respectively, as compared with the data measured at 5 cm depth.
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Figure 4. Peripheral dose(PD) in phantom from 6 MV X-rays measured at 5 cm, 10 cm and 15 cm depth with target to chamber distance of 100 cm. Effective field size of 10 cm x 10 cm having 1 cm sweeping gap width and 0° collimator is used. PD is normalized to 100% on the central axis at depth of maximum dose of static open field.
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Discussion
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Pre-measured PD data from static fields [1, 2, 13–17], and an empirical relationship [18], are available as a guideline to evaluate approximate out-of-field organ dose from conventional treatment. The qualitative and quantitative behaviour of our jaw-shaped open field (10 cm x 10 cm, 0° collimation and 5 cm depth) PD data is in agreement with other findings [2, 17]. But beyond 30 cm out-of field distance, our PD data for MLC shaped static field disagrees with that of Mutic et al [17] who have reported reduction in PD at all out-of-field distances. However, when the collimator is rotated to 90° our data agrees with the findings of Mutic et al [17].
Although IMRT offers significant advantages for dose conformality to irregular target volumes with sharp dose gradient beyond the target, it also increases MU/Gy and hence whole body dose. This may be a concern for long-term radiation sequelae in patients treated with this technique [8, 19]. Whole-body dose data are available from tomotherapy [5–8]. However, the same data cannot be applied to sliding window IMRT technique as the collimator scatter and transmission, head leakage and internal scatter, which are the main components of PD, depend on the collimator design of treatment machine and beam delivery technique. Our study was carried out to find PD from different DMLC fields which closely simulated sliding window IMRT. When static fields are simulated in DMLC mode, PD was found to increase by a factor of 2 to 15, similar to the increase in MU for DMLC fields over corresponding static open fields. Increase in MU due to the use of a universal wedge have also shown to increase PD by a factor of 2–4 compared with PD resulting from unwedged beam [14–16].
Even though the intensity of radiation is not modulated in our dynamic fields, different strip field width can be thought to represent the different complexity of intensity profile required in IMRT. As the degree of modulation increases, i.e. peaks and valleys in the intensity profile are sought to be closely spaced, these highly modulated beams require narrow MLC openings leading to sharp increase in MU. With sliding window IMRT, higher MU becomes an absolute corollary and the planned MU tends to be larger in cases where the fluence distribution is more modulated.
Even though PD from DMLC field is generally observed to be more than that of jaw-shaped static field, a slight reduction in PD is observed near the field edge for DMLC field with larger sweeping gap width. This may be attributed to reduction in collimator scatter and transmission of DMLC fields offered by the stationary portion of the MLC. The increase in MU for smaller sweeping gap widths for the same field and dose, increases both scatter and leakage dose leading to the increase in PD.
Besides internal scatter and leakage dose, PD in the proximal region (5–30 cm) seems to be a complex interplay of leaf scatter, leaf end design, MLC placement in treatment head, MLC motion and beam-on time. Similar to our crest and trough effect appearing within 30 cm of the field edge, Greene et al [20] have reported a sharp peak at 20 cm from the edge of 10 cm x 10 cm field due to maximum scatter coming from the face of the beam-defining jaws, which disappears for large field sizes as the point that sees maximum scatter from the face of the jaws shifts into the primary beam itself. In our case, this situation never realises completely, even for large DMLC fields, probably because scatter radiation arises both from the face of jaws and from the rounded end face of the sweeping MLCs.
The crest and trough effect appearing prominently for smaller dynamic fields may be clinically significant especially when a critical organ is lying close to the primary field. If an organ at risk is lying at the distance of 15 cm from the field edge (field size 6 cm x 6 cm) it would received 4.5 times more dose than the case where this effect is not taken into account. Moreover, it is not known if commercially available treatment planning systems take into account this pattern of PD during optimization and dose computation. Further study is required to segregate the different components of scatter and leakage. Inclusion of PD characteristics in dose calculation algorithms is suggested.
Most published data for static field shows small variation in PD with depth of measurement [1, 2, 15]. However, similar to the finding of Tatcher et al, our measurement for DMLC field shows decrease in PD with depth of measurement.
The largest field studied is 14 cm x 14 cm as limited by the maximum range of travel of individual leaves relative to the carriage. While normalizing the dose at depth of dose maximum to 100%, central axis depth dose characteristics of the DMLC field of a particular size is assumed to remain same as that for a static field. The uncertainty associated with low dose measurement especially at 60 cm with lower MU generally for the static fields are not considered in the presented data. For open fields where the MU is small, MU was scaled by a factor of 2–3 to get good signal at largest out-of-field distance. The reproducibility of meter reading for DMLC fields was observed to be less than 2% during the measurement.
As the IMRT field employed in the clinical situations consists of a mixture of multiple strip fields having different gap width (beamlet size), validation of the phantom measured PD from uniform DMLC field needs to be carried out using the clinically employed fluence.
Received for publication June 16, 2004.
Revision received May 31, 2005.
Accepted for publication July 19, 2005.
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Abstract
Introduction
