Peripheral dose from uniform dynamic multileaf collimation fields: implications for sliding window intensity-modulated radiotherapy

Abstract

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 degrees) 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.

A review of the impact of photon and proton external beam radiotherapy treatment modalities on the dose distribution in field and out-of-field; implications for the long-term morbidity of cancer survivors

“…This might be explained by the measurement technique used by Stern et al, as diodes are likely to over-respond outside the field where the photon energy is lower than in the center of the field. The data by Mazonakis et al [20] is for a 13.5 )17.5 cm field, and is in good agreement with data for a 14 )14 cm field [21].…”

“…This is expected as increased beamon time leads to increased leakage dose, which is of greater relevance further away from the field; while scatter in the treatment volume, which is the main component of peripheral dose close to the field edge, is less affected. At large distances (!30 cm) the factor by which the peripheral dose for dynamic IMRT fields increase compared with static fields is roughly the same as the increase in monitor units (MU) [21]. For dynamic IMRT, assuming that the degree of modulation is similar resulting in similar average gap widths, the increase in peripheral dose with field size is not only due to a larger contribution of scatter from the target volume, but also due to a larger relative increase in MU.…”

“…For dynamic IMRT, assuming that the degree of modulation is similar resulting in similar average gap widths, the increase in peripheral dose with field size is not only due to a larger contribution of scatter from the target volume, but also due to a larger relative increase in MU. This is represented in Figure 6 by Sharma et al [21], for a 6 )6 cm and a 14 )14 cm field delivered with slit fields of 2 cm width. The same authors also show a considerable increase in peripheral dose as the degree of modulation increases, represented by smaller slit field widths (data not shown).…”

Major reduction of monitor units with the avoidance of leaf-sequencing step by direct aperture based IMRT optimisation

“…In the investigated commercial IMRT techniques the number of MUs was more than two-fold compared with conventional 3D-CRT and DABO (Table I). The increase in MUs leads to higher head leakage and scatter [3]. This is expected to increase radiation induced malignancies [19].…”

“…Unfortunately, the delivery of the IMRT treatment fields requires 2 to 5 times more monitor units (MUs) than 3D-conformal radiotherapy (3D-CRT) [1,2]. An increase in the number of MUs is correlated with an increase in outof-field radiation dose [3]. Hall and Wuu [4] estimated that if the number of MUs in IMRT is increased with a factor of 2Á3 compared with 3D-CRT, IMRT is likely to almost double the incidence of second malignancies for patients surviving 10 years.…”