Does intensity-modulated stereotactic radiotherapy achieve superior target conformity than conventional stereotactic radiotherapy in different intracranial tumours?

AIMS: To compare the dosimetric outcome of various conventional stereotactic radiotherapy (SRT) techniques with intensity-modulated stereotactic radiotherapy (IMSRT) in brain tumours of varying shape, size, location and proximity to organs at risk (OARs). MATERIALS AND METHODS: Fused computed tomography and magnetic resonance imaging datasets of four patients with different brain tumours previously treated with non-coplanar static conformal fields (SCF) were re-planned on the BrainScan treatment planning system using non-coplanar conformal arcs (CA), dynamic conformal arcs (DCA) and IMSRT with coplanar (IMSRT_CP) or non-coplanar (IMSRT_NCP) beam arrangement. Beam shaping and intensity modulation were carried out using a BrainLab micromultileaf collimator. The primary objective for each plan was to encompass >or=99% of the planning target volume (PTV) by >95% of the prescribed dose while minimising the dose to OARs. RESULTS: The mean PTV coverage in SCF, CA, DCA, IMSRT_NCP and IMSRT_CP was 99.2, 99.5, 99.4, 99.2 and 99.2%, respectively. The highest dose within the target was <107% of the prescribed dose in all plans. Conformity was found to vary depending on the shape and location of the target. The best mean conformity index, ranging from 0.74 (CA) to 0.84 (IMSRT_NCP) was observed in spherical tumours. Among the three conventional SRT techniques, DCA and SCF appeared comparable (mean conformity index 0.72 and 0.71, respectively) and more conformal than CA (mean conformity index 0.67). In all cases, IMSRT showed better target conformity than conventional SRT techniques with a mean conformity index of 0.83 for non-coplanar and 0.81 for coplanar beam arrangement. The maximum improvement in conformity index was observed for IMSRT_NCP in complex, concave and irregularly shaped targets. The volume of normal brain and other OARs irradiated to high (>or=80%) and low (>or=30%) dose varied depending on the tumour shape, size, and location, but was essentially comparable in all three conventional SRT techniques. IMSRT (both coplanar as well as non-coplanar) reduced the volume of normal brain being irradiated to moderate to high doses compared with conventional SRT techniques, more so for large and irregular targets. CONCLUSIONS: DCA and SCF are preferred conventional SRT techniques in terms of target conformity and reduction of doses to OARs. The use of IMSRT_NCP further improves conformity and reduces doses to OARs in a range of brain tumours commonly considered for stereotactic irradiation.

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

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.

High-precision radiotherapy for craniospinal irradiation: evaluation of three-dimensional conformal radiotherapy, intensity-modulated radiation therapy and helical TomoTherapy.

This study aimed to establish the feasibility of intensity-modulated radiation therapy (IMRT) in craniospinal irradiation (CSI) using conventional linear accelerator (IMRT_LA) and compare it dosimetrically with helical TomoTherapy (IMRT_Tomo) and three-dimensional conformal radiotherapy (3DCRT). CT datasets of four previously treated patients with medulloblastoma were used to generate 3DCRT, IMRT_LA and IMRT_Tomo plans. A CSI dose of 35 Gy was prescribed to the planning target volume (PTV). IMRT_LA plans for tall patients were generated using an intensity feathering technique. All plans were compared dosimetrically using standardised parameters. The mean volume of each PTV receiving at least 95% of the prescribed dose (V(95%)) was >98% for all plans. All plans resulted in a comparable dose homogeneity index (DHI) for PTV_brain. For PTV_spine, IMRT_Tomo achieved the highest mean DHI of 0.96, compared with 0.91 for IMRT_LA and 0.84 for 3DCRT. The best dose conformity index was achieved by IMRT_Tomo for PTV_brain (0.96) and IMRT_LA for PTV_spine (0.83). The IMRT_Tomo plan was superior in terms of reduction of the maximum, mean and integral doses to almost all organs at risk (OARs). It also reduced the volume of each OAR irradiated to various dose levels, except for the lowest dose volume. The beam-on time was significantly longer in IMRT_Tomo. In conclusion, IMRT_Tomo for CSI is technically easier and potentially dosimetrically favourable compared with IMRT_LA and 3DCRT. IMRT for CSI can also be realised on a conventional linear accelerator even for spinal lengths exceeding maximum allowable field sizes. The longer beam-on time in IMRT_Tomo raises concerns about intrafraction motion and whole-body integral doses.