Loose helical delivery is a potential solution to account for respiration-driven tumour motion in helical tomotherapy (HT). In this approach, a treatment is divided into a set of interlaced 'loose' helices commencing at different gantry angles. Each loose helix covers the entire target length in one gantry rotation during a single breath-hold. The dosimetric characteristics of loose helical delivery were investigated by delivering a 6 MV photon beam in a HT-like manner. Multiple scenarios of conventional 'tight' HT and loose helical deliveries were modelled in treatment planning software, and carried out experimentally with Kodak EDR2 film. The advantage of loose helical delivery lies in its ability to produce a more homogeneous dose distribution by eliminating the 'thread' effect-an inherent characteristic of HT, which results in dose modulations away from the axis of gantry rotation. However, loose helical delivery was also subjected to undesirable dose modulations in the direction of couch motion (termed 'beating' effect), when the ratio between the number of beam projections per gantry rotation (n) and pitch factor (p) was a non-integer. The magnitude of dose modulations decreased with an increasing n/p ratio. The results suggest that for the current HT unit (n = 51), dose modulations could be kept under 5% by selecting a pitch factor smaller than 7. A pitch factor of this magnitude should be able to treat a target up to 30 cm in length. Loose helical delivery should increase the total session time only by a factor of 2, while the planning time should stay the same since the total number of beam projections remains unchanged. Considering its dosimetric advantage and clinical practicality, loose helical delivery is a promising solution for the future HT treatments of respiration-driven targets.
Loose helical delivery is a potential solution to account for respiration-driven tumour motion inpan> helical tomotherapy (HT). In this approach, a treatmenpan>t is divided inpan>to a set of inpan>terlaced 'loose' helices commenpan>cinpan>g at differenpan>t ganpan>try anpan>gles. Each loose helix covers the enpan>tire target lenpan>gth inpan> onpan>e ganpan>try rotationpan> durinpan>g a sinpan>gle breath-hold. The dosimetric characteristics of loose helical delivery were inpan>vestigated by deliverinpan>g a 6 MV photonpan> beam inpan> a HT-like manpan>ner. Multiple scenpan>arios of conpan>venpan>tionpan>al 'tight' HT anpan>d loose helical deliveries were modelled inpan> treatmenpan>t planpan>ninpan>g software, anpan>d carried out experimenpan>tally with Kodak pan> class="Gene">EDR2 film. The advantage of loose helical delivery lies in its ability to produce a more homogeneous dose distribution by eliminating the 'thread' effect-an inherent characteristic of HT, which results in dose modulations away from the axis of gantry rotation. However, loose helical delivery was also subjected to undesirable dose modulations in the direction of couch motion (termed 'beating' effect), when the ratio between the number of beam projections per gantry rotation (n) and pitch factor (p) was a non-integer. The magnitude of dose modulations decreased with an increasing n/p ratio. The results suggest that for the current HT unit (n = 51), dose modulations could be kept under 5% by selecting a pitch factor smaller than 7. A pitch factor of this magnitude should be able to treat a target up to 30 cm in length. Loose helical delivery should increase the total session time only by a factor of 2, while the planning time should stay the same since the total number of beam projections remains unchanged. Considering its dosimetric advantage and clinical practicality, loose helical delivery is a promising solution for the future HT treatments of respiration-driven targets.
Authors: Michael W Kissick; Ryan T Flynn; David C Westerly; Peter W Hoban; Xiaohu Mo; Emilie T Soisson; Keisha C McCall; Thomas R Mackie; Robert Jeraj Journal: Phys Med Biol Date: 2008-08-18 Impact factor: 3.609