Abstract
PurposeHybridArc is a novel treatment technique blending aperture-enhanced optimized arcs with discrete IMRT-elements, allowing selection of arcs with a set of static IMRT-beams. This study compared this new technique to helical Tomotherapy, and RapidArc, in preoperative radiotherapy of rectal cancer.Material and methodsTwelve rectal cancer patients treated consecutively with Tomotherapy Hi-Art II system were simulated with HybridArc and RapidArc. Treatment plans were designed to deliver homogeneous dose of 46.0Gy to mesorectum and draining lymph nodes, with a simultaneous-integrated-boost to the primary tumor up to a total dose of 55.2Gy. Planning objectives were 95% of prescribed dose to 95% of PTVs, while minimizing the volume of small bowel receiving more than 15Gy (V15) and the mean bladder dose. Dose gradient towards simultaneous-integrated-boost (GI), calculated by dividing the volume receiving more then 52.4Gy (95% of PTV55.2Gy)to the volume of PTV55.2Gy, was kept below 1.5. Mean beam-on time and amount of MUs were also analyzed.ResultsPTV swere adequately covered by all plans. Significant advantage was found for Tomotherapy in sparing small bowel (V15 = 112.7cm3SD73.4cm3) compared to RapidArc (133.4cm3SD75.3cm3) and HybridArc (143.7cm3SD74.4cm3) (p < 0.01). The mean bladder dose was better with RapidArc (20.6GySD2.2Gy) compared to HybridArc (24.2Gy SD4.3Gy) and Tomotherapy (23.0GySD4.7Gy) (p < 0.01). The mean beam-on time was significantly lower (p < 0.01) for HybridArc (2.7min SD0.8) and RapidArc (2.5min SD0.5) compared to Tomotherapy (11.0min SD0.7). The total amount of MUs was significantly (p < 0.01) lower for RapidArc (547SD44)compared to HybridArc (949 SD153).ConclusionsHybridArc is a feasible solution for preoperative RT with a simultaneous-integrated-boost in rectal cancer patients. It achieved similar PTV coverage with significant lower beam-on time, but less efficient in sparing small bowel and bladder compared to Tomotherapy and RapidArc. The added value of HybridArc is that the treatment modality can be implemented on every LINAC equipped with Dynamic-Conform-Arc and IMRT treatment techniques, while maintaining the same QA-schemes.
Highlights
In patients with locally advanced rectal cancer, preoperative radiotherapy (RT) is considered standard of care by improving local tumor control and overall survival [1,2,3,4]
It achieved similar planning target volume (PTV) coverage with significant lower beam-on time, but less efficient in sparing small bowel and bladder compared to Tomotherapy and RapidArc
The added value of HybridArc is that the treatment modality can be implemented on every LINAC equipped with Dynamic-Conform-Arc and intensity-modulated RT (IMRT) treatment techniques, while maintaining the same QA-schemes
Summary
In patients with locally advanced rectal cancer, preoperative (chemo) radiotherapy (RT) is considered standard of care by improving local tumor control and overall survival [1,2,3,4]. The German Rectal Cancer Study Group reported any grade 3 acute and late toxicity in 27% and 14% of the patients undergoing conventional RT, respectively [5]. The treatment is delivered in arc rotation during which the MLCs are moving dynamically while the dose rate and gantry speed are varying [10,11]. HybridArc is novel treatment technique and a different interpretation of the IMAT concept, blending aperture-enhanced optimized arcs with discrete IMRT-elements. This method allows selection of Aperture-optimized-Arcs, delivered with constant gantry speed and dose rate, with a set of static IMRT-elements at specified intervals along each arc. By weighting the contribution of arcs versus IMRT-elements, HybridArc aims achieving an optimal dose distribution, while limiting the use of IMRT
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