Characteristics of gated treatment using an optical surface imaging and gating system on an Elekta linac.

BACKGROUND: Knowing the technical characteristics of gated radiotherapy equipment is crucial for ensuring precise and accurate treatment when using techniques such as Deep-Inspiration Breath-Hold and gating under free breathing. With one of the first installations of the novel surface imaging system Catalyst™ (C-RAD AB, Sweden) in connection with an Elekta Synergy linear accelerator (Elekta AB, Sweden) via the Elekta Response Interface, characteristics like dose delivery accuracy and time delay were investigated prior to clinical implementation of gated treatments in our institution. METHODS: In this study a moving phantom was used to simulate respiratory motion which was registered by the Catalyst™ system. The gating level was set manually. Within this gating window a trigger signal is automatically sent to the linac initiating treatment delivery. Dose measurements of gated linac treatment beams with different gating levels were recorded with a static 2D-Diode Array (MapCheck2, Sun Nuclear Co., USA) and compared to ungated reference measurements for different field sizes. In addition, the time delay of gated treatment beams was measured using radiographic film. RESULTS: The difference in dose delivery between gated and ungated treatment decreases with the size of the chosen gating level. For clinically relevant gating levels of about 30%, the differences in dose delivery accuracy remain below 1%. In comparison with other system configurations in literature, the beam-on time delay shows a large deviation of 851 ms ± 100 ms. CONCLUSIONS: When performing gated treatment, especially for free-breathing gating, factors as time delay and dose delivery have to be evaluated regularly in terms of a quality assurance process. Once these parameters are known they can be accounted and compensated for, e.g. by adjusting the pre-selected gating level or the internal target volume margins and by using prediction algorithms for breathing curves. The usage of prediction algorithms becomes inevitable with the high beam-on time delay which is reported here.

Optimized radiation of pelvic volumes in the clinical setting by using a novel bellyboard with integrated gonadal shielding.

The purpose of this study was to determine the feasibility of a custom-made, modified bellyboard to reduce radiotherapy side effects on small bowel, bladder, skin, and male gonads. Two groups of 10 consecutive patients each were treated from January 2003 through April 2003 with neoadjuvant (45 Gy) or adjuvant (54 Gy) radio(chemo)therapy in single fractions of 5 days a week 1.8 Gy for rectal carcinoma, using a photon energy of 15 MV. One group was positioned in a prone position without an immobilization device, the other group was positioned on our bellyboard. Treatment planning was calculated by using a 4- and a 3-field box technique. Differences in the dose of organs of risk were calculated. For 1 male patient, a gonadal shielding was developed and integrated. All patients examined with the bellyboard demonstrated an anterior and cranial dislocation of the small bowel. Using a 4-field box, the mean dose to the small bowel of patients treated on our bellyboard was 56.5% as compared to 63.1% when treated without the bellyboard. When a 3-field box was used, the mean dose to the small bowel was 52.4% when the bellyboard was used, as compared to a mean dose of 63.1% without the bellyboard. Regarding the dose volume effects to the bladder, the mean dose for patients treated with a 4-field box was about 14.5% higher as compared to patients treated with a 3-field box. The mean dose to the hip joints and skin also depended on the radiation technique. The patient who received gonadal shielding received a maximal total gonadal dose of about 75.0 cGy in single fractions of maximal 3.0 cGy (TL-dosimeters). Daily setup variations evaluated by a beam's-eye view were similar in both groups and ranged from 0.5 cm 1.0 cm. For daily use, our bellyboard appears to be an ideal compromise due to effectiveness, its easy handling, and reproductive positioning; moreover, it can also be used in combination with gonadal shielding.