L Fortan

3D conformal intensity-modulated radiotherapy planning: interactive optimization by constrained matrix inversion.

BACKGROUND AND PURPOSE This paper presents a method for interactive optimization of 3D conformal intensity-modulated radiotherapy plans employing a quadratic objective that also contains dose limitations in the organs at risk. This objective function is minimized by constrained matrix inversion (CMI) that follows the same approach as the gradient technique using matrix notation. MATERIALS AND METHODS Sherouses GRATIS radiotherapy design system is used to determine the outlines of the target volume and the organs at risk and to input beam segments which are given by the beam segmentation technique. This technique defines the beam incidences and the beam segmentation. The weights of the segments are then calculated using a quadratic objective function and CMI. The objective function to be minimized consists of two components based on the planning target volume (PTV) and the organ at risk (OAR) with an importance factor w associated with the OAR. RESULTS Optimization is tested for concave targets in the head and neck region wrapping around the spinal cord. For a predefined w-value, segment weights are optimized within a few seconds on a DEC Alpha 3000. In practice, 5-10 w-values have to be tested, making optimization a less than 5 min procedure. This optimization procedure predicts the possibility of target dose escalation for a tumour in the lower neck to 120-150 Gy without exceeding the spinal cord tolerance, whereas human planners could not increase the dose above 65-80 Gy. CONCLUSIONS Treatment plans optimized using a quadratic objective function and the CMI algorithm are superior to those which are generated by human planners. The optimization algorithm is very fast and allows interactive use. Quadratic optimization by CMI is routinely used by clinicians at the Division of Radiotherapy, U.Z.-Gent.

HIGH-DOSE PREOPERATIVE RADIOTHERAPY DOES NOT ALTER THE STRENGTH OF UNILATERALLY IRRADIATED COLON ANASTOMOSES IN RATS

PURPOSE We studied the influence of preoperative radiotherapy on the strength of colon anastomoses in rats. We compared a conventional (2 Gy/fraction; 1 fraction/day; 5 days/week; cumulative doses of 40.0, 60.0, and 80.0 Gy) and a hyperfractionated schedule (1.6 Gy/fraction, 2 fractions/day, 5 days/week, cumulative doses of 41.6, 60.8, and 80.0 Gy). We compared unilaterally with bilaterally irradiated anastomoses for two conventional radiation schedules. METHODS AND MATERIALS The rectosigmoid was always irradiated. Depending on the experiment, the cecum was irradiated or not. A side-to-side anastomosis between rectosigmoid and cecum was constructed the day following the last irradiation. The strength of the anastomosis was evaluated by means of a bursting pressure (BP) measurement after 10 days. A control group and a sham-treated group were carried out. RESULTS Compared to controls, the strength of unilaterally irradiated anastomoses was not altered and BP values were independent of the radiation schedule and of the cumulative dose. In case of bilaterally irradiated colon anastomoses, anastomotic strength was significantly reduced at 80 Gy, but not at 40 Gy. CONCLUSIONS After high doses of preoperative radiotherapy, colon anastomoses in rats can be safely constructed if only one anastomotic segment is irradiated. The strength of bilaterally irradiated colon anastomoses is dose-dependent.

Execution of a single-isocenter three-field technique, using a multileaf collimator or tray-mounted cerrobend blocks: Effect on treatment time.

PURPOSE In this article, we studied the total treatment time of a single-isocenter three-field irradiation of breast and axilla, using either tray-mounted cerrobend blocks, or a multileaf collimator (MLC) for field shaping. METHODS AND MATERIALS A total of 20 female, unselected patients were given 50 Gy (2 Gy/fraction) on breast and 46 Gy on axilla and supraclavicular region (2 Gy/fraction). Patients were randomized between two different treatment groups. The first group (n = 10) was treated on a Philips SL-75 linear accelerator (SL-75), using 5 MV photons with tray-mounted cerrobend blocks. The second group (n = 10) was treated on a Philips SL-25 linear accelerator, using 6 MV photons and a MLC (SL-25-MLC). RESULTS Although the beam-on time on the SL-25-MLC was significantly higher (p < 0.0001) compared to the SL-75, overall treatment time was significantly shorter using a MLC instead of tray-mounted cerrobend blocks (p < 0.0001). The difference in total treatment time was in the range of 100 s per patient per day. The main difference between the two accelerators was observed when setup of the second and third field was done using the automatic setup facility of the SL-25-MLC (avoids entering the treatment room). A mean time gain of 124 s per treatment session was observed using automatic setup. Considering the yearly number of patients receiving this treatment, a total time gain equivalent to 16.15 8-h workdays was calculated. CONCLUSIONS Compared to a technique using tray-mounted cerrobend blocks in the single-isocenter three-field irradiation of a breast and axilla, a MLC combined with automatic field setup provides a significant time advantage, by reducing the number of manipulations inside the treatment room.

Off-line verification of the day-to-day three-dimensional table position variation for radiation treatments of the head and neck region using an immobilization mask

Variation in the table height position for 175 treatments of 167 patients was calculated as a measure for day-to-day set-up precision in 2063 treatment sessions and resulted in a median standard deviation of 1 mm. The median standard deviations of table longitudinal and lateral position were 3 and 5 mm, respectively.

717 Does high-dose pre-operative radiotherapy alter the strength of colon anastomosis in rats?

The purpose was to study the effect of pre-operative radiotherapy on the strength of colon anastomosis, constructed of one irradiated and non-irradiated segment, as well as the difference between a conventional and a hyperfractionated radiation treatment. Groups of 15 animals were treated with conventional (2.0 Gy/day to total doses of 40.0, 60.0 or 80.0 Gy) or hyper-fractionated (1.6 Gy, 2×/day to total doses of 41.6,60.8 or 80.0 Gy) radiotherapy and compared to control and sham treated groups. The strength of the colon anastomosis (constructed the day after ending the radiotherapy) was measured by a bursting experiment. End results were bursting pressure (BP) and bursting wall-tension (BWT). There was no significant difference in BP or BWT between the irradiated animals and the control groups. We conclude that high-dose pre-operative radiotherapy to one end of a colon anastomosis does not affect its anastomotic strength.