Dosimetric Influence Of Lung Tumor For Respiratory Movement In Three Dimensional Conformal Radiation Therapy (3DCRT)

Objective: Three dimensional conformal radiation therapy (3DCRT) is a precise advanced radiotherapy technique, which can elevate dose of target tumor and decrease extent of normal tissue. Because the technique need rather conformal request, it need high-precision target location. Respiratory movement would mostly depress treatment efficacy for lung cancer by 3DCRT. For counteracting the effect, usually add margin from CTV to PTV to ensue not to leak tumor. The margin in the ICRU62 report is called internal margin (IM). Now the margin altitude has not final conclusion, so most think tumor movement altitude by simulation equipment is outside margin. But, we always think if practical dose distribution (emphasis to consider 90% isodose distribution) for target movement concord 3D motion attitude of tumor from simulation equipment. To study dosimetric influence of different size lung cancer for respiratory movement in 3D-RT, for exactly understanding clinical real dose distribution and providing dosimetric information for definition IM to compensate respiratory movement.Methods: Motion phantom system was used to simulate normal respiratory movement with similar motion condition (motion amplitude 15mm, motion frequency 21/minute),which can simulate target tumor movement along Z axle. By CMS treatment plan system we default three rectangle simulation tumor target ,which sizes are 2×2cm, 5×5cm and 8×8cm. TPS request: 1.isocenter radiation for 3cm; 2.simulate tumor target contained by 90% isodose, which request 98%comfortbal inspection; 3.to suppose no position error; 4.to use single nearing field. By CMS treatment plan system we work out every irradiation field and transmit to accelerator by network. Then we work out expanding irradiation field, which request expanding 0.5cm, 1.0cm, 1.5cm along motor direction for retrieve motion effect and not expanding at vertical motor direction. We put Kodak-omat film under 3.0cm depth. isocenter irradiate every field in 300MU/min and 100cGy/F at 0 gantry angle. In quiescent condition every field irradiate three times and In motion condition every field irradiate three times. Pencil penumbra and dosimetric distribution (especially 90% isodose line) of different size CTV were measured with low photosensitize film. Then all kind of IM(IM at motor direction 0mm, 0.5mm, 1.0mm and 1.5mm separately)are installed to analyze the 90% isodose line distribution under similar motion condition. Then we set up calibration trace with other films, which field is 3×3cm and dose rate is 300MU/min. We obtain film result by VXR-16 laser scanner and RIT113 film calculating system software, which is 10%, 20%,30%,50%,70%,80%,90%,100% isodose line color art. We measure distance from 20% isodose line to 80% isodose line at midpoint. Every isodose curve area is measured by AutoCAD software. Statistical analysis method adopt negative parameter test, which P<0.05 is statistical significance.Result: (1) Different targets all enlarge their pencil penumbra only in motor direction ,which penumbra make broaden average 6cm.(2) But change of 90% isodose line is different, which the less target (2*2cm) change 27.53% and the largest target change 7.56%. It is illustrated that distribution of 90% isodose line is different by statistical analysis which P=0.024<0.05.(3) After all kind of internet margin are installed, we could watch area of 90% isodose line with 0.5cm margin can exactly contain CTV target and area of 90% isodose line with 1.0cm or 1.5cm margin exceed CTV target.Conclusion: Different targets all enlarge their pencil penumbra only in motor direction, which penumbra make broaden and dose gradient change mild. Distribution of 90% isodose line is different with similar motion condition and is mostly influenced on motion direction. Now it is obviously otiose for profuse expand because achieving motion amplitude for internet margin is too large for expanding standard. We consider that it should carry out individuation principle and different handle when defining internet margin. We should sufficiently know dose distribution of target for respiratory movement in clinical work.