Superficial Dose Study Of Chest Wall IMRT After Radical Mastectomy

Superficial dose is of major significance in IMRT(intensity-modulated radiation therapy), especially for the breast or head and neck cancer treatment where the target is near skin. High dose can make skin being inflamed or festering, and low dose may lead to local recurrence. As to chest wall IMRT after radical radiotherapy, the real dose delivered to superficial region is different from that calculated in TPS,which may be caused by the error of TPS algorithm and the influence of respiratory. Multiple experimental techniques, such as multilayer film extrapolation method, Monte carlo simulation and respiratory simulation with 6 dimension motion platform Hexa Motion, were used to evaluate the accuracy of four commonly used algorithms in TPS and the influence of respiratory on superficial dose of chest wall IMRT after radical mastectomy.1. Quantitative analysis of the superficial dose calculation accuracy offour commonly used algorithmsEGSnrc(BEAMnrc\DOSXYZnrc) program was performed to simulate the central axis dose distribution in a 30 cm×30 cm×30 cm water phantom under the conditions of different incident gantry angles(0°, 30°and 60°) of Varian Trilogy accelerator, 10 cm×10 cm open field and SSD(source surface distance) of 100 cm to evaluate the calculation accuracyof four algorithms(AXB,CCC,AAA,PBC)in the superficial region,combined with the multilayer film extrapolation method(5 stacked pieces of radiochromic films whose effective measuring points were at 0.189,0.567, 0.945, 1.323 and 1.701 mm depth). It was indicated that, in superficial region, good agreement was achieved between MC simulation and film extrapolation method, with the mean differences less than 1%,2% and 5% for 0°, 30° and 60°, respectively. The relative skin dose errors were 0.84%, 1.88% and 3.90%; the mean dose discrepancies(0°, 30° and60°) between each of four algorithms and MC simulation were(2.41±1.55%, 3.11±2.40%, 1.53±1.05%),(3.09±3.00%, 3.10±3.01%,3.77±3.59%),(3.16±1.50%,8.70±2.84%,18.20±4.10%) and(14.45±4.66%, 10.74±4.54%, 3.34±3.26%) for AXB, CCC, AAA and PBC respectively. In conclusion, AXB performs best in the calculation of the superficial dose, and CCC dose a good job too, but the pencil beam-like AAA and PBC show large incident angle dependence and calculation discrepancies, thus care should be taken to evaluate superficial dose by the two algorithms.2. Quantitative analysis of the influence of the respiratory motion to chest wall superficial doseMeta analysis was used to acquire the average, maximum and minimum respiratory amplitudes of chest wall in three dimension. The mean respiratory period was obtained from 8 radical mastectomy patientswith the RPM system in our department. All the required parameters including amplitudes and periods were put into respiratory trace generator(RTG) software tool provided by Quirk's team to make three dimensional respiratory curve with different amplitudes, to simulate the chest wall motion by driving the Hexa Motion platform. Real treatment plans(inverse IMRT) of 8 patients mentioned above were delivered to the motion phantom and the multilayer Gafchromic film extrapolated method was used to measure superficial dose of different respiratory amplitudes and static state at positions of 0°, 30° and 60° and survey the influence of three dimensional respiratory motion to the superficial region(0-1.5 mm).For the respiratory curve with mean amplitude, the superficial dose of dynamic state was higher than that of static state by 0.15-7.97%, with statistical significance(p<0.05); the extrapolated skin dose(70 ?m) of dynamic state was also higher than that of static state, with statistical significance at positions of 0° and 60°(p<0.05). The minimum amplitude induced the maximum relative dose errors which were 4.9±2.6 %,8.0±7.2% and 5.4±2.5% at positions of 0°, 30° and 60°, respectively.There was no obvious variation trend of relative dose difference with the growth of respiratory amplitude at positions of 0° and 30°, At the position of 60°, the average relative dose differences were 5.4±2.5%,1.9±1.3%,1.8±1.6% for minimum, mean and maximum amplitudes, respectively,which meant that the average relative dose difference was reduced withthe increase of respiratory amplitude. To sum up, the superficial dose of chest wall IMRT after radical mastectomy can be increased by the respiratory motion, but the influence of small amplitude variation to the superficial dose presents no obvious trend.