To Investigate The Changes Of Volume And Spatial Location In Target And Organs At Risk During Intensity-modulated Radiotherapy For Cervical Cancer

Objective To monitor the changes of target and organs at risk during IMRT(Intensity Modulated Radiation Therapy) for cervical cancer by MRI, analyze the change law of tumor regression, space shape and spatial displacement of organs at risk, acquire the quantitative critria of the changes, to investigate the time of change IMRT plan.Methods Totally 16 patients first diagnosed with cervical cancer were recruited. All cases received a comprehensive treatment, namely IMRT combined with a synchronized chemotherapy and an intracavitary brachytherapy from the behind. A two-way positioning CT scan was performed and a two-way radiation therapy planning was made when the radiation dose in the primary tumor reached 23.4-27Gy/13-15 f. When the radiation dose reached 30.6Gy/17 f, the two-way IMRT planning was put into use till the radiation dose reached 48.6Gy/27 f. A pelvic MRI scan was performed before the radiation therapy, and when the radiation dose of primary tumor reached 9Gy/5f, 18Gy/10 f, 27Gy/15 f, 36Gy/20 f, and 48.6Gy/27 f. Acquired MR images named MR0 f, MR5 f, MR10 f, MR15 f, MR20 f, MR27 f, respectively. Those MR images were registrated with CT0 f, respectively. Then the target area and the organs at risk were outlined and were mapped to CT0 f by means of information interaction. Calculated volume(V), volume change rate(△V%) and DSC of CT/MR0,5,10,15,20,27 f, respectively. A cone beam CT(CBCT) scan was performed during the first 3 consecutive days after the onset of radiation therapy and was repeated once every week after that.Results 1. Volume(V) change. 1) tumor target: GTV-T decreased gradually with an increase in the number of radiation and had a linear correlation with the number of radiotherapy(P<0.001), GTV-T decreased from 79.62cm3 to 20.86cm3, the average change was 73.80%. There were significant difference in GTV-T among different groups(P<0.001), There was significant difference between V0 f and V10 f, V15 f, V20 f, V27f(P<0.05),except the difference between V0 f and V5f(P>0.05). CTV change little, the average change was 9.56%, there were no significant difference in among different groups(P>0.05). CTV-T had the minimum volume change. 2)Organs at risk: The biggest change was bladder, intestine volume was the most stable. There were no significant difference in among different groups(P>0.05). 2.Volume change rate(△V%). 1) tumor target: GTV-T △V%5f、△V%10f、△V%15f、△V%20f、△V%27f of GTV-T was:(23.05±12.53)%、(40.35±13.63)%、(55.57±12.07)%、(64.06±9.57)% and(70.85±8.64)%,There was significant difference between △V%5f and △V%10f 、 △V%15f 、△V%20f、△V%27f(P<0.05), △V% increased gradually, tumor shrink gradually. Tumor regression was the most obvious during the 2nd and 3rd week, followed by the 4th week, and almost came to a plateau during the 5th week( Radiotherapy Section 20-27f). CTV and CTV-T had a small △V%, the mean was 8.02% and 5.78%. 2)Organs at risk: Bladder had the maximum △V%(mean, 53.71%; maximum, 325.94%) while small intestine had the minimum △V%(mean, 16.30%). There was no significant difference among different groups(p>0.05). 3. DICE Similarity Coefficient(DSC). 1) tumor target: DSC5f、DSC10f、DSC15f、DSC20f、DSC27f of GTV-T was:0.65±0.16、0.56±0.11、0.54±0.12、0.48±0.10 and 0.37±0.12. GTV-T decreased gradually with an increase in the number of radiation and had a linear correlation with the number of radiotherapy(P<0.001). There was significant difference between DSC5 f and DSC10f、DSC15f、DSC20f、DSC27f(P<0.05). The DSC of CTV-T had the maximum change scope with an average value of 0.52 and a minimum value of 0, 40%(32/80)DSC less than 0.5. The DSC of CTV had the maximum change scope with an average value of 0.96 and a minimum value of 0.75. There was no significant difference among different groups of CTV-T and CTV(p>0.05). 2)Organs at risk: DSC sort steeled were sigmoid, bladder, rectum and intestine. There was no significant difference among different groups(p>0.05).4. Correlation between △V% and DSC. 1) Correlation between target △V% and DSC of OAR: △V% of GTV-T was negatively correlated with the DSC of both the bladder and rectum(r=-0.295,-0.231, P<0.05). △V% of CTV was negatively correlated with the DSC of the bladder, rectum as well as intestine(r=-0.306,-0.357, and-0.376, respectively; P<0.05). 2) Correlation between OAR △V% and target DSC: △V% of bladder was negatively correlated with the DSC of both the GTV-T and CTV-T(r=-0.263,-0.310, P<0.055). △V% of rectum was negatively correlated with the DSC of the GTV-T, CTV-T and CTV(r=-0.284,-0.300 and-0.344, respectively; P<0.01).Conclusions 1. The V, △V% and DSC of GTV-T were all negatively correlated with the increase of radiation times. Tumor regression and spatial location changs were the most obvious during the 2~3 week, suggesting that the optimum time for modifying radiation treatment planning is the 2week/10 f. 2. △V% of bladder was negatively correlated with the DSC of both the GTV-T and CTV-T, △V% of rectum also was negatively correlated with the DSC of the GTV-T, CTV-T and CTV. Clear, bladder and rectum volume change will result in a change of the spatial position of the target. 3. △V% of GTV-T was negatively correlated with the DSC of both the bladder and rectum, △V% of CTV was negatively correlated with the DSC of the bladder, rectum as well as intestine. Visible, GTV-T and CTV volume change will result in a change of the spatial position of OAR. 4. △V%, DSC of target and organs at risk may be potential evaluation indexes for modifing the radiotherapy planning in ART, the threshold need further studies.