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L. Moretti
Author of
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MINI 18 - Radiation Topics in Localized NSCLC (ID 139)
- Event: WCLC 2015
- Type: Mini Oral
- Track: Treatment of Localized Disease - NSCLC
- Presentations: 1
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MINI18.04 - Tumor Volume Variations and Related Dosimetric Impact During Stereotactic Body Radiation Therapy for Lung Cancer (ID 958)
16:45 - 18:15 | Author(s): L. Moretti
- Abstract
Background:
This study aimed to evaluate the importance of interfraction variations in gross tumor volume (GTV) during stereotactic body radiotherapy (SBRT) for early lung cancer patients and assess its impact on dosimetric GTV coverage
Methods:
Forty-seven consecutive patients undergoing SBRT were treated with 48 Gy in 4 fractions (group 1: n=35) or 60 Gy in 8 fractions (group 2: n=12). For each patient, Cone-Beam Computed Tomography (CBCT) imaging obtained at each fraction and initial planning 4DC (CT) were analyzed. GTVs were delineated on all CBCTs, and individual treatment planning was recalculated on each CBCT. Statistical analyses were performed to compare differences between independent samples: the Mann-Whitney U test was used for non-normal continuous variables analyses between groups and the χ2 test for proportions within each SBRT group. Wilcoxon signed rank test was also used to assess changes in volume, dosimetric parameters, and tumor localization. All significance tests were two-tailed and p<0.05 was considered significant
Results:
A total of 236 CBCTs were processed and analyzed. Median total treatment times were 8 days for group 1 and 19.5 days for group 2. There was a significant tumor volume change between the initial CT and the 1st CBCT (p=0.003) in group 1. This was not found in group 2 (p=0.67). GTV was significantly larger at the 2nd CBCT (p=0.003 for group 1 and p=0.049 for group 2) compared to the 1st CBCT. Volume changes were not significantly different at the 3rd fraction compared to 1st CBCT. In group 1, GTV volume significantly decreases at the 4th fraction compared to the 2nd (p=0.047). In group 2, the significant decrease in volume occurs at the 6th fraction (p=0.026). There was no association between the overall treatment time and tumor volume variations. Taken individually (n=47) 83% of tumors have at least one occurrence of a greater than 15% volume change during SBRT compared to the 1[st] CBCT. Variations of more than 20%, 30% and even 40% were observed in ~60%, 40%, and 17% of treatments, respectively. No factor that would predict a significant volume change during SBRT for the patients analyzed could be identified. In group 1, tumor coverage factor (>95%) for any given fraction deviated no more than 5% from optimised coverage obtained in the initial treatment plan. Although sample size is smaller, there was a trend towards lower tumor coverage factors in group 2 compared to group 1. Conformity index for all tumors still ranged from 3.41 to 13.35 in group 1, and 0.95 to 10.48 in group2, without any association with tumor volume variations or treatment time
Conclusion:
There was considerable variation in tumor volumes and more frequent than initially expected for patients undergoing lung SBRT. However, these volume changes did not significantly impact dosimetric parameters. Whether these results affect treatment and/or patient outcome remains to be investigated in prospective clinical trials
