Joseph Awotwi-Pratt, PhD
BC Cancer Agency
Purpose/Objective(s): To investigate the feasibility of using the first fraction plan for subsequent fractions by comparing and contrasting a standard plan (SP) with fraction specific plans (FSP) in a retrospective study.
Materials/Methods: 84 plans in total for 17 patients inserted with the ring and tandem (R&T) applicator only were evaluated. All the plans were computed tomography (CT) based and each patient received 6Gy per fraction with the exception of three patient who had 6.5Gy per fraction for four fractions. Each patient had both CT and magnetic resonance Imaging (MRI) during the second fraction for an improved delineation of clinical target volume (CTV) and organs at risk (OAR) (i.e., bladder, rectum, sigmoid and bowel). In scenario A, FSP was used i.e., dwell positions and times were re-optimised for each fraction. In scenario B, SP was used i.e., the same dwell positions and times were used for all fractions. A comparison of the CTV coverage, international commission on radiological units and measurement (ICRU) points, D2cc (dose to 2cc volume) and D1cc (dose to 1cc volume) of OAR was carried out to evaluate the two scenarios.
Results: The CTV coverage for FSP ranged from 92.5% to 99.5% while that of SP ranged from 88.3 to 99.9%. Only one patient’s CTV coverage using SP was below the 90% minimum coverage limit. The OAR D2cc were marginally higher for SP but most were within the dose constraints. The mean of the ratios of D2ccFSP/D2ccSP for the bladder, rectum, sigmoid and bowel are 0.97, 0.97, 0.99 and 0.98 respectively. A similar trend was obtained for the D1ccFSP/D1ccSP.
Conclusion: This study has shown that there is virtually no difference between SP and FSP when using the R&T applicator. The doses to OAR are comparable and most are within the dose limit and also the minimum dose coverage of the CTV was achieved.
BC Cancer Agency
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