Purpose: This study explores the feasibility of limiting dose optimisation to the first brachytherapy fraction (BT1) and applying the same plan for the second fraction (BT2).
Material and methods: Seventy one consecutive patients were analysed. Pulsed dose rate (PDR) BT was initiated after about 4 weeks of external beam radiotherapy (EBRT). Thirty eight patients had only intracavitary applicator (IC), and 33 had combined IC with interstitial needles (IC/IS). The optimised BT1 plan was copied to BT2 images with no further optimisation (single plan scenario) and dose volume histogram (DVH) parameters were compared with those of full dose optimisation for every fraction (optimised plan scenario).
Results: 31/38 IC patients had similar applicator geometry in both fractions and mean DVH parameters were comparable between full optimisation and single plan. The mean HR CTV D90 in total EQD2 with optimisation was 94.5 Gy and with single plan scenario was 94.4 Gy (p=0.89). Organs at risk (OARs) planning aims were fulfilled with the single plan, although 5/31 patients would receive 3-10 Gy extra to the D(2cm(3)). The mean doses in total EQD2 for the D(2cm(3)) of the bladder, rectum, sigmoid and bowel were respectively 68.5, 61.0, 64.9 and 60.6 Gy for the optimised plan, and for the single plan scenario were 69.0, 61.3, 65.1 and 60.8 Gy respectively. The difference was statistically not significant. The standard deviation (SD) of the difference between the single plan and the optimised plan was 3.2 Gy for HR CTV and 2.9, 1.4, 1.2, 1.6 Gy for the bladder, rectum, sigmoid and bowel D(2cm(3)), respectively. Only 4/33 IC/IS patients had the same applicator geometry and single plan was therefore not feasible for the majority of these patients.
Conclusion: For IC BT in small volume tumours (primarily stage IB-IIB) with mean HR CTV volume at BT1=24±12 cm(3), comparable mean DVH parameters resulted when applying a single plan, but with considerable variations in individual patients. Yet since in our population the applied target doses are high and the OARs doses are lower than the dose volume constraints these variations may not have considerable clinical consequences. Individual optimisation for each BT fraction is recommended when interstitial needles are used.
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