The Changes Of Target Volumes Between External Beam Radiotherapy And Brachytherapy And Between Each Fraction Of Mri-Guided Brachytherapy For Gyn Cancer

The recent advent of high resolution (3.0 Tesla) MRI in brachytherapy (BT) allows for more accurate delineation of macroscopic tumor volumes for gynecological cancer. Tumor volume is known to be inversely related to local control and actuarial survival. We investigate the changes of tumor volumes (GTV and high risk (HR)-CTV) between external beam radiotherapy (EBRT) and BT, and between each fraction (fx) of MRI-guided BT. Inter-observer variations in HR-CTV contouring are also determined. We retrospectively studied 32 high dose rate (HDR) and 6 EBRT plans of eight patients with FIGO stage IB1-IV cervical cancer. All underwent EBRT; mean 49Gy10 in EQD2 (Equivalent Dose in 2Gy fx), 45Gy with 1.8Gy/fx with or without 5.4 ˜ 9 Gy boost. All received concurrent chemotherapy followed by HDR; mean 31Gy10 in EQD2, 5.5Gy×5fx ˜ 7Gy×3fx. Titanium Fletcher-Suit-Delclos-style tandem-and-ovoids applicators were used. High resolution (3.0 Tesla) MRI were scanned. GTV and HR-CTV were contoured on T2-weighted MRI in EBRT. For the HDR planning of each fx, two physicians independently contoured GTV and HR-CTV. Target volume changes were assessed after EBRT. Target volume changes and conformity index (CI) were assessed for each fx of HDR. For the three patients (stage IIIA-IV) with PET confirmed lymph node (LN) involvement, all GTV on the first HDR were found to be larger than 35cc (mean 49 ± 20) and smaller than 18cc (15 ± 3) for non-LN involvement cases. Their GTV before EBRT were larger than 63 cc (79 ± 23) with LN involvement versus smaller than 54 cc (27 ± 21) without LN involvement. All HR-CTV, on the first HDR, were greater than 77cc (104 ± 32) versus smaller than 45cc (35 ± 13). For large volume of HR-CTV (>40cc), the D90 (minimum dose delivered to 90% of the volume) of HR-CTV was on average 64 ± 17% of prescription dose in conventional, Point-A based plans. In MRI-Guided HDR plans, the D90 of HR-CTV was improved but still showed on average 77 ± 16% of prescription dose, showing the difficulties on large tumor volumes with BT. The volumes of GTV (HR-CTV) changed on average -25 ± 12cc (-42 ± 35) after EBRT but before BT. Through each fx of HDR, HR-CTV (GTV) changed on average -12 ± 12cc (-11 ± 7) and -21 ± 29cc (-7 ± 10) after 1st and 2nd fx while on average less than -9cc after 3rd and 4th fx. Interobserver CI did not differ significantly (0.75 ± 0.11) for HR-CTV. Smaller CI (0.56 ± 0.21) was found for GTV. Patients with lymph node involvement were found to show larger tumor volumes on the first HDR as well as on EBRT. Interobserver inconsistencies on tumor contouring can be minimized by the use of high resolution (3.0 Tesla) MRI.