ESTRO 38 Abstract book

S85 ESTRO 38

EQD2 10 smaller by 19.2cm 3 at any given target volume (Fig.1b); mean Bladder ICRU and Rectovaginal Point doses (D2cm 3 is given in parenthesis) smaller by 9.1 (7.5) and 5.6 (3.3) Gy EQD2 3, respectively; mean vaginal dose higher by 17.1GyEQD2 3 . IC/IS compared to IC centres had : significantly improved target coverage and smaller V85Gy EQD2 for intermediate and large CTV HR volumes (Fig. 1c- f); decreased OAR doses, mostly in T&O. T&R IC/IS compared to T&O IC/IS centres had : mean CTV HR D 90% higher by 2.7Gy EQD2 10 ; mean V85Gy EQD2 10 smaller by 8.3cm 3 at any given CTV HR volume; mean Bladder D2cm 3 and Rectovaginal Point doses smaller by 4.3Gy and 4.8Gy EQD2 3 , respectively; mean vaginal dose larger by 22.5Gy EQD2 3 . All above mentioned differences were statistically significant (p<0.01). For patients treated with IC/IS, needles were used for all fractions in 85% and 55% of cases in T&R IC/IS and T&O IC/IS centres, respectively. Average number of needles/patient was 4.9 (SD:2.5) and 3.5 (SD:1.8) in T&R IC/IS and T&O IC/IS centres, respectively.

OC-0173 Dosimetric comparison of T-O brachytherapy with/without interstitial component in FIGO I-IIB tumors M. Federico 1 , A. Perez Fustero 1 , C. Catarina 2 , I. Fernandez 3 , J.L. Perez Molina 3 , I. Morales Orue 1 , M. Lloret 1 1 Hospital Universitario de Gran Canaria Dr. Negrín, Radiation Oncology, Las Palmas de Gran Canaria- Ca, Spain ; 2 ULPGC, Centro de Tecnologías de la Imagen CTIM, Las Palmas de Gran Canaria, Spain; 3 Hospital Universitario de Gran Canaria Dr. Negrín, Medical Physics, Las Palmas de Gran Canaria- Ca, Spain Purpose or Objective Intracavitary/interstitial (IC/IS) brachytherapy (BT) improves HRCTV dose coverage in large locally advanced tumors resulting in improved local control and survival. In small tumors IC/IS brachytherapy may be useful in selected situations due to difficult OAR/tumor topography. A prospective trial was designed in order to evaluate in a series of consecutive patients with small cervical tumors potential gains of IC/IS vs. Intracavitary (IC) brachytherapy Material and Methods 200 consecutive patients with histologically proven cervical cancers treated between 2013-18 with 45 Gy 3D CRT and MR-based IGABT (4 fraction of 7 Gy each in 2 different applications) were analyzed. Out of the original 200 patients the ones receiving TO application were selected (175 pts). To further select only the tumors potentially suited for IC BT application a standard IC 7 Gy point A plan was generated and patients with a HRCTV coverage <86 Gy EQD2 were discarded (69 pts). Out of the 106 remaining patients 79 had IC/IS application in all 4 BT fractions and were selected for the present study. All pts had a Figo stage I (20 pts) or II (59 pts). At the time of their treatment all patients received IC/IS T-O application (Elekta Utrecht applicator) optimized upon the following constraints: HRCTV≥86Gy EQD2 ; Bladder≤90 Gy EQD2 ; Rectum≤70 Gy EQD2 ; Sigmoid ≤70 Gy EQD2 . For the present study an IC plan was recalculated. To compare IC or IC/IS plans a cost function was generated Fig1a. To assign a specific cost to each individual planification a function was created with a linear and quadratic component. The zero of the function for each ROI was set to its specific constraint (86 Gy EQD2 for HRCTV, 90 Gy EQD2 for bladder, etc) or in case of D0.1 cc values to the median value of the delivered IC/IS planifications. The final cost of each plan was the sum of the individual ROI cost function normalized and weighted as follow (2/6 HRCTV cost, 1/6 for each OAR D2cc value and finally 1/6 for the D0.1cc values of the 3 OARs) (Fig 1b)

Conclusion The T&R IC applicator plans show better coverage and dose conformity than the T&O IC applicator plans for CTV HR <45cm 3 . For large target volumes, both T&R and T&O IC plans fail to ensure adequate target coverage. Bladder and rectum are, in general, better spared with T&R IC, while the 5mm lateral vaginal dose is higher. Routine application of IC/IS improves target coverage and dose conformity, and better spares the OARs. With the addition of needles, the differences seen in target/OAR doses and in V85Gy EQD2 between the IC applicators become smaller. These differences are also related to both the number of needles and fractions with needles used.