ESTRO 36 Abstract Book

S627 ESTRO 36 2017 _______________________________________________________________________________________________

2 Ospedale San Bortolo, SURGERY, Vicenza, Italy 3 Ospedale San Bortolo, RADIOTHERAPY, Vicenza, Italy Purpose or Objective In IORT of the breast cancer using electron beams (IOERT), the beam energy should be properly chosen, as recommended by both ICRU 71 (2004) and AAPM TG72 (2006), to ensure that the entire PTV be covered by the 90% of the maximum dose (D max ) and the ICRU reference point be positioned as near as possible to D max . Due to the physical characteristics of these beams, the measurement of the mammary gland thickness can be critical. In fact, usually it is measured before docking using a needle and a ruler ('needle method”), or ultrasounds. Nevertheless the measured thickness can differ from the real one after docking completion, thus affecting the accuracy of the subsequent dose release. To allow accurate measurements of the gland thickness under treatment conditions, a new measurement system (MARK's) was developed at Vicenza Hospital. The aim of this work is to compare the needle method to MARK's in terms of surgeon- surgeon variability and dosimetry impact. Material and Methods A mobile IOERT-dedicated linac (LIAC,SIT) with four electron energies (4 to 10 MeV) is used at Vicenza Hospital. MARK’s is a sterilizable manual pointer with integrated ruler. After radioprotective disk positioning, the surgeon stitches the mammary gland to prepare the PTV. Then he inserts the terminal part of the applicator, after applying a thin patch layer underneath to prevent target herniation and, while keeping it pressed, he inserts the pointer inside the applicator allowing direct thickness measurements in treatment conditions.

B. Barney 1 , G. Martinez 1 , R. Hecox 1 , J. Clark 1 1 Intermountain Health Care, Radiation Oncology, Provo, USA Purpose or Objective A significant percentage of female post-lumpectomy breast cancer patients treated with whole breast radiotherapy (WBRT) have a lumpectomy cavity seroma on the initial CT simulation. Our purpose was to prospectively evaluate for changes in the size of the postoperative tumor bed during a course of WBRT, prior to the lumpectomy cavity boost (LCB). Material and Methods This prospective study was approved by the IRB, and informed consent was given by 20 women prior to study enrollment. All patients underwent breast conserving surgery and received a recommendation for LCB following WBRT by the treating physician. The median patient age was 63 years (range, 41-84). Most patients (n=19, 95%) had Stage 0, I, or II breast cancer. There was no standardized dose or fractionation for WBRT or the boost; these decisions were left to the treating physician’s discretion. Regional lymph nodes were treated as per standardized guidelines. When chemotherapy was required, it preceded WBRT. Each patient underwent initial CT simulation (CT1) at a median 39 days (range, 11-216) from surgery. Twelve women (60%) had a lumpectomy cavity seroma on CT1, and 8 (40%) did not. All patients underwent a second CT simulation (CT2) approximately 1 week before the LCB began. Median time from CT1 and CT2 was 30 days (range, 21-42). The LCB volume was immediately contoured on CT1 based on surgical clips, presence/location of seroma, and surgical findings. Without referencing CT1 LCB contours, the treating physician then contoured a modified LCB volume once CT2 was obtained, using the same factors for CT1 LCB delineation. We prospectively compared LCB volumes from CT1 and CT2 across the cohort and within seroma/no seroma subgroups. Univariate analysis of several factors potentially associated with a change in LCB volume from CT1 to CT2, including time from surgery to CT1 (≤40 days vs >40 days), time from CT1 to CT2 (≤30 days vs >30 days), and presence of seroma on CT1, was performed. Results The median LCB volumes on CT1 and CT2 for the entire cohort were 20.1 and 8.5 cm 3 , respectively. Most patients (n=17, 85%) experienced a reduction (rather than increase) in the LCB volume from CT1 to CT2. For patients with seromas, median LCB volumes on CT1 and CT2 were 36.0 and 8.8cm 3 , respectively, representing a volume reduction of >75% over the course of WBRT. For patients without seromas, median LCB volumes on CT1 and CT2 were 11.8 and 8.0cm 3 , respectively, representing a volume reduction of 32% during WBRT. On univariate analysis, only the presence of seroma was associated with a significant change in LCB volume during WBRT. Conclusion Most patients experienced a change in the size of the LCB volume during WBRT. Patients with seroma experienced a more dramatic volume reduction than those without. We recommend that women who will undergo LCB and have a seroma at the time of initial CT simulation undergo a re-simulation to plan the LCB boost towards the end of the WBRT course. EP-1164 Improved accuracy in IORT with electron beams by a new measuring system of mammary gland thickness P. Scalchi 1 , A. Marchesin 2 , G. Scalco 2 , S. Bacchiddu 3 , C. Mari 3 , L. Grandin 3 , P. Francescon 1 , F. De Marchi 2 , C. Baiocchi 3 1 Ospedale San Bortolo, MEDICAL PHYSICS, Vicenza, Italy

14 patients were studied. The measurements were taken first by the needle method, and then by MARK's. Five measurements points were always taken, one at the center of the PTV and four marginal positions (cranio- caudal and lateral). The electron energies were chosen based on the resulting thickness. The two systems were compared in terms of both the choice of the electron energy, as resulting by following ICRU and AAPM recommendations, and the surgeon-surgeon variability. Results As shown in the following Table, the needle method systematically overestimates the PTV thickness and surgeon-surgeon reproducibility is better for MARK’s. Following ICRU71 and AAPM TG72 the needle method would cause 11 erroneous energy choices and 5 treatments to be wrongly canceled. N. of erroneous N. of

Surgeon- surgeon variability (needle method)

possible treatment cancelations following ICRU 71 and

Surgeon- surgeon variability (MARK's)

Thickness difference between methods

energy choices (needle method)