ESTRO 2020 Abstract Book

S804 ESTRO 2020

lumbar tract ± the third proximal femour(s). An option to conventional 3D isocentric radiotherapy modalities could be the VMAT modality in order to conform the dose and reduce toxicity to intrapelvic organs in frail , opioid analgesics refractory -pretreated patients (pts). Material and Methods Fifteen pts with widespread bone metastatic cancers entered into this study. Bone metastases from prostate cancer were found in 6 pts, from breast and lung cancers were recorded in 4 and 5 respectively. Palliative radiotherapy 3 Gy for 5 consecutive days was prescribed on a whole volume including the whole pelvis alone (2 pts) ± other bone districts : the lumbar spine from the top of L1 to the anatomic femoral neck (left 3 pts and right 2 pts); from the top of L3 to both superior third of femoral bone (5 pts); from the top of L2 to both femoral neck under 3 cm the intertotrochanter femoral bones (3pts). Three delivery modalities were adopted and compared in each pts : a 3D technique with 2 opposed isocentric AP-PA (3D- AP-PA) MLC customized fields and 10 MV photon beams ; a 3D isocentric MLC customized 4 fields box technique with 6-10 MV wedged photon beams (3D 4 F box) and a dual arch 6 MV photon VMAT( d-VMAT). PTV coverage and the median mean dose (MMD) to rectum , bladder and the peritoneal bag were analysed and compared in EQD2 system. To assess the effect on pain, the Numeric Pain Rating Scale was applied. Results The best dosimetry was observed in the d-VMAT modality. The median V95 in d-VMAT was 99.5% ± 0.3 vs 96.7% ± 0.5 in 3D-4F box vs 95% ± 0.2.5 in 3D-AP-PA (p < 0.03). According the EQD2 estimation , the MMD to rectum was 11. 5 Gy ± 0.5 in d-VMAT vs 13 Gy ± 0.8 in 3D-4F box vs 15 Gy ± 0.6 in 3D-AP-PA ( p < 0.04); the MMD to bladder was 14 Gy ± 0.6 in d-VMAT vs 17.5 Gy ± 0.5 in 3D-4F box vs 18.75 Gy ± 0.3 in 3D AP-PA. For the peritoneal bag V15 (EQD2) the MMD was 1.960 cc±1.50 in d-VMAT vs 2250 cc ± 100 for 3D- 4F box vs 2800 cc ± 150 for 3D-AP-PA ( p < 0.02). VMAT modality showed a dose distribution tight conformed to the target. This modality was chosen to treat all patients. No genitourinary and gastrointestinal acute toxicities were recorded. Pain and narcotic assumption were improved. Conclusion In terms of response and toxicity VMAT based complex whole pelvis radiation could be considered as a faster and much more easy HBI delivery in palliation of widespread complex pelvic bone metastatic cancers. PO-1494 Dose integration method of intensity- modulated arc therapy and HDR brachytherapy of prostate cancer G. Fröhlich 1 , P. Ágoston 1 , K. Jorgo 1 , C. Polgár 1 , T. Major 1 1 National Institute of Oncology, Centre of Radiotherapy, Budapest, Hungary Purpose or Objective To validate an alternative method for summing biologically effective doses of intensity-modulated-arc-therapy (IMAT) with interstitial HDR brachytherapy (BT) boost in prostate cancer. Material and Methods Fifteen intermediate or high-risk prostate cancer patients treated with IMAT and interstitial HDR BT boost were included in the study and additional plans using IMAT boost instead of BT were created. The prescribed dose was 2/44 Gy to the whole pelvis, 2/60 Gy to the prostate and vesicle seminals and 1x10 Gy BT or 2/18 Gy IMAT to the prostate gland. As the critical organs receive the maximal total dose in the region where the dose maximum is in BT, the teletherapy planning CT was registered with the US of the BT for every patient, and the most exposed volume of critical organs in BT were identified on these CT images. The minimal dose of these from IMAT was summed with their BT dose, and these biological total doses (EQD2) were

compared using BT vs. IMAT boost with Wilcoxon-matched pairs test. This method was compared with the conventional uniform dose conception (UDC). Results The EQD2 D90 of the prostate was significantly higher with BT than with IMAT boost: 99.3 Gy vs. 77.9 Gy, p=0.0034. The D2 to rectum, bladder and hips were lower with BT, than with TT boost 50.3 Gy vs. 76.8 Gy (p=0.0117), 73.1 Gy vs. 78.3 Gy (p=0.1614) and 41.9 Gy vs. 50.6 Gy (p=0.0044), while D0.1 to urethra was higher, 96.1 Gy vs. 79.3 Gy (p=0.0180), respectively.

*p- value

EQD2

TT + BT boost TT + TT boost

99.3

(96.8-

77.9 (76.4- 78.5) 76.8 (65.8- 79.3) 79.3 (78.6- 80.4) 78.3 (77.2- 79.8) 50.6 (43.6- 58.1)

D90 (Gy)

0.0034

101.9)

(29.8-

D 2

(rectum) (Gy) 50.3

0.0017

65.8) 96.1 96.9) 73.1

D 0.1 (Gy)

(urethra)

(95.5-

0.0180

(46.0-

D 2

(bladder) (Gy)

0.1614

140.5)

41.9 58.3)

(33.5-

D 2

(hips) (Gy)

0.0044

Table. The EQD2 total doses of intensity-modulated arc therapy with interstitial HDR BT boost (TT + BT boost) and intensity-modulated arc therapy with teletherapy boost (TT + TT boost). D90: the minimum dose delivered to 90% of the prostate (Gy), D 2 (rectum), D 2 (bladder), D 2 (hips): the minimal dose of the most exposed 2 ccm of the rectum, the bladder and the hips (Gy), D 0.1 (urethra): the minimal dose of the most exposed 0.1 ccm of the urethra (Gy). *Wilcoxon-matched pairs test. UDC overestimates D2(rectum) by 37% (p=0.0117) and underestimates D0.1(urethra) by 1% (p=0.0277) and D2(bladder) by 7% (p=0.0614). Conclusion Based on our individual biological dose summation method, total dose to the prostate is higher with BT vs. IMAT boost. BT boost yields lower rectum, bladder and hip doses, but higher dose to the urethra. UDC overestimates rectum dose and underestimates the dose to the urethra and to the bladder. The potential advantage of our dose integration method is that it takes into account the most exposed part of the OARs and thus sparing these parts from higher doses in TT (Figure).

PO-1495 Effect of prescription isodose line on the brain dose for radiosurgery of brain metastasis J. Krayenbuehl 1 , Z. Mariangela 1 , T. Stephanie 1 , G. Matthias 1 , A. Nicolaus 1 1 University Hospital Zürich, Department of Radiation Oncology, Zurich, Switzerland Purpose or Objective To reduce the risk of radiation necrosis during the treatment of brain metastasis (BM) with radiosurgery, steep dose gradients around the targets are desirable. Multiple factors may affect the dose gradient such as tumor size, number of BM, technique used and prescription percentage isodose line (PIL). In this study, we evaluated the impact of the prescription PIL on the dose distribution