Abstract Book

S803

ESTRO 37

Results Factors

(VMAT). Three planning target volumes (PTV) were defined for each patient: The original target volume (PTV0), the PTV0 minus 1 cm longitudinal (PTV-1) and the PTV0 minus 2 cm longitudinal (PTV-2) (FIG. 1). For each PTV one treatment plan with a prescribed dose of 50.4 Gy in 28 fractions was calculated. Dose to the peritoneal cavity as a surrogate for the small bowel were evaluated and compared between different PTVs. Results The mean volumes of the PTV0, the PTV-1 and PTV-2 were 1524 cm 3 , 1460 cm 3 , and 1359 cm 3 . When using a cranial longitudinal margin of minus 1 cm (PTV-1) compared with the origin PTV (PTV0), all dose parameters (Dmean, D830cc, V10, V15, V20, V25, V30, V35, V40, V45, V50) could be significantly reduced for the peritoneal cavity. All of these dose parameters were also significantly reduced for the PTV-2 compared with the PTV0 and the PTV-2 compared with PTV-1. The NTCP- based side effects will be calculated soon. Conclusion Reduction of the cranial PTV of 1 and 2 cm can significantly reduce the dose to the small bowel (defined as peritoneal cavity). Previous studies have shown a highly significant dose-volume relationship for the development of Grade 3+ acute small bowel toxicity in patients receiving neoadjuvant CRT for rectal cancer. Despite clinical benefit and oncologic outcome remain unclear, reduction of the longitudinal margins might provide the opportunity to reduce side effects of chemoradiation (CRT) in upcoming studies. EP-1482 Lateral pelvic lymph node metastasis in rectal cancer receiving NACRT – a time for serious relook R. Engineer 1 , M. Karthik 1 , S. Lewis 1 , A. D'souza 2 , K. Suman 3 , S. Arya 3 , V. Ostwal 4 , A. Ramaswamy 4 , P. Patil 5 , S. Mehta 5 , S. Chopra 1 , M. Ramadwar 6 , A. Saklani 2 1 Tata Memorial Hospital, Radiation Oncology, Mumbai, India 2 Tata Memorial Hospital, Surgical Oncology, Mumbai, India 3 Tata Memorial Hospital, Radiology, Mumbai, India 4 Tata Memorial Hospital, Medical Oncology, Mumbai, India 5 Tata Memorial Hospital, Gastroenerology, Mumbai, India 6 Tata Memorial Hospital, Pathology, Mumbai, India Purpose or Objective Patients with locally advanced rectal caners having suspicious lateral pelvic lymph nodes (LPLN) are associated with have poor local control and higher rate of distant metastasis. There is wide variability in management of these patients regarding the role of LPLN dissection (LPLND). LPLND is not routinely recommended due to excessive surgical morbidity. The aim of this study was to identify the set of patients in whom LPLND is indicated. In this study we analysed the outcomes of all consecutive patients with locally advanced rectal cancer with suspicious LPLN treated at our center Material and Methods Patients having locally advanced rectal cancer having suspicious LPLN on diagnostic MRI treated between June 2013 to 2016 were studied. All received neoadjuvant chemoradiotherapy (NACRT) to pelvis to a dose of 5000 cGy in 25 fractions over 5 weeks with concurrent Tab Capecitabine (825 mg/m2 bid for 5 weeks). Response evaluation MRI pelvis was done at 6 – 8 weeks post radiotherapy. They were stratified as per the response to NACRT as complete responders (CR) in whom the LPLN were not seen or were < 5 mm. Patients having persistent or suspicious looking LPLN >5mm were labelled as having sub-optimal responders (SR). TME alone was done in patients with CR and LPLND + TME was done in those with SR.

CR (Complete response) n=21 (45%)

SR (Partial or suboptimal response) n=27 (55%)

affecting response

Significance

at

LPLN n=48

Gender

13(27) 8 (17)

22(46) 5 (10)

0.18

Male

Female

Initial T stage

T2

0 20(42) 0 1 (2)

1

(2)

23(48) 2

T3

0.47

(4)

1 (2)

T4a

T4b

Initial lateral pelvic lymph node size (mm)

12(25) 6

4 1

(8) (2)

5-8

(12)

0.001

3 (6)

22(46)

8-10

>10mm

Lateral pelvic lymph node dissection

0 21(44)

22(46) 5 (10)

0.001

Yes

No

ypT

Stage

13(27) 8 (17)

12(25) 15(31)

0.25

Lateral pelvic lymph nodes positive on pathology

0.001

- -

2

(4)

20(42)

Positive

Negative

Tumor regression grade

13(27) 8 (17)

13(27) 14(29)

0.45

1-2