Session Item

For rectal cancer dose to small bowel has been shown to be associated with acute toxicity. As PTV margin reduction results in smaller treatment volumes, dose to organs at risk (OAR) will also decrease. There is limited evidence of the effect of reduced PTV margins on dose to OAR in short course radiotherapy (SCRT). Aim-To evaluate the impact of reducing PTV margins on OAR dose in SCRT of rectal cancer.

Patients treated with SCRT between May-July 2020 were included in the analysis. A planning CT was acquired in supine position with bowel and bladder preparation. GTV was macroscopic tumour including rectal lumen. CTVA was GTV + 10mm margin. CTVB was elective nodes. PTV_5mm, PTV _7mm and PTV_10mm were created from CTVF (CTVA + CTVB) + 5mm, + 7mm and + 10mm respectively . OAR (bladder, small bowel loops) were delineated as per RTOG definitions. All structures were delineated by a Clinical Oncologist and peer reviewed by a multi-disciplinary team. For each plan the dose prescribed to PTV was 25Gy in 5 fractions. 

Treatment plans were created using Eclipse Treatment planning system (TPS) v15.5. Three plans were calculated for each patient with PTV_5mm, PTV_7mm and PTV_10mm . The same planning objectives and priorities were used for each plan within the photon optimiser (PO) and recalculated using ACUROS v15.5. All plans consisted of 2 VMAT full rotational arcs, 6 MV at 600 MU/min and delivered on a Varian Truebeam (2.7) [Varian Medical Systems, Palo Alto, Ca, USA]. Dose metrics were extracted from dose volume histograms (DVH) for all plans.

20 rectal cancer SCRT patients were included in the analysis. Median age was 70 [IQR 60.75 -77], female (n=5), male (n=15). Rectum level was low (n=11), mid (n=4) and upper (n= 5). A total of 60 treatment plans were analysed.

The mean volume for PTV _5mm was 790.7 cm³  (SD 239.7), PTV_7mm 930.1 cm³ (SD 267.3), PTV_10mm was 1103 cm³ (SD 296.5). The mean volume for small bowel and bladder was 206 cm³ (SD 154.5) and 221.1 cm³ (SD 154.5) respectively.  All plans complied with the planning constraints for each structure (Table 1).  Figure 1. Shows the population DVH for bladder and small bowel with bladder showing most variation towards higher dose. For small bowel, variation in dose was observed across the median volume for all PTV margins.

In SCRT for rectal cancer, smaller PTV margins result in reduced dose to OAR. This is observed at the objectives and across the dose volume histogram for this population.