ESTRO 36 Abstract Book

S551

ESTRO 36 2017

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This study showed that VMAT treatment plans were

relatively robust during the treatment course. In this

patient cohort small changes in dose to OAR were not

significant, despite a reduction in PTV.

PO-1008 Feasibility of stereotactic ablative

radiotherapy for locally-advanced non-small cell lung

cancer

K. Woodford

, V. Panettieri

, T. Tran Le

, S. Senthi

The Alfred Hospital, Alfred Health Radiation Oncology,

Melbourne, Australia

Purpose or Objective

Stereotactic ablative radiotherapy (SABR) has enabled a

curative treatment for elderly patients or those with

significant comorbidities diagnosed with early-stage non-

small cell lung carcinoma (NSCLC) who would have

otherwise gone untreated. As a result population-based

survival has improved. If SABR could be utilized in the

treatment of locally-advanced NSCLC in the same way, the

public health impact would be greater, as twice as many

patients are diagnosed with advanced disease. We

assessed the feasibility of SABR for locally-advanced

NSCLC.

Material and Methods

Twenty three patients with N2 and/or N3 locally-advanced

lung cancer were retrospectively replanned. Targets and

organs-at-risk (OAR) were delineated using 4DCT and

replanned with RapidArc delivery (AcurosXB Vn13.6).

Three planning approaches were assessed; conventional

approach (1.0cm ITV to PTV expansion, prescribed to

100%); SABR approach (0.5cm ITV to PTV expansion,

prescribed to 80%) and a hybrid approach (0.5cm ITV to

PTV expansion, prescribed to 100%). We assessed the

feasibility of three dose regimes, with PTV doses all having

a biologic equivalence of 60Gy in 30 fractions (α/β=10).

The planning aim was to determine the least number of

fractions to deliver an acceptable plan. Acceptable was

defined as ≥95% target coverage by the prescribed dose

whilst maintaining the OAR tolerances below. Marginally

acceptable was defined as 90-95% target coverage with

lung V20 <30% and other OAR tolerances met. Descriptive

statistics were used. We assessed doses to the PRVs (2mm

expansion) of each OAR to determine the IGRT

requirements for each strategy.

Results

Fourteen patients had N2 involvement whilst nine had N3

involvement. Mean ITV size was 207.7cc (range 31-

706.1cc). The hybrid approach generated acceptable

plans in 48% of patients (11/23), while the conventional

and SABR approaches achieved 26% (6/23) and 4% (1/23)

respectively. If acceptable was defined by >90% target

coverage by the required dose and lung V20 was less than

30%, 70% (16/23) of patients had acceptable plans with

the hybrid approach. Those that failed the hybrid

approach did so due to poor PTV coverage (n=5) or

unacceptable lung dose (n=2). Of the 18 patients who had

an acceptable plan generated (regardless of planning

approach), one was achieved with the 8-fraction regime,

with the remaining needing the 12-fraction regime. OAR

PRV max doses were 2-3.5% over the OAR dose for the

conventional and hybrid approaches and 6% for the SABR

approach, highlighting the need for IGRT.

Conclusion

SABR was feasible for approximately half of the locally-

advanced NSCLC patients we assessed and for almost all

of these cases only a 12-fraction scheme was feasible. If

the alternative to SABR is no treatment at all,

compromises to tumour coverage or OAR tolerances may

be acceptable, increasing feasibility. This data will inform

a phase I study testing the safety of SABR for locally

advanced NSCLC.

Poster: RTT track: Image guided radiotherapy and

verification protocols

PO-1009 Evaluation of setup margins using cone-beam

CT for prostate and pelvic nodes irradiation

A. Van Nunen

, T. Budiharto

, B. De Vocht

, D. Schuring

Catharina Ziekenhuis, Radiotherapie, Eindhoven, The

Netherlands

Purpose or Objective

In 2014 radiotherapy for prostate and pelvic nodes was

introduced in the Catharina hospital. For this tumour site,

CBCT is used for position verification. Due to variation in

prostate position in relation to lymph nodes, large setup

margins are required to deliver the correct target dose to

both volumes. A CTV-PTV margin of 1 cm is used for both

prostate and lymph nodes. The aim of this study was to

evaluate the required setup margins using different

correction and registration strategies.

Material and Methods

CBCT-scans of 20 patients were included in this study. 220

scans were analysed retrospectively. Patients were

treated with an offline SAL correction protocol with an

initial action level of 10 mm and a maximum number of 3

measurements. When large day-to-day variations were

observed, an online correction protocol was performed.

All CBCT-scans were registered automatically using a grey

value, seed or bone match algorithm of the XVI software

(Elekta, Crawley, UK). For these automatic matches either

a clipbox containing bony structures and the entire PTV, a

mask consisting of the prostate or a mask consisting of

lymph nodes CTV was used (figure 1). Registration of the

lymph node area was performed to determine the

correlation between bony anatomy and the position of the

pelvic lymph nodes. For all these registrations all

translations, rotations and table corrections were

collected. From these results the random and systematic

setup errors were determined. The required setup

margins were then calculated using the margin recipe M =

2.5Σ+0.7σ (Σ: systematic error, σ: random error).

Results

There was a large correlation between bony structures and

lymph nodes in all directions (correlation coefficient >

0.82). Correlation between bony structures and the

prostate position was large in lateral direction and small

in longitudinal and vertical direction due to large variation

in rectal filling. This resulted in larger margins in this

direction. The

required setup margins are summarised in

Table 1. In this margin calculation, we did not account for