Dosimetric differences between field and volume-based regional nodal RT in the POSNOC breast trial
Hannah Price,
United Kingdom
PD-0335
Abstract
Dosimetric differences between field and volume-based regional nodal RT in the POSNOC breast trial
Authors: Hannah Price1, Zohal Nabi1, Roeum Butt1, Romaana Mir2, Patricia Diez1, David Dodwell3, Shabina Sadiq4, Mickey Lewis4, Amit Goyal5
1National Radiotherapy Trials Quality Assurance Group, Mount Vernon Cancer Centre, Radiotherapy Physics, Northwood, United Kingdom; 2Mount Vernon Cancer Centre, Clinical Oncology, Northwood, United Kingdom; 3Oxford University, Nuffield Department of Population Health, Oxford, United Kingdom; 4University of Nottingham, Nottingham Clinical Trials Unit, Nottingham, United Kingdom; 5Royal Derby Hospital, Oncoloplastic Breast Surgery, Derby, United Kingdom
Show Affiliations
Hide Affiliations
Purpose or Objective
POSNOC (NCT02401685) is a randomised, multicentre, non-inferiority trial comparing
adjuvant RT to adjuvant RT plus axillary treatment in patients with early-stage
breast cancer. Field-based RT techniques were used by most centres upon opening. Subsequent
adoption of a nodal volume-based approach has been slow. Dosimetric differences
between field-based and volume-based nodal plans in patients recruited to
POSNOC are reported.
Material and Methods
A left-sided breast cancer patient with axillary RT
from 20 institutions planned with field-based techniques was evaluated for dose
coverage to axillary lymph node levels 1-4 (L1-4). Breast
specialists retrospectively delineated L1-4
in line with ESTRO consensus guidelines (Offersen et al, 2015); contours were checked independently by a radiation oncologist.
The PTV was defined as a 5mm expansion of the CTV, then grown by 6mm to account for penumbra, allowing for direct
comparison with conventional field placement.
POSNOC pre-defined planning objectives: Field-based plans
aimed for >90% dose to the supra-clavicular fossa (SCF) and >80% dose to mid axilla; volume-based
used PTV V90%≥90%. In addition, coverage of L1-4 by field-based techniques
was evaluated by suitability of field border placement and PTV V80%
≥ 90%.
Results
14/20 centres met POSNOC field-based
objectives.
The cranial border extended beyond L3 and L4 by 18 mm, on average. Field borders were short caudally, laterally
and medially by a mean of 7, 2 and 10 mm, respectively.
The PTV V80%
objective was consistently achieved for L2 and L3 (96.5% and 92.9%,
respectively), however not so for L1 and L4 (88.3% and 78.8%, respectively),
where results were variable (Figure 1). Loss of L1 coverage was mostly in the
cranial aspect due to short lateral axillary borders; L4 coverage was
compromised medially where borders were shortened to reduce dose to lung and spinal
cord. Use of oblique fields at some centres improved coverage.
V90%≥90% was met only by 4/20 centres; although
possible with field-based techniques, absence of volumes made it difficult.
One patient was
identified as an outlier (>2 standard deviations from the mean) but was included in the analysis.
The centre was informed.
Conclusion
POSNOC aimed
to treat the SCF and axillary regions, whereas ESTRO consensus guidelines
extend nodal inclusion to the caudal aspect of L4. As such, the POSNOC-defined
field borders failed to encompass L4 in its entirety.
A
field-based approach for left-sided axillary RT provided acceptable dose
coverage in terms of V80% for all combined nodal levels and for L2 and
L3, specifically. However, this technique resulted in under-coverage of L1 and
L4. In addition, excess normal tissue was irradiated cranially. V90%≥90% was often not met.
Field-based
planning for breast and axillary treatment may fail to fully encompass L1-4. A volume-based approach would provide a target for which parameters
such as field size, beam orientation, and energy could be optimised to achieve
adequate coverage of the nodal region.