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Conclusion
The CTV and/or surrogate target structures in recurrent
rectal cancer are visible on all CBCTs from our on-board
imaging system, enabling volumetric image-guided
adaptive strategies. A 5 mm margin was found to be
sufficient to account for the deformations of the target in
the majority of treatment fractions; there is therefore a
considerable potential for reduction of the treated
(normal tissue) volume compared to current wide margins.
PV-0134 Isotoxic stereotactic radiotherapy for central
pelvic recurrence in gynecological cancer
M. Llewelyn
1
, A. Taylor
1
1
The Institute of Cancer Research and The Royal Marsden
NHS Foundation Trust, Clinical Oncology, Sutton, United
Kingdom
Purpose or Objective
Radical radiotherapy is the treatment of choice for central
pelvic recurrence in gynaecological cancer. Following
whole pelvic radiotherapy, a high dose boost is given to
macroscopic disease. When brachytherapy is not feasible,
local control with EBRT alone is only 30-50%. Stereotactic
radiotherapy offers potential for dose escalation to
improve outcomes. Cumulative OAR dose tolerances are
internationally
established
for
intrauterine
brachytherapy, and similar principles can be applied with
SBRT. This can be delivered with Cyberknife or linear
accelerator VMAT, and the GTV-PTV margin depends on
whether real-time motion tracking of tumour is utilised.
The aims were to compare dose escalation options with
the two stereotactic techniques and the impact of variable
GTV-PTV margins.
Material and Methods
The scans of 10 patients with central pelvic recurrence
were used for comparison of techniques, delivering EBRT
45 Gy in 25 fractions to pelvis followed by a SBRT boost.
Cumulative dose limits for bowel, bladder and rectum
were developed using GEC-ESTRO guidelines. Cyberknife
and VMAT SBRT plans were produced: initially 20 Gy in 5
fractions with GTV-PTV margins of 3mm, 5mm and 7mm.
Plans were normalized for 95% coverage by prescription
isodose and Dmax 125-140%. Dose was then escalated or
de-escalated in 2.5 Gy increments until the OAR dose
limits were exceeded. The highest dose level meeting
OAR criteria was compared between techniques for each
GTV-PTV margin, with assessment of boost dose and total
cumulative dose including the phase one EBRT (EQD2-10).
Results
With 20 Gy in 5 fractions and 5mm margin, mean GTV dose
with Cyberknife was 23.0 Gy (total 72 Gy) and VMAT-SBRT
24.3 Gy (74.1 Gy). Conformity index was 1.1 vs 1.2 and
dose drop off Cyberknife 5.0, VMAT 4.5. Using isotoxic
planning to OAR tolerances, CK 3mm was 23.8 Gy (total
73.3 Gy), VMAT 3mm using equivalent prescription 25.7 Gy
(76.5 Gy) and highest deliverable with VMAT 3mm 28.7 Gy
(81.6 Gy). With 5mm margin, CK 21.6 Gy (69.8 Gy), VMAT
5mm equivalent dose 22.6 Gy (71.3 Gy) and highest
deliverable 26.1 Gy (77.2 Gy), while a 7mm margin with
VMAT is 24.0 Gy (73.6 Gy).
Conclusion
SBRT can significantly increase total dose to GTV
compared to conventional radiotherapy techniques. With
an isotoxic approach VMAT-SBRT can deliver higher doses
to GTV than Cyberknife, even when a larger GTV-PTV
margin is used to allow for the lack of real time tracking.
We plan to proceed with a clinical trial to evaluate long-
term outcomes.
PV-0135 Short tangential arcs in VMAT based breast and
chest wall radiotherapy planning
A. Munshi
1
, B. Sarkar
1
, S. Roy
1
, T. Ganesh
1
, B.K. Mohanti
1
1
Fortis Memorial Research institute, Radiation Oncology,
Haryana, India
Purpose or Objective
The study aimed to analyze partial tangential arc
Volumetric Modulated Arc Therapy (VMAT) treatment
planning and delivery, including analyzing the cardiac and
contralateral breast doses resulting from this technique.
Material and Methods
A total of 104 consecutively treated breast cancer
(conservation as well as mastectomy) patients were taken
for this dosimetric study. All patients were planned using
partial arc volumetric modulated arc therapy (VMAT) in
the Monaco treatment planning system (TPS) using two
partial arc beams. All patients were divided into seven
different categories: 1) All the patients in the study
(OVERALL), 2) Left sided whole breast and chest wall
patients (LWBCW), 3) Left chest wall patients (LCW), 4)
Left whole breast patients (LWB), 5) Right sided whole
breast and chest wall patients (RWBCW), 6) Right chest
wall (RCW) patients, and 7) Right whole breast (RWB)
patients. We evaluated each treatment plan for PTV
coverage and doses to OARs. SPSSversion 16.0 software
was used for statistical analysis.
Results
There were 62 left sided and 42 right sided breast cancer
patients in the overall analysis. The percentage of PTV
volume receiving 95% of the prescription dose (PTV V95%,
mean±SD) varied in the range of 91.2±5.2% to 94.8±2.1
with mean dose of 92.4±5.2% for all cases. The (mean ±SD)
cardiac dose for all the patients was 289±23 cGy. The
(mean±SD) cardiac doses were higher for left sided
patients (424±33.8 cGy) as compared to right sided
patients (123.9 ± 80 cGy) (p<0.001). Cardiac mean doses
were higher with arc angles> 30 degrees versus 30
degrees (324.5±247.1 cGy versus 234.4±188.4 cGy) (p=
0.001). Similarly contralateral breast mean dose was
higher with arc angles > 30 degrees versus 30 degrees
(126±115 cGy vs 88.6±76.1 cGy) (p =0.001). However
cardiac V20Gy, V30Gy and V40Gy did not exhibit any
statistical difference between the two groups (p= 0.26,
0.057 and 0.054 respectively).
Conclusion
This is the first large study of its kind that assesses the
dosimetric outcome of tangential partial arc VMAT
treatments in a large group of mastectomy and breast
conservation patients. Our study demonstrates the
efficacy of this technique in dose coverage of PTV as well
as in minimizing dose to OARs. Further, based on our
results, we conclude that the arc length for the bi-
tangential arcs should be 30
⁰
since it helps to achieve the
most optimal balance between target coverage and
acceptable OAR doses.
PV-0136 Linear energy transfer in normal tissues in
spot scanning proton therapy of pro state cancer
J. Pedersen
1
, J. BB Petersen
1
, C. H. Stokkevåg
2
, K. S.
Ytre-Hauge
3
, O. Casares -Magaz
1
, N. Mendenhall
4
, L. P.
Muren
1
1
Aarhus University Hospital, Department of Medical
Physics, Aarhus C, Denmark
2
Haukeland University Hospital, Department of Oncology
and Medical Physics, Bergen, Norway
3
University of Bergen, Department of Physics and
Technology, Bergen, Norway
4
University of Florida Proton Therapy Institute,
Department of Radiation Oncology, Gainesville- FL, USA