Estimation of second cancer risk after radiotherapy for rectal cancer: comparison of 3D conformal radiotherapy and volumetric modulated arc therapy using different high dose fractionation schemes.
OBJECTIVE
To investigate second cancer risk (SCR) comparing volumetric modulated arc therapy (VMAT) and 3D conformal radiotherapy (3DCRT) with different high dose fractionation schemes.


METHODS
VMAT and 3DCRT virtual treatment plans for 25 patients previously treated with radiotherapy for rectal cancer were evaluated retrospectively. Doses prescribed were 25 × 1.8 Gy and 5 × 5 Gy, respectively. SCR was estimated using a carcinogenesis model and epidemiological data for carcinoma and sarcoma induction. SCR was determined by lifetime attributable risk (LAR).


RESULTS
Mean excess LAR was highest for organs adjacent to the PTV. Total LAR for VMAT and 3DCRT was 2.3-3.0 and 2.0-2.7 %, respectively. For 5 × 5 Gy, LAR was 1.4-1.9 % for VMAT and 1.2-1.6 % for 3DCRT. Organ-specific excess LAR was significantly higher for VMAT, and highest for bladder and colon. Size and shape of the PTV influenced SCR and was highest for age ≤ 40 years. For a patient with an additional lifetime risk of 60 years, LAR was 10 % for 25 × 1.8 Gy and 6 % for 5 × 5 Gy.


CONCLUSIONS
No statistically significant difference was detected in SCR using VMAT or 3DCRT. For bladder and colon, organ-specific excess LAR was statistically lower using 3DCRT, however the difference was small. Compared to epidemiological data, SCR was smaller when using a hypofractionated schedule. SCR was 2 % higher at normal life expectancy.


BACKGROUND
ClinicalTrials.gov Identifier NCT02572362 . Registered 4 October 2015. Retrospectively registered.
Department of Radiation Oncology, Kantonsspital Graubuenden, Chur, Switzerland. daniel.zwahlen@gmx.ch.|Department of Physics, University of Zurich, Zurich, Switzerland. daniel.zwahlen@gmx.ch.|Department of Radiation Oncology, Kantonsspital Graubuenden, Loestrasse 170, Chur, CH-7000, Switzerland. daniel.zwahlen@gmx.ch.
Daniel R
DR
Zwahlen
Department of Physics, University of Zurich, Zurich, Switzerland.
Laura I
LI
Bischoff
Institute of Radiotherapy, Klinik Hirslanden, Zurich, Switzerland.
Günther
G
Gruber
Institute of Radiotherapy, Klinik Hirslanden, Zurich, Switzerland.
Marcin
M
Sumila
Department of Physics, University of Zurich, Zurich, Switzerland.|Institute of Radiotherapy, Klinik Hirslanden, Zurich, Switzerland.
Uwe
U
Schneider
D000368:Aged; D000369:Aged, 80 and over; D019583:Dose Fractionation, Radiation; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009381:Neoplasms, Radiation-Induced / Q000453:epidemiology*; D016609:Neoplasms, Second Primary / Q000453:epidemiology; D058958:Organs at Risk / Q000528:radiation effects; D011880:Radiotherapy Planning, Computer-Assisted; D020266:Radiotherapy, Conformal / Q000009:adverse effects*; D050397:Radiotherapy, Intensity-Modulated / Q000009:adverse effects*; D012004:Rectal Neoplasms / Q000532:radiotherapy*; D012189:Retrospective Studies; D012306:Risk
Intensity-modulated radiotherapy; Radiotherapy; Rectal cancer; Second malignancies; Volumetric-modulated arc therapy