Dose integration method of intensity-modulated arc therapy and HDR brachytherapy of prostate cancer
PO-1494
Abstract
Dose integration method of intensity-modulated arc therapy and HDR brachytherapy of prostate cancer
Authors: Fröhlich|, Georgina(1)*[turul49@yahoo.com];Ágoston|, Péter(1);Jorgo|, Kliton(1);Polgár|, Csaba(1);Major|, Tibor(1);
(1)National Institute of Oncology, Centre of Radiotherapy, Budapest, Hungary;
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Purpose or Objective
To validate an alternative method for summing biologically effective doses of intensity-modulated-arc-therapy (IMAT) with interstitial HDR brachytherapy (BT) boost in prostate cancer.
Material and Methods
Fifteen intermediate or high-risk prostate cancer patients treated with IMAT and interstitial HDR BT boost were included in the study and additional plans using IMAT boost instead of BT were created. The prescribed dose was 2/44 Gy to the whole pelvis, 2/60 Gy to the prostate and vesicle seminals and 1x10 Gy BT or 2/18 Gy IMAT to the prostate gland. As the critical organs receive the maximal total dose in the region where the dose maximum is in BT, the teletherapy planning CT was registered with the US of the BT for every patient, and the most exposed volume of critical organs in BT were identified on these CT images. The minimal dose of these from IMAT was summed with their BT dose, and these biological total doses (EQD2) were compared using BT vs. IMAT boost with Wilcoxon-matched pairs test. This method was compared with the conventional uniform dose conception (UDC).
Results
The EQD2 D90 of the prostate was significantly higher with BT than with IMAT boost: 99.3 Gy vs. 77.9 Gy, p=0.0034. The D2 to rectum, bladder and hips were lower with BT, than with TT boost 50.3 Gy vs. 76.8 Gy (p=0.0117), 73.1 Gy vs. 78.3 Gy (p=0.1614) and 41.9 Gy vs. 50.6 Gy (p=0.0044), while D0.1 to urethra was higher, 96.1 Gy vs. 79.3 Gy (p=0.0180), respectively.
| EQD2 | TT + BT boost | TT + TT boost | *p-value |
| D90 (Gy) | 99.3 (96.8-101.9) | 77.9 (76.4-78.5) | 0.0034 |
| D2(rectum) (Gy) | 50.3 (29.8-65.8) | 76.8 (65.8-79.3) | 0.0017 |
| D0.1(urethra) (Gy) | 96.1 (95.5-96.9) | 79.3 (78.6-80.4) | 0.0180 |
| D2(bladder) (Gy) | 73.1 (46.0-140.5) | 78.3 (77.2-79.8) | 0.1614 |
| D2(hips) (Gy) | 41.9 (33.5-58.3) | 50.6 (43.6-58.1) | 0.0044 |
Table. The EQD2 total doses of intensity-modulated arc therapy with interstitial HDR BT boost (TT + BT boost) and intensity-modulated arc therapy with teletherapy boost (TT + TT boost). D90: the minimum dose delivered to 90% of the prostate (Gy), D2(rectum), D2(bladder), D2(hips): the minimal dose of the most exposed 2 ccm of the rectum, the bladder and the hips (Gy), D0.1(urethra): the minimal dose of the most exposed 0.1 ccm of the urethra (Gy). *Wilcoxon-matched pairs test.
UDC overestimates D2(rectum) by 37% (p=0.0117) and underestimates D0.1(urethra) by 1% (p=0.0277) and D2(bladder) by 7% (p=0.0614).
Conclusion
Based on our individual biological dose summation method, total dose to the prostate is higher with BT vs. IMAT boost. BT boost yields lower rectum, bladder and hip doses, but higher dose to the urethra. UDC overestimates rectum dose and underestimates the dose to the urethra and to the bladder. The potential advantage of our dose integration method is that it takes into account the most exposed part of the OARs and thus sparing these parts from higher doses in TT (Figure).