Room: Exhibit Hall | Forum 3
Purpose: To evaluated the appropriateness of full parameter addition (FPA) based methods for computing EQD2 for DVH parameters of HDR ICBT/ISBT and EBRT dose composite.
Methods: Radiotherapy plans of 10 patients, who received 27.5-30Gy intracavitary brachytherapy (ICBT) in 5 fractions, 45Gy whole pelvis EBRT and 5.4-18Gy boost EBRT, were retrospectively evaluated. Split-ring &Tandem applicator was used while one interstitial needle was inserted for three patients. CT of the third ICBT was selected as the reference image set. EBRT and ICBT doses were deformed by registering the CT of each ICBT fraction and EBRT planning CT to the reference, then scaled to EQD2 and combined to create the dose composite (bDDC_all). Two FPA based methods, hWSA_wp and bWSA_abs, were used for EQD2 dose composite computation using equations from the previous and current ABS worksheets, respectively. HR-CTV D90, OAR D2cc and D0.1cc were calculated using hWSA_wp, bWSA_abs and bDDC_all methods. Different EBRT boost dose contribution factors were evaluated for EQD2 equations applied in the current ABS worksheet.
Results: The differences in D2cc of accumulated doses for rectum, bladder, sigmoid and bowel between hWSA_wp and bDDC_all were 0.44Â±0.92 GyÎ±/Î²=3, 0.11Â±0.94 GyÎ±/Î²=3, -0.64Â±1.35 GyÎ±/Î²=3, and -1.61Â±0.73 GyÎ±/Î²=3, respectively. Differences in DVH parameter values among bDDC_all and FPA methods were not statistically significant (p>0.05). By using 0.25 in EQD2 equations, difference in HR-CTV between bWSA_abs and bDDC_all reduced to 0.1% (0.13Gy) while difference in OAR D2cc and D0.1cc reduced from 2.12 Gy to 1.13 Gy and 3.09 Gy to 1.78 Gy, respectively.
Conclusion: Compared to FPA methods, bDDCall demonstrated lower OAR D2cc and D0.1cc, however the differences were not statistically significant. Current ABS recommended FPA based worksheet can serve as an acceptable plan evaluation tool for clinical purposes. DVH parameters evaluation tends to be more accurate by using the factor of 0.25 for EBRT boost.