Radiobiological effectiveness and its role in modelling secondary cancer risk for proton therapy
Madkhali, A . 2016
Radiobiological effectiveness and its role in modelling secondary cancer risk for proton therapy
Purpose/Objective(s):
In proton therapy, a radiobiological effectiveness ratio (RBE) of 1.1 (RBE1.1) is often used. In reality, RBE depends on dose, linear energy transfer (LET), biological end point, and tissue type. Using a value of RBE that may be not accurate may affect dose calculation and hence, outcome.
Materials/Methods:
We used an in-house built code for modelling malignant induction probability (MIP) from voxel-by-voxel dose map (Timlin 2014) and implement a published model to calculate structure-specific RBE, recalculate dose and MIP, and compare the outcomes with initial calculations using RBE1.1. MIP was calculated using linear quadratic (LQ), linear (LIN), and linear-no-threshold (LNT) models for proton therapy plans for an adult and a teenage patient diagnosed with medulloblastoma (MB). The MIP was then re-calculated using the RBE model by Dale and Jones which is a function of dose (d), α and β and RBEmin and RBEmax:
RBE=(-α+√(α^2+4βd(〖RBE〗_max α+〖RBE〗_(min )^2 βd)))/2βd
Results:
Results are shown in Table 1. The difference in MIP by using RBE1.1 and RBEMinMax is ~2-3%. The effect on mean dose varies between different organs and is between 6% and 8%. Clinical implications due to difference in RBE depend on beam characteristics, dose, structures concerned, and the volume irradiate
| Medulloblastoma - Adult | |||
| Model | RBE1.1 | RBEMinMax | RBE1.1:RBEMinMax |
| LQ | 0.099 | 0.097 | 1.03 |
| LIN | 0.078 | 0.076 | 1.03 |
| LNT | 0.643 | 0.655 | 0.98 |
| (a) | |||
| Medulloblastoma -Teen | |||
| Model | RBE1.1 | RBEMinMax | RBE1.1:RBEMinMax |
| LQ | 0.068 | 0.066 | 1.02 |
| LIN | 0.057 | 0.056 | 1.02 |
| LNT | 0.554 | 0.567 | 0.98 |
| (b) | |||
| Mean Dose (Gy) - Adult | |||
| Name | RBE1.1 | RBEMinMax | RBE1.1:RBEMinMax |
| Right lung | 1.41 | 1.50 | 0.94 |
| Left lung | 1.36 | 1.45 | 0.94 |
| Nasopharynx | 4.16 | 4.48 | 0.93 |
| Right kidney | 0.54 | 0.58 | 0.94 |
| Left kidney | 1.10 | 1.16 | 0.94 |
| Left parotid | 4.44 | 4.75 | 0.94 |
| Right parotid | 2.15 | 2.30 | 0.93 |
| Thyroid | 0.19 | 0.20 | 0.92 |
| Oral cavity | 0.04 | 0.04 | 0.92 |
(c)
Table 1: Values of whole body MIP using RBE of 1.1 (RBE1.1 )and RBE using the described model (RBEMinMax ) and the relationship between them for the adult (a) and the teenage patient (b). (c) Mean dose for selected structures from the adult patient’s plan using RBE1.1 and RBEMinMax and the relation between them.
Conclusion:
Using RBE1.1 makes proton therapy dose and dose-dependent predictions less accurate. Our results using a RBE calculation model show that decreased accuracy may have clinical implications, which agrees with published literature (Jones 2012; Jones, 2014), and may affect secondary cancer risk and normal tissue complication probability calculations as well.
Radiobiological effectiveness and its role in modelling secondary cancer risk for proton therapy
Purpose/Objective(s):