Prostate Stereotactic Ablative Radiation Therapy Using Volumetric Modulated Arc Therapy to Dominant Intraprostatic Lesions

Abstract

PurposeTo investigate boosting dominant intraprostatic lesions (DILs) in the context of stereotactic ablative radiation therapy (SABR) and to examine the impact on tumor control probability (TCP) and normal tissue complication probability (NTCP).Methods and MaterialsTen prostate datasets were selected. DILs were defined using T2-weighted, dynamic contrast-enhanced and diffusion-weighted magnetic resonance imaging. Four plans were produced for each dataset: (1) no boost to DILs; (2) boost to DILs, no seminal vesicles in prescription; (3) boost to DILs, proximal seminal vesicles (proxSV) prescribed intermediate dose; and (4) boost to DILs, proxSV prescribed higher dose. The prostate planning target volume (PTV) prescription was 42.7 Gy in 7 fractions. DILs were initially prescribed 115% of the PTVProstate prescription, and PTVDIL prescriptions were increased in 5% increments until organ-at-risk constraints were reached. TCP and NTCP calculations used the LQ-Poisson Marsden, and Lyman-Kutcher-Burman models respectively.ResultsWhen treating the prostate alone, the median PTVDIL prescription was 125% (range: 110%-140%) of the PTVProstate prescription. Median PTVDIL D50% was 55.1 Gy (range: 49.6-62.6 Gy). The same PTVDIL prescriptions and similar PTVDIL median doses were possible when including the proxSV within the prescription. TCP depended on prostate α/β ratio and was highest with an α/β ratio = 1.5 Gy, where the additional TCP benefit of DIL boosting was least. Rectal NTCP increased with DIL boosting and was considered unacceptably high in 5 cases, which, when replanned with an emphasis on reducing maximum dose to 0.5 cm3 of rectum (Dmax0.5cc), as well as meeting existing constraints, resulted in considerable rectal NTCP reductions.ConclusionsBoosting DILs in the context of SABR is technically feasible but should be approached with caution. If this therapy is adopted, strict rectal constraints are required including Dmax0.5cc. If the α/β ratio of prostate cancer is 1.5 Gy or less, then high TCP and low NTCP can be achieved by prescribing SABR to the whole prostate, without the need for DIL boosting.

A validated tumor control probability model based on a meta‐analysis of low, intermediate, and high‐risk prostate cancer patients treated by photon, proton, or carbon‐ion radiotherapy

“…This has led to the development of dose painting approaches [66][67][68][69] and consequently TCP models that require this feature. 70,71 Other TCP models that do not account for this feature (whilst perfectly appropriate for the prediction of uniform doses) are intrinsically lacking in the context of dose and/or LET painting scenarios, 72,73 which will likely become more prevalent in the future. It should be noted that the assumption of clongenic densities (P CTV and P DIL ) influences the model fit and that the model fit cannot determine the distribution of clongenic densities.…”

Efficacy of a rectal spacer with prostate SABR—first UK experience

“…Hutchinson et al evaluated the 10-year costs associated with rectal toxicity complications across different RT modalities and found that spacer use was immediately cost-effective for high dose (50 Gy) SABR [34]. This recommendation will be of particular relevance to future dose-painting trials where radiation boosts to the dominant intra-prostatic lesions (DILs) may be limited by rectum dose constraints [35,36], with UK radiotherapy centres keenly awaiting NICE guidance for rectal spacer use in prostate radiotherapy [37].…”

Volumetric Modulated Arc Therapy Planning for Primary Prostate Cancer With Selective Intraprostatic Boost Determined by 18F-Choline PET/CT

“…TCP was calculated via the LQ-Poisson Marsden Model using DVH data for IDL suv60% , IDL suv70% , and non-IDL prostate (i.e., GTV prostate - IDL suv60% ) [25-27]. NCTPs for the rectum, bladder, and femoral heads were calculated using the Lyman-Kutcher-Burman formula with Nirmierko’s equivalent uniform dose (EUD) [28-32].…”