What is the T(pot) for prostate cancer? Radiobiological implications of the equivalent outcome with (125)I or (103)Pd

[Practical update of total dose compensation in case of temporary interruption of external radiotherapy in the COVID-19 pandemic context]

L’étalement est un facteur important de récidive locale et indirectement d’évolution à distance, notamment, en cas de durée de traitement allongée. La pandémie actuelle a un impact sur les patients en cours de radiothérapie qui doivent interrompre leur traitement de manière parfois prolongée du fait de la nécessité de soins respiratoires induits par le COVID-19. Les modèles utilisés de compensation de la dose totale en cas d’interruption prolongée de la radiothérapie sont connus, mais il nous a semblé important de synthétiser afin que chaque oncologue radiothérapeute puisse proposer un traitement le plus optimal possible tant en termes de risque de récidive locale que de protection des tissus sains. L’objectif de ce type de recommandation est d’homogénéiser les pratiques de l’ensemble de la discipline.

Single Course IMRT Plan to Deliver 45 Gy to Seminal Vesicles and 81 Gy to Prostate in 45 Fractions

We treat prostate and seminal vesicles (SV) to 45 Gy in 25 fractions (course 1) and boost prostate to 81 Gy in 20 more fractions (course 2) with Intensity Modulated Radiation Therapy (IMRT). This two-course IMRT with 45 fractions delivered a non-uniform dose to SV and required two plans and two QA procedures. We used Linear Quadratic (LQ) model to develop a single course IMRT plan to treat SV to a uniform dose, which has the same biological effective dose (BED) as that of 45 Gy in 25 fractions and prostate to 81 Gy, in 45 fractions. Single course IMRT plans were compared with two-course IMRT plans, retrospectively for 14 patients. With two-course IMRT, prescription to prostate and SV was 45 Gy in 25 fractions and to prostate only was 36 Gy in 20 fractions, at 1.8 Gy/fraction. With 45-fraction single course IMRT plan, prescription to prostate was 81 Gy and to SV was 52 or 56 Gy for a α/β of 1 and 3, respectively. 52 Gy delivered in 45 fractions has the same BED of 72 Gy3 as that of delivering 45 Gy in 25 fractions, and is called Matched Effective Dose (MED). LQ model was used to calculate the BED and MED to SV for α/β values of 1–10. Comparison between two-course and single course IMRT plans was in terms of MUs, dose-max, and dose volume constraints (DVC). DVC were: 95% PTV to be covered by at least 95% of prescription dose; and 70, 50, and 30% of bladder and rectum should not receive more than 40, 60, and 70% of 81 Gy. SV Volumes ranged from 2.9–30 cc. With two-course IMRT plans, mean dose to SV was non-uniform and varied between patients by 48% (54 to 80 Gy). With single-course IMRT plan, mean dose to SV was more uniform and varied between patients by only 9.6% (58.2 to 63.8 Gy), to deliver MED of 56 Gy for α/β − 1. Single course IMRT plan MUs were slightly larger than those for two-course IMRT plans, but within the range seen for two-course plans (549–959 MUs, n=51). Dose max for single-course plans were similar to two-course plans. Doses to PTV, rectum and bladder with single course plans were as per DVC and comparable to two-course plans. Single course IMRT plan reduces IMRT planning and QA time to half.

Status and prospects of percutaneous vertebroplasty combined with ¹²⁵I seed implantation for the treatment of spinal metastases

Metastatic spinal tumours are the most common type of bone metastasis. Various methods have been used to treat metastatic spinal lesions, including radiotherapy, chemotherapy, isotope therapy, bisphosphonate therapy, analgesics, and surgery. Conservative treatments such as radiotherapy and chemotherapy are not appropriate and usually are ineffective in patients with vertebral fractures and/or spinal instability. Minimally invasive surgical treatments using non-vascular interventional technology, such as percutaneous vertebroplasty (PVP), have been successfully performed in the clinical setting. PVP is a non-invasive procedure that creates small wounds and is usually associated with only minor complications. In the present study, we will review the clinical status and prospects for the use PVP combined with ¹²⁵I seed implantation (PVPI) to treat spinal metastases. The scientific evidence for this treatment, including safety, efficacy, and outcome measures, as well as comparisons with other therapies, was analysed in detail. PVPI effectively alleviates pain in metastatic spinal tumour patients, and the use of interstitial ¹²⁵I seed implants can enhance the clinical outcomes. In conclusion, PVPI is a safe, reliable, effective, and minimally invasive treatment. The techniques of PVP and ¹²⁵I seed implantation complement each other and strengthen the treatment’s effect, presenting a new alternative treatment for spinal metastases with potentially wide application.

Changes in radiobiological parameters in 131Cs permanent prostate implants

In prostate permanent implants using 131Cs seeds, the prostatic edema developed during the implantation procedure, increases the separation between the seeds. This leads to a decrease in the prostate coverage and thus causes an edema induced dose reduction, which results in an increase in tumour cell surviving fraction (SF) with a corresponding decrease in tumour control probability (TCP). To investigate the impact of edema on the SF and the TCP, the expression of the SF of the linear quadratic (LQ) model was extended to account for the effects of edema using the exponential nature of edema resolution and the dose delivered to the edematous prostate. The SF and the TCP for edematous prostate implants were calculated for 31 patients who underwent real time 131Cs permanent seed implantation. The dose delivered to the edematous prostate was calculated to compute the SF and the TCP for these patients for edema half lives (EHL) ranging from 4 days to 34 days and for edemas of magnitudes (M0) varying from 5 to 60% of the actual prostate volume.

A reduction in the dose delivered to the edematous prostate was found with the increase of EHL and edema magnitude which results in an increase of the SF, and corresponding decrease in the TCP. The dose reductions in 131Cs implants varied from 1.1% (for EHL = 4 days and M0 = 5%) to 32.3% (for EHL = 34 days and M0 = 60%). These are higher than the dose reduction in 125I implants, which vary from 0.3% (for EHL = 4 days and M0 = 5%) to 17.5% (for EHL = 34 days and M0 = 60%). As edema half life increased from 4 days to 34 days and edema magnitude increased from 5 to 60% the SF increased by 4.57 log, and the TCP decreased by 0.80. Compensation of edema induced increase in the SF and decrease in the TCP in 131Cs seed implants should be carefully done by redefining seed positions with the guidance of post-needle plans. The presented model in this study can be used to estimate the SF or the TCP for pre plan or real time permanent prostate implants using day 0 post-implant CT images.

Radiation therapy in prostate cancer

Adenocarcinoma of the prostate is one of the most frequently diagnosed cancers of men in the Western hemisphere and is second only to lung cancer for male cancer mortality. Most patients are diagnosed in the early/clinically localized stage, which can be treated curatively with radiation therapy alone. Innovative methods such as brachytherapy, three-dimensional conformal radiotherapy (3D-CRT), and IMRT (intensity modulated radiotherapy) are able to deliver very high tumoricidal doses to the diseased prostate, with minimal side effects to the surrounding tissue. Radiation therapy combined with hormonal treatment can be curative in locally advanced disease. Radiation therapy is also very effective in alleviating symptoms of metastatic prostate cancer (bone metastases, spinal cord compression, and bladder outlet obstruction).

External beam radiotherapy boosts to reduce the impact caused by edema in prostate permanent seed implants

In prostate permanent seed implants, it has been shown that edema caused by the surgical procedure decreases dose coverage and hence may reduce treatment efficacy. This reduction in treatment efficacy has been characterized by an increase in tumour cell survival, and biomathematical models have been developed to calculate the tumour cell survival increases in seed implanted prostates of different edema magnitudes and durations. External beam boosts can be utilized to neutralize the negative impact of edema so that originally desired treatment efficacy can be achieved. In this study, a linear quadratic model is used to determine fractionation sizes of the external beam boosts for both (125)I and (103)Pd seed implants. Calculations were performed for prostates of different edema magnitudes and durations, and for tumour cells of different repair rates and repopulation rates.

Prescription dose in permanent Cs131 seed prostate implants

Recently, 131Cs seeds have been introduced for prostate permanent seed implants. This type of seed has a relatively short half-life of 9.7 days and has its most prominent emitted photon energy peaks in the 29-34 keV region. Traditionally, 145 and 125 Gy have been prescribed for 125I and 103Pd seed prostate implants, respectively. Since both the half-life and dosimetry characteristics of 131Cs seed are quite different from those of 125I and 103Pd, the appropriate prescription dose for 131Cs seed prostate implant may well be different. This study was designed to use a linear quadratic radiobiological model to determine an appropriate dose prescription scheme for permanent 131Cs seed prostate implants. In this model, prostate edema was taken into consideration. Calculations were also performed for tumors of different doubling times and for other related radiobiological parameters of different values. As expected, the derived prescription dose values were dependent on type of tumors and types of edema. However, for prostate cancers in which tumor cells are relatively slow growing and are reported to have a mean potential doubling time of around 40 days, the appropriate prescription dose for permanent 131Cs seed prostate implants was determined to be: 127(+5)(-12)Gy if the experiences of 125I seed implants were followed and 121(+0)(-3)Gy if the experiences of 103Pd followed.

Biological effective dose for comparison and combination of external beam and low-dose rate interstitial brachytherapy prostate cancer treatment plans

We report a methodology for comparing and combining dose information from external beam radiotherapy (EBRT) and interstitial brachytherapy (IB) components of prostate cancer treatment using the biological effective dose (BED). On a prototype early-stage prostate cancer patient treated with EBRT and low-dose rate I-125 brachytherapy, a 3-dimensional dose distribution was calculated for each of the EBRT and IB portions of treatment. For each component of treatment, the BED was calculated on a point-by-point basis to produce a BED distribution. These individual BED distributions could then be summed for combined therapies. BED dose-volume histograms (DVHs) of the prostate, urethra, rectum, and bladder were produced and compared for various combinations of EBRT and IB. Transformation to BED enabled computation of the relative contribution of each modality to the prostate dose, as the relative weighting of EBRT and IB was varied. The BED-DVHs of the prostate and urethra demonstrated dramatically increased inhomogeneity with the introduction of even a small component of IB. However, increasing the IB portion relative to the EBRT component resulted in lower dose to the surrounding normal structures, as evidenced by the BED-DVHs of the bladder and rectum. Conformal EBRT and low-dose rate IB conventional dose distributions were successfully transformed to the common "language" of BED distributions for comparison and for merging prostate cancer radiation treatment plans. The results of this analysis can assist physicians in quantitatively determining the best combination and weighting of radiation treatment modalities for individual patients.

Clinical and physical quality assurance for intensity modulated radiotherapy of prostate cancer

The implementation of intensity modulated radiotherapy (IMRT) for patients with prostate cancer in daily routine has been elaborated at our department. Our quality assurance (QA) concept is one method to pave the way for initiating IMRT treatments for starting institutions. A clinical quality assurance (CQA) procedure has been set-up for all patients before and throughout the course of radiotherapy. Simultaneously medical physicists established a physical quality assurance (PQA) concept that has been followed for all patients as well. Alternative CQA and PQA procedures are discussed. The literature is reviewed and discussed with special respect to quality assurance in IMRT of prostate cancer patients.

Biological-effective versus conventional dose volume histograms correlated with late genitourinary and gastrointestinal toxicity after external beam radiotherapy for prostate cancer: a matched pair analysis

BACKGROUND

To determine whether the dose-volume histograms (DVH's) for the rectum and bladder constructed using biological-effective dose (BED-DVH's) better correlate with late gastrointestinal (GI) and genitourinary (GU) toxicity after treatment with external beam radiotherapy for prostate cancer than conventional DVH's (C-DVH's).

METHODS

The charts of 190 patients treated with external beam radiotherapy with a minimum follow-up of 2 years were reviewed. Six patients (3.2%) were found to have RTOG grade 3 GI toxicity, and similarly 6 patients (3.2%) were found to have RTOG grade 3 GU toxicity. Average late C-DVH's and BED-DVH's of the bladder and rectum were computed for these patients as well as for matched-pair control patients. For each matched pair the following measures of normalized difference in the DVH's were computed: (a) deltaAUC = (Area Under Curve [AUC] in grade 3 patient--AUC in grade 0 patient)/(AUC in grade 0 patient) and (b) deltaV60 = (Percent volume receiving = 60 Gy [V60] in grade 3 patient--V60 in grade 0 patient)/(V60 in grade 0 patient).

RESULTS

As expected, the grade 3 curve is to the right of and above the grade 0 curve for all four sets of average DVH's--suggesting that both the C-DVH and the BED-DVH can be used for predicting late toxicity. deltaAUC was higher for the BED-DVH's than for the C-DVH's--0.27 vs 0.23 (p = 0.036) for the rectum and 0.24 vs 0.20 (p = 0.065) for the bladder. deltaV60 was also higher for the BED-DVH's than for the C-DVH's--2.73 vs 1.49 for the rectum (p = 0.021) and 1.64 vs 0.71 (p = 0.021) for the bladder.

CONCLUSIONS

When considering well-established dosimetric endpoints used in evaluating treatment plans, BED-DVH's for the rectum and bladder correlate better with late toxicity than C-DVH's and should be considered when attempting to minimize late GI and GU toxicity after external beam radiotherapy for prostate cancer.

Practical methods for compensating for missed treatment days in radiotherapy, with particular reference to head and neck schedules

Unscheduled interruption of a radiotherapy treatment can lead to significant loss in local tumour control, particularly in tumours that repopulate rapidly. General guidelines for dealing with such treatment gaps have been issued by the Royal College of Radiologists and more specific advice on the use of compensation methods has been published previously [Hendry et al., Clin Oncol 1996;8:297-307; Slevin et al., Radiother Oncol 1992;24:215-220]. This article further elaborates on the practical application of these methods. It sets out the main considerations arising in the especially critical case of head and neck treatments and simple calculations are used to illustrate the approaches which may be adapted for particular situations. Radiobiological parameter values are suggested for use in the calculations, but these may require modification in the light of further research in this important area.

Biologically effective dose for permanent prostate brachytherapy taking into account postimplant edema

PURPOSE

To study the influence of radiobiologic and physical parameters and parameters related to edema on the biologically effective dose (BED) for permanent prostate implants and to determine the optimal timing of seed reconstruction for BED calculation.

METHODS AND MATERIALS

On the basis of the linear-quadratic model, an expression for the BED was derived, including the edema parameters. A set of parameter values was defined, and these parameter values were varied one at a time to examine the effect on the BED and the theoretically effective treatment time (t(eff)). A ratio epsilon was defined to investigate the optimal timing of seed reconstruction.

RESULTS

The maximal BED decreases when the extent of lethal damage is smaller, the potential tumor doubling time is smaller, the half-life time of the seeds is shorter, and the magnitude of prostate volume increase is larger. For 125I, the optimal timing of seed reconstruction is 25 days after implantation. Seed reconstruction 1 day after the implantation results in an underestimation of the BED of at most 43%, depending on the magnitude and half-life of edema. An overestimation of the BED of at most 22% is calculated when seed reconstruction took place at the effective treatment time.

CONCLUSION

The maximal BED depends strongly on the value of alpha, the potential tumor doubling time, and the choice of isotope. If prostate volume increase due to edema is not taken into account, the BED will be underestimated shortly after the implantation and overestimated if the calculations are based on images taken several months after implantation. The optimal timing of BED evaluation for 125I seed implants and typical prostate edema values is 25 days after implantation.

Edema-induced increase in tumour cell survival for 125I and 103Pd prostate permanent seed implants--a bio-mathematical model

Edema caused by the surgical procedure of prostate seed implantation expands the source-to-point distances within the prostate and hence decreases the dose coverage. The decrease of dose coverage results in an increase in tumour cell survival. To investigate the effects of edema on tumour cell survival, a bio-mathematical model of edema and the corresponding cell killing by continuous low dose rate irradiation (CLDRI) was developed so that tumour cell surviving fractions can be estimated in an edematous prostate for both 125I and 103Pd seed implants. The dynamic nature of edema and its resolution were modelled with an exponential function V(T) = V(p)(1 + M exp(-0.693T/ T(e))) where V(p) is the prostate volume before implantation, M is the edema magnitude and T(e) is edema half-life (EHL). The dose rate of a radioactive seed was calculated according to AAPM TG43, i.e. D = SkAg(r)phi(an)/r2, where r is the distance between a seed and a given point. The distance r is now a function of time because of edema. The g(r) was approximated as 1/r(0,4) and 1/r(0.8) for 125I and 103Pd, respectively. By expanding the mathematical expression of the resultant dose rate in a Taylor series of exponential functions of time, the dose rate was made equivalent to that produced from multiple fictitious radionuclides of different decay constants and strengths. The biologically effective dose (BED) for an edematous prostate implant was then calculated using a generalized Dale equation. The cell surviving fraction was computed as exp(-alphaBED), where alpha is the linear coefficient of the survival curve. The tumour cell survival was calculated for both 125I and 103Pd seed implants and for different tumour potential doubling time (TPDT) (from 5 days to 30 days) and for edemas of different magnitudes (from 0% to 95%) and edema half-lives (from 4 days to 30 days). Tumour cell survival increased with the increase of edema magnitude and EHL. For a typical edema of a half-life of 10 days and a magnitude of 50%. the edema increased tumour cell survival by about 1 and 2 orders of magnitude for 125I and 103Pd seed implants respectively. At the extreme (95% edema magnitude and an edema half-life of 30 days), the increase was more than 3 and 5 orders of magnitude for 125I and I03Pd seed implants respectively. The absolute increases were almost independent of TPDT and the prostate edema did not significantly change the effective treatment time. Tumour cell survival for prostate undergoing CLDRI using 125I or 103Pd seeds may be increased substantially due to the presence of edema caused by surgical trauma. This effect appears to be more pronounced for 103Pd than 125I because of the shorter half-life of 103Pd. If significant edema is observed post implantation, then a boost to the prostate using external beam radiotherapy may be considered as a part of the treatment strategy.

A theoretical investigation into the role of tumour radiosensitivity, clonogen repopulation, tumour shrinkage and radionuclide RBE in permanent brachytherapy implants of 125I and 103Pd

There is growing clinical interest in the use of 125I (half-life 59.4 days) and 103Pd (half-life 16.97 days) for permanent brachytherapy implants. These radionuclides pose interesting radiobiological challenges because, even with slowly growing tumours, significant tumour cell repopulation may occur during the long period taken to deliver the full radiation dose. This results in a considerable amount of the prescribed dose being wasted. There may also be changes in the tumour volume during treatment (due to oedema and/or shrinkage), thus altering the relative geometry of the implanted seeds and causing additional dose rate variations. This assessment examines the interaction between the above effects and additionally includes allowance for the influence of the relative biological effectiveness (RBE) of the radiations emitted by the two radionuclides. The results are presented in terms of the biologically effective doses (BEDs) and likely tumour control probabilities (TCPs) associated with the various parameter combinations. The overall BED enhancement due to the RBE effect is shown always to be greater than the RBE itself and is greatest in tumours which are radio-resistive and/or fast growing. The biological dose uncertainties are found to be less with 103Pd and the TCPs associated with this radionuclide are expected to be significantly higher in the treatment of some 'difficult' tumours. Using typically prescribed doses 125i appears to be better for treating radiosensitive tumours with long doubling times and which shrink fairly rapidly. However, unless 125I doses are reduced, this advantage may well be offset by the greatly enhanced biological doses delivered to adjacent normal structures.

Is alpha/beta for prostate tumors really low?

PURPOSE

Brenner and Hall's 1999 paper estimating an alpha/beta value of 1.5 Gy for prostate tumors has stimulated much interest in the question of whether this ratio (of intrinsic radiosensitivity to repair capacity) is much lower in prostate tumors than in other types of tumors that proliferate faster. The implications for possibly treating prostatic cancer using fewer and larger fractions are important. In this paper we review updated clinical data and present somewhat different calculations to estimate alpha/beta.

METHODS AND MATERIALS

Seventeen clinical papers published from 1995 to 2000 were reviewed to obtain estimates of biochemical control from radiotherapy alone using external beam, I-125 implants, or Pd-103 implants. The focus was on intermediate risk patients. Three methods of estimating alpha/beta were employed. First, a simple two-step graphical comparison of isoeffective doses from external beam and implant modalities was made, to see which value of alpha/beta predicted the observed identity of biologic effect. Second, the same data were subjected to Direct Analysis (maximum likelihood estimation), from which an estimate of alpha/beta and also of the T(12) of repair of sublethal damage in the tumors (both with confidence intervals) were obtained. Third, preliminary clinical data comparing two different sizes of high-dose boost doses were analyzed in which significantly different bNED was observed at 2 years.

RESULTS

The second method gave the definitive result of alpha/beta = 1.49 Gy (95% CI 1.25-1.76) and T(12) = 1.90 h (95% CI 1.42-2.86 h). The first method gave a range from 1.4 to 1.9 Gy and showed that if mean or median dose were used instead of prescribed dose, the estimate of alpha/beta would be substantially below 1 Gy. The third method, although based on early follow-up, was consistent with low values of alpha/beta in the region of 2 Gy or below. The estimate for T(12) is the first value reported for prostate tumors in situ.

CONCLUSIONS

All the estimates point toward low values of alpha/beta, at least as low as the estimates of Brenner and Hall, and possibly lower than the expected values of about 3 Gy for late complications. Hypofractionation trials for intermediate-risk prostatic cancer appear to be indicated.

Radiation for prostate cancer

The balance between tumour control and normal tissue damage with conventional radiotherapy is critical to outcome and morbidity in the treatment of localised prostate cancer. Recent technological advances have allowed a reduction in the amount of normal tissue included in target treatment volumes. This reduces morbidity and allows dose escalation, theoretically increasing the likelihood of tumour control. The methods used to achieve dose escalation are discussed and the available evidence for their safety and efficacy, relative to conventional treatment, is reviewed. Although there are no randomised studies to provide evidence of increased survival, the available evidence supports the hypothesis that dose escalation produces survival rates equivalent to surgical series and provides a realistic choice for patients.